[From the U.S. Government Printing Office, www.gpo.gov]
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H BEST MANAGEMENT PRACTICES
H DESIGN GUIDANCE MANUAL
I                     FOR HAMPTON ROADS

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               HAMPTON ROADS PLANNING DISTRICT COMMISSION
                          DECEMBER 1991


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   HAMPTON ROADS PLANNING DISTRICT COMMISSION

CHESAPEAKE                                 POQUOSON
ï¿½ ROBERT G. BAGLEY                          L. CORNELL BURCHER
 DR. ALAN P. KRASNOFF                     , ROBERT M. MURPHY
 JAMES W. REIN
                                           PORTSMOUTH
FRANKLIN                                     JOHNNY M. CLEMONS
 ROBERT E. HARRELL                        ) V. WAYNE ORTON
ï¿½ JOHN J. JACKSON                           GLORIA O. WEBB

HAMPTON                                   SOUTHAMPTON COUNTY
 T. MELVIN BUTLER                         p ROWLAND L. TAYLOR
 JAMES L. EASON                             C. HARRELL TURNER
I ROBERT J.O'NEILL, JR
                                           SUFFOLK
ISLE OF WIGHT COUNTY                        S. CHRIS JONES
p MYLES E. STANDISH                       p RICHARD L. HEDRICK
 A. O. SPADY
                                           VIRGINIA BEACH
JAMES CITY COUNTY                            JOHN A. BAUM
 DAVID B. NORMAN                            ROBERT E. FENTRESS
 DAVID L. SISK                              HAROLD HEISCHOBER
                                             JAMES K. SPORE
NEWPORT NEWS                                WALTER E. MATHER
 JOE S. FRANK                             k REBA S. MCCLANAN
 DR. VINCENT T. JOSEPH                      MEYERA E. OBERNDORF
 EDGAR E. MARONEY
                                           WILLIAMSBURG
NORFOLK                                    o JACKSON C. TUTTLE, II
 MASON C. ANDREWS, M.D.                     JOHN HODGES
  PAUL D. FRAIM
 JOSEPH A. LEAFE                          YORK COUNTY
  JAMES B. OLIVER, JR.                       PAUL W. GARMAN
  G. CONOLY PHILLIPS                       , DANIEL M. STUCK

    ,EXECUTIVE COMMITTEE MEMBER

                             PROJECT STAFF

    ARTHUR L. COLLINS                      EXECUTIVE DIRECTOR/SECRETARY

    JOHN M. CARLOCK                        DIRECTOROF PHYSICALAND
                                             ENVIRONMENTAL PLANNING
    WILLIAM P. WICKHAM                     PHYSICAL PLANNER, (1990-1991)
    TODD A. GRISSOM                        PHYSICAL PLANNER
    JERYL G. ROSE                          PHYSICAL PLANNER

    JOYCE M. COOK                          DIRECTOR, SECRETARIAL SERVICES
    DANA F. SHEARER                        WORD PROCESSING OPERATOR

    ROBERT C. JACOBS                       DIRECTOR OF GRAPHIC AND
                                             PRINTING SERVICES
    JOSEPH MARHEFKA, JR.                   GRAPHICS TECHNICIAN
    JEANNE L. MUNDEN                       GRAPHICS TECHNICIAN
    MICHAEL R. LONG                        GRAPHICS TECHNICIAN
    RACHAEL V. PATCHETT                    REPROGRAPHIC SUPERVISOR


              Hampton Roads Planning District Commission
                       The Regional Building
                        723 Woodlake Drive
                     Chesapeake, Virginia 23320
                          (804) 420-8300










 HAM PTON   ROADS  REBA S. McCLANAN, CHAIRMAN * ROBERT M. MURPHY. VICE CHAIRMAN * ROBERT G. BAGLEY, TREASURER
 PLANNING DISTRICT COMMISSION                                                     ARTHUR L. COLLINS, EXECUTIVE DIRECTOR/SECRETARY
*                                                                                                  February 18, 1992
         CHESAPEAKE
    RobertG. Bagley. City Councilman  Chief Administrative Officers
   Dr. AlanP. Krasnoff, CityCouncilman  Cities, Counties and Towns
      James W. Rein, City Manager  Hampton Roads, Virginia
                               H a mpton Roadas, Virgini a
          FRANKLIN
   Robert E. Harrell, City Councilman
     John J. Jackson, City Manager                                          Re:   Regional BMP Design Manual

          HAMPTON
      T. Melvin Butler, Vice Mayor
       James L. Eason, Mayor
    .o..rtJ. o'..,,,.Jr.,City Manger            , 
            RoertJ.ONeil.Jr.CiyManager  Enclosed for your use is one  (1) copy of the  Best Manaaement
      ISLE OF WIGHT COUNTY     Practices Desian Guidance Manual  for Hampton  Roads,  prepared  with
    O.A.Spady.BoardotSupervisors  consultant assistance by the Hampton Roads Planning District Commission,
   MylesE Standish,CountyAdministrator  in cooperation with the staffs of the region's fourteen cities and counties
       JAMES CITY COUNTY       and the Town of Smithfield. Completion of the Manual was made possible
   David B. Norman. CountyAdministrator  by the cooperation and assistance of your staff involved in the Chesapeake
    David L Sisk. Board oSupevisors  Bay and Stormwater Management Programs.

         NEWPORT NEWS
                NEWPORTNEWS           The  Manual,  which  provides guidance  on  the  design  of  Best
     Joe S. Frank, City Councilman
   Dr. Vincenlt T. Joseph, CityCouncilman  Management  Practices for stormwater management, is divided into two
     Edgar E. Maroney, City Manager  parts. Part I focuses on a wide variety of practices that are suitable for small
                NORFOLK        sites, especially single lot residential sites. Part II focuses on larger sites and
 Mason C. Andrews, M.D., City Councilman  emphasizes detention and  retention facilities and the incorporation of
     Paul D. FraimCity Councilman  wetlands into stormwater management  facilities.  Both  Parts include
       Joseph A. Leafe, Mayor
     JaSa. olierJ..Cty Maor    discussion of maintenance requirements, other operational considerations,
    G.Conoly Phillips, City Councilman  facility construction and operational costs and life expectancy.
          POQUOSON
       L Cornell Bucher, Mayor        The Manual is one element of the Commission's regional stormwater
     Robert M. Murphy. City Manager  management program.  It is designed to assist the local governments in
               PORTMOT complying with the requirements of the Chesapeake Bay Preservation Act,
     JIMoUhn    Mtas well as the requirements of the Virginia Stormwater Management Act
   Johnny M. Clemons. City Councilman
     V. Wayne Orlon, City Manager  and the federal Stormwater NPDES Regulations. The Manual is intended to
       Gloria 0. Webb, Mayor   supplement, not replace, existing local government criteria and standards
  |   SOUTHAMPTONCOUNTY         for design, installation and  maintenance of stormwater management
  RowlandL. Taylor, County Administrator  facilities and Best Management Practices.
   C. Harrell Turner, Board of Supervisors
                                     The staff of the Hampton Roads Planning District Commission hopes
           SUFFOLK  tha   t           you and your staffs will find the Best Manaqcement Practices DesicIn
    Richard L. Hedlrick, City Manager
     S. Chris Jones, City Councilman  Guidance Manual for Hampton Roads to be useful in implementing your
                               local stormwater management  program.   If you  have any questions or
        VIRGINIA BEACH         concerns, please do not hesitate to call./    )
     John A. Baum, City Councilman
     Robert E. Fentress, Vice Mayor
   Harold Heischober., City Councilman /ncere 
   Walter E. Mather. Citizen Appointee
   Reba S. McClanan, City Councilwoman
      Mayera E Oberndorf, Mayor
     James K. Spore. City Manager
                                                                          Arthur L. Co  ins
          WILLIAMSBURG                                                       Executive Drector/Secretary
        John Hodges, Mayor
    Jackson C. Tuttle, 11, City Manager
                               JMC:dfs
         YORK COUNTY            Enclosure
   Paul W. Geranman, Board of Supervisors
   Daniel M. Stuck, County Administrator
                                        HEADQUARTERS * THE REGIONAL BUILDING * 723 WOODLAKE DRIVE * CHESAPEAKE. VIRGINIA 23320 * (804) 420-8300
                                         PENINSULA OFFICE * HARBOUR CENTRE, 2 EATON STREET * SUITE 502 * HAMPTON, VIRGINIA 23669 * (804) 728-2067













                  BEST MANAGEMENT PRACTICES

                    DESIGN GUIDANCE MANUAL

                              FOR

                    HAMPTON ROADS VIRGINIA





This report was produced, in part, through financial support from the
Chesapeake Bay Local Assistance Department pursuant to Contract No. 91-42
of August 1990 and Unnumbered Contract of June 20, 1990 and from the
Virginia Council on the Environment pursuant to Coastal Resources
Management Program Grant No. NA90AA- H-CZ796 from the National Oceanic
and Atmospheric Administration.



Preparation of this report was included in the HRPDC Work Program for FY
1990-91, approved by the Commission at its Executive Committee Meeting of
March 21, 1990 and in the HRPDC Work Program for FY 1991-92, approved by
the Commission at its Executive Committee Meeting of March 20, 1991.



                     U  D. S. DEPARTMENT OF COMMERCE NOAA
                     CA,,STAL SERVICES CENTER
                     2234 SOUTH HOBSON AVENUE
                     CHARLESTON, SC 29405-2413





                     Prepared by the Staff of the
             Hampton Roads Planning District Commission

                          December 1991


                      Property of CSC Library








       I                            ~~~~~~ACKNOWLEDGEMENTS

             From its inception, this project has entailed extensive involvement by staff
I     ~    ~representatives of the participating local jurisdictions, including the members of the
        Hampton Roads Chesapeake Bay Committee (Local CBPA Staff Contacts) and
        supporting staff from other local Departments. Staff from the Chesapeake Bay Local
        Assistance Department and from the Virginia Council on the Environment also
        participated heavily in both the technical and administrative aspects of the project.
        Their participation has been instrumental in the successful completion of this
        document.

3       ~CHESAPEAKE

              Brent Neilson, Director, Department of Planning
 I      *   ~~Lee Dydiw, Planner, Department of Planning
              D. Ray Stout, City Engineer, Department of Public Works
              Joan Fowler, P.E., Engineer, Department of Public Works

         HAMPTON

 3      *   ~~Patricia Thomas, Senior City Planner, Department of Planning
              Mike Hodges, Senior Civil Engineer, Department of Public
              Works

         ISLE OF WIGHT COUNTY

 3          ~~~W. Douglas Caskey, Assistant County Administrator/Community Development
           * William L. McKay, County Planner (1990)
           * R. Bryan David, County Planner
 I         ~ ~~Mary Ann Welton, Environmental Planner (1 99 1)

3      ~JAMES CITY COUNTY

           * Wayland Bass, P.E., County Engineer

I      ~NEWPORT NEWS

              Paul F. Miller, Director, Department of Planning and Development
           * Robert B. Gardner, Manager of Planning Services, Department of Planning and
              Development
              Cathy James, Senior District Planner, Department of Planning and
  I         ~ ~~Development
              Lambert Logan, P.E., Civil Engineer Department of Engineering








NORFOLK

     R.W. Miner, Jr., Assistant Director, Department of City Planning and Codes
     Administration
     Edwin L. Rosenberg, Manager, Division of Environmental Affairs
     W. Keith Cannady, Environmental Engineer, Division of Environmental Affairs
     Suzanne P. Allan, Project Coordinator, Division of Environmental Affairs
     Chris L. Chambers, P.E., Design Engineer, Department of Public Works
     Forrest Robertson, Civil Engineer, Department of Utilities (1990)

POQUOSON

     Robert Goumas, Director, Department of Planning
     Kristen Lawrence, P.E., Director, Department of Engineering and Utilities

PORTSMOUTH

     Norman Whitaker, Director, Department of Planning
     Stephen White, Principal Planner (through October 1990)
*    Martha Little, Planner, Department of Planning
I   *    Michael Kelly, Planner, Department of Planning
*    Rock Bell, P.E., Principal Civil Engineer, Department of Public Works

SMITHFIELD

     Colonel Elsey Harris, Jr., Town Manager
     Kenneth L. McLawhon, Assistant Town Manager/Planner

SUFFOLK

     Robert A. Baldwin, Assistant Director of Community Development
     Scott Mills, Environmental Planner, Department of Community
     Development

VIRGINIA BEACH

     H. Clayton Bernick, Ill, Administrator of Environmental Management
     Mary R. Morris, Environmental Programs Coordinator, Division of
     Environmental Management (1990)
     Mary M. Heinricht, Environmental Information Coordinator, Division of
      Environmental Management
     John Fowler, P.E., Civil Engineer, Department of Public Works
     James Flechner, Civil Engineer, Department of Public Works
     June Barrett-McDaniels, P.E., Civil Engineer, Department of Public Works
      Barbara Duke, Planner, Department of Planning








WILLIAMSBURG

*    Reed T. Nester, Director, Department of Planning
*    Carolyn A. Murphy, Code Compliance Officer, Department of Planning

YORK COUNTY

     Cynthia Taylor, Senior Planner, Department of Community   Development
     J. Blair Wilson, P.E., Civil Engineer, Department of Community Development

CHESAPEAKE BAY LOCAL ASSISTANCE DEPARTMENT

     R. Keith Bull, Executive Director
*    C. Scott Crafton, Chief, Engineering Division
    Darryl M. Glover, Senior Environmental Engineer
*    Marlene F. Hale, Civil Engineer
     Michael L. Smiley, Landscape Architect
     John J. Zeugner, Principal Environmental Planner
     Brian S. Parsons, Grants Coordinator

VIRGINIA COUNCIL ON THE ENVIRONMENT

     Keith J. Buttleman, Administrator
     Ann D. Brooks, Assistant Administrator
     Laura M. Lower, Grants Coordinator

SDN WATER RESOURCES

     Jerome M. Normann, P.E., Director and Partner
     Ved P. Malhotra, P.E., Assistant Director of Water Resources
     Kyle F. Fortin, Systems Analyst

URS CONSULTANTS, INC.

     Lamont W. Curtis, P.E., Vice-President
     Robert Arnold,, P.E.
     Timothy Clarke
      Philip Rhinehart
      Mark Kraus, Environmental Concern, Inc.

*Denotes principal participant.








                               INTRODUCTION


     Previous studies have identified nonpoint source pollution and stormwater
management as key water quality issues in many of the estuaries, lakes and rivers of
the Hampton Roads area. These issues were addressed in a comprehensive and
cooperative fashion by the Hampton Roads Water Quality Agency during the 1970s
and early 1980s. As a result of those studies and related programs undertaken
throughout the country during the same period, increased attention has been
placed on nonpoint source pollution and stormwater management by state and
federal regulatory agencies. The cooperative state-EPA Chesapeake Bay Program
underscores the importance of these issues insofar as the Chesapeake Bay is
concerned. As regulatory programs evolved in the late 1980s, it was determined that
a comprehensive and integrated approach to complying with these regulations must
be developed.

     The Hampton Roads Planning District Commission (HRPDC), through its
predecessor the Southeastern Virginia Planning District Commission (SVPDC),
addressed these issues through the Reaional Stormwater Management Strategy for
Southeastern Virginia and the Elizabeth River Basin Environmental Management
Program, completed in 1989. Those studies recommended a comprehensive
program to be used by the region's local governments to satisfy the requirements of
the Chesapeake Bay Preservation Act, the Virginia Stormwater Management Act, the
Stormwater Permitting Program of the U.S. Environmental Protection Agency and
the State/EPA Nonpoint Source Management Programs.  Many of the
recommendations were aimed at cooperative approaches to satisfying these
requirements.  Both studies recommended uniform implementation of Best
Management Practices for nonpoint source pollution control.

     The Chesapeake Bay Preservation Act (CBPA) was passed by the Virginia
General Assembly in 1988. It recognizes the contribution of nonpoint source
pollution to the water quality problems of the Chesapeake Bay and its tributaries.
The CBPA requires Tidewater localities to designate Preservation Areas, which if
improperly developed would lead to water quality degradation, and to incorporate
measures into their comprehensive plans and land use controls to protect these
areas. The "Chesapeake Bay Preservation Area Designation and Management
Regulations" (VR-173-02-01) establish criteria for designating such areas and
performance criteria for managing the impacts of development in those areas. This
program is mandatory for all localities in Tidewater Virginia. Thus, in Hampton
Roads, only the City of Franklin, Southampton County and the six towns in
Southampton County are not governed by the program.








     Specific performance criteria for stormwater management have been
established. They are:

    * 0  To prevent a net increase in nonpointsource pollution in runoff from new
          development;

    *    To achieve a ten (10) percent reduction in nonpointsource pollution from
          redevelopment; and,

    *    To achieve a forty (40) percent reduction in nonpoint source pollution
          from agricultural and silvicultural uses.

The Best Manaqement Practices Desiqn Guidance Manual for Hampton Roads
provides methods for local governments to use in addressing the first two of these
criteria. It does not address the criteria that apply to agricultural and silvicultural
uses.

     The Virginia Stormwater Management Act was enacted by the General
Assembly in 1989 and implementing regulations were promulgated by the
Department of Conservation and Recreation, Division of Soil and Water
Conservation, in 1990.  These regulations require that local stormwater
management ordinances accomplish the following:

     *    Require regulated development activities to maintain post-development
          peak runoff rates at or below pre-development runoff rates;

     * 0  Establish minimum technical criteria to control nonpoint source pollution
          and localized flooding;

     0    Require the provision of long-term responsibility for and maintenance of
          stormwater management facilities; and,

     *    Require local programs to include certain minimum administrative
          procedures.

The Virginia Stormwater Management Regulations provide for a voluntary program
and are only applicable to localities that adopt a stormwater management program.
In providing guidance on meeting the stormwater management performance
criteria of the CBPA regulations, the Guidance Manual also provides guidance that
will assist local governments in satisfying the Stormwater Management Regulations.

     The Clean Water Act of 1987 directed the U.S. Environmental Protection
Agency to establish a program for permitting municipal and industrial stormwater
discharges through the National Pollutant Discharge Elimination System (NPDES)
permit program.  The Regulations currently apply to stormwater systems in
municipalities with populations greater than 100,000, but may apply in the future to








smaller municipalities as well. The Regulations require that permit applications be
submitted and that management plans be developed as part of that application
process. Specific performance standards or discharge limits have not yet been
established. The Guidance Manual should facilitate local efforts to comply with such
standards when they are promulgated.

     As the region's localities have devised specific approaches to implementing
these various stormwater and nonpoint source management requirements, the need
for comparable and consistent approaches to facility design has been underscored.
The development community appears to agree, at least in concept, with this idea.

     Based on this well-documented need, in 1990 the HRPDC, in cooperation with
the participating localities, undertook a project to develop a Regional Design
Manual for Best Management Practices (BMPs). This Manual was to document the
most appropriate and effective BMPs for use in the Hampton Roads region. The
selected BMPs were to be sufficient to enable development in the region to comply
with the requirements of the Chesapeake Bay Preservation Act, as well as with other
state and federal stormwater management regulations. Completion of the project
was facilitated by financial assistance from the Chesapeake Bay Local Assistance
Department and the Virginia Council on the Environment. Engineering consultant
assistance was also obtained.

     The following objectives were established for this project. They are:

     *    To develop a uniform, regional approach to implementation of state and
          federal stormwater and nonpoint source management programs.

          To develop BMPs which satisfy the CBPA stormwater management
          performance criteria.

     *    To determine the most cost-effective, preferred BMPS for application in
          Southeastern Virginia.

     *    To develop standardized engineering design standards and specifications
          for BMPs to be used in Southeastern Virginia.

As the Manual evolved over the last two years, the focus shifted to providing
common guidance for use by both local planning and engineering staffs and the
development community in designing and developing stormwater management
facilities.

     The Best Manaqement Practices Desiqn Guidance Manual for Hampton Roads
has been completed in two Phases. The Guidance Manual is not intended to
supercede the public facility and stormwater management design requirements
established by local governments. It is intended to supplement those requirements
and provide additional guidance for use in designing stormwater facilities that








comply with evolving state and federal stormwater management requirements.
Users are referred to the local public facility and stormwater management facility
design manuals for locality-specific design requirements. In addition, users are
referred to standard engineering texts and handbooks for detailed design
information on hydraulics and hydrology.

     Both phases of the Guidance Manual address general planning considerations,
including an overview of the Chesapeake Bay Preservation Act and implementing
regulations, the Virginia Stormwater Management Act and its implementing
regulations and the EPA Stormwater NPDES Permit Regulations. For each Best
Management Practice addressed, design guidance is provided, as is information
about construction and maintenance requirements and costs and life expectancy.

     Phase I of the Guidance Manual focuses on on-site BMPs and, in particular, on
BMPs which are suitable for single family dwelling units and small commercial sites.
It includes detailed design criteria and specifications, as well as examples, on each of
the practices. Practices included in this phase include Biofiltration, Grassed Swales,
and Filter Strips. They also include Dry Wells, Infiltration Trenches and Basins,
Underground Storage Trenches, Porous and Modular Pavements, Grit-Oil Separators
and Water Quality Inlets. Suggestions on combining one or more of these practices
in certain situations are also provided.

     Phase II encompasses regional BMPs. It addresses detention and retention
facilities, the incorporation of wetlands features into such facilities and the
retrofitting of existing stormwater management facilities so that they provide
additional water quality benefit. Phase II of the Guidance Manual stresses the value
of multi-objective stormwater management planning and facility design. Detailed
checklists are provided for construction, inspection and maintenance of such
facilities. Because these facilities are typically large-scale, subject to a wide variety of
site-specific and watershed-specific constraints, and require design flexibility and
innovation, detailed design examples are not included. Detailed guidance on the
establishment of wetlands in stormwater management facilities is included.

     The Guidance Manual is structured to facilitate updating as new research is
completed and as additional experience with BMP installation and operation in the
Hampton Roads area is gained. Each chapter on an individual practice is structured
as a stand- alone section that can be reproduced and provided to property owners
and designers. This organization also facilitates replacement of current chapters
with new information as it becomes available.

     Preparation of the Best Manaqement Practices Design Guidance Manual for
Hampton Roads is one element of a comprehensive regional program in stormwater
and environmental management. Other elements of this program include:

      1.   Reqional Stormwater Manaqement Stratecv for Southeastern Vircinia,
           SVPDC staff, 1989. This study documented the evolving state of
           stormwater management regulation at the state and federal level. It








    outlines a technical and institutional strategy that can be used by the
     region's local governments to comply with these regulations. While
    specifically applicable to the eight jurisdictions that, at that time, were
     part of the Southeastern Virginia Planning District Commission, it is
    generally applicable to all of the localities in the Hampton Roads Planning
     District.

2.   Stormwater Management Financing StrateQY for Hampton  Roads
     Virginia, HRPDC staff, 1991. This study documented the need for
     additional authority for local governments to use in financing stormwater
     management programs to meet state and federal regulations. It
     recommends use of stormwater utilities as an equitable means of
     accomplishing that.

3.   Model Environmental Assessment Procedure, HRPDC staff, 1992. This
     study outlines one approach to evaluating the environmental impacts of
     development proposals. It also provides guidance on conducting the
     water quality impact assessments required under the Chesapeake Bay
     Preservation Act.

4.   Vegetative Practices Design Guidance is being prepared by the HRPDC
     staff in cooperation with staff from the region's localities. This study will
     constitute Phase III of the BMP Manual and will be incorporated into the
     Manual when it is completed. It reflects a recognition on the part of local
     government staffs that structural BMPs may not be the most cost-
     effective approach to stormwater management for small, residential sites
     in the Hampton Roads region.

5.   BMP Tracking System is being developed by the HRPDC staff in
     cooperation with staff members from the region's localities. This project
     will provide a computerized system for monitoring BMP installation and
     maintenance to assist localities in ensuring that property owners comply
     with the CBPA requirements for BMP maintenance.

6.   Regional Stormwater Management Coordination Process. Through this
     activity, the staff of the HRPDC is facilitating regular meetings of the local
     government staff involved in stormwater management programs. These
     meetings provide an opportunity for exchange of ideas and experience.
     In a related activity, the Hampton Roads Municipal Communicators, in
     cooperation with the HRPDC and local government staffs, has developed
     educational materials, including brochures and a video, on stormwater
     management.









U DESIGN GUIDANCE MANUAL
 I          ~~FOR HAMPTON ROADS



     ... _-.".II ! ,x

       I~~~~~~~~~~~~~~~~~~~~~'             '"..


I~ ~: I ..'~i I~,
      -' *                               --'
              .  A  "
        '~~ ~ '        A.:i' 
   *  PHASE I BMPS"FO  SMALL STES
I~c.. .-.i''L''-"






                   DECEMBER 1991












                               BEST MANAGEMENT PRACTICES

                                DESIGN GUIDANCE MANUAL

       *                                ~~~~~~~~~FOR

                                 HAMPTON ROADS VIRGINIA

                              PHASE I: BMPS FOR SMALL SITES





N          ~~~This report was produced, in part, through financial support from the
               ChesaeakeBay Local Assistance Department pursuant to Contract No. 9 1-42
            of August 1990 and Unnumbered Contract of June 20, 1990 and from the
            Virginia Council on the Environment pursuant to Coastal Resources
            Management Program Grant No. NA9OAA- H-CZ796 from the National Oceanic

            and Atmospheric Administration.


            Preparation of this report was included in the HRPDC Work Program for FY
            1990-9 1, approved by the Commission at its Executive Committee Meeting of
I        ~~~March 21, 1990 and in the HRPDC Work Program for FY 1991-92, approved by
            the Commission at its Executive Committee Meeting of March 20, 1991.










     I                      ~~~~~~~~Prepared by SDN Water Resources
                              in cooperation with the Staff of the
    *                  ~~~~~~Hampton Roads Planning District Commission

                                       December 1990








                                DISTRIBUTION

Distribution of this report and the accompanying software is governed by the
provisions of the December 7, 1991 Letter Agreement between the Hampton Roads
Planning District Commission (HRPDC), Chesapeake Bay Local Assistance Department
(CBLAD) and Smith Demer Normann, Ltd. (SDN). Specifically:

     "HRPDC and CBLAD shall have the full, complete and perpetual right to
     reproduce and distribute the Manual and Software within Virginia to
     political subdivisions subject to or interested in the Chesapeake Bay
     Preservation Act. HRPDC, CBLAD and such political subdivisions shall
     have unrestricted use of the Manual and Software for purposes of
     implementing the Chesapeake Bay Preservation Act, related laws and/or
     regulations and other governmental purposes; provided, however, that
     the rights of all nongovernmental agents and contractors to use the
     Manual and Software shall be limited to services performed for or on
     behalf of HRPDC, CBLAD and/or such political subdivisions. HRPDC and
     CBLAD shall not distribute the Software outside of the Commonwealth
     of Virginia or to any person, entity or political subdivision other than the
     foregoing. HRPDC and CBLAD shall contractually restrict their
     distributees from making any further copy of, distribution of, or
     otherwise making use of, the Software inconsistent with this paragraph.
     Any diskettes distributed hereunder will have an appropriate label
     permanently affixed to reflect the foregoing restrictions.

     "SDN will have the full, complete and perpetual right to distribute the
     Software outside of the Commonwealth of Virginia, provided that the
     programs contained therein shall be modified such that they are
     inapplicable to the Chesapeake Bay Preservation Act requirements in
     effect in Virginia. SDN may only use the Manual and Software in
     Virginia as a contractor for HRPDC, CBLAD or a political subdivision to
     whom the Manual and Software have been distributed pursuant to
     Paragraph 5 above and for the purpose for which the Manual and
     Software were so distributed. The foregoing notwithstanding, SDN shall
     have the right to conduct research and development of the Manual and
     Software for marketing and distribution outside of the Commonwealth
     of Virginia. Notwithstanding anything herein contained to the contrary,
     SDN shall not be precluded from using its engineering and programming
     expertise, technical know-how, formulations and designs in the
     performance of best management practices services for other clients and
     customers as long as such services do not involve a prohibited use of the
     Software and Manual. SDN shall not distribute the Software in Virginia
     or take any other action which could reasonably be foreseen as adversely
     impacting the efficacy of the Software for the implementation of the
     Chesapeake Bay Preservation Act and related purposes.  SDN shall
     contractually restrict its distributees from making any further copy of,








 U          ~~~distribution of, or otherwise making use of, the Software inconsistent
             with this paragraph. Any diskettes distributed hereunder will have an
             appropriate label permanently affixed to reflect the foregoing
             restrictions.

*      ~~The Hampton Roads Planning District Commission and the Chesapeake Bay Local
        Assistance Department request the cooperation of recipients of this Manual in
        complying with these restrictions.
















I                         ~~~~BMP DESIGN GUIDANCE MANUAL



                                      JANUARY, 1991




                                          Prepared for:

I                          ~~~~~~~~Hampton Roads Planning District Commission
                                       Chesapeake, Virginia


                                          Prepared by:





                                      SDN Water Resources
    U                                     ~ ~~~~~~~~~~~Central Park
                                 Six Manhattan Square, Suite 102
                                     Hampton, Virginia 23666
                                          (804) 865-9610









                               TABLE OF CONTENTS

               N                                                                                         ~~~~~~~~~~~~~~~~~~~~~~~Page

           REFERENCES                                               ............................                vi
         1.INTRODUCTION ............................1
           1.1 Purpose and Scope........................                                                        1
           1.2 General Planning Considerations....................2
 I              ~~~~~1.3 Biofiltration...........................2
           1.4 Grassed Swale with Check Dam....................3
           1.5 Filter Strip ...........................3
           1.6 Dry Well............................3
 U                ~~~~~1.7 Infiltration Trench.........................3
              1. nitainBasin .........................3
             1.9Underground Storage Trench.....................4
           1.10 Porous Pavement .........................4
           1.11 Grid/Modular Pavement.......................4
           1.12 Grit-Oil Separator.........................4
           1.13 Water Quality Inlet ........................4
            1.14 Regional BM1Ps.........................                                                         5

I          ~~~2.  GENERAL PLANNING CONSIDERATIONS                                             ..................6
           2.1 Soil Information .........................6
           2.2 Groundwater Information......................                                                     8
 I              ~~~~~2.3 Other Site Selection Criteria.....................                                              8
           2.4  Pollutant Removal Efficiency of BMPs ..1..............1 
           2.5 Design Storm..........................13

      3. BIOFILTRATION                                                   ...........................15
           3.1 Description...........................15
           3.2 Applicability ..........................15
 U                ~~~~~3.3 Design Criteria.........................15
              3. einExamples ........................18
              35Maintenance Requirements .....................21
            3.6 Life Expectancy.........................21
            3.7 Cost .............................21
            3.8 Construction Specifications .....................21

       4.  GRASSED SWALE WITH CHECK DAM...................23
            4.1 Description...........................23
 I              ~~~~~4.2 Applicability ..........................23
            4.3 Design Criteria .........................23
            4.4 Design Examples ........................27
 I              ~~~~~~4.5 Maintenance Requirements .....................34
            4.6 Life Expectancy.........................34
            4.7 Cost .............................34

                                       i







          4.8 Construction Specifications ..................... 35

        5.FILTER STRIP............................36
          5.1 Description ...........................36
I              ~~~~~5.2 Applicability...........................36
          5.3 Design Criteria..........................36
          5.4 Design Examples.........................40
I              ~~~~~5.5 Maintenance Requirements......................44
          5.6 Life Expectancy .........................44
          5.7 Cost..............................44
          5.8 Construction Specifications......................45
      6. DRY WELL.............................55
*                ~~~~~6.1 Description ...........................
          6.2 Applicability...........................55
          6.3 Design Criteria .........................5
1                ~~~~~6.4 Design Examples ........................58
          6.5 Maintenance Requirements......................63
          6.6 Life Expectancy                                         .........................64
          6.7 Cost..............................64
          6.8 Construction Specifications......................65
      7. INFILTRATION TRENCH........................68
I              ~~~~~7.1 Description...........................68
          7.2 Applicability ..........................68
          7.3 Design Criteria .........................68
I              ~~~~~7.4 Design Examples ........................72
          7.5 Maintenance Requirements......................77
          7.6 Life Expectancy .........................79
 I              ~~~~~7.7 Cost .............................79
          7.8 Construction Specifications .....................79

      8 INFILTRATION BASIN                                                 .........................83
          8.1 Description...........................83
          8.2 Applicability ..........................83
 3                ~~~~~8.3 Design Criteria .........................84
          8.4 Design Examples ........................87
          8.5 Maintenance Requirements .....................93
          8.6 Life Expectancy.........................94
          8.7 Cost .............................94
          8.8 Construction Specifications .....................95

1          ~~~9 UNDERGROUND STORAGE TRENCH ...................96
          9.1 Description...........................96
          9.2 Applicability ..........................96
 I              ~~~~~9.3 Design Criteria .........................96
          9.4 Design Examples ........................99
          9.5 Maintenance Requirements.....................103

                                 ii







         9.6 Life Expectancy ........................103
         9.7 Cost.............................103
         9.8 Construction Specifications.....................103

U    ~~~10 POROUS PAVEMENT.........................107
         10.1 Description ..........................107
         10.2 Applicability..1........................0I
 I              ~~~~~10.3 Design Criteria.........................110
         10.4 Design Examples........................115
         10.5 Maintenance Requirements.....................118
 I              ~~~~~10.6 Life Expectancy ........................118
          10.7 Cost.............................118
          10.8 Construction Methods and Specifications................119

     11 GRID/MODULAR PAVEM4ENT......................122
          11.1 Description ..........................122
 I              ~~~~~11.2 Applicability..........................122
          11.3 Design Criteria.........................122
          11.4 Design Example ........................127
          11.5 Maintenance Requirements.....................130
          11.6 Life Expectancy ........................130
          11.7 Cost.............................130
          11.8 Construction Specifications.....................130

     12 GRIT-OIL SEPARATOR ........................133
          12.1 Description ..........................133
 I              ~~~~~12.2 Applicability..........................133
          12.3 Design Criteria.........................133
          12.4 Design Examples........................135
 I              ~~~~~12.5 Maintenance Requirements.....................140
          12.6 Life Expectancy ........................140
          12.7 Cost.............................140
 I              ~~~~~12.8 Construction Specifications.....................140
      13 WATER QUALITY INLET.......................141
          13.1 Description ..........................141
          13.2 Applicability..........................141
          13.3 Design Criteria.........................141
          13.4 Design Example ........................143
          13.5 Maintenance Requirements.....................147
          13.6 Life Expectancy ........................147
          13.7 Cost.............................147
          13.8 Construction Specifications.....................147
      14 BMP COMBINATIONS .........................148






                                LIST OF FIGURES


      Figure I - Biofiltration Schematic ........................20
      Figure 2 - Grassed Swale w/Check Darns Schematic..................31
      Figure 3 - Grassed Swale w/Check Dams Detail ...................32
      Figure 4 - Grassed Swale w/Check Dams......................33
I       ~~~Figure 5 - Effective Filter Strip Length Determination .................39
      Figure 6 - Effective Filter Strip Length Determination.................42
      Figure 7 - Vegetative Filter Detail ........................43
I   ~~~Figure 8 - Dry Well Schematic .........................61
      Figure 9 - Dry Well Detail...........................62
      Figure 10 - Infiltration Trench Schematic......................74
      Figure I1I - Observation Well Detail/Infiltration Trench Detail ..............75
      Figure 12 - Parking Lot Perimeter Trench .....................76
      Figure 13 - Infiltration Basin Schematic ......................89
      Figure 14 - Infiltration Basin ..9........................g
      Figure 15 - Infiltration Basin ..........................91
      Figure 16 - Two-Level Infiltration Basin......................92
      Figure 17 - Underground Storage Schematic ....................101
U    ~~~Figure 18 - Underground Storage Detail......................102
      Figure 19 - Porous Pavement Detail ..1.....................9o
      Figure 20 - Porous Pavement Schematic......................117
I       ~~~Figure 21 - Representative Grid Pavements.....................126
       Figure 22 - Grid/Modular Pavement Schematic ...................128
       Figure 23 - Modular Pavement Detail ......................129
I       ~~~Figure 24 - Grit-Oil Separator Schematic......................136
       Figure 25 - Grit-Oil Separator Detail.......................137
       Figure 26 - Grit-Oil Separator Detail.......................138
*         ~~~Figure 27 -Grit-Oil Separator Detail.......................139
       Figure 28 - Water Quality Inlet Schematic.....................144
       Figure 29 - Water Quality Inlet Detail ......................145
       Figure 30 - Water Quality Inlet Detail ......................146
       Figure 31 - Concrete Diversion Detail ......................149
       Figure 32 - Diversion Box Detail........................150
       Figure 33 - Under-the-Swale Trench.......................151
       Figure 34 - Filter Strip Porous Pavement .....................152
       Figure 35 - Swale/Trench...........................153
       Figure 36 - Underground Trench with Grit/Oil Separator................154
U    ~~~Figure 37 - Off-line Infiltration Basin ......................155
       Figure 38 - Off-line Trench System .......................156







                                      iv







                                   LIST OF TABLES

Table 1 - Hydrologic Soil Properties ............................................. 7
Table 2 - BMPs Selection Criteria .............................................. 10
Table 3 - BMP Removal Efficiency ............................................. 12
Table 4 - Manning's Value of Vegetation ......................................... 17
Table 5 - Permissible Velocities for Various Ground Covers ............................ 25
Table 6 - Typical Costs for Establishing Vegetative Cover ............................. 34
Table 7 - Gabion Sizing Criteria ............................................... 35
Table 8 - Effective Buffer Strip Length .......................................... 38
Table 9 - Permanent Seeding and Seeding Dates .................................... 49
Table 10 - Maintenance Fertilization for Permanent Seedings ........................... 54
Table 11 - Approved Geo-Textiles For Use in Dry Wells ............................. 66
Table 12 - Approved Geo-Textiles For Use in Infiltration Trenches ....................... 81
Table 13 - Approved Geo-Textiles For Use in Underground Storage Trenches .............. 105
Table 14 - Aggregate Gradation for Porous Pavement ............................... 112
Table 15 - Porous (Open-Graded) Asphalt Concrete Formulation ....................... 121
Table 16 - Types of Grid Pavements ........................................... 122

















 I~~~~~~~~~~~~~~~~







                                          REFERENCES


Controlling Urban Runoff, A Practical Manual for Planning and Designing Urban BMPs - Metropolitan
Washington Council of Governments, July 1987.

Standards and Specifications for Infiltration Practices - Maryland Department of Natural Resources, February
1984.

Local Assistance Manual - Chesapeake Bay Local Assistance Department, November 1989, as amended.

Biofiltration Systems for Storm Runoff Water Quality Control - Richard R. Homer, December 1988.

BMPs for Urban Stormwater Management in Virginia - C.Y. Kuo and G. V. Loganathan. Critical Water
Issues and Computer Applications, American Society of Civil Engineers, June 1-3, 1988.

Phosphorus and Nitrogen Removal Efficiencies of Infiltration Trenches - Chin Y. Kuo, G. D. Boardman,
K.T. Laptos, May 1990.

Urban Hydrology for Small Watershed, TR-55, Soil Conservation Service, June 1986.

Urban Stormwater Management, Special Report No. 49 - American Public Works Association, 1981.

Public Works Design Guide for Stormwater Management and the Use of Best Management Practices in the
City of Virginia Beach, March 1990.

Stormwater Management Ordinance - City of Virginia Beach, June 1990.

Ordinance to amend City Zoning Ordinance, pertaining to Chesapeake Bay Preservation Areas - City of
Virginia Beach, August 1990.

Executive Summary - Stormwater Management Plan, City of Virginia Beach, August 1988.

Water Quality Impacts from Stormwater Runoff Pollution, City of Virginia Beach, December 1987.

Erosion and Sedimentation Control - Chapter 6, Isle of Wight County, April 1984.

Ordinance amending certain sections of Chapter 9 - Erosion and Sedimentation Control, City of Suffolk,
May 1989.

Proposed Chesapeake Bay Preservation Areas Program, Summary of Overlay District and Regulations, City
of Norfolk, July 1990.

Chesapeake Bay Preservation Areas Program, First Year Program Proposal, City of Norfolk, July 1990.

Ordinance amending Chapter 29, to provide for the establishment of a Chesapeake Bay Preservation Area
Overlay District (Draft), City of Chesapeake, August 1990.


                                                i







Erosion and Sediment Control Handbook, City of Portsmouth, January 1976.

Results of the Nationwide Urban Runoff Program - Executive Summary, December 1983.

Regional Stormwater Management Strategy for Southeastern Virginia, SVPDC, May 1989.

Best Management Practices Handbook for the Occoquan Watershed, Northern Virginia Planning District
Commission, August 1987.

A Simplified Technique for Developing Site Specific Nonpoint Source Control Plans, SVPDC 1983.

Guidebook for Screening Urban Nonpoint Pollution Management Strategies, NVPDC, November 1979.

Best Management Practices Handbook - Virginia State Water Control Board, Planning Bulletin 321, 1979.

A Study of Infiltration Trenches - Virginia Water Resources Research Center - Bulletin 163, April 1989.

Effectiveness of Best Management Practices for Stormwater Management in Urbanized Watersheds -
Virginia Water Resources Research Center - Bulletin 159, January 1988.

Runoff and Pollution Abatement Characteristics of Concrete Grid Pavements - Virginia Water Resources
Center, Bulletin 135, October 1981.

Long-Term Effectiveness and Maintenance of Vegetative Filter Strips - Virginia Water Resources Research
Center - Bulletin 153, 1986.

Hampton Roads Water Quality Management Plan - Preliminary Nonpoint Source Control Plan and
Management Practice Effectiveness and Cost Studies, December 1982.

Hampton Roads Water Quality Management Plan - Management Practice Inventory and Cost Evaluation,
1982.

Hampton Roads Water Quality Management Plan - Management Practice Inventory and Cost Evaluation -
Appendix C - Volume I to Volume HI, 1982.
















                                                vii






                                          APPENDICES


Appendix I - Sizes of Coarse Aggregates - Open Graded

Appendix II - Seeding Mixtures, Rates and Dates: Northern Piedmont and Mountain Region

Appendix III - Chesapeake Bay Local Assistance Department Guidance Calculation Procedure






































                                                oo.
 I~~~~~~~~~~~~~~~~~~~~Vl







INTRODUCTION

1.1    Purpose and Scope

       The Hampton Roads Planning District Commission (HRPDC), on behalf of its member local

governments, has undertaken development of a cooperative regional stormwater management

program. A variety of regional efforts in this regard have been underway for several years to assist

the localities to respond to various state and federal programs as well as local needs. They have

ranged from studies of stormwater quantity issues in the early 1970s to studies of stormwater

quality and nonpoint source management issues in the late 1970s and 1980s.

       In 1988, the Virginia General Assembly enacted the Chesapeake Bay Preservation Act

(CBPA), which established requirements for local land use planning and regulation to protect water

quality.  These requirements include fairly stringent performance criteria for stormwater quality

management on a development specific basis. The CBPA and its requirements affect development

activities in several Southeastern Virginia localities. They include the County of Isle of Wight, the

Cities of Chesapeake, Norfolk, Portsmouth, Suffolk, and Virginia Beach, and the Towns of

Smithfield and Windsor.

        To facilitate local compliance with the stormwater management requirements of the CBPA,

the localities requested the HRPDC to undertake development of a Best Management Practices

(BMPs) Design Guidance Manual for Southeastern Virginia. SDN Water Resources was contracted

in August 1990 to provide assistance in the development of this Manual. The following objectives

were established for preparing the Design Guidance Manual:

               To develop BMPs which satisfy the CBPA stormwater management performance

                criteria.

                To  determine  the most  cost-effective, preferred  BMPs  for application in

                Southeastern Virginia.







               To develop standardized engineering standards and specifications for BMPs to be

               used in Southeastern Virginia.

       As the project evolved, the Manual has become a design guidance document. It provides

guidance to the localities on those BMPs which are most appropriate for use in Southeastern

Virginia. While the primary purpose is to assist the localities in complying with the CBPA and its

implementing regulations, the Manual should also assist in addressing the quality aspects of the

Virginia Stormwater Management Act and the National Pollution Discharge Elimination System

stormwater permitting requirements.

       The Manual is not intended to be a comprehensive stormwater management Manual, insofar

as hydrology and hydraulics are concerned. Users should consult standard engineering texts and

handbooks for detailed information on these subjects.

        SDN Water Resources has prepared planning guidelines, design criteria, and standard details

for the following BMPs. A brief description of each BMP in the Design Guidance Manual follows.

Section numbers are keyed to the chapter numbers for each BMP. Detailed discussion of each BMP

including design criteria, maintenance requirements, costs, and construction specifications can be

found in subsequent chapters.

1.2     General Planning Considerations

        To select a BMP for a site, factors to be considered are the infiltration rate of the soils of

the site and the ground water table. The total contributing area to the BMP is also a factor. Other

physical factors to be considered for selection of a BMP for a site are proximity to water supply

wells and foundations, slope of the site, and specific use of the site.

1.3    Biofiltration

        Biofiltration as a BMlP utilizes the interaction of soil and vegetative cover to remove

pollutants from surface runoff. It functions like a swale or filter strip.



                                           2







      1.4     Grassed Swale with Check Dam

              Grassed swales are typically used in low density areas as an alternative to curb and gutter

      drainage systems.  The pollutants are filtered out by the grass and subsoil. Check dams may be

     used to temporarily pond runoff, allowing infiltration over a period of time. They cannot, however,

      accommodate major runoff events and may lead to other downstream BMPs.

      1.5     Filter Strip

              Also known as buffer zones, filter strips are similar to grassed swales except that they are

      wider.  They should be at least 20 feet wide and not be used on slopes greater than 15%. Filter

      strips are usually vegetated and accept evenly distributed sheet flow. There are secondary benefits

      including aesthetics, wildlife habitat, and noise screening.

      1.6     Dry Well

              A dry well utilizes the concept of infiltration to remove pollutants from surface runoff from

      rooftops.   The dry well is a variation of the infiltration trench and is designed exclusively for

      runoff from rooftops. Roof leaders are extended to a stone filled trench located a minimum of ten

      (10) feet from the building foundation.

      1.7     Infiltration Trench

              An infiltration trench is typically three (3) to eight (8) feet deep and filled with stone to

      create an underground reservoir. Runoff can either drain from the reservoir into the underlying soil

      (exfiltration) or be collected by underdrains and directed to an  outflow.  Typically, infiltration

      trenches can only accommodate limited quantities of runoff and are used for sites of less than ten

      (10) acres in size.

      1.8     Infiltration Basin

              Whereas infiltration trenches serve small sites, infiltration basins can serve drainage areas

      up to 50 acres. They are designed to promote exfiltration through the underlying material. They




I                                                   ~~~~~~~~~~~~~~~~3







                 should be vegetated and often include devices which prevent coarse sediment from entering the

                 basin as well as emergency spiliways for extreme storm events.

1               ~~~~1.9  Underground Storage Trench

                         An underground storage trench is designed to remove sediments and hydrocarbons from

                 parking lots and commercial sites where there is not enough space for infiltration systems.

                 1.10   Porous Pavement

                         Porous pavement detains and minimizes the effects of runoff containing traffic generated

I               ~~~~~pollutants. Both soluble and very fine grained pollutants are removed by infiltration through the

                 stone reservoir and into the underlying soil. This BMIP has a number of shortcomings which

                 generally confine it to low volume traffic areas such as parking lots. It consists of a graded

I               ~~~~aggregate cemented with asphalt cement, with numerous voids to provide a high rate of

*               ~~~~~permeability.

                 1.11   Grid/Modular Pavement

                         Using the same concept as porous pavement, this type of pervious pavement consists of a

                 grid made of concrete, clay bricks, or granite sets. 'Be void areas of the grid are filled with a

H               ~~~~~pervious material such as sod, gravel, or sand.

                 1.12    Grit-Oil Separator

                         A grit-oil separator is used to remove oil and grit deposits from runoff from parking lot

I               ~~~~~areas and commercial sites.

                 1.13   Water Quality Inlet

                         Water quality inlets are typically used to serve parking lots one (1) acre or less in size, and

                 are primarily used in combination with other BMPs as a pretreatment facility to remove coarse

                 sediment particles.






           I                                               ~~~~~~~~~~~~~~4







1.14   Regional BMPs

       Phase II of the BMP Design Guidance Manual will incorporate regional BMPs like

Detention and Retention Ponds, Extended Detention/Retention Ponds, and Detention/Retention

Ponds with wetland bottoms.







2.   GENERAL PLANNING CONSIDERATIONS

       The planning of BMPs requires collection of information about underlying soils and the groundwater

table. All BMPs which utilize infiltration are dependent on the ability of the underlying soil to infiltrate

storm runoff and not be inundated by groundwater. The following paragraphs provide guidelines for

selecting suitable BMPs for a specific site.

        2.1     Soil Information

               A critical element in selecting a BMP for a specific site is collecting and analyzing the soil

        information. An initial indication of the soil can be made from existing Soil Survey Maps prepared

        by the U.S. Soil Conservation Service. A detailed soil survey for each site where an infiltration

        BMP is to be located should be conducted. This is normally performed by taking samples from a

        drilled hole. The hole should be drilled at least four (4) feet below the anticipated design depth of

        the BMP. The collected samples should be graded in the laboratory and the infiltration rate

        determined. The minimum infiltration rate is the rate at which the water passes through the soil

        profile during saturated conditions. It is measured in inches per hour. The hydrologic soil

        properties are obtained by identifying the soil textures (gradation test). Table 1 lists soil texture

        classes and their typical infiltration rates.











 I~~~~~~~~~~~~~~~~










                               Table 1 - Hydrologic Soil Properties

                                      Minimum Infiltration Rate
         Texture Class                       (Inches/Hour)                   Hydrologic Soil Group

             Sand                                8.27                                  A
         Loamy Sand                              2.41                                  A
         Sandy Loam                              1.02                                  B

             Loam                                 0.52                                 B

           Silt Loam                              0.27                                  C

       Sandy Clay Loam                            0.17                                  C

          Clay Loam                               0.09                                 D
       Silty Clay Loam                           0.06                                  D

          Sandy Clay                              0.05                                 D

           Silty Clay                             0.04                                  D

             Clay                                0.02                                  D

Source:         "Controlling Urban Runoff' - Metropolitan Washington Council of Governments.
Note:           The CBPA regulations define soil with an infiltration rate greater than six (6) inches
                per hour as highly permeable.


              Soil textures with a minimum infiltration rate greater than or equal to 0.27 inches per hour

      are generally suitable for infiltration practices. Soil textures with a minimum infiltration rate close

      to, but less than 0.27 inches per hour may be used for infiltration practices with careful analysis of

      the soil profile.

              Hydrologic Soil Group classification indicates the minimum rate of infiltration obtained for

      bare soils after prolonged wetting. These groups are classified as A, B, C, and D. Group A soils

      have low runoff potential and high infiltration rates even when thoroughly wetted. Group B soils

      have moderate infiltration rates when thoroughly wetted. Group C soils have low infiltration rates





                                                   7







when thoroughly wetted. Group D soils have high runoff potential and have very low infiltration

rates when thoroughly wetted.

2.2     Groundwater Information

        Another critical element in selecting a BMP for a specific site is the location of the seasonal

high groundwater table. This can be determined by observing static water elevation in borings. The

groundwater elevations should be determined after a period of eight (8) to 12 hours and not after

the boring is taken. Generalized information about the groundwater table may also be obtained from

local health departments and the SCS.

        Groundwater information is important to determine the safe distance between the bottom

of the BMP structure and the seasonal high groundwater table. Infiltration BMPs should be located

in areas where the bottom of the structure is two (2) to four (4) feet above the seasonal high

groundwater table. This distance should also protect against the flooding of the structure due to the

rise of the water table. A flooded infiltration BMP will be ineffective. The Virginia Stormwater

Management Regulations require that the invert of the infiltration BMPs should be four (4) feet

above the seasonal high groundwater table.

2.3     Other Site Selection Criteria

        Selection of a BMP for a specific site depends on the contributing area of that BMP.

Infiltration practices such as infiltration trenches, porous pavement, grid/modular pavement, and

underground storage trenches are practical and economical for contributing areas up to five (5)

acres. Dry wells are suitable for rooftop areas up to one (1) acre. Infiltration basins may serve

areas up to 50 acres. Grassed swales with check dams are suitable for areas up to 30 acres.

Biofiltration can be used for areas up to ten (10) acres. Filter strips are suitable for areas up to five

(5) acres.

        Topographic conditions also determine the feasibility of a BMP for a specific site. These

site conditions include slopes, proximity of water supply wells, and building foundations. The use







of infiltration BMPs on fill material is not recommended due to the possibility of slope failures

when the fill material is saturated. Table 2 presents a matrix that shows the site selection criteria

for all BMPs. A solid dot indicates that a BMP is feasible.  Open space in the matrix indicates a

restriction. Other site selection restrictions for each BMP are also indicated.

        Maximum feasible depth of a BMP is a function of the minimum infiltration rate of the soil.

The permeability of the soil underlying a BMP and the void ratio of the stone aggregate reservoir

dictates the maximum feasible depths of infiltration BMPs.








































                                             9











                    TABLE-2 BMPs SELECTION CRTERMA




                                    AREA SERVED (ACRES              SOIL TYPE AND MINIMUM OHRRSRCIN
             BEST                                                INFILTRATION RATE (INCHIES/HiR.)     OHRRSRCIN
                                                                LOAMY SANDY  SILT SANDY CLAY SILTY SANDY SILTY
                                                             SAND SAND LOAM LOAM LOAM CLAY LOAM CLAY CLAY CLAY CLAY l 
      MANAGEMENT                     LOMLA_ --~=~ 

   PRACTICES (BMPs)                                      + a)0  Aw Ho R. DCC  DO                              29DD
                                          U)  -  ('S   ('S   C~~~~~t   8.27  2.41  1.02 0.52 0.27  0.17 0.09 0.06 0.05 0.04 0.02  ca. ~ a    ~     ~ 

  BIOFILTRATION                *                                          0                        1-2 < 4

  DRY WELL                                                         02-4 <20 >100 >10 >20 ROO-F:-

  INFILTRATION TRENCH           @@0 0                                                              2-4 <20 >100 >10 >20   2-6

  INFILTRATION BASIN               0        0011**                                                 2-4<20 >100 >10>20    2-5

  GRASSED SWALES                                            12<                                               1         /-
  (WITH CHECK DAMS)             0     0      0                                                      -<        1         /-

  FILTER STRIPS*                                                                                   1.2 <P0

  POROUS PAVEMENT              0                                      02-4 <5                                   >20 HEAVY 1-4
                                                                                                                       TRlAMC

  UNDERGROUND STORAGE          0                                      02-4   >100 > 10 >20

  GRID/MODULAR PAVEMENT    AES                              0      0                               -<
                                                                                                                       ONLY IN
  GRIT-OIL SEPARATOR          M0IA
                                                                                                                       AREA
                                                      -                                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ONLY IN
  WATER QUALITY INLET                                                                                              MECIA
                                                                                                                       AREA

 DETENTION PONDS

 RETENTION PONDS

 EXTENDED DETENTION/                     *      *      *     *    *      *     *      
 RETENTION PONDS
 DETENTION/RETENTION
 WITH WETLAND BOTTOMS


 NOTE: A FILTER STRIP, DESIGNED AS A BMP, DIFFERS FROM THE BUFFER AREA REQUIRED BY THE GBPA REGULATIONS. REFER TO SECTION 5.







2.4     Pollutant Removal Efficiency of BMPs

       Pollutants exist in particulate or soluble forms, or as a mix of both forms. Particulate

pollutants, such as sediment and lead, are removed by settling and filtering.  Soluble pollutants,

such as nitrate, phosphate, and trace metals, are removed through biological uptake by bacteria,

algae, rooted aquatic plants, or vegetation.

       Total phosphorus has been selected by the Chesapeake Bay Local Assistance Department

(CBLAD) as the keystone pollutant to be removed. By removing total phosphorus from the urban

runoff, other urban pollutants are also removed.

       Table 3 lists an estimated removal efficiency of each BMP. Each removal efficiency has

been selected based on review of current literature and is intended as a general guideline only.

More precise numbers for removal efficiency will be forthcoming in the future as a result of more

monitoring and evaluation of BMPs.










                      Table 3 - BMP Removal Efficiency 

                                                  I  REMOVAL EFFICIENCY,
                      BMP                              PERCENT

BIOFILTRATION                                           40- 80
DRY WELL                                                50 - 70
INFILTRATION TRENCH                                     50 - 70

INFILTRATION BASIN                                      50 - 70
GRASS SWALES (W/CHECK DAMS)                             10 - 20
FILTER STRIPS                                           20 - 50

POROUS PAVEMENT                                         50 - 70
UNDERGROUND STORAGE                                     50 - 70

GRID/MODULAR PAVEMENT                                   50 - 70

GRIT-OIL SEPARATOR                                      10 - 25

WATER QUALITY INLET                                     10 -25
DETENTION PONDS                                         20 - 50
RETENTION PONDS                                         35 - 65
EXTENDED DETENTION/RETENTION PONDS                      25 - 60
DETENTION/RETENTION PONDS WITH WETLAND BOTTOMS          40 - 75

Source:   Smith Demer Normann, 1990
Note:     Removal efficiencies refer to Total Phosphorus.

















                                   12







2.5    Design Storm

       The CBPA has established the following technical criteria:

                       For new development, the post-development nonpoint source pollution

               runoff load cannot exceed the pre-development load based on average land cover

               conditions. This is referred to as a "no net increase" standard.

                       For redevelopment sites not served by BMPs, the post-development

               nonpoint source pollution runoff load must be 90 percent or less than the pre-

               development load for that site. This is referred to as a "10 percent reduction"

               standard.

       A 100-foot vegetative buffer is required landward of specified sensitive shoreline features.

Under certain circumstances, the width of the buffer area can be reduced if equivalent water quality

benefits are achieved by installing BMPs on site.

       Pollution runoff loads are computed for an average annual rainfall depth of 45 inches for

Tidewater.

        Virginia Stormwater Management Regulations established the following technical criteria:

                       A stormwater management plan for a land development project shall be

               developed so that from the site, the post-development peak runoff rate from a two-

               year (2 yr.) storm and a ten-year (10 yr.) storm, considered individually, shall not

                exceed their respective pre-development rates.

                       For infiltration facilities, the water quality volume must be completely

                infiltrated within 48 hours. Water quality volume means the volume equal to the

               first 0.5 inch of runoff multiplied by the total area of the land development project.

        All BMP's can be designed for a specific storm or for the first flush runoff volume. The

storage volume of all BMPs can also be sized based on:



                                          13







       (1)    The runoff produced by a one-inch (1") storm over the contributing site

area.

       (2)     0.5 inch of runoff per impervious acre in the contributing site area (first

flush).

       (3)     The runoff per impervious acre produced by a one-inch (1") storm.

       (4)     0.5 inch of runoff in the contributing site area (Virginia Stormwater

Management Regulations).








































                                   14







3.      BIOFILTRATION

       3.1     Description

               Biofiltration is a BMP which utilizes the concept of treating stormwater runoff by direct

       contact with the soil and vegetation. Biofiltration involves storm runoff being transported over a

       vegetative surface and is similar to a filter strip or a swale. The storm runoff can also be ponded

       in an area containing emergent wetland plants. Pollutants such as sediments and trace elements in

       the runoff are removed by biological uptake and infiltration through the soil. By providing

       sufficient residence time for the storm runoff, pollutant removal and infiltration can be

       accomplished. A schematic of biofiltration is shown on Figure 1.

       3.2     Applicability

               Biofiltration in its many forms is used as a BMP for removing pollutants from storm runoff.

       It can be used in residential areas and adjacent to highways.

       3.3     Design Criteria

               3.3.1   Soil Permeability

                       Biofiltration can be used with soils having infiltration rates ranging from 1.02

               inches per hour to 0.06 inches per hour. These infiltration rates are associated with soil

               textural groups of sandy loam, silt-loam, sandy clay loam, clay loam, and silty clay loam.

               3.3.2   Length

                       This BMP should have a hydraulic length of at least 200 feet. If length is less,

               width needs to be made larger to provide the equivalent residence times. Sufficient

               residence time is needed     "  , sediments to be removed through the mechanism of

                settling.

                3.3.3   Side Slopes

                       Side slopes should be as flat as feasible with three to one (3:1) being the

                recommended value.







3.3.4  Longitudinal Slone

       Longitudinal slopes should be in the range of two (2) to four (4) percent. Slopes

less than two (2) percent can be used with underdrains to avoid persistent pooling of runoff.

The longitudinal slopes of two (2) to four (4) percent are impossible to attain in a large

portion of Southeastern Virginia.

3.3.5   Vegetation Cover

        Vegetation cover as outlined in chapters 4 and 5 should be used for biofaiters.

Vegetation selected should be suitable for the site. The type of vegetation selected depends

on the type of underlying soil. Maintenance requirements should also be considered in

selecting the type of vegetation.

3.3.6   Shape

        A parabolic shape for biofi/ters is preferred. Initially, the swale channel can be

constructed as a trapezoid. With time, trapezoidal shapes tend to become parabolic due to

the growth of vegetation and settlement of solids.

3.3.7   Depth of Flow

        The design depth of flow should be at least two (2) inches less than the winter

vegetation height.

3.3.8   Groundwater Table

        The seasonal high groundwater table should be between one (1) and two (2) feet

below the ground surface.  A groundwater table high enough to provide moisture to the

vegetation during the dry season but not high enough to create long periods of saturation

is ideal. In areas with high saturation, emergent wetland plants can be planted.

 3.3.9   Velocity

         Based on the slope parameters of the biofilter selected, the velocity of storm runoff

 should not exceed 1.5 feet per second. This velocity will assist in the removal of

                                   16







suspended solids in the storm runoff. If the velocity exceeds 1.5 feet per second, suspended

solids in the storm runoff cannot be easily settled.

3.3.10 Manning's Value of Vegetation

       The following Manning's n values are recommended:


                         Table 4 - Manning's Value of Vegetation

                      Height of Vegetation                         n Value

         Dense grass up to 6 inches tall                             0.07
         Dense grass 6-12 inches tall                                0.10
         Dense grass > 12 inches tall                                0.20

         Wetland Plants                                              0.07

         Source:        Biofiltration Systems for Storm Runoff, Water Quality
                        Control - Richard R. Homer, December 1988.
         Note:          Refer also to Chapter 5 in the Virginia Erosion and Sediment
                        Control Handbook.


3.3.11  Depth of Flow

        Biofiltration can be designed for a specific storm or the first flush runoff volume.

The storage volume can be sized based on 0.5 inches of runoff per impervious acre in the

contributing site area (first flush).

        Biofiltration is normally designed as a water quality trench. As such, a significant

portion of the runoff volume will bypass the trench and is not infiltrated. An earthen berm

protected by stone should be installed at the end of the biofilter. Height of the berm should

be the design depth of flow and freeboard. The berm shall facilitate ponding of the storm

runoff. Stone protection of the berm should prevent erosion from overflow. Provision for

overflow can also be made by providing a notch in the berm.






                                   17





I


|                                                                3.4    Design Examples

                                                                                         Design a biofilter swale for a discharge of two (2) CFS. This discharge is computed to be

                                                                  generated from a commercial site of one (1) acre. The vegetative cover selected is dense grass with

                                                                  a Manning's n value of 0.07.  The swale is to be designed for a depth of four (4) inches.

                                                                  Longitudinal slope of the Biofilter swale should not exceed two (2) percent. Design the swale for

||                                                               lengths of 200 feet and 150 feet.



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1 19



1






                                                                                           I  FIGURE 1

                                                                                    Ac
                                                                   I





                                            I






                                        SCHEMATIC SECTION A-A

 BERM AT END
 OF SWALE                                                  A





                                                 SIDE VIEW

                               O  =  Discharge Rate Q (cfs)
                               n =  Manning's Coefficient of Vegetation Cover

                               dF = Depth of Flow (In)

                               S = Longitudinal Slope (%)
                                L =  Length of Swale (Ft)
                               W =  Design Width of Swale (Ft)

                                V =  Design Velocity of Swole (Ft/Sec)

                                A =  Design Area of Swale (Sq Ft)
                                T =  Resident Time (Sec)

     For Length of Swale = 200 Ft:                       For Length of Swole other than 200 Ft
                                                      calculate for length 200 ft and also do:
                            On
              W =          A   5       1
                    0.76 (  )         (   )2                        200 Ft          V = 






               V= 
                    A
                                                      |  BIOFILTRATION SCHEMATIC


                                                                           REVS'D:             DRAWN:  K.F.F.
                                                                           CHK'D:              CHK'D:   V.P.M
                                                     X/v/  ' -~   f    '~ \DATE:                DATE:  OCT. 1990

Smith Demer Normam                                                              REVS'D:             SCALE:  N.T.S.
Engineers - Plnner - Surveyor. - Lndpe Architec  HAMPTON   ROADS               CHK
  Centro Part  Six             12o.ott Sq.re  S,  HAM PTON  ROADS              CHK'D:
          Hampton, VriniPa 23666
 (804)865-9610 (804)627-6900 F  (804)865-1533  PLANNING DISTRICT COMMISSION     DATE:







3.5     Maintenance Requirements

       Maintenance requirements of biofilters are minimal, except for mowing and removal of

sediments. Once the vegetation has been established, mowing should be performed on an as needed

basis. It should be performed at least once a year and the vegetation clippings should not be

allowed to decay in the biofiltration facility. Sediments should be removed whenever the volume

of the facility is determined to be inadequate and ponding occurs. Continuous ponding may require

mosquito control.

        Proper inspection should be performed during the construction of the biofiltration facility.

It should be inspected regularly during the period when vegetation is being established. After the

vegetation has been established, once a year inspection should be enough.

        Maintenance costs of biofilters depend on various factors such as size of the BMP, type of

vegetation, frequency of mowing, and can vary from facility to facility.

3.6    Life Expectancy

        Biofiltration, if properly constructed, inspected, and maintained, can have a life expectancy

of several years. With regular maintenance, it can function properly for 20 years. After more

examples of this BMP have been constructed and monitored a more exact number for life

expectancy can be established.

3.7     Cost

        Bioftlter establishment costs are similar to swale or filter strip costs.  Costs for planting

emergent wetlands could range from $1,000-$3,000/acre for material plus labor.

3.8     Construction Specifications

        3.8.1  Site Preparation

                Install needed erosion and sediment control practices such as silt fences, dikes,

         contour ripping, erosion stops, channel lines, sediment traps, and sediment basins.



                                           21







3.8.2   Vegetation

        Select vegetation according to the site conditions. Use guidelines and specifications

outlined in section 5.8. Select fine, close-growing, water-resistent grasses.

3.8.3   Slope

       The minimum slope for biofilters should normally be two (2) percent and the

maximum four (4) percent. A flatter slope can be specified if it is known that the ponded

runoff will drain and will not be subject to persistent water pooling.

3.8.4 Compaction

        Avoid compaction during construction. If compaction occurs, till before planting

vegetation to restore soil infiltration capacity.

































                                   22







4.   GRASSED SWALE WITH CHECK DAM

       4.1     Description

               Concentrated storm runoff can be impounded behind check dams constructed of railroad

       ties or stone berms to induce infiltration. As the storm runoff overtops the check dams, it can be

       directed to flow over vegetated drainage swales with gentle slopes. The gentle slopes with

       vegetative cover provide non-erosive flow velocities. The combination of low velocities and

       vegetative cover provide an opportunity for sediments to settle out. Grassed swale with check dam

       schematic is shown on Figure 2. Typical grassed swale details are shown on Figures 3 and 4.

       4.2     Applicability

               Grassed swales with check dams are mostly applicable in residential developments of low

       to moderate density where the impervious cover is relatively small. Swales are usually located in

        a drainage easement at the side or back of residential lots or in highway medians. Swales should

       not be designed for large, infrequent storms.

               Swales with check dams can be used in combination with infiltration trenches. The trench

        should be constructed under the swale. The pool created by the check dam increases the volume

        of surface runoff infiltrating into the trench. The grassed swale helps to impede the transport of

        suspended solids downstream. Swales are not generally capable of removing soluble pollutants,

        such as nutrients, because of insufficient residence time and vegetation types.

               This BMP is an alternate to curb and gutter sections and is less expensive.

        4.3     Design Criteria

               4.3.1   Soil Permeability

                       The permeability or final infiltration rate of the underlying soil should be equal to

                or greater than 0.17 inches per hour. Those soil textural classes that have slow infiltration

                rates should not be considered for grassed swales. Thus, the suitable textural classes of the

                soil underlying the swale are sand, loamy sand, sandy loam, loam, and sandy clay loam.

                                                  23







4.3.2   Swale Gradient

       Grassed swales with check dams should not be constructed with bottom slopes of

greater than five (5) percent. Minimum slopes can be as close to zero (0) as drainage will

permit.

4.3.3 Groundwater Table

       The seasonal high groundwater table should be at least one (1) to two (2) feet

below the bottom of the grassed swale.

4.3.4   Design Storm

       Grassed swales with check dams can be designed for a specific storm or for the first

flush runoff volume. The storm runoff volume to b.. ~:cd can be based on 0.5 inch of

runoff per impervious acre in the contributing site area (first flush).

       Runoff associated with less frequent large storms will bypass the grassed swales

without being treated and overtop the check dams.

4.3.5 Storage Time/Maximum Draining Time

       It is recommended in the literature that the maximum allowable ponding time in

swales be 24 hours.

4.3.6 Permissible Velocity

        If a large design storm is used to design the swale, the velocity of flow expected

from the design storm should not exceed the permissible velocity for the type of vegetative

lining used for the swale. Table 5 lists the permissible velocities for various covers.












                                  24










                  Table 5 - Permissible Velocities for Various Ground Covers

                                                              Permissible Velocity (feet/second)
                                          Slope Range
   No.                Cover                Percent (%)       Erosion Resistant    Easily Eroded
                                                                  Soils               Soils

    1       Bermudagrass (Bynodon              0-5                   8                   6
             Dactylon)

    2       Kentucky 31 Tall                   0-5                   7                   5
             Fescue (Festuca
             Arundinacea)

    3       Grass-legume mixture               0-5                   4                   3

    4        Red Fescue                        0-5                  3.5                 2.5
             Redtop (Agrostis Alba)
             Lespedeza Servicea
             Alfalfa

    5       Annuals*                           0-5                   3                   2
             Common Lespedeza
             Sudan Grass
             Small Grain Ryegrass

    6       Rock Riprap Section               5-10                   8                  6.5
             (for temporary
             construction)

*              Annuals are used on mild slopes (less than three (3) percent) or as temporary
               protection until permanent covers are established. Use on slopes steeper than five
               (5) percent is not recommended.
Source:        Maryland Standards and Specifications for Soil Erosion and Sediment Control, 1983.

















                                                25







       4.3.7   Capacity

               The swale must have sufficient capacity to pass the peak discharge rate of the

       design storm. The grassed channel should be designed in accordance with the Manning

       formula.


                                   O..1.49 AR2/3ax/_
                                       n



where  Q =    peak flow rate of the design storm, in cubic feet per second (cfs)

       n=    Manning's roughness coefficient

       A =    Cross-sectional area of the swale (ft.2)

       R =    Hydraulic Radius (ft.)

       S =    Longitudinal slope (ft./ft.)



       4.3.8   Side Slope

               The side slopes of the vegetated swale should not exceed three to one (3:1) and for

        swales lined with riprap two to one (2:1).

       4.3.9   Cross Section

               Swale channel cross-sections can be trapezoid, parabolic, or V-shaped. The

        trapezoidal swale shape is the preferred section due to its ease of construction. With time,

        trapezoidal shapes tend to become parabolic due to the growth of vegetation and settlement

        of solids.










                                          26







4.4     Design Examples

                A one-acre (1 ac.) lot is to be developed for constructing a house with a roof-top

        area of 2,000 square feet. The soil borings on the site indicate that the soil is silt loam with

        an infiltration rate of 0.27 inches per hour. The depth of the seasonal high water table is

        determined to be five (5) feet deep.

                Design a swale to treat the runoff. Swale is assumed to be constructed adjacent to

        the width of the one-acre lot assumed to be 500 feet. The side slopes of the swale should

        be three to one (3:1) and the bottom width of the check dams are assumed to be ten (10)

        feet. The longitudinal slope of the swale is two (2) percent.



































                                            27





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1 28



1







       Design a swale for a row of houses in a residential development. Runoff from this

residential development is computed to generate 0.4 inches of runoff from a one-inch (1")

storm. The contributing site area is two (2) acres.  The soil characteristics at the site

suggest an infiltration rate of 1.2 inches per hour. Groundwater information reveals the

seasonal depth of water to be three (3) feet. Side slopes for the swale to be designed are

three to one (3:1) and the bottom width of check dams is 10 feet. The longitudinal slope

of the swale is three (3) percent.







































                                    29



















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                  ~~~~~~~~~~~~~~~~~I          ~FIGURE 21

      I                                        ~~~~~~~~~~~~P                                   P










           3                                                           ~~~~~~~~~~~~~~~~~~~~DT





                          f  =  Infiltration Rate(ln/Hr)
 3    ~ ~ ~ ~ ~~~T=  Maximum Allowable Ponding Time(Hrs)
                          DT =  Depth to Seasonal High Groundwater Table(Ft)
 3                         ~~~~~~Dul=  Min Dist from Swale bottom to Groundwater Table(Ft)
                           d= Depth of Swale(Ft)
                          WBe=  Bottom Width of Check Dom  (Ft)

                          Z =  Side Slope Ratio (N:1)
                          S =  Longitudinal Slope (%

                           L= Length Behind Check Darns (Ft)
                          LH = Total Hydraulic Length of Swale (Ft)
 3  ~ ~ ~ ~ ~~~A=  Increase in Runoff Depth(ln)
                          AC = Contributing Drainage Area(Sq Ft)
 3                         ~~~~~~~T  =  Swale Filling Time(Hrs)
                          P  =  Amount of Rainfall(ln)
 3                         ~~~~~~~WT =  Top Width of Check Dom (Ft) = We + 2 ds Z

        To Compute L, if not known:

              L  =            ~~~A,                                                         ds (WT +W13LH
         H, = s       1      2     fI                      Volume of Storage (Cu Ft)4
                    4            ~~~12    12

          I  _   d5                             LH              ~~~~~~~~~~~~Adjusted LB    L,
             LB  =  ds       Check Dams =       LH_                                Check Dams
                -  SLB

                                                          GRASSED SWALE WI CHK DAMS SCHEMATICJ

                                                                             REVS'D:____   DRAWN:   K.F.F.
                                                                             CHK'D: ___ _ CHK'D:   v.P.m
                                                                              DATE: ______ DATE:  OCT. 1990
                           SmM   rL-')X,,n~~~~~~r10ff  Normaw                    REVS'D: ~~~~~~~ SCALE:   N.T.S.
    Engineers - Plonnen: - Surveyors - Landscape Architects
      Central Park  Six Manhattan Square Suite 102  HAMPTON   ROADS               CHK'D: ______
        (80)85-910(84)827-60  ar 848512      PLANNING DISTRICT COMMISSION          DT:_______ 
I~~~~~~~~~-90 a 8485i3






                                                                               I  FIGURE 3


















                        SIDE-SLOPES
                       3:1 OR LESS

     SWALE SLOPES
     AS CLOSE TO   . .: :.
     ZERO AS DRAINAGE   'I T
     WILL PERMIT - ,                                                   RA ROAD TIEAM
                                                                  (INCREASES INFILTRATION)

  DENSE GROWTH       ' '  DIER 
  OF GRASS (REED                                                              FILTER FABRIC
  CANARY OR KY-31
  TALL FESCUE
            RAILROAD SPIKE 


                                ï¿½I,..'..' S,-




                                                   STONE PREVENTS
                                                   DOWNSTREAM SCOUR















          I \~~~ |  ~GRASSED SWALE W/ CHK DAMS DETAIL


                                                                REVS'D:           DRAWN:  R.E.C.
                                                   ( r/X/J  f    " /  CHK'D:       CHK'D:  V.P.M.
                                                                DDATE_           DATE:  OCT. 1990

minth Demer Norma                                                   n                     / REVS'D SCALE:  N.T.S.
Engineers - Plonnen - Surveyor - Landscape Architects  TA1  Tr T 1    A T  CHK'D:
 Cantrl Park  5Y Ucnhottan Square Suite 102                      HAMTON  ROADS  DA: PAG  3 7
        Hampton  Virginia 23666   -     
(804)865-96 0 (804)627-6900 Fa (804)865-1533  PLANNING DISTRICT COMMISSION    DATE: 






                                                                | FIGURE 4 






                                    REAR 
                                   SLOPES
                                    TO
                                   SWALE  \

         STR EET SWALE STR EET








                                  SIDE VIEW













                                   U)     y
                +____-____    _ ____  _
           LO CD











                                  TOP VIEW




                           I| GRASSED SWALE W/ CHK DAMS


                                                    REVS'D:          DRAWN: K.F.F.
                                                    CHK'D:           CHK'D:  V.E.M.
                                                    DATE:            DATE: OCT. 1990
Smith Demer Normann                                                   REVS'D:           SCALE:  N.TS.
Engineen - Pannonr - Surveyor - Land.cope Architcts   R               CHKD:
 Centrol Park  Six MonhattCn Square Suite 102  HLAVLP 1 TON  R ADS   CHKD: 
       Fbmplton, vilinic 2365       PAITCMSO                        DEPAGE   33
(804)865-59610 (804)627-6900 F: (804)865-533  PLANNING DISTRICT COMMISSION 







              4.5    Maintenance Requirements

                      Swale maintenance is very minor if the swale is properly installed. An annual inspection

              of the site is recommended to assure that the swale is functioning properly. Swale maintenance

              consists mainly of keeping the vegetative cover dense and vigorous and involves periodic mowing,

              and reseeding of bare spots. Mowing of the swale too close to the ground (scalping) should be

              avoided. Vegetative cover should remain at least three (3) inches. Any sediments collected behind

              the check dam should be periodically removed. Some temporary nuisance problems like mosquito

              breeding may develop and can be alleviated by periodic mowing. Persistent ponding in swales can

              be eliminated by debris cleaning, mowing, and drilling holes in the bottom of the swale.

                      Maintenance costs for swales depend on various factors such as size of the swale, type of

              vegetation, number of check dams, frequency of mowing, and can vary for each swale.

              4.6    Life Expectancy

                      Grassed swales have long been used as ditches for highways. Their life expectancy as water

               quality facilities has not been monitored sufficiently. If properly constructed, inspected, and

               maintained, it is estimated that they can last for 20 years.

               4.7    Cost

                      The typical costs for establishing the vegetative cover by various seeding methods are given

               below:

I~~~~~~~~~~~~~~~~~~~~~~~~~
                                    Table 6 - Typical Costs for Establishing Vegetative Cover

                         Grading                                      $3 - $8 per cubic yard

                         Hydroseeding (with mulch and fertilizer      $ 1,500 - $1,750 per acre

                         Conventional Seeding                         $1,200 - $1,600 per acre
                         Seed/straw mulching cost for a swale         $2 - $4 per linear foot
                         Railroad Ties 6" x 8"                        $4 - $6 per linear foot




                                                         34







4.8     Construction Specifications

       Check dams can be constructed of railroad ties, wood logs, gabions or other suitable

material. Wood logs should be pressure treated logs or made of water resistant tree species such

as cedar, hemlock, swamp oak or locust. Earthen check dams are not recommended as they can

be easily eroded. Check dams should be installed perpendicular to the direction of flow and can

be anchored into the sloping sides of the channel. The toe of the check dam should be protected

by riprap which should be placed over a suitable geotextile fabric.

       Gabions used as check dams should be made of hexagonal triple twist mesh with PVC

coated galvanized steel wire. The maximum linear dimension of the mesh opening shall not exceed

4.5 inches and the area of the mesh opening shall not exceed ten (10) square inches.

        Stone or riprap for gabions should be sized according to the following criteria:



                           Table 7 - Gabion Sizing Criteria

            Basket Thickness                                 Stone Size
                (Inches)                                      (Inches)
                    6                                           3-5
                   9                                           4-7
                   12                                          4 - 7
                   18                                          4-7
                   36                                          4- 12



        The stone or riprap shall consist of field stone or rough unhewn quarry stone. The stone

shall be hard and angular and of a quality that will not disintegrate on exposure to water or

weathering. The specific gravity of the individual stones shall be at least 2.5.

        A small notch or depression shall be provided in the gabion dam to create a flow channel

in the center of the dam for ovcrflows.





                                          35







FILTER STRIP

5.1     Description

        A vegetative filter strip is an area of vegetative cover through which the storm runoff flows

before it leaves the site. The storm runoff must be evenly distributed across the filter strip and the

flow velocity of the runoff should be reduced. Concentrated flow from the site across the filter strip

should be avoided. Concentrated flows tend to form a channel. Once a channel is formed, filter

strips will not perform as designed. The flow can be evenly distributed across the filter strip by

using level spreaders.  A vegetated filter strip detail with spreader is shown on Figure 7. A

vegetated filter strip can provide the following benefits.

        ï¿½       Serves as an effective method of reducing sediment yield by protecting the soil

        from rainfall impact energy.

        ï¿½ Reduces runoff by reducing overland flow velocities, increasing the time of

        concentration, and increasing infiltration.

        ï¿½       Removes suspended sediment iJroverland flow by filtering, absorption, and gravity

        sedimentation as the flow velocity is reduced.

5.2     Applicability

        Vegetative filter strips can be used as the sole BMP or in combination with other BMPs.

All BMP structures should be surrounded by vegetative filter strips to alleviate the sediment load

being delivered to the BMP.

5.3     Design Criteria

        5.3.1   Flow

                The vegetative filter should be used to control overland sheet flow only. If the

        filter will be subject to any concentrated flows, such as found at low points in parking lots

        or grass areas, then a level spreader should be used to establish sheet flow.



                                          36







5.3.2   Selecting the Tvwe of Vegetation

        The selection of vegetative materials ranges from using existing vegetation to

specifying a vegetation mix tailored to suit the characteristics of the site. Table 9 should

be used as a guide in selecting the vegetation type.

5.3.3   Slope Characteristics

        The effectiveness of vegetative filters as sediment control devices decreases with

increasing slope. Filter strips are not effective on slopes greater than 15 percent.

5.3.4   Runoff

        When filter strips are used in treating sediment-laden runoff, the following shall be

considered:

        (1)     Good drainage to ensure satisfactory performance.

        (2)     A level spreader at the inlet to ensure uniform distribution of flow.

        (3)     An adequate filter area and length of flow to provide the desired treatment.

        (4)     Slopes less than five (5) percent are more effective; steeper slopes require

        a greater area and length of flow to achieve the same effectiveness.

        (5)     Provisions for mowing and removing undesirable vegetation to maintain the

        effectiveness of the filter area.

5.3.5   Length of Filter Strip

        The minimum length of filter strip used in conjunction with all other BMPs should

be 20 feet.

        Additional guidelines to assist the designer in calculating the trap efficiency of an

existing vegetative buffer strip, or the length of vegetative filter required to provide a

specific trap efficiency are provided below.





                                    37







             5.3.6   Graphical Solution

                     A solution for computing the sediment trap efficiency of a vegetative filter strip can

             be represented graphically. Figure 5 shows the relationship between trap efficiency (TR)

             and the length and slope of the filter strip, as well as the roughness coefficient of the

             vegetation (Manning's n). The required length of a filter strip is very sensitive to variation

             in the trap efficiency as it approaches 100 percent, indicating that a small incremental

             increase in the trap efficiency requires a considerable addition in the filter strip. The curves

             also suggest that a significant trap efficiency (up to 75 percent) may be achieved at

             relatively short filter strip lengths. Figure 5 assumes a coarse silt material.

                     The trap efficiency for other soil textures may also be determined using Figure 5.

             The settling velocity of sediment particles manifests the appropriate trap efficiencies that

              are attainable using filter strips for a particular particle size. In general, the greater the

              settling velocity, the higher the trap efficiency per length of filter strip. For example, the

              ratio of the settling velocities for a coarse silt and a fine silt is 4.9. Thus, the filter strip

              length obtained from Figure 5 should be multiplied by this ratio to obtain the filter strip

              length for a fine silt. This would provide the same trap efficiency indicated on Figure 5.

              The settling velocity ratio of coarse silt to medium silt, fine sands, and medium sands are

              1.3, 0.02, and 0.005 respectively. These ratios are shown on Table 8.


                              Table 8 - Effective Buffer Strip Length
         Type of Soil                Ratio of Settling Velocity        Effective Buffer Strip Length

Coarse Silt                                        1                   1 x length from Figure 5
Fine Silt                                         4.9                  4.9 x length from Figure 5
Medium Silt                                       1.3                  1.3 x length from Figure 5

Fine Sands                                       0.02                  0.02 x length from Figure 5
Medium Sands                                     0.005                 0.005 x length from Figure 5


                                                  38






  14-
  13-                               /

  1 2-

  11- Soo
  *  10-
   i    n:0.80                                                   -1500
E-                    n:0.35


: 6.-          //    /0141400


   5-        //                                                    1300
   4i









                     I                           /           - %ooR 
                     1                          / r1









                                                     -600 0


                                               TR: 9  O%



                                                     /  00


                      /I                                     -400
                      I                                :85%


                                                 | X 




               0.33           0.67           1.00            1.33

                    RUNOFF VELOCITY (ft/sec)


Figure  5    Effective Buffer Length Determination for Trap Efficiencies (TR) of 75 to 99
          percent (Buffer length for coarse silt.)

Source:  Wong and McCuen, 1982 39







5.4     Design Examples

Example 1

        Design the length of a vegetated filter strip required to remove 95% of sediments from the

runoff. The type of vegetation selected for filter strip is dense grass of a height greater than 12

inches. Slope of the filter strip will be two (2) percent. The type of soil for the filter strip is coarse

silt. Use Figure 5 to determine the effective length of the filter strip.

        From Table 4, select the Manning's value (n) for dense grass greater than 12 inches. For

a slope of two (2) percent, draw a straight line to intersect the curve for selected n value of 0.20.

Draw a vertical line from the point of intersection to intersect 95 percent trap efficiency (TR) curve.

Draw a straight line from this point to find the effective filter strip length of 200 feet.

































                                             40






                Example 2

                        Design a vegetative filter strip for underlying soil of medium silt. The slope of the filter

                strip is four (4) percent and the type of vegetation selected is dense grass with a Manning's

                roughness coefficient (n) of 0.20. The filter strip is assumed to trap 75% of sediments from the

                runoff. Use Figure 6 to determine the effective length of the filter strip.

                        By using the procedure outlined in example 1, effective filter strip length is approximately

                55 feet. As the underlying soil is medium silt, multiply this length by 1.3 (from Table 8) to arrive

I               ~~~~~at effective filter strip length of 72 feet.
























          ~~~~~~~~~~~~~~41









         12-

         '11-

          1  901  n:0.80                                        -1500

                           n.n:O.35
       I  7-                                                    -1400
       0                               n:0.20
       -, 5-
I          5                                                     - -1300

          4------
          3-                        I                           -1200 D
          2- -

                               Ta  9 9 P 


                        1-    A,                                ~~~~~~-1100  zQ
                                                                        W
                                                         TR:95%







                                                        ~~-1000





             1                                                  ~~~~~~~~~~~~~~-900
               U                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~IO



















                                                       T, t  a 5
                                            I                   -8~~~~~00 













                        0.3    a 0 7              1 .00         1.33

                             RUNOFF VELOCITY (ft/sec)
                                 EXAMPLE 2
       Figure  6  Effective Buffer Length Determination for Trap Efficiencies UpR) of 75 to 99
                  percent (Buffer Length for coarse silt.) 4

       Source: Wong and McCuen, 1982 4





                     3-                                                                      ~~~~~~~~~~~~~~~FIGURE 71








                                                                              FLOW SPREADER

          I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~fl





                I~~~~~~~~~~~~~~~~~~~~~~~~~~~~A
     I~~~~~~~AUA ORMNMD
     ~~~~~~~~~VG ILE








             ~~~~~~~~~EEAIVFITRDAL








                 U                                            VEGET~~~~~~~~~~~~~~~~~~~~~ATIE FILTE: OCT.AIL9

        Smrfff Denier Nom-am                                                   REV        _D SCALE:  N.T.S.
U       ~~~~Engineers  Planners - Surveyors - Landscape Architecta              EsD_     ___
          Central Park  Six W~nhottam Square Suite 102   HA M PON   RAD   CK'D
                    mnpo.Virginao 23856              PT NR AGE                43D:P
         (B104)BGS-WO1 (804)627-6900 Foocn (804)665-1533  PLANNING DISTRICT COMMISSION DAE______







5.5     Maintenance Requirements

        Maintenance is a vital factor in maintaining an adequate vegetative erosion control cover.

See Table 10 to obtain the maintenance fertilization program for permanent seedings. When the

filter strip is established and is functioning properly, fertilization should be minimized.

        5.5.1   Irrigation: If soil moisture becomes deficient, irrigate to prevent loss of stand of

        protective vegetation.

        5.5.2  Repairs:  Inspect all seeded areas for failures and make necessary repairs,

        replacements, and reseedings within the current planting season, if possible.

                (1)    If a stand is inadequate for erosion control, overseed and fertilize using half

                of the rates originally applied.

                (2)    If stand is over 60 percent damaged, reestablish following original lime,

                fertilizer, seedbed preparation, and seeding recommendations.

        Maintenance costs for filter strips depend on various factors such as length of the filter strip,

type of vegetation, frequency of mowing, and can vary for each filter strip.

5.6     Life Expectancy

        Filter strips are similar to grassed swales. As a BMP, filter strips can last for a long time,

probably 10 to 20 years, if ideal conditions are maintained on site. Life expectancy of the filter

strip may be only six (6) months if evenly distributed sheet flow and uniform, dense, and vigorous

vegetation is not maintained.

5.7     Cost

        Vegetative filter strip costs are similar to grassed swales costs. Costs are minimal when

existing grass or meadow area is reserved at the site before development begins. If a filter strip is

used as an on-site erosion control practice during the construction phase of development, it can be

rehabilitated with a small expenditure after the development is complete.



                                           44







5.8     Construction Specifications

       5.8.1   Site Preparation

                (1)    Install needed erosion and sediment control practices such as silt fences,

               dikes, and contour ripping, erosion stops, channel lines, sediment traps, and

                sediment basins.

                (2)     If grading is required and topsoil is suitable for use, remove and stockpile

                the topsoil.

                       Note: Topsoil salvaged from the existing site may often be used but it

                should meet the same standards as set forth in these specifications. The depth of

                topsoil to be salvaged shall be six (6) inches unless the depth described as a

                representative profile for the particular soil type in the soil survey is less than six

                (6) inches, in which case the lesser depth shall be removed.

                (3)     Grade as needed and feasible to permit the use of conventional equipment

                for seedbed preparation, seeding, mulch application, anchoring, and maintenance.

                (4)     Liming: Where the subsoil is either highly acid or composed of heavy

                clays, ground dolomite limestone shall be spread at the rate of two (2) tons per acre

                (100 pounds per 1,000 square feet).  Lime shall be distributed uniformly over

                designated areas and worked into the soil in conjunction with tillage operations as

                described in the following procedures.

                (5)    Tilling:  After the area to be topsoiled has been brought to grade, and

                immediately prior to dumping and spreading the top-soil, the subgrade shall be

                loosened by discing and by scarifying to a depth of at least three (3) inches to

                permit bonding of the topsoil to the subsoil. The track of a bulldozer moving

                perpendicular to the contour will create small horizontal check dams to help prevent

                top soil from sliding down the slope.

                                           45









5.8.2  Soil Preparation and Amendments

       (1)    Materials:  Topsoil shall be loamy sand, sandy loam, loam, or silt loam,

       only, and in that respective order of preference. It shall not have a mixture of

       contrasting textured subsoil and contain no more than five (5) percent by volume

       of cinders, stones, slag, coarse fragments, gravel, sticks, roots, trash, or other

       extraneous materials larger than 1-1/2 inches in diameter. Topsoil must be free of

       plants or plant parts of bermudagrass, quackgrass, Johnsongrass, nutsedge, poison

       ivy, or Canada thistle. All topsoil shall be tested by a recognized laboratory for

        organic matter content, pH and soluble salts. A pH of 6.0 to 7.5 and an organic

        content of not less than 1.5 percent by weight is required. If the pH value is less

        than 6.0, lime shall be applied and incorporated with the topsoil to adjust the pH

        to 6.5 or higher. Topsoil containing soluble salts greater than 500 parts per million

        shall not be used.

               No sod or seed shall be placed on soil which has been treated with soil

        sterilants or chemicals used for weed control until sufficient time according to

        manufacturer guidelines has elapsed to permit dissipation of toxic materials.

                Note: Top,,.il substitutes or amendments as approved by a qualified

        agronomist or soil scientist may be used in lieu of natural topsoil.

        (2)    Grading: The topsoil shall be uniformly distributed and tracked and shall

        be a minimum compacted depth of six (6) inches. Spreading shall be performed

        in such a manner that sodding or seeding can proceed with a minimum of

        additional soil preparation and tillage. Any irregularities in the surface resulting

        from topsoiling or other operations shall be corrected in order to prevent the

        formation of depressions or water pockets. Topsoil shall not be placed while in a

                                   46







       frozen or muddy condition, when the subgrade is excessively wet, or in a condition

       that may otherwise be detrimental to proper grading and seedbed preparation.

       (3)     Lime and fertilize according to soil tests: Lime and fertilizer needs can be

       determined by a qualified soil testing laboratory.

       (4)     In lieu of soil tests, apply 1,000 pounds of 10-10-10 (basic fertilizer) or

       equivalent per acre if ureaform fertilizer is not used, and 600 pounds of 10-10-10

       or equivalent per acre if ureaform fertilizer is used. Apply the lime and basic

       fertilizer before seeding and harrow or disc uniformly into the soil to a minimum

       depth of three (3) inches on slopes flatter than three to one (3:1). On slopes steeper

       than three to one (3:1), the lime and fertilizer shall be worked in as well as

       possible. On sloping land, the final harrowing or discing operation should be on

       the contour wherever feasible. No attempt should be made to drag any disced area

       to make the soil surface very smooth after discing. When the 600 pounds per acre

       rate of 10-10-10 basic fertilizer application is used, then at the time of seeding,

       apply 30-0-0 ureaform fertilizer at a rate of 400 pounds per acre.

               Note: The slow release ureaform fertilizer will supply nitrogen over a

       longer period of time and will result in a healthier stand of grass.

5.8.3   Seeding

        (1)     Select a mixture from Table 9.

        (2)     Apply seed uniformly with a cyclone seeder, drill, cultipacker seeder, or

        hydroseeder (slurry includes seed and fertilizer) on a firm, moist, seedbed.

        Maximum seeding depth should be 1/4-inch on clayey soils, when using other than

        hydroseeder method of application.

                Note: If hydroseeding is used and the seed and fertilizer is mixed, they

        will be mixed on site and the seeding shall be immediate without interruption.

                                   47







       Locally accepted and approved seeding mixtures can also be used. CBLAD

recommends the permanent seeding guidelines contained in the Virginia Erosion

and Sediment Control Handbook.

















































                         48










                                         Table 9 - Permanent Seeding and Seeding Dates

                                                                     Lbs/                        Coastal Plain
 Mix      Seeding Mixtures (Use Certified Seed if         Lbs/       1000         -                             1
 No.      Available)                                     Acre      Sq. Ft.     2/1- 4/30          5/1 - 814         8/15 - 1031

   1      'Kentucky 31' Tall Fescue*                       60        1.38           X                  -                 X

  2      'Kentucky 31' Tall Fescue*                       60         1.38                            X
          'Boer' 'Lehmans' (a)
          Weeping Lovegrass                                2         .05

  3      'Kentucky 31' Tall Fescue*                       50         1.15           X
          'Korean' lespedeza (b) inoculated (h)           15         .34

  4       'Kentucky 31' Tall Fescue*                      40         .92           X                                    X
          'Interstate' Serices lespedeza (b)(h)           20         .46

  5       'Kentucky 31' Tall Fescue*                      40         .92
          Birdsfoot trefoil, inoculated(h)                10         .23

  6       'Kentucky 31' Tall Fescue (759%')                                         X                                   X
          Redtop (5%)
          Canada Bluegrass (10%)                          90          2
          Kentucky Bluegrass (10%)(e)

  7      Kentucky Bluegrass (50%)                                                   X                                   X
          "Pennlawn' Creeping Red Fescue (40%)            90          2
          Redtop (10%)

  8      Droughty Areas
          'Kentucky 31' Tall Fescue*                      30         .69            X                                   X
          Redtop                                           5         .11

  9      Weeping lovegrass                                 2         .05            X                 X
          Serecia lespedeza(b) inoculated(h)              20         .46

  10      Poorly Drained Areas
          'Kentucky 31' Tall Fescue* 30                              .69            X                 -                 X

  11      Reed canarygrass (c)                             10         .23            -                 -                 X

  12      Shaded Areas
          'Kentucky 31' Tall Fescue*                      60         1.38           X                 -                 X

  13      Red Fescue 'Jamestown' or 'Pennlawn'            40          .92           X                 -                  X

  14      Lawns & High Maintenance Areas
          'Plush', 'Birka', 'Parade', 'Vantage'
          'Columbia', 'Merion', 'Adelphi', 'South')**     90          2             X                 -                 X
          'Dakota', 'Kenblue', Kentucky Bluegrass,
          Red Fescue, 'Pennlawn' or 'Jamesto,'n'          10         .23

  15      'Kentucky 31' Tall Fescue* (g)                  220-
                                                          260        5-6            X               X(f)                x

Source:   Maryland Standards and Specifications for Stormwater Management Infiltration Practices.
*         Certified Seed Only
**        Three (3) varieties at 30 lb. each to make the 90 lb. mix.









                                                               49







Footnotes for Table 9.

(a)     Use Weeping lovegrass to provide a stand of grass for erosion control during summer.
(b)     Use hulless seed.
(c)     Preferable to seed in fall with seed from current year's crop.
(d)     All mixtures except 2 and 9 may be seeded during winter months in an emergency if two (2) tons
        per acre of a well-anchored mulch is used.
(e)     Approved State Highway Administration Mixtures.
(f)     Can be seeded during this period if irrigation water is used.  Use two (2) tons per acre of well-
        anchored straw mulch.
(g)     Can use ten (10) percent Kentucky bluegrass.
(h)    Lemuminous Seeds. All leguminous seeds shall be inoculated or treated with unexpired approved
        culture for the specific legume in the proper proportions as specified on the package label. The
        inoculant shall be stored at room temperatures, out of direct sunlight and away from heating units.
        When seeding dry with mechanical seeders, the following method of mixing the inoculant with the
        seed shall be followed. The culture in powder form is preferred and shall be thoroughly mixed with
        the seed by using a very small quantity of water-, just enough to dampen the seeds before the culture
        is powdered on. The leguminous seed is then mixed with the other seeds of the formula. Seeds
        inoculated with the powder shall be sown within 48 hours after treatment. Seeds inoculated with
        the liquid culture shall be sown within 24 hours after treatment. Inoculated seed not used within
        these time periods shall be reinoculated. Inoculant and seed treated with inoculant shall not be
        exposed to sunlight for more than one (1) hour prior to seeding. When seed is applied by hydraulic
        seeders, ten (10) times the quantity of inoculant recommended for dry leguminous seed application
        shall be used. Inoculated seed shall not be held in a slurry with fertilizer for more than one (1)
        hour, otherwise reinoculation will be required before applying the seed.















                                                   50







5.8.4 Mulching

               Mulch materials are listed in order of their effectiveness. Mulch mattings

       are normally only used on critical areas such as waterways or steep slopes.

5.8.5  Materials and Amounts

       (1)    Mulch mattings: Mattings such as jute or excelsior blanket shall be stapled

       to the surface in waterways and on steep slopes. Lighter materials of paper, plastic,

       and cotton mulch mattings may be used where erosion hazard is not severe. If the

       area is to be mowed, do not use metal staples.

       (2)    Straw: Straw shall be unrotted small grain applied at the rate of 1-112 to

       two (2) tons per acre, or 70 - 90 (two bales) pounds per 1,000 square feet. Mulch

       materials shall be relatively free of all kinds of weeds and shall be free of

       prohibited noxious weeds such as thistles, Johnsongrass, and quackgrass.

               Spread uniformly by hand or mechanically. For uniform distribution of

       hand spread mulch, divide area into approximately 1 00n  square foot sections and

       place 70-90 pounds of mulch in each section.

        (3)    Wood cellulose fiber: Mulch at the rate of 1,500 pounds per acre or 35

       pounds  per 1,000 square feet.   Wood  cellulose fiber may  be  applied by

       hydroseeding.

5.8.6   Mulch anchoring:  Anchoring shall be accomplished immediately after mulch

placement to minimize loss by wind or water. This may be accomplished by one of the

following methods, (listed by preference) depending upon size of area, erosion hazard, and

cost.  On sloping land, practice No. 1 below, should be accomplished on the contour

whenever possible. Contouring of all operations applies to all straw and to wood chip

practices on more critical sites, except "tracking" should be done up and down the slope

with 1-1/2 inch cleat marks running across the slope.

                                  51







(1)     Mulch anchoring tool and tracking:  A mulch anchoring tool is a tractor

drawn implement designed to punch and anchor mulch into the surface two (2)

inches of the soil. This practice affords maximum erosion control but is limited to

flatter slopes where equipment can operate safely. "Tracking" is primarily used on

three to 1 (3:1) or steeper cut and fill slopes to cut and mulch into the soil by 1-1/2

inch track cleats of a bulldozer making groves across the slope.

(2)     Mulch netting: Staple lightweight biodegradable paper, plastic, or cotton

nettings over the mulch according to the manufacturer's recommendations. Netting

is usually available in rolls four (4) feet wide and up to 300 feet long.

(3)     LiQuid Mulch Binders:  Applications of liquid binders should be heavier

at edges where wind catches mulch, in valleys, and at crests of banks.  The

remainder of the area should be uniform in appearance. Caution should be used

with asphalt in residential and similar areas.

        (a)     Cutback asphalt - rapid curing (RC-70, RC-250, and RC-800) or

        medium curing (MC-250 or MC-800). Apply five (5) gallons per 1,000

        square feet or 200 gallons per acre on flat areas and on slopes less than

        eight (8) feet high.  On slopes eight (8) feet or more high, use eight (8)

        gallons per 1,000 square feet or 348 gallons per acre.

        (b)    Emulsified asphalt - (SS-1, CSS-1, CMS-2, MS-2, RS-1, RS-2,

        CRS-1, and CRS-2). Apply five (5) gallons per 1,000 square feet or 200

        gallons per acre on flat areas and on slopes less than eight (8) feet high.

        On slopes eight (8) feet or more high, use eight (8) gallons per 1,000

        square feet or 348 gallons per acre.

                All asphalt designations are from the Asphalt Institute

        Specifications.

                           52







               (c)     Synthetic binders: Synthetic binders such as Acrylic DI (Agri-Tax,

               DCA-70, Petroset or Terra Tac may be used at rates recommended by the

               manufacturer to anchor mulch material.

       (4)     Wood Cellulose Fiber: Wood cellulose fiber may be used for anchoring

       straw. The fiber binder shall be applied at a net dry weight of 750 pounds/acre.

       The wood cellulose fiber shall be mixed with water and the mixture shall contain

       a maximum of 50 pounds of wood cellulose fiber per 100 gallons.

       (5)     Peg and Twine: Drive eight- to ten-inch (8" to 10") wooden pegs to within

       two (2) to three (3) inches of the soil surface every four (4) feet in all directions.

       Stakes may be driven before or after applying mulch. Secure mulch to soil surface

       by stretching twine between pegs in a criss-cross within a square pattern. Secure

       twine around each peg with two (2) or more complete turns.

               Note: All names given above are registered trade names. This does not

       constitute a recommendation of these products to the exclusion of other products.

5.8.7   Irrigation

       If soil moisture is deficient, supply new seedings with adequate water for plant

growth until they are firmly established. This is especially true when seedings are made

late in the planting season, in abnormally dry or hot seasons, or on adverse sites.

















                                  53









                                    Table 10 - Maintenance Fertilization for Permanent Seedings
                                        Use Soil Test Recommendations or Rates Shown Below

                                                                      Lbs. Per
 Mixture                                                 Lbs. Per       1,000
   No.         Seeding Mixture         Formulation         Acre         Sq.Ft.              Time                      Mowing

1,2,3,7,8,  Tall fescue makes up        10-10-10           500          11.5      Yearly or as needed       * Not closer than 3" if
10         70% or more of cover             or                                                              occasional mowing is
                                          30-0-0           400           9.2      Fall                      desired.
                                         10-10-10          600          13.8      Yearly or as needed

4,5        Fairl' , ,iform stand         5-10-10           500           11.5      Fall the year following   Not required. Not closer
            of t,   icue and                                                       establishment and         than 4" if occasional
            sericea icspedeza, or                                                  every 4-5 years           mowing is desired,and then
            birdsfood trefoil                                                     thereafter.                in fall after seed has
                                                                                                            matured.

11         Weeping lovegrass and         5-10-10           500          11.5      Spring the year           Not required. Not closer
            sericea lespedeza.                                                     following                 than 4" if occasional
           Fairly uniform plant                                                   establishment and         mowing is desired and then
            distribution                                                           every 3-4 years           in fall after sericea has
                                                                                  thereafter                matured.

9,12,13,1   Red fescue                  20-10-10           250           5.8       September, 30 days        Mow no closer than 2" for
4                                                                                  later                     red fescue and Kentucky
                                                                                                           bluegrass; and no closer
15,17      Kentucky bluegrass-          20-10-10           250           5.8      December,                 than 3" for fescue.
            red fescue mixture;         20-10-10           250           5.8      May 20-June 30 if
            'Ky-31' tall fescue         20-10-10           100           2.3      needed

Source:        Maryland Standards and Specifications for Soil Erosion and Sediment Control.
Note:          Under Mixture No., refer to Table 9.





                                                                   54







6.   DRY WELL

        6.1     Description

                A dry well is an excavated pit lined with engineering filter fabric and backfilled with stone

        aggregate. The dry well is generally a much smaller structure than an infiltration trench. Inflow

        to the dry well is mostly through an inflow pipe. A typical dry well schematic is shown on Figure

        8 and a dry well detail is shown on Figure 9.

        6.2     Applicability

                A dry well is generally used to capture the runoff from roof top areas of less than one (1)

        acre in surface area. This BMP is used to store runoff from residential, commercial, and industrial

        buildings.

        6.3     Design Criteria

                 6.3.1   Soil Permeability

                         Soil textural classes with infiltration rates greater than, or equal to, 0.27 inches per

                 hour should be used for the design of dr9 wells. This infitration rate is associated with soil

                 textural groups of sand, loamy sand, sandy loam, loam, and silt loam. The infiltration rate

                 of the underlying soil where the well is located is the major limiting factor in the selection

                 and feasibility of the dry well as a BMP.

                 6.3.2   Depth of Well

                         The final infiltration rate of the soil below the dry well determines the maximum

                 allowable well depth. A well with a grass covered surface should have at least one (1) foot

                 of overlying soil above the stone aggregate reservoir. In case a dry well is installed under

                 a driveway or a patio deck, the depth of overlying soil should be considered zero. The

                 surface area of the well can be minimized by making the dry well as deep as feasible. On

                 the other hand, the dry well can be made shallow and broad. The increased surface area

                 of the bottom of the dry well increases exfiltration and provides more area for soil filtering

                                                    55







of pollutants. The larger well bottoms also help in reducing clogging at the soil/filter cloth

interface by providing exfiltration over a wide area.

6.3.3   Groundwater Table

       The seasonal high groundwater table should be located at least two (2) to four (4)

feet below the bottom of the well. The soil permeability (infiltration rate) and the

groundwater table are the two parameters which determine the maximum allowable depth

of the well.

6.3.4   Proximity to Wells and Foundations

        Dry wells should be located at least 100 feet upgradient from any drinking water

supply well to minimize the possibility of groundwater contamination. Also the dry well

should be located at least ten (10) feet downgradient and 100 feet upgradient from building

foundations.

6.3.5   Design Storm

        Dry wells can be designed for a storm of a specific recurrence interval or for the

first flush runoff volume. If for first flush, storage volume can be sized based on 0.5 inches

of runoff per acre of rooftop area.

        Dry wells are normally designed as water quality facilities for runoff generated from

rooftops. As such, a significant portion of the runoff volume from the site will bypass the

dry well and is not infiltrated.  Additional BMPs should be installed to infiltrate storm

runoff from other impervious areas on the site.

6.3.6 Storage Time/Maximum Draining Time

        All dry wells should be designed to drain within a maximum time of three (3) days

(72 hours), or a minimum time of two (2) days (48 hours). These values are derived from

existing literature. The Virginia Stormwater Management Regulations recommend two (2)

days (48 hours).

                                   56







6.3.7  Stone Amregate

       The stone aggregate which fills the dry well forms the reservoir through which the

storm runoff passes and is filtered. The aggregate material should be clean, washed stone.

Wash run gravel is preferred. The City of Virginia Beach recommends using James River

stone as aggregate. The clean washed stone aggregate should have a maximum diameter

of three (3) inches and a minimum diameter of one (1) inch. Void spaces for the stone

aggregate should fall within the range of 30 to 40 percent. A table showing open graded

coarse aggregates is included in the Appendix.

6.3.8   Observation Well

       An observation well should be installed in the dry well. The observation well can

be a perforated PVC pipe, four (4) to six (6) inches in diameter.  The pipe should be

located in the center of the well with the bottom resting on a plate. The top of the

observation well should be capped to prevent vandalism.

       The observation well helps in monitoring the function of this BMP. The water

level in the observation well should be measured after a storm event. If the dry well does

not drain completely after three (3) days, the well is not functioning properly and remedial

steps may need to be taken to improve its performance.

6.3.9   Runoff Filtering

        Since the dry well is designed to capture the runoff from rooftops, screens should

be placed at the top of the roof downdrains to prevent leaves and other debris from entering

the dry well.

6.3.10 Overflow Reauirements

        The overflow path of the surface runoff exceeding the capacity of the dry well

should be evaluated. A surcharge pipe above the dry well should be installed to allow

drainage in extreme events.

                                  57







6.4     Design Examples

        A one-acre (1 ac.) lot is to be developed for constructing a house with a roof-top area of

2,000 square feet. The soil borings on the site indicate that the soil is silt loam with an infiltration

rate of 0.27 inches per hour. The depth of the seasonal high water table is determined to be five

(5) feet deep.

        Design a dry well to capture the runoff from a one-inch (1") rainfall and for first flush. The

runoff from a one-inch (1") rainfall is computed to be 0.30 inch. The depth of soil over the dry

well is one (1) foot and runoff depth from area over dry well is computed to be 0.03 inch.





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I I.



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1 60



1






                                      P                                                      | FIGURE 8















                                                       Q dw



                                                   VR
                                IXP~~~f DM









                         f  =  Infiltration Rate(In/Hr)

                         Ts =  Maximum Allowable Storage Time of Well(Hrs)
                         VR =  Void Rotio Well Medium
                         Dr =  Depth to Seasonal High Groundwater Table(Ft)

                         Dm =  Min Dist from Well bottom to Groundwater Table(Ft)

                         Ac =  Rooftop Area of Runoff to Dry Well(Sq Ft)

                         Qc =  Runoff Depth from Rooftop Area (In)
                         Qo =  Runoff Depth from Area over Dry Well(ln)
                         dw =  Depth of Well(Ft)

                         do =  Depth of Soil Overlying Dry Well(Ft)

                         P  =  Rainfall(In)
                         T  =  Dry Well Filling Time(Hrs)

                         C  =  Water Capacity of Overlying Soil(In/In)
                                                                              Qc
                                                                          Ac f2
          For First Flush Design:                      Area of Well =                     12
                                                                VR dw  - P +  + deC+  f T
                             Qc                                                 12   12           12
        Area of Well -=                                                              P              Qo
                           VR dw                   NOTE:  IF doC >                   SET    2 --+   +  doC  = O


                                                          DRY WELL SCHEMATIC


                                                                     REVS'D:             DRAWN:   K.F.F.
                                                                     CHK'D:              CHK'D:   V EuM.
                                                                     DATE:               DATE:   OCT. 1990

Smff Der er Norma tREVS'D:                                                                            SCALE: N.T.S.
Engineer - Plonns - Surveyors - LandscApe Architect) sIS,
 Central Park  Six Manhattn Square Suite 102  HAMPTON   ROADCHK'D:61
         Hampton. virginio 2356 DATE: PAGE1
(804)865-s610  (804)627-6900 For (804)865-1533  PLANNING DISTRICT COMMISSION     DATE:






                                                                                   FIGURE 9








                   ROOF LEADER





                            OVERFLOW PIPE

                                           SPLASH BLOCK

                                                      *  CAP WITH LOCK





                                                                          /   COVER VARIES
                                                                              OVER DRY WELL
                                              OOOOooop

                                           000 0ocIDooo
                                        ip--o-0  0000-00 C)00-00 (0 00
                   /DRYT PI    '    2              00
                    ZDYWELL                        0
                     INLET PIPE                     00                           CAP END OF PIPE
I~~~~~~~~~~~~~~~~~11                                 00
 BUILDING                            STONE FILL     00oo  TEST WELL
 OUNDATION                           STONE      F    I L00    PERFORATED
 FOUNDATION                                         00/ -  C PIPE
                                    1.5-3.0 N 00    PVC PIPE    )&>6    FILTER FABRIC LINES
                                                   00                          TOP, BOTVOM, AND
                                                   00                           SIDES OF DRY WELL
                                                   00
                                                   00
                                                   go       REBAR
                   10' MINIMUM                      00        ANCHOR
                    SETBACK                         00                  4ï¿½; REBAR






                                    FOOT PLATE







                                                            DRY WELL DETAIL


                                                                   REVS'D:          DRAWN: K.F.F.
                                                                   CHK'D: ____ CHK'D-  V.E.M
                                                                   DATE:            DATE:  OCT. 1990
                                                                                    SCALE: N.TS.
Smrtf Denier Normam                                                  RS'D:S
Engioemrn - Pianomar- Sureyors - LandBcope Architectz                CHK'D:
  Contrl Pork  SMa Mnhotton Sqoors S.Its 102   H AMPTON  ROADSPAGE:62
         Honpton. ViWrqro 22668
 (604)885-9810 (804)627-8900 For (804)885-1553  PLANNING DISTRICT COMMISSION    DATE: _______







6.5     Maintenance Requirements

        All dry wells are prone to clogging even if they are properly designed and constructed.

However, routine maintenance requirements for dry wells are minimal. The major maintenance item

is to clean the roof leader of tree leaves, pine needles, any separated shingle particles, and other

debris from the roof. The cleaning of gutters should be performed on an as needed basis.

        6.5.1  Inspection

                The dry well should be inspected several times in the early months of operation.

        It should be inspected once a month initially for a period of six (6) months. The inspection

        should be conducted after large or frequent storms to determine the water level in the

        observation well. A log book should be maintained to indicate the inspection visits and the

        rate at which the dry well dewaters or exfiltrates. Once the performance characteristics of

        the dry well have been determined, monitoring can be performed on a semi-annual or

        annual basis.

        6.5.2   Non-Routine Maintenance

                Despite careful design, construction, and maintenance, some dry wells will get

        clogged and need rehabilitation. Clogging in dry wells is most likely to occur near the top

        of the dry well, between the interface of stone and filter fabric. Surface clogging can be

        fixed by carefully removing the top layer of vegetation and stone, removing the clogged

        filter fabric, installing new filter fabric, and cleaning or replacing the top layer of stone.

        Clogging can also occur at the bottom of the dry well at the filter fabric/soil interface.

        Rehabilitation of the dry well then requires the removal of the top layer of vegetation and

        stone, the filter fabric, the entire stone aggregate reservoir, and the bottom filter fabric layer.

        Before the dry well is reconstructed, the subsoil layer should be scarified to promote better

        infiltration.



                                           63







       6.5.3 Total Maintenance Costs

               Rehabilitation of dry wells with complete reconstruction will cost the same as the

       initial construction cost. Partial dry well rehabilitation may cost approximately 20 percent

       of the initial construction cost. An annual set-aside of five (5) to ten (10) percent of the

       initial construction cost should be accumulated to cover routine/non-routine maintenance

       expenditures. These estimates are based on existing information and may vary from site

       to site and differ for each jurisdiction. Reliable maintenance costs and life expectancies of

       dry wells will become more accurate with experience and time.

6.6     Life Expectancy

       Dry well as a BMP has not been in use for a long enough time to determine its life

expectancy. More monitoring of existing facilities needs to be done before reliable information for

life expectancy can be determined. Based on experience in the State of Maryland, a dry well may

function properly anywhere from six (6) months to two (2) years. Proper construction, inspection,

and regular maintenance in terms of removing debris, leaves, and other materials from rooftop

gutters may enhance the useful life of drywell as a BMP.

6.7     Cost

        A general planning estimate for infiltration trench costs can be obtained by using the

following relationship:

                               C = 32.7 VI0ï¿½63

                where           C       = construction cost in 1990 dollars

                               Vs = storage volume in cubic feet.

        The above planning equation to estimate dry wells costs should not be used for storage

volumes greater than 10,000 cubic feet. Costs associated with other appurtenances are not included

in the above relationship. An additional 25% should be added to the above derived planning cost

to cover contingency costs.

                                           64







Construction Specifications

6.8.1   Trench Preparation

       Excavate the dry well to the design dimensions.  Excavated materials should be

placed away from the excvated sides for wall stability. Large tree roots must be trimmed

flush with the sides in order to prevent fabric puncturing or tearing during subsequent

installation procedures. The side walls of the dry well should be roughened where sheared

and scaled by heavy equipment.

6.8.2 Fabric Laydown

       The filter fabric roll must be cut to the proper width prior to installation. This

width must include sufficient material to conform to dry well perimeter irregularities and

for a six-inch minimum top overlap. Place the fabric roll over the dry well and unroll a

sufficient length to allow placement of the fabric down into the dry well. Stones or other

anchoring objects should be placed on the fabric at the edge of the dry well to keep the

lined dry well open during windy periods. When overlaps are required between rolls, the

upstream roll should lap a minimum of two (2) feet over the downstream roll in order to

provide a shingled effect. The overlap ensures fabric continuity and ensures that the fabric

conforms to the excavation surface during aggregate placement and compaction.


















                                   65







                     A partial list of suggested filter fabric brands is listed in Table 11.



                   Table 11 - Approved Geo-Textiles For Use in Dry Wells

Mirafi 140-N                                       Note:  This is a partial list of acceptable filter
                                                          fabrics available from suppliers for use
              Supac 4NP, 4.5NP, SNP, and 8NP              in infiltration trenches. The use of a
Typar 3401                                                 brand name does not constitute an
                                                          endorsement by HRPDC of any
AMOCO 4545                                                 particular product or company.

EXXON Geo-textiles No. 125D, 130D, and
150D

TerraTex SD

ISource:        "Controlling Urban Runoff' - Metropolitan Washington Council of Governments. 


             6.8.3   Stone Aggregate Placement and Compaction

                     The stone aggregate should be placed in lifts and compacted using plate

             compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches is

             recommended. The compaction process ensures fabric conformity to the excavation sides,

             thereby reducing the potential for soil piping, fabric clogging, and settlement problems.

              6.8.4 Overlaopping and Covering

                     Following the stone aggregate placement, the filter fabric should be folded over the

              stone aggregate to form a six-inch (6") minimum longitudinal lap. The desired fill soil or

              stone aggregate should be placed over the lap at sufficient intervals to maintain the lap

              during subsequent backfilling.

              6.8.5   Contamination

                     Care shall be exercised to prevent natural or fill soils from intermixing with the

              stone aggregate. All contaminated stone aggregate must be removed and replaced with

              uncontaminated stone aggregate.




                                                66







6.8.6 Voids Behind Fabric

       Voids should be avoided between the fabric and excavation sides. Removing

boulders or other obstacles from the walls is one source of such voids. Natural soils should

be placed in these voids at the most convenient time during construction but prior to

installing fabric to ensure fabric conformity to the excavation sides. Soil piping, fabric

clogging, and possible surface subsidence will be avoided by this remedial process.

6.8.7   Unstable Excavation Sides

        Vertically excavated walls may be difficult to maintain in areas where the soil

moisture is high or where soft cohesive or cohesionless soils predominate. These conditions

may require laying back of the side slopes to maintain stability; trapezoidal rather than

rectangular cross sections may result.

6.8.8   Veaetative Buffer

        A vegetative buffer of at least 20 feet wide (wider if possible) should be used to

intercept surface runoff from all impervious areas.

6.8.9   Traffic Control

        Heavy equipment and traffic shall be restricted from travelling over the infiltration

areas to minimize compaction of the soil.

6.8.10 Observation Well

        An observation well, as described in subsection 6.3.8 and Figure 9 should be

provided. The depth of the well at the time of installation should be clearly marked on the

well cap.










                                   67







7.   INFILTRATION TRENCH

        7.1     Description

                An infiltration trench is a shallow excavated pit, generally two (2) to ten (10) feet in depth,

       backfilled with coarse stone aggregate. Stormwater runoff is temporarily stored in the voids in the

        aggregate material and gradually infiltrates into the surrounding and underlying soil. Infiltration

       trenches are a viable BMP for permeable soils when the water table is two (2) to four (4) feet below

       the bottom of the trench.

                Infiltration trenches can remove both soluble and particulate pollutants. Stormwater runoff

        is generally laden with sediments and coarse material which should be prevented from entering the

       trench. The runoff should enter the trench through a minimum 20-foot wide vegetative buffer strip

        for surface trenches or through structures such as water quality inlets or grit-oil separators for

        underground trenches. By capturing the sediments before the runoff enters the trench, the life of

        the trench can be increased.

                An infiltration trench schematic is shown on Figure 10. Infiltration trench details are shown

        on Figures 11 and 12.

        7.2     Applicability

                Infiltration trenches are primarily on-site control BMPs and are generally applicable to

        small drainage areas (1 to 10 acres). This BMP can be installed in residential developments and

        open space areas as a surface trench, and in commercial areas as an underground trench with special

        inlets. An infiltration trench can also be installed under a grass swale.

        7.3     Design Criteria

                7.3.1   Soil Permeability

                        Soil textural classes with infiltration rates greater than or equal to 0.27 inches per

                hour should be used for the installation of infiltration trenches. This infiltration rate is

                associated with soil textural groups of sand, loamy sand, sandy loam, loam, and silt loam.

                                                   68







The infiltration rate of the underlying soil and the depth of the groundwater table are the

major limiting factors in the selection and feasibility of the infiltration trench as a BMP.

7.3.2   Denth of Trench

        The final infiltration rate of the soil below the infiltration trench determines the

maximum allowable trench depth. A trench with a grass covered surface should have at

least one (1) foot of overlying soil above the stone aggregate reservoir. The surface area

of the trench can be minimized by making the trench as deep as feasible. The trench can

also be made shallow and broad. The increased surface area of the bottom of the trench

increases exfiltration rates and provides more area for soil filtering of pollutants. A larger

trench bottom also helps in reducing clogging at the soil/filter cloth interface by providing

exflltration over a wide area.

7.3.3   Groundwater Table

        The seasonal high groundwater table should be located at least two (2) to four (4)

feet below the bottom of the trench.  The soil permeability (infiltration rate) and the

groundwater table are the two parameters which determine the maximum allowable depth

of the trench.

7.3.4   Proximity To Wells and Foundations

        Infiltration trenches should be located at least 100 feet upgradient from any drinking

water supply well to minimize the possibility of groundwater contamination.  Also the

trenches should be located at least ten (10) feet downgradient and 100 feet upgradient from

building foundations.

 7.3.5   Design Storm

         Infiltration trenches can be designed for a specific storm or for the first flush runoff

 volume. If for first flush, the trench storage volume can be sized based on 0.5 inches of

 runoff per impervious acre in the contributing site area.

                                    69







       Infiltration trenches are normally designed for water quality. As such, a significant

portion of the runoff volume (storms producing more than 0.5 inches of runoff) will bypass

the trench and is not infiltrated.

7.3.6 Storage Time/Maximum Drainin2 Time

       All infiltration trenches should be designed to drain within a maximum time of

three (3) days (72 hours), or a minimum time of two (2) days (48 hours). These values are

derived from literature. The Virginia Stormwater Management Regulations recommend two

(2) days (48 hours).

7.3.7   Stone Azareate

       The stone aggregate which fills the infiltration trench forms the reservoir through

which the storm runoff passes and is filtered. The aggregate material should be clean,

washed stone. Wash run gravel is preferred. The City of Virginia Beach recommends

using James River stone as aggregate. The clean washed stone aggregate should have a

maximum diameter of three (3) inches 'and a minimum diameter of one (1) inch. Void

spaces for the stone aggregate are normally within the range of 30 to 40 percent. A table

showing open graded coarse aggregates is included in the Appendix.

7.3.8   Observation Well

       An observation well shall be installed in the infiltration trench. The observation

well should be a perforated PVC pipe, four (4) to six (6) inches in diameter.  The pipe

should be located in the center of the trench and the bottom should rest on a plate. The top

of the well should be capped to prevent vandalism and tampering.

       The observation well helps in monitoring the function of the trench.  The water

level in the trench should be measured after a storm event.  If the trench does not

completely drain after three (3) days, it indicates that the trench is not functioning properly

and remedial steps may need to be taken to improve the performance.

                                  70







7.3.9   Runoff Filtering

        It is important to prevent any floatable material, settleable solids, grease, and oil

from entering the infiltration trench. Runoff filtering devices such as vegetative filter strips

(minimum of 20 feet) and water quality inlets can be used in front of the trench to prevent

objectionable materials from entering the trench. All trenches with surface inlets shall be

designed to capture the sediments and other material before the storm runoff discharges into

the stone aggregate reservoir.

        Infiltration trenches in combination with grass swales with check dams are feasible

combinations to increase the volume of infiltration into the trench. In this alternative, the

trench can be constructed under the swale with check dam to create a pool of water.

        The sides of the trench should be lined with filter fabric to prevent the entry of

sediments into the trench. The bottom of the trench, if constructed in good permeable soil,

can be lined with a six-inch layer of sand or filter fabric.

        In addition to the vegetative filter strip (minimum 20 feet), filter fabric placed one

(1) or two (2) feet below the top of the trench can be used to prevent sediments from

entering the trench.

7.3.10 Overflow Reouirements

        In all cases, the overflow path of storm runoff exceeding the capacity of the trench

should be evaluated and accommodated. The trenches are designed to treat the first flush

volume of runoff and control small drainage areas.












                                    71







                                                                      7.4    Design Examples

                                                                                              A one-acre (1 ac.) lot is to be developed for constructing a house with a roof-top area of

|                                                                           2,000 square feet. The soil borings on the site indicate that the soil is silt loam with an infiltration

I rate of 0.27 inches per hour. The depth of the seasonal high water table is determined to be five

                                                                      (5) feet deep.

I Design an infiltration trench to capture the runoff rmaoeic  1)rifl  n   o  is

                                                                      flush. The runoff from a one-inch (1") storm is computed to be 0.30 inch.

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                                                                                          1 FIGURE 101




                                                                                  P

                                                                                   I

                                                 /I-   -                 Ac 




     DT


           Dm             f




                      f  =  Infiltration Rate(ln/Hr)
                      Ts =  Maximum Allowable Storage Time of Trench(Hrs)

                      VR =  Void Ratio Trench Medium

                      DT =  Depth to Seasonal High Groundwater Table(Ft)
                      DM =  Min Dist from Trench bottom to Groundwater Table(Ft)
                      Ac =  Contributing Drainage Area(Sq Ft)
                      !Q =  Increase in Runoff Depth(In)
                      dT =  Depth of Trench(Ft)
                      P  =  Amount of Rainfall(In)
                      T  =  Trench Filling Time(Hrs)





             For First Flush Design:
                                  AQ A                                                 AQ A
             Area of Trench =    12                         Area of Trench =               A2
                                  VR dT                                                   P    f
                                                                                      12   12





                                                     IINFILTRATION TRENCH SCHEMATIC


                                                                          REVS'D:             DRAWN:  K.F.F.
                                                                          CHK'D:              CHK'D:   V.E.M.
                                                                          DDATE:               ATE:  OCT. 1990
Smnin     DemerNomarn                                                          REVS'DM             SCALE:  N.T.S.
Engineers - Planners - Surveyors - Landscape Architects
  entrol Pork   Si. MaUnhottan Square Suite 102CHK'D:
  C(a     to (s04)627       )            PLA2NNING DISTRICT COMMISSION    CDATE:                    PAGE    74
(804)8- 9610   7-69    0 F  (604)865-1533  PLANNING DISTRICT COMMISSION






                                                                                     FIGURE  11

                                                    WELL CAP






              OBSERVATION WELL                 oo                 FILTER FABRIC
                                                00
              PERFORATED                        gg
              PVC PIPE                          00
                                      STONE    00
                                                00
                                       FILL    00
                                                00
                                                00
                                                I0             REBAR
                                                               ANCHOR
                                                00
                                                00
                                                00




                                FOOT PLATE



                                                   OBSERVATION WELL DETAIL




                                                              OBSERVATION WELL
                                                              (SEE DETAIL ABOVE)
                    EMERGENCY
                    OVERFLOW
                                                              4'R  4'  4' RUNOFF'

                                                          2,  MIN.*4
                                                    4'  VEGETATED STRIP 4'  4'

                  STONE
                 SURFACE                                        y


                FILTER
                FABRIC


                AGGREGATE



                    FILTER
                    FABRIC





                                                 IINFILTRATION TRENCH DETAILj


                                                                    REVS'D:          DRAWN:  K.F.F.
                                                                    CHK'D:           CHK'D:  V. E.VM.
                                                                    DATE:            DATE:  OCT. 1990
                                                                                      SCALE: N.TS.
Smrth Demer Normam                                                    REVS'D:SCALE 75S
Engimnor - Plnnen - Sureywrs - Lands.tap ArWhile ta                   CHKPD:
  Control Park  So Monhotton Squore Swot. 102               HAP O NRA
         Hampton, Virginio 23686                                      DATE: PAGE          75
 (804)B65-961o (804)627-6900 Fox: (804)865-1533  PLANNING DISTRICT COMMISSION






                                                                                FIGURE 12 
                                          .    ....  ... ... ...  .. .........        ....E    2 
                              SLOPE .......... 


                                        .  . ..  ... ... ....r
                                _  _  _     ~~..........              .....  ...    


                              .           .PE  ' ' ' ' tE ' '         '.'. ..  . .....
                                           . . . .   . .   X ).. .. ..........



                           Ia~~e  _                           i....
                              SLOPE .... L






                                            uif
                                         ... .. . . .......







                                 SLOPE,~i.  b.O VI





                   AS A LEVEL SPREADER                  D
                                                   ,-''SHOULD NOT EXTEND
                                                   OVER FILTER STRIP



                                        / 
                          ,,=-----s       ,    /X\/>    TRENCH    /\'

                                      ,'  \    ,    g F              >        FILTER FABRIC






            "    SLOTTED CURB,,,'/ I  /\ S'IDEV




                                           SIDE VIEW

                                    I PARKING LOT


                       s t                    _       _                ~~~~~~~     ~     ~~~REVS'D:____  DRAWN:  R.E.C.
          ( ( ,7J                           s    s         \              CHK'D: _CHK'D    VPM
                                                                \Y/    s   S    tRDATE:  DATE   OCT 1990
                                           .IDE ., .   .   r   _SCALE    NTS
*  Sm  Demer Normam                                                          REVS'D: ___
Enginers - Planners - SurayeRs - Landscape Architects  TT A  R   A TDA       nKI
  Centnpl Pork  SY Hnhotton Soure Suite 102  HAMP 1 N  ROAS                 v        v.
 (804)ps5-610ï¿½o (804)27-6900 F~ca(804)8B5-1533  PLANNING DISTRICT COMMISSION  DATE:              PAGEh76 h
   I~~~~~~~~~~~~~~~~~~~..........







7.5     Maintenance Requirements

       All Infiltration Trenches are prone to clogging by sediments even if they are properly

designed and constructed. However, routine maintenance requirements for trenches are minimal.

       7.5.1   Inspection

               The trench should be inspected several times in the early months of operation. The

       inspection should be conducted after large or frequent storms to determine the water level

       in the observation well. A log book should be maintained to indicate the inspection visits

       and the rate at which the trench dewaters or exfiltrates.   Once  the performance

       characteristics of the trench have been determined, monitoring can be performed on a semi-

        annual or annual basis.

        7.5.2   Sediment Removal

               Infiltration trenches installed with water quality inlets for pre-treatment of runoff

        should be cleaned of sediments periodically, with a minimum interval of six (6) months.

        Built-up sediment, if not removed, will reduce the storage capacity of the water quality

        inlets. The removal of sediments and other objectionable material can be performed

        manually or by using a vacuum pump.

        7.5.3   Buffer Maintenance

               Vegetative filter strips should be mowed at least twice a year. Grass in the filter

        strip should not be mowed less than three (3) inches.  Grass clippings should be either

        bagged or disposed of away from the trench.  The condition of the vegetative filter strip

        should be inspected annually. Bare spots or eroded areas should be reseeded or re-sodded.

                Trees should not be allowed to grow in the vicinity of the trench to prevent the

        roots from puncturing the filter fabric. Branches extending over the trench should be

        trimmed so that the tree leaves do not clog the trench.



                                          77







7.5.4 Non-Routine Maintenance

        Despite careful design, construction, and maintenance, some trenches will get

clogged and need rehabilitation. In surface trenches, clogging is most likely to occur near

the top of the trench, between the interface of stone and filter fabric. Surface clogging can

be fixed by carefully removing the top layer of vegetation and stone, removing the clogged

filter fabric, installing new filter fabric, and cleaning or replacing the top layer of stone.

Clogging can also occur at the bottom of the trench at the filter fabric/soil interface.

Rehabilitation of the trench then requires the removal of the top layer of vegetation and

stone, the filter fabric, the entire stone aggregate reservoir, and the bottom filter fabric layer.

Before the trench is reconstructed, the subsoil layer should be scarified to promote better

infiltration.

        Clogging of underground trenches should be alleviated in the same manner as

surface trenches. If pavement or concrete are used as a surface layer for the trench instead

of grass or vegetation, reconstruction of the trench can be costly and difficult.

7.5.5   Total Maintenance Costs

        Rehabilitation of underground trenches including complete reconstruction of surface

trenches may cost the same as the initial construction cost.  Surface trench rehabilitation

may cost approximately 20% of the initial construction cost. An annual set-aside of 5-10%

of the initial construction cost for surface trenches and 10-15% for underground trenches

may be required to cover routine/non-routine maintenance expenditures. These estimates

are based on existing infonnation and may vary from site to site and differ for each

jurisdiction. Reliable maintenance costs and life expectancy of trenches will become more

accurate with experience and time.





                                   78







7.6     Life Expectancy

       Infiltration trenches as BMPs have not been in use for a long enough time to determine life

expectancy. More monitoring of existing facilities needs to be done before reliable information for

life expectancy can  be determined.   An  infiltration trench should be constructed after the

contributing area has been stabilized to prevent the sediments in the runoff from clogging the

trench. Based on experience in the State of Maryland, an infiltration trench may function properly

anywhere from six (6) months to two (2) years. Proper construction, inspection, and regular

maintenance may enhance the useful life of infiltration trench as a BMP.  Life expectancy of

infiltration trenches may vary from site to site depending upon the land use in the contributing area

and the contents of the storm runoff.

7.7     Cost

        A general planning estimate of infiltration trench costs can be made by using the following

relationship:

                               C       = 32.7 Vs0.63

        where                   C       = construction cost in 1990 dollars

                                Vs     = storage volume in cubic feet.

        This planning equation for estimating trench costs should not be used for storage volumes

greater than 10,000 cubic feet. Costs associated with pretreatment of runoff, like vegetative filter

strips or water quality inlets, and other appurtenances are not included in the above relationship.

An additional 25% should be added to the above derived cost to cover contingency costs.

7.8     Construction Specifications

        7.8.1   Timing

                An Infiltration trench should not be constructed or placed in service until all of the

        contributing drainage area has been stabilized and approved by the responsible inspector.



                                           79







7.8.2   Trench Prer)aration

       Excavate the trench to the design dimensions. Excavated materials should be

placed away from the trench sides to enhance trench wall stability. Large tree roots must

be trimmed flush with the trench sides in order to prevent fabric puncturing or tearing

during subsequent installation procedures. The side walls of the trench should be

roughened where sheared and scaled by heavy equipment.

7.8.3 Fabric Laydown

       The filter fabric roll must be cut to the proper width prior to installation. This

width must include sufficient material to conform to trench perimeter irregularities and for

a six-inch minimum top overlap. Place the fabric roll over the trench and unroll a sufficient

length to allow placement of the fabric down into the trench.  Stones or other anchoring

objects should be placed on the fabric at the edge of the trench to keep the lined trench

open during windy periods. When overlaps are required between rolls, the upstream roll

should lap a minimum of two (2) feet over the downstream roll in order to provide a

shingled effect. The overlap ensures fabric continuity and ensures that the fabric conforms

to the excavation surface during aggregate placement and compaction.




















                                   so







                      A partial list of suggested filter fabric brands is listed in Table 12.



                Table 12 - Approved Geo-Textiles For Use in Infiltration Trenches

 Miraft 140-N                                      Note:  This is a partial list of acceptable filter
                                                           fabrics available from suppliers for use
                                                           in infiltration trenches. The use of a
 Typar 3401                                                brand name does not constitute an
                                                           endorsement by HRPDC of any
 AMOCO 4545                                                particular product or company.

 EXXON Geo-textiles No. 125D, 130D, and
 150D

 TerraTex SD

3  Source:       "Controlling Urban Runoff' - Metropolitan Washington Council of Governments.


               7.8.4  Stone Agaregate Placement and Comnaction

                      The stone aggregate should be placed in lifts and compacted using plate

               compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches is

               recommended. The compaction process ensures fabric conformity to the excavation sides,

               thereby reducing the potential for soil piping, fabric clogging, and settlement problems.

               7.8.5   Overlaiming and Covering

                      Following the stone aggregate placement, the filter fabric should be folded over the

               stone aggregate to form a six-inch (6") minimum longitudinal lap. The desired fill soil or

               stone aggregate should be placed over the lap at sufficient intervals to maintain the lap

               during subsequent backfilling.

               7.8.6   Contamination

                       Care shall be exercised to prevent natural or fill soils from intermixing with the

               stone aggregate.  All contaminated stone aggregate must be removed and replaced with

               uncontaminated stone aggregate.




                                                 81







7.8.7 Voids Behind Fabric

       Voids should be avoided between the fabric and excavation sides. Removing

boulders or other obstacles from the trench walls is one source of such voids. Natural soils

should be placed in these voids at the most convenient time during construction but prior

to installing fabric to ensure fabric conformity to the excavation sides. Soil piping, fabric

clogging, and possible surface subsidence will be avoided by this remedial process.

7.8.8   Unstable Excavation Sides

       Vertically excavated walls may be difficult to maintain in areas where the soil

moisture is high or where soft cohesive or cohesionless soils predominate. These conditions

may require laying back of the side slopes to maintain stability; trapezoidal rather than

rectangular cross sections may result.

7.8.9   Vegetative Buffer

       A vegetative buffer of at least 20 feet wide (wider if possible) should be used to

intercept surface runoff from all impervious areas.

7.8.10 Traffic Control

        Heavy equipment and traffic shall be restricted from travelling over the infiltration

areas to minimize compaction of the soil.

7.8.11 Observation Well

        An observation well, as described in subsection 7.3.8 and Figure 11 should be

provided. The depth of the well at the time of installation should be clearly marked on the

well cap.










                                   82







        8    INFILTRATION BASIN

                8.1     Description

 I                     ~~~~~~~An infiltration basin is a storm runoff impoundment made by excavating a pit in, or down

                to, good permeable soils. The purpose of the basin is to store the storm runoff for a selected design

                 storm or for the first flush and to slowly infiltrate it through the permeable bottom of the basin.

                 An infiltration basin schematic is shown on Figure 13. Figure 14 shows a typical infiltration basin

                 which receives the concentrated storm runoff. A riprap apron near the inlet is required to reduce

                 incoming velocity. The flat basin floor with dense grass turf is used to trap sediments. An

                 emergency spillway should be provided to bypass overflows.

                         Figure 15 shows an infiltration basin with a riprap pilot channel on one side of the basin

I               ~~~~~which extends from the outfall of the pipe to the riser. B aseflow can flow through the pilot channel

                 and leave the basin through a low flow pipe at the base of the riser. Any flow larger than the base

                 flow will spread to the entire basin floor which is covered with dense grass turf. Riprap in the pilot

I               ~~~~~channel should have a layer of filter fabric under it. A riser outlet can be designed to pond the

                *runoff from a specific storm to facilitate settling of the sediments from the runoff. An emergency

                 spiliway should be provided to handle overflows.

                         Figure 16 shows a two-level infiltration basin and is a modification of the basin in Figure

                 14. The sediment forebay helps in settling heavier sediments and other objectionable materials, thus

I               ~~~~~enhancing the infiltration capacity of the remaining portion of the basin.

                         For all designs of infiltration basins with infow from a storm drain pipe, the hydraulic

                 grade line of the storm drain system should be checked.

I               ~~~~~8.2  Applicability

                          An infiltration basin can typically be constructed for drainage areas of five (5) to fifty (50)

                  acres. For drainage areas less than five (5) acres, it is more appropriate to use an infiltration trench


                                                              83







or dry well. An infiltration basin has relatively large surface area requirements when compared to

a trench or a dry well.

8.3     Design Criteria

        8.3.1   Soil Permeability

                The permeability or final infiltration rate of the underlying soil in the infiltration

        basin will determine how rapidly the storm runoff ponded in the basin will infiltrate into

        the ground. Soil textural classes with infiltration rates greater than or equal to 0.52 inches

        per hour should be used for the installation of the basin. This infiltration rate is associated

        with soil textural groups of sand, loamy sand, sandy loam, and loam.

        8.3.2   Depth of Basin

                A typical infiltration basin can range from three (3) feet to six (6) feet in depth.

        The rate and quantity of exfiltration is increased by increasing the surface area of the basin.

        Thus, wide and long basins with shallow depths are preferable to small and deep basins.

        With the passage of time, the bottom surface area of the basins will get clogged and will

        diminish the exfiltration rate. Excess bottom surface area can compensate for the loss of

        infiltration capacity.

         8.3.3   Groundwater Table

                The seasonal high groundwater table should be located at least two (2) to four (4)

         feet below the bottom of the basin. The soil permeability (infiltration rate) and the depth

         to the groundwater table are the two parameters which are used to determine the maximum

         allowable depth of the basin.

         8.3.4 Proximity To Wells and Foundations

                 Infiltration basins should be located at least 100 feet upgradient from any drinking

         water supply well to minimize the possibility of groundwater contamination. Also, the


                                            84







basins should be located at least ten (10) feet down-gradient and 100 feet up-gradient from

building foundations.

8.3.5   Basin Slopes

       The main objective in the infiltration basin design is to achieve a uniform ponding

depth over the entire surface area of the basin. The uniform ponding depth can be achieved

by grading the floor of the basin as close to a zero slope as possible. Any low spots should

be avoided to prevent ponding of the surface runoff. Deposition of solids in low areas

may cause clogging of the underlying soil.

        The storm runoff entering the basin should be spread out evenly over the bottom

surface of the basin to promote better infiltration. This can ',- achieved by providing an

apron or a level spreader.

        The side slopes of the basin should be three to one (3:1) or flatter to help in

establishing proper vegetation and grass cover. Water tolerant grass should be planted on

the bottom and sides of the basin. A dense growth of grass will help to prevent scouring

of the basin floor and sides.

8.3.6 Design Storm

        Infiltration Basins can be designed for a specific storm or for the first flush runoff

volume. If for first flush, the basin storage volume can be based on 0.5 inches of runoff

per impervious acre in the contributing site area.

        Larger and less frequent storms will bypass the basin. Additional storage will be

needed to provide control of storm runoff from these larger storms. This can be achieved

by providing a conventional riser pipe in the basin.

        Concentrated flows with erosive velocities should be prevented from entering the

 basin.   An  emergency  spillway  should be provided for all basins created by  an


                                    85







embankment.   The emergency spillway design should comply with state and local

requirements.

8.3.7 Storage Time/Maximum Draining Time

       All infiltration basins should be designed to drain within a maximum time of three

(3) days (72 hours), or a minimum time of two (2) days (48 hours). These values are

derived from literature. The Virginia Stormwater Management Regulations recommend two

(2) days (48 hours).

8.3.8   Runoff Filtering

        Sediments in the storm runoff and objectionable floating materials should be

prevented from entering the basin. The longevity of the basin can be increased by installing

sediment forebays near the inlets to trap incoming sediment loads. It is recommended to

provide a minimum of 20 feet vegetative buffer around the basin to prevent sediments from

entering the basin.

























                                  86





I

|                                                                          8.4    Design Examples

                                                                                                                     Design an infiltration basin for a residential development of 5.2 acres. Soil borings

|                                                                                                      indicate that the soil is sandy loam with an infiltratio  aeo   .2ice   e   or   h


                       I ~~~~~~~increase in runoff depth is 0.30 inch for a one-inch (1"  anal    h   et   fsaoa
                                                                                             ground water table is determined to be five (5) feet. Side slopes of the basins should be

I  three to one (3:1) and top width of the basin should be4fet



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                                                                                                -      __: i'E   '    -i''0: ...S0-  'R.4:::    - :.-::t -::::::.--::-: -::,:-:-,:-:00-i~ES   -:    - t.... -:t.:   -- --                                                                                                                                     ... .   - 00-. .--...... -......       .
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                     b ::- --i- iR ::E:-E iDE~~.S.-. .:...........-. -. -. - .. .....-.....-........................"......."....,.... 0::-,.....,."... -- ....-. ::::::I: . -: i....... . - _` '...........-.'....
                                         i t-t:-t i -: -:g ;~~~. j                                                                                                                                                                                                                                            -:                        .  ... :.ii-. i. . .  !.. .  i.,iri .   .i... -....- .- ......  .  _ _  ..E..- 11 .   S. .
                      s                                                                   ~ ~~~~~~~~..-_-1. -            ..-... .. . t . ...S-X0- E  .E.! -....S t .    . .     .      --i-L.:...-.-.. . :::: - i    :  --S-i  ::::::       -::X ...-..i.......i-.......                                                                  ...
                     *~~~~~~~~~~~~~~..~.....~....~~. ,.                                                                   - S: i- iE .,                                        .  .  ..                                                                                                                              .:                   ,. _ . .  :.:.. .,.......d... . ~   . -. :i:::):L: :::::!::!:::: ::-:::::  :-::::  ::::: : : :- : :-   i   : --::::
                                                                                                            :::--,-:E-:...::::: :..:::-. ....... .-- ....-............:.. . ...::.. ... E:.:,:.:.::::::.:::.'::,.:.:::::::::..4:.:: .........-..... ,_
                     |~~~~~~~~~~~.                                                                .  . .   - 11'   .    -       ...   ....   ...  ..    Drain  geI.   ..   .   ....a.    .  j..    -..  2N1.  1. I  -. .  .  .-                                                                                    - -      ..   ..  ;  .   .       -     -; -    -;--  .....


                                                                                      :: -: -: ..........,.,..y...-............:: ,  -.:::: :::::-E :: :E: : -: E.._ :    X::   -:  -  : .: i :     :   :E:  :-:: .E:: -:  . ::    i:   : : -. ..: . . .::   .- ::  .~. -: :ii ,:  i:  E ~. . ~ . .1 1 --: : :! ~   :.    .. . . .    d     , ~ . .    . .               ..... ...
                     - ~~~~~~~~~~~~..                                                             . .  .  .  .  .  .  .  .  . .  .I...     ..   . : :   -...   ..   ..    -  --   ...   :.   :,-                                 ..  ...    .:.   E   ::: W-.. . .t . ..  .       -    ;       :     i - i -  . X.-  i :4-t i, .. i- -. X .. . .. .  i.-X..-.. ...     -i.-
                                                                                        ;-   -   -  -      - .idth.    o f ..   '.    .Basin(Ft.    _  _0 ...;  ..  . ..   .            ::   :    ,:-:                 I         -      -    .    .. .. .  -       ..    I.. . -    ..   .I.     ...  ..    ..      -      .  . ..   .   ..  -. ; . ..    I...    -    .. ., .....-.,,...,..X....    .1
                                                                                       -.....   ___   ....:0;:-    .:.: .    :.   -   : ..I.....  :1-. :- ::. :I:.. -:   :::. I:.:: -.. -0::::-....   .. ::.-:::-          I... ::.: :: ::..... -           -- l:-;y::g-     . . 0: -  -  ..'.I....                                                                                     .
                                                                                        .-ttt ....     40 i-:.1 t~~ti ...   ..   t00;-.....  --S...   .  I- X~- ...;0.:;-il-git ,      4 .1 , ~    -. .i:000  I....t-.. 000                                                                        .1 .... ty..:X0   ..  - ,, 0. -.
                      l~ ~ ~ ~~~~    ..  .....~-                                                                                                                                                                 -                       1    .-  ... ..  ,.,
                      |~~.~. 87                                                                                                                                                                                                                                                                                                                                                        :   :!::X


                                                                                    l~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - ..







|                             Example 2

                                              Design an infiltration basin for a commercial site of 5.0 acres with an increase in

I runoff depth of 0.50 inch associated with a rainfall of 1.3 inches. Soil borings indicate that

 ~the soil is sandy loam with an infiltration rate of 0.9 inches per hour. The depth of the

                                    seasonal ground water table is determined to be 4.5 feet. Side slopes of the basin should

|                             be three to one (3:1) and top width of the basin should be 35 feet.





                                    --ï¿½  Feaiblt ,nu  Paaee.< .E---i-.   
              ;~~~~~~.--'-'.'.-'.---."-'''~i5i.:`'   '   '  '''               - -'--..- 

                   *   '       -  '"''j't'.'"'t:'.  ''' ''""'""'-'t''"T'lf   ''--: .' -   -'  ':' ',':' ""'-       '' : '' "'--'  '':"''"  W-"-:  '  'i-',':,"'    '-' 0 -0--:  - 
                                           InfiltationRate~hI~r):0.90.:

* iiMai mum'Al lowablePonidng Timeiiii'of Basinriiiiiiiii.i;-i:i'ili

                                   D;epth -to- Seasonl Hig  Goundwater Tale(Ft)

                                  t0Miii; Dist. 0from 'Basin 'bottom' to Groundate  Table(Ft):  2.00002a      -0W0 00t0XX0t0   0:tS

         E                   0't*****:0 Maxium Basin tlDepth(Ft:  2.500 0 ***** 000 
                                          iSE~~~~~~~fiEE.~~~~~~ ~  . id, d2    E-     E --- i   i  EiX Eig,    iEi   i-i    i  ::.;- iERER   - : i S   _ _::i  i iDE  E E _:i :i::i:-i ilj    ï¿½-'ï¿½ii      .
                                  *..---ï¿½>>; :Design Input: Par eters--g- ï¿½- c      .0---4-.~:0;;;-':0t-;;t-.g



I                           ti00Contributing _ iDrainage tArea(Sq :t):    217800.            ï¿½     ï¿½ -    .

                                   .Depth  of.Basi (Ft):  -2.50044'0';"";l:t '-  'tt-"00-':0tt;;.X  ;;0'  i    --'       :''    '           ~''ttt

| -Amount of Rainfall(In): 1.V30-  :::



         I''~~~~~ Basin. Legh(t:  4
        | ti~~~~~~t~~~id ;;S~~~~~Io~p e~  'jof Bas inr-(Ra ):'-03.00 't-0 -00't '' ... ............................ 0 ---ii-i ij'j' i' t:- ..,. .. '0.  :'' ij-l': il-i-'- ... :j'l-i0l00l-' j-:-ilt'tj , I:-.-;.0.0.00-0-0t-00.t-.0.00-tt00.t~i~it






1                           _11:~~18ï¿½5~.I.88
                         I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:--i-:::::: :::::::






                 I                                                                                  ~~~~~~~~~~~~~~~FIGURE 131


             I                                                                      ~~~~~~~~~~~~~~~~TOP  VIEW


*                             ~~~~~~~SIDE  VIEW


       I~~~~~~~~~~~











               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~IN


                         f = Infiltration Rate(ln/Hr)
                           Tp=  Maximum Allowable Ponding Time(Hrs)
                         DT =  Depth to Seasonal High Groundwater Table(Ft)

                           DM=  Min Dist from Bosin bottom to Groundwater Table(Ft)
                         AO =  Increase in Runoff Depth(In)

I                         ~~~~~~~~~Ar =  Contributing Drainage Area(Sq Ft)
                           De=  Depth of Basin(Ft)
                          P =  Amount of Rainfall(ln)
                          Z =  Side Slope Ratio (N:1)
                          W  =  Top Width of Basin (Ft)

                          L  =  Top Length of Basin (Ft)




                                             AO
                                          12 yAc +  Z De(W- 2Z De)
                                                W(DB  --     Z D2
                                                        1 2




                                                            INFILTRATION BASIN SCHEMATIC


                                                                              REVS'D:____         DRAWN: K.F.F,
                                                                              CHK'D: ____ CHK'D; -vPm
                                                                              DATE: ______ DATE: OCT  1990
                           RnM Demer No~~~~~~~am   REVS'D:  ~~~~SCALE: N.TS.
    Engineers  Planners - Su,,eyars - Landscape Architects  A        ______
      Contral-Park  Six Manhattan Square Suits 102    HA M P T O      ROADCHK'D
                       Ham~~~pton. ROADSo 25                                      DATE:                 PAGE 89
     (B04)B85-961O (804)627-6900 Fox (604)565-1533  PLANNING DISTRICT COMMISSION    DT:_____






                                                                                     I  FIGURE  14


                      MAINTENANCE
                     RIGHT-OF-WAY


                             I>~~ \          SIDE-SLOPES
                                             MAXIMUM OF 3:1






      /     f /                            ;~ EMBANKMENT
               -,',, ,  .....-., ........ .................... .......... .... ........ ............................... ..........,   -, ,
STORM                                                                                          ERGENY
STORM .....                               I".AT. ....... FLJTEMERGENCY





  THROUGH RIPRAP APRON















                                                                       RUNOFF D EN ........SPILLWAY
I  TO  BASS  L

























                       LRRIPRAP APRON
                      ALSO SERVES TO
                       TRAP SEDIMENTS


                                              SIDE VIEW










           \ vRUNOFF/DATE: DATE: OCT. 1990
 Smrth Demer Nornar                                                                      SCALE:  N.T.S.
                                              SIDE VIEW













 Engineer - Plcnnero  - Sur'aoyrs -  Londscape ArchitoctTN              AT
   Coentrl Pcrk  Six Manhrattmn Sq..re Suite 102  H AMTO _rvOADf CHK ' D:
          .H5_6oqmpton. we r668,,,ï¿½ 6_1  PLNNN.DSRITCOMSSO            DATE: PA  G       E         YU






                    I   -        -                                                          ~~~~~~~~~~~~~FIGURE 151

     I                      ~~~~~~~MAINTENANCE
                           RIGHT-OF-WAY

          I                                        ~~~~~~~~~~~~~~~~~~~~~SIDE-SLOPES          RIP'RAP OUTFALL
                                                  MAXIMUM OF 3:1                           PROTET~ION
                                                                                              MEOCY
                                      xxl~~~~~~~~~~~~~~  ~~~SPILLW Y


                      STOR~~~~4              FLAT BASIN FLOOR ~~~~~~~~~~~.:iK**::......
               I  RUNO~~~~~~~~~~~~~~~~~~~~~~~~~~~~..F....



                  ~~~~~~~~~~~FLTO BASIN FLOOR....

I                                                     TOP ~STORN--




                                                                .......... SPILWA



         R NOFF                  BLEED

                     ~~~~~~~~~~TROUGH RIPAPREVE
I~~~ BASIN FLOOR


                                                   SIDE VIEW








         I~~~~~~~~~~~~~~~~~EFLRTO ISTFIRAGE..ON  BASIN.-.-



                                                         SP~~~~~REASDER__AND    DRAWN TOE 





                                                                          CHK'D: _______   CHKD      M     j
                                                                          DATE: _____ DATE: OCT. 1990

*     ~~~Smithi Demier Normamn                                                REVSCAL  NT
        Engineers - planners - Surveyars - landvmap  Aftehte ts               CKD
          Central Park .Six Manhattan Square Suit. 102  H A    M     P      T     N      _RA     PAGE   91
         (804)865-9610 (804)627-6900 Fain (604)665-1533  PLANNING DISTRICT COMMISSION    DT:_______ 






                                                                                          | FIGURE  16 


                            MAINTENANCE
                            RIGHT-OF-WAY
                                    \   \       ~                        - -
                                   \I~ ~ W         SIDE-SLOPES
                                                   MAXIMUM OF 3:1






            R OFF4 0 t SEDIENT FREEAY.    .    . . . . . . . . . . . . . ............ ......... ....................... ...
                       .... ..-....... . ... ......... ..  
      STORM
      RUNOFF SPILLWAY


         RUNOFF BLEEDS
         THROUGH RIPRAP
         TO BASIN FLOOR
                                             FLAT BASIN FLOOR/
                                               WITH DENSE                                 & RIPRAP OUTFALL
                                                GRASS TURF                                   PROTECTION

                                                    TOP VIEW






                                                                 EMBANKMENT                      EMERGENCY
                                                                                            SPI LLWAY
                    ï¿½  z, ~~~~............................ ................................................'2












            UTRA N /  /     ' ...........SEDIMENT A ......................    ...
            RUNOFF .-"~>',X' '~.............ï¿½. ...................":~:"fl~..............."~'t"'l"''t/i'~  
                               x',~~~~~~~~~~~.,. ........... ...... ....~,%. W/,,, ,. ~.,,/,,%. ~. '(. ' /  
                           ï¿½ ~',;~~~~ ....,.. .. rl.H .,t .t-.f~.I    7v  x~~~,














                           LRIPRAP APRON
                             ALSO SERVES TO
     *                       ~~~~~~~TRAP SEDIMENTS


                                                    SIDE VIEW





                                                   I  TWO-LEVEL INFILTRATION BASIN


                               z                     _      _              ~~~~~~~     ~     ~~~~REVS'D:____    DRAWN-  R.E.C
                                                                           CHK'D:            CHK'D:  V.P.M.
                                                                           DATE:             DATE:  OCT, 1990
        R Deer                     BL n                 -                 .EVS:    SCALE N.T.S.
*      ~~~~~~Ecgineers - Planners - Sw-ceyore - Landscape Architects TTA1, ~T i  )C uo
         Central Perk H5iptn.W.r~niote 23868  Suit, tO2  HAMPvi  1   ROl ,ADS  CHK  ______u_    n  n 
                              rcmamoni~lrgic 236ti6   PLANNING DISTRICT ..   DATE:
                         (804)6"-90   )6276900   (04)8-R  PLANNING DISTRICT COMMISSION
                                                                 EMBANTEMERGENCY







8.5     Maintenance Requirements

       8.5.1  Inspection

               The infiltration basin initially should be inspected after every major storm to

       determine how long the storm runoff remains in the basin. Standing water in the basin for

       more than 72 hours after a storm indicates problems with infiltration. Semi-annual or

       annual inspections should be performed to determine proper functioning of the basin.

       8.5.2 Mowing

               The basin floor and sides should be mowed as needed to prevent unsightly growth

       of grass or weeds. Mowing operations should be performed during dry periods because at

       other times the bottom of the basin may be soggy. If the infiltration basin is being used

       as a passive recreational area, more frequent mowing may be needed.

       8.5.3   Erosion Control

               Eroding or barren areas at the bottom of the basin or on the sides should be

       immediately re-vegetated.

               During the regular semiannual or annual inspection, other items like condition of

       the embankment, riprap at the inlet, outlet, and pilot channel (if used) should be checked.

       Trash should be removed from basins with no outlets.

        8.5.4 Non-Routine Maintenance

               Infiltration basins with risers and outlet pipes may not need replacement for a long

        time depending on the type of materials used for their construction. Concrete outlet pipes

        may last for 40-50 years and corrugated metal outlet pipe longevity may be about the same.

               The life expectancy of the basin depends on the amount of sediment being trapped

        and the infiltration capacity of the underlying soil. The literature suggests that a well-

        maintained basin may remain functional for five (5) to ten (10) years before the trapped


                                          93







       sediments need to be removed. Sediment removal should be undertaken when the bottom

       of the basin is dry. Trapped sediments can be used as a fill material for new sites or used

       as spreading soil in gardens.  Sediments should be removed with light equipment.  The

       bottom of the basin should be scarified to restore its infiltration rate and re-vegetated.

       8.5.5 Total Maintenance Costs

               Based on current experience with infiltration basins in other parts of the country,

       it is reasonable to assume the total annual maintenance costs to be three (3) to five (5)

       percent of the initial construction cost.

8.6     Life Expectancy

       The main cause of failure of an infiltration basin as a BMP is clogging of the basin bottom

with sediments from the storm runoff.   Infiltration basins should be constructed after the

contributing area serving the basin has been stabilized. Based on experience in the State of

Maryland, infiltration basins may typically function properly anywhere from six (6) months to two

(2) years.  Life expectancy may also depend on the land use in the contributing area and the

contents of the storm runoff. Proper construction inspection and maintenance may enhance the

useful life of the infiltration basin as a BMP.

8.7     Cost

        A general planning estimate of infiltration basin costs can be made by using the following

equation:

                               C       = 13.2 Vs0.69

        where                   C       = construction cost in 1990 dollars

                               Vs      = basin storage volume in cubic feet.






                                           94







       The above equation to estimate basin construction costs does not include land acquisition

costs or the cost of any pretreatment structures. An additional 25% should be added to the

construction cost to cover contingency costs.

8.8     Construction Specifications

        As with other infiltration practices, infiltration basins, if not constructed properly may have

a high rate of failure. The most common cause of failure is poor sediment control on upland

contributing areas and incorrect location of the basin in soils with poor infiltration rates. The

infiltration basin should be constructed after the contributing area has been stabilized.

        8.8.1   Excavation

                Compaction of underlying soils by heavy equipment should be prevented. After

        final grading of the basin bottom is completed, the bottom should be scarified to provide

        a porous surface texture for better infiltration rates.

        8.8.2   Lining Material

                Infiltration basins should be lined with a 6-inch to 12-inch layer of filter material

        such as coarse sand to help prevent the buildup of impervious deposits on the soil surface.

        The filter layer should be replaced or cleaned when it becomes clogged.

                Establishing dense vegetation on the basin side slopes and floor is recommended.

        A dense vegetative stand will not only prevent erosion and sloughing (side slope erosion),

        but will also provide a natural means of maintaining relatively high infiltration rates.

        Erosion protection for inflow points to the basin should also be provided.

                The basin should be stabilized with vegetation within one (1) week after

        construction.







                                           95







9   UNDERGROUND STORAGE TRENCH

       9.1     Description

               An underground storage BMP functions like an infiltration trench, except that it can accept

       concentrated runoff. The concentrated runoff should be pretreated before it enters the underground

       storage trench. In some situations where there is good permeable underlying soil and there is not

       enough space available to install an infiltration trench, an underground storage trench can be

       constructed under paved areas. This is not a recommended practice for all sites, because it is

       extremely costly; it is hard to replace the trench if the pavement fails; and, maintenance of the

       trench is a problem. An underground storage trench can be installed under a grass lined swale to

        augment infiltration of runoff. A schematic of an underground storage trench is shown on Figure

        17. Detail of an underground storage trench is shown on Figure 18.

       9.2     Applicability

               As mentioned above, the application of an underground storage trench is similar to an

       infiltration trench. Underground storage trenches are primarily on-site control BMPs and are

        generally applicable to small drainage areas up to five (5) acres. This BMP can be installed in

        residential developments or commercial areas.

        9.3     Design Criteria

               9.3.1   Soil Permeability

                       Soil textural classes with infiltration rates greater than or equal to 0.27 inches per

               hour should be used for the installation of underground storage trenches. This infiltration

               rate is associated with soil textural groups of sand, loamy sand, sandy loam, loam, and silt

               loam. The infiltration rate of the underlying soil and the depth of the groundwater table

                are the major limiting factors in the selection and feasibility of the underground storage

                trench as a BMP.


                                                  96







9.3.2   Dewth of Trench

       The final infiltration rate of the soil below the underground storage trench

determines the maximum allowable trench depth.  A trench with a grass covered surface

should have at least one (1) foot of overlying soil above the stone aggregate reservoir. The

surface area of the trench can be minimized by making the trench as deep as feasible. The

trench can also be made shallow and broad. The increased surface area of the bottom of

the trench increases exfiltration rates and provides more area for soil filtering of pollutants.

A larger trench bottom also helps in reducing clogging at the soil/filter cloth interface by

providing exfiltration over a wide area.

9.3.3 Groundwater Table

       The seasonal high groundwater table should be located at least two (2) to four (4)

feet below the bottom of the trench. The soil permeability (infiltration rate) and the

groundwater table are the two parameters which determine the maximum allowable depth

of the trench.

9.3.4 Proximity To Wells and Foundations

        Underground storage trenches should be located at least 100 feet upgradient from

any drinking water supply well to minimize the possibility of groundwater contamination.

Also the trenches should be located at least ten (10) feet downgradient and 100 feet

upgradient from building foundations.

9.3.5   Design Storm

        Underground storage trenches can be designed for a specific storm or for the first

flush runoff volume. If for first flush, the trench storage volume can be sized based on 0.5

inches of runoff per impervious acre in the contributing site area.




                                   97







       Underground storage trenches are normally designed for water quality. As such,

a significant portion of the runoff volume (storms producing more than 0.5 inches of

runoff) will bypass the trench and is not infiltrated.

9.3.6   Storage Time/Maximum Draining Time

       All underground storage trenches should be designed to drain within a maximum

time of three (3) days (72 hours), or a minimum time of two (2) days (48 hours). These

values are derived from literature. The Virginia Stormwater Management Regulations

recommend two (2) days (48 hours).

9.3.7   Stone Aggregate

       The stone aggregate which fills the underground storage trench forms the reservoir

through which the storm runoff passes and is filtered. The aggregate material should be

clean, washed stone. Wash run gravel is preferred. The City of Virginia Beach

recommends using James River Stone as aggregate. The clean washed stone aggregate

should have a maximum diameter of thlee (3) inches and a minimum diameter of one (1)

inch. Void spaces for the stone aggregate are normally within the range of 30 to 40

percent. A table showing open graded coarse aggregates is included in the Appendix.

9.3.8   Observation Well

        An observation well shall be installed in the underground storage trench. The

observation well should be a perforated PVC pipe, four (4) to six (6) inches in diameter.

The pipe should be located in the center of the trench and the bottom should rest on a plate.

The top of the well should be capped to prevent vandalism and tampering.

        The observation well helps in monitoring the function of the trench.  The water

level in the trench should be measured after a storm event. If the trench does not




                                  98







        completely drain after three (3) days, it indicates that the trench is not functioning properly

        and remedial steps may need to be taken to improve the performance.

        9.3.9   Runoff Filterina

                It is important to prevent any floatable material, settleable solids, grease, and oil

       from entering the underground storage trench. Runoff filtering devices such as water

       quality inlets can be used in front of the trench to prevent objectionable materials from

       entering the trench. All trenches with surface inlets shall be designed to capture the

        sediments and other material before the storm runoff discharges into the stone aggregate

        reservoir.

               The sides of the trench should be lined with filter fabric to prevent the entry of

        sediments into the trench. The bottom of the trench, if constructed in good permeable soil,

        can be lined with a six-inch (6") layer of sand or filter fabric.

        9.3.10 Overflow Renuirements

                In all cases, the overflow path of storm runoff exceeding the capacity of the trench

        should be evaluated and accommodated. The trenches are designed to treat the first flush

        volume of runoff and control small drainage areas.

9.4     Design Examples

        Design an underground storage trench for a one (1) acre commercial site with a paved area

of 20,000 square feet generating 0.4 inches of runoff for a one-inch (1") rainfall. The runoff should

be pretreated before entering the underground storage trench. Underlying soil is silt loam with an

infiltration rate of 0.27 inches per hour. Groundwater table is at ten (10) feet from the ground. The

trench will be installed under a grassed area one (1) foot deep. Depth of the trench should not be

more than three (3) feet.




                                           99





I
                                                       - . .. I I . . I I I-- . - - I - 1.
                                                      ...,l.,..---.l,-.  I.. 1. I.. . --....,.....,.l....,...,..  --.- -, I.." 1. . 1. 1-1.
                                                                        1. II I. 11.1. 1- I 11,11--l...........-...
                                                      .....  .-I . ---,-
                                                      ........
                                                    .-::::::: :
                                                      .,..''.:i::iIiDESIGNOFUNDERGROUND:STORAGEFORI"RAINFALLI.,..,....''.,.,..,....I
                                                                                                                                                                                                                .".......-
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                                                      .... I......, III I.
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I 100


I


I







                                                                                          IFIGURE 17



                                                                                  P









                I_ D  .--f.
            DT   dT








                     f  =  Infiltration Rate(ln/Hr)

                     Ts =  Maximum Allowable Storage Time of Trench(Hrs)

                     VR =  Void Ratio Trench Medium

                     DT =  Depth to Seasonal High Groundwater Table(Ft)
                     Du =  Min Dist from Trench bottom to Groundwater Table(Ft)
                     Ac =  Contributing Drainage Area(Sq Ft)
                     AQ =  Increase in Runoff Depth(ln)

                     dT =  Depth of Trench(Ft)

                     do =  Depth over Trench(Ft)

                     P  =  Amount of R'ainfall(ln)

                     T  =  Trench Filling Time(Hrs)



             For First Flush Design:
                                 AQ                                                  AQ Ac
             Area of Trench -    12                         Area of Trench = 
                                  VVIRT                                                  P    f
                                                                            V, d1 ----- T
                                                                                     12   12





                                                    IUNDERGROUND STORAGE SCHEMATIC


                                                                        REVS'D:             DRAWN:   K.F.F.
                                                                        CHK'D:              CHK'D:   V.E.M.
                                                                        DATE:               DATE:  OCT. 1990
           Cient Demeral aNorrnmam                                                RE VS : SCALE:   N.T.S.
            ma-Tlrffi Derer Normam                                          REVS'D:
Engineers - Planners - Surveyors - Landscape Architects  HAMPTON              R  SAD  CHK :D
 Centl Por   Six Manhatton Squre Suite 102 HAMPTON ROAD                                                       10CHK'D:
         Hompton. Virginia 23666                                             DATE: PAGE 101
(804)865-9510 (604)627-6900 Foe (804)865-1533  PLANNING DISTRICT COMMISSION






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                                       D ~ ~    ~     ~    ~     _     _      PAGo10
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      (804)865   (804)627   Poe, (804)B65 s~~~.  PLANIN  DITITCMISON DT:_____
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                9.5     Maintenance Requirements

                        An underground storage BMIP may require more inspection visits and need more

I              ~~~~~enforcement than an infiltration trench. The water quality inlet should be inspected at least twice

                a year to ensure proper functioning. If needed, contents of the chambers should be pumped out and

                properly disposed of at least twice a year.

                        Rehabilitation of underground storage trenches may cost the same as the initial construction

                cost An annual set-aside of 10-15% of the initial construction cost for underground storage

I              ~~~~~trenches may be required to cover routine/non-routine maintenance expenditures. These estimates

                 are based on existing information and may vary from site to site and differ for each jurisdiction.

                 Reliable maintenance costs and life expectancy of underground storage trenches will become more

I               ~~~~~accurate with experience and time.

                 9.6     Life Expectancy

                         Enough information on the useful life expectancy of underground storage trench is not

                 available. A rough estimate of the life expectancy of an underground storage trench is six (6)

                 months to two (2) years. Proper construction inspection, and maintenance may enhance useful life

                 expectancy.

*               ~~~~9.7  Cost

                         The cost of installing an underground storage trench could be 30 to 40 percent more than

I               ~~~~~an infiltration trench because a pretreatment facility should be constructed before this BM[P. An

I               ~~~~underground storage trench may range from $5,000 to $15,000.

                 9.8     Construction Specifications

                         The specifications listed for infiltration trenches are applicable for underground storage

                 trenches. They are listed here for ease of use.




           1                                               ~~~~~~~~~~~~~~103







9.8.1   Timing

       An underground storage trench should not be constructed or placed in service until

all of the contributing drainage area has been stabilized and approved by the responsible

inspector.

9.8.2   Trench Preparation

       Excavate the trench to the design dimensions. Excavated materials should be

placed away from the trench sides to enhance trench wall stability. Large tree roots must

be trimmed flush with the trench sides in order to prevent fabric puncturing or tearing

during subsequent installation procedures. The side walls of the trench should be

roughened where sheared and scaled by heavy equipment.

9.8.3   Fabric Laydown

       The filter fabric roll must be cut to the proper width prior to installation. This

width must include sufficient material to conform to trench perimeter irregularities and for

a six-inch minimum top overlap. Place the fabric roll over the trench and unroll a sufficient

length to allow placement of the fabric down into the trench. Stones or other anchoring

objects should be placed on the fabric at the edge of the trench to keep the lined trench

open during windy periods. When overlaps are required between rolls, the upstream roll

should lap a minimum of two (2) feet over the downstream roll in order to provide a

shingled effect. The overlap ensures fabric continuity and ensures that the fabric conforms

to the excavation surface during aggregate placement and compaction.











                                  104







                      A partial list of suggested filter fabric brands is given below:



I         Table 13 - Approved Geo-Textiles For Use in Underground Storage Trenches

 Mirafi 140-N                                      Note:  This is a partial list of acceptable filter
                                                           fabrics available from suppliers for use
                                                           in underground storage trenches. The
 Typar 3401                                                use of a brand name does not constitute
                                                           an endorsement by HRPDC of any
 AMOCO 4545                                                particular product or company.

 EXXON Geo-textiles No. 125D, 130D, and
 150D
 TerraTex SD

| Source:        "Controlling Urban Runoff' - Metropolitan Washington Council of Governments.


               9.8.4   Stone Aggregate Placement and Compaction

                      The stone aggregate should be placed in lifts and compacted using plate

               compactors. As a rule of thumb, a maximum loose lift thickness of 12 inches is

               recommended. The compaction process ensures fabric conformity to the excavation sides,

               thereby reducing the potential for soil piping, fabric clogging, and settlement problems.

               9.8.5   OverlavDing and Covering

                       Following the stone aggregate placement, the filter fabric should be folded over the

               stone aggregate to form a six-inch (6") minimum longitudinal lap. The desired fill soil or

               stone aggregate should be placed over the lap at sufficient intervals to maintain the lap

               during subsequent backfilling.

               9.8.6   Contamination

                       Care shall be exercised to prevent natural or fill soils from intermixing with the

               stone aggregate. All contaminated stone aggregate must be removed and replaced with

               uncontaminated stone aggregate.


                                                 105







                      9.8.7 Voids Behind Fabric

                               Voids should be avoided between the fabric and excavation sides. Removing

I                     ~~~~~~~boulders or other obstacles from the trench walls is one source of such voids. Natural soils

                       should be placed in these voids at the most convenient time during construction but prior

                       to installing fabric to ensure fabric conformity to the excavation sides. Soil piping, fabric

                       clogging, and possible surface subsidence will be avoided by Uhs remedial process.

                       9.8.8   Unstable Excavation Sides

                               Vertically excavated walls may be difficult to maintain in areas where the soil

                       moisture is high or where soft cohesive or cohiesionless soils predominate. These conditions

                       may require laying back of the side slopes to maintain stability; trapezoidal rather than

I                     ~~~~~~~rectangular cross sections may result.

                       9.8.9 Vegetative Buffer

                               A vegetative buffer of at least 20 feet wide (wider if possible) should be used to

I                     ~~~~~~~intercept surface runoff from all impervious areas.

                       9.8.10 Traffic Control

                               Heavy equipment and traffic shall be restricted from travelling over the infiltration

*                     ~~~~~~~areas to minimize compaction of the soil.

                       9.8.11 Observation Well

  U                           ~~~~~~~~An observation well, as described in subsection 9.3.8 and Figure 18 should be

                       provided. The depth of the well at the time of installation should be clearly marked on the

                       well cap.









                                                           106







POROUS PAVEMENT

10.1   Description

       Porous pavement is comprised of porous asphalt material and a high void aggregate base

that temporarily stores the storm runoff and the rain falling onto the paved asphalt surface. The

stored runoff in the aggregate base is then infiltrated into the permeable underlying soil. In most

cases, porous pavement consists of four (4) layers as described below (from bottom to top):

        a Minimally compacted subbase consisting of undisturbed existing soil. Two-inch

        (2") thick layer of 0.5 inch diameter gravel on top of a filter fabric layer. This layer acts

        as a filter course.

                Reservoir base course consisting of 1.5 - 3.0 inch diameter stone aggregate

        (aggregate subbase). The thickness of this layer is determined from the runoff volume that

        needs to be stored. This course acts as a stone reservoir.

        a Two-inch (2") thick layer of 0.5-inch diameter gravel to stabilize the reservoir base

        course. This layer acts as a filter course.

                Porous asphalt pavement course with a thickness that is based on bearing strength

        and pavement design requirements. In most applications 2.5 - 4.0 inches thick is found to

        be sufficient.

                Based on the infiltration capacity of the underlying soil, auxiliary drainage

        structures like pipe drains, french drains, etc. may also need to be installed at the bottom

        of the porous pavement.

                The storm runoff infiltrates through the pores of the porous asphalt pavement course

        into the void spaces in the underground aggregate base course. Runoff then exfiltrates out

        of the reservoir base course into the underlying subbase. For underlying subsoils with




                                           107







marginal or low infiltration rates, the exfiltrated runoff may be collected by underdrain

pipes and routed to an outfall structure.

       Porous pavements should be restricted to sites with contributing drainage areas of

1/4 to ten (10) acres.

        A detail of typical porous pavement illustrating the four (4) layers is shown on

Figure 19. A porous pavement schematic is shown on Figure 20.









































                                   108






                                                                                      | FIGURE  19





















     POROUS PAVEMENT COURSE
     2.5 - 4.0 INCHES THICK                                                       FILTER COURSE
                                                                               0.5 INCH DIAMETER GRAVEL
                                              ... :::?::::: :? ?:.  - ~: /    ~2.0 INCH THICK

                                                                               STONE RESERVOIR
                                                                                1.5 - 3.0
                                                                                INCH DIAMETER STONE
                                                                               (AGGREGATE SUBBASE)

        FILTER COURSE
        2.0 INCH THICK GRAVEL              ....... ......                        FILTER FABRIC LAYER
                                                                               UNDISTURBED SOIL




















                                                        POROUS PAVEMENT DETAIL


                                                                        REVS'D:          DRAWN:  K.F.F.
                                                          (S~t (.~ ,,CHK'D:        , , _'  CHK'D:  V.E.M.
                                                                        DATE:            DATE:  OCT. 1990
     Smith  Demer Normam                                                  REVS'D           SCALE:  N.T.S.
      Enginear - Plann, - Survaor  - Landscape o rlt.ects                CHK'D:.h,,,
        Cetrol Park . Sio Manhattan Square Suits 102  HAMP TON  R ADS
        (     H04)- 0pt)6.7-900 Vrg (n4) 2 5-36 53 DATE: PAGE  109
      (804)865-9610 (804)627-6900 For. (804)85-1533  PLANNING DISTRICT COMMISSION    DATE: 
I~ ~~~~~~~HmpD.Vrii 35







10.2   Applicability

       The use of porous pavement as a BMP is restricted to low traffic volume parking areas.

It is a substitute for conventional asphalt pavement. It is feasible on sites with gentle to flat slopes

and subsoils with moderate permeability. Possible areas for use of this BMP include:

        ï¿½ Overflow parking areas.

        ï¿½      Emergency stopping areas, parking lanes, and cross-over lanes on divided highways.

        ï¿½      Low traffic volume roads

        I       Driveways attached to residential lots.

10.3   Design Criteria

        10.3.1  Soil Permeability

                The permeability or infiltration rate of the underlying soil will determine the depth

        of the aggregate subbase of the porous pavement. Soil textural classes with infiltration rates

        greater than or equal to 0.52 inches per hour should be used for the design of the porous

        pavement. This infiltration rate is associated with soil textural groups of sand, loamy sand,

        sandy loam, and loam. A thorough examination of the permeability of the underlying soils

        is the key element in the design of the porous pavement. At least two (2) test soil borings

        should be taken to determine the permeability of the underlying soil.

        10.3.2 Groundwater Table

                The seasonal high groundwater table should be located at least two (2) to four (4)

        feet below the bottom of the reservoir base course (aggregate subbase).

        10.3.3  Depth of Aggregate Base Course

                A typical porous pavement reservoir base course can range from two (2) to four (4)

        feet. A shallow reservoir base course with a large bottom surface area is preferred over a

        deep and smaller bottom surface area base course.


                                           110







10.3.4 Proximity of Wells and Foundations

       Porous pavement should be located at least 100 feet upgradient from a drinking

water supply well and should be at least ten (10) feet downgradient from building

foundations.

10.3.5 Pavement Slope

       Porous pavement should be used on sites with slopes less than five (5) percent.

10.3.6 Storage Time/Maximum Draining Time

       The reservoir base course should be designed to completely drain within a

maximum of three (3) days (72 hours), and a minimum of two (2) days (48 hours).

10.3.7 Frost Heave

       If a soil with a high susceptibility to frost heaving is being considered (for example

silt loam), the reservoir base course should extend below the frost line to allow for proper

drainage. In most cases, this depth below the frost line will exceed the depth of storage

required to control the runoff volume from the site.

10.3.8 Pavement Design

       The traffic intensity over the porous pavement is the key factor in determining the

thickness of the porous asphalt pavement course. The traffic intensity is defined by the

average daily Equivalent Axle Load (EAL), which is based on the equivalent of 18,000

pounds (18 Kips) axle load in the design lane.

10.3.9  Calculation of Void Space

        Void space should be calculated according to the testing procedure recommended

in Federal Highway Administration Report No. FHWA-RD-74-2. Design of Open-Graded

Asphalt Friction Courses (Smith, Rice, and Spelman, 1974). The volume of the sample







             should be measured mechanically rather than calculated from a water displacement method

            because a great deal of water is absorbed.

             10.3.10 Aggregate Gradation

                    The gradation required to obtain a porous asphaltic pavement is of the "open"

             graded type as contrasted to the "dense" graded type which is capable of close packing.

             The following aggregate specification is recommended.


                     Table 14 - Aggregate Gradation for Porous Pavement

    U.S. Sieve Series Size                Opening (mm)                Specification: Percent Passing
                                                                             by Weight

           1/2-inch                            12.70                               100

           3/8-inch                            9.51                              95-100
             #4                                4.76                              30-50

             #8                                2.38                               5-15

            #200*                              0.074                               2-5

* Aggregate should be uniformly graded between the #8 and #200 sieve.
Source:        Maryland Standard and Specification for Infiltration Practices.


                     Open graded mixes, due to their relatively high permeability to air and water,

             provide good resistance and durability to freeze/thaw conditions and to asphalt film

             oxidation.

             10.3.11 Type and Ouality of Aggregate

                     The aggregates selected for porous pavement construction should meet requirements

             of the standard specification for "Crushed Stone, Crushed Slag, and Crushed Gravel for

             Dry- or Water-Bound Macadam Base and Surface Courses of Pavements," ASTM D 693-

             77, with two (2) exceptions.  First, the gradation test must be of the open graded type

             described here. Second, a soundness test is required, as specified in ASTM D 692-79,


                                               112







"Coarse Aggregate for Bituminous Paving Mixtures," to determine if the aggregate is

susceptible to disintegration by water.

10.3.12 Asihalt Cement Grade in Mix

       The suggested viscosity grade of asphalt cement to be used is AC-20 of AASHTO

M-226-73 I. This grade is to be considered a tentative starting point because test results

obtained from the design process may indicate an advantage or a necessity to alter the

asphalt grade.

10.3.13 Mixina Temperatures

       To ensure that the individual aggregate particles are completely surrounded by

asphalt, and that the asphalt is tightly bound to each particle, the temperature of mixing at

the hot mix plant shall be rigidly controlled. Too low a mixing temperature will result in

inadequate asphalt binding and coverage of the aggregate, while too high a mixing

temperature will allow asphalt to drain from the mix, resulting in a lower asphalt content

and decreased strength. Suitable mixing temperatures range from 230 to 260 degrees

Fahrenheit, but the lower end of that range (2300 to 240ï¿½F) is recommended.

10.3.14 Asphalt Content in Mix

        For road paving durability and to prevent too rapid hardening of the asphalt, it is

desirable to have the highest asphalt content possible in the mix. Too much asphalt would

separate out under traffic, so that maximum asphalt content is generally limited by that

factor. Experience has shown that 5.5 percent by weight is the minimum recommended

asphalt content. Asphalt content should be determined according to the testing procedure

recommended in Federal Highway Administration Report No. FHWA-RD-74-2, already

cited.  The Marshall Design method for determining mix content is not recommended.

Using a 5.5 percent asphalt content and the recommended six-inch (6") minimum surface


                                  113







                       course, a 0.6-inch rainfall reservoir capacity is obtained with an inifiltration rate of 176

                       inches per hour. A four-inch (4") minimum surface course is recommended by the Asphalt

I                     ~~~~~~~~Institute.

                       10.3.15 Traffic Control

                               Experience has shown the need for close control of contractor vehicles on newly

                       installed areas of porous asphalt pavement. Damage to pavement porosity results chiefly

                       from abuse during the early life of the pavement. Normally, paving is done while heavy

                       construction or earth moving is continuing in an area. The pavement is thus subjected to

                       mud and dirt from contractor vehicles for up to several months, and the continual passage

                       of these vehicles compacts the dirt into the pores. Only if caked mud is cleaned from

I                      ~~~~~~vehicle wheels and the pavement is cleaned daily by sweeping and high-pressure water

                       washing can porosity be retained. Clogging can be further minimized by proper use of

                       curbing to prevent surrounding soils from washing onto the pavement surface.













        I~~~~~~~~~~~~~~~1





I


1 10.4 Design Examples

                                               Design porous pavement for one (1) acre of parking lot to control runoff of 0.8 inch

I contributed by a one-inch (1 ") rainfall. Underlying


I rate of 0.58 inches per hour. Depth to groundwater t
                                      pavement should be two (2) feet. Design the pavement for first flush also.
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                                                                                        .I..   - . I ''...  .-.1...-.-.-. - 1.  I I---
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                                                                                                            --. ,X: ::::::: ::%::-... ..
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                                     -- -- ''....1--- . .... - - ...-...
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                                    I. -1-1 . - -.1-I-, -11-1. 1.1-1 .11 1-1.  1.11-1. .... ... I.... -.
                                      .. ... - 1-1-1-. .. , - , __ , ..., - . -1 .....1-1 . . - I  - .1.. . I. .. .... ..- _1 I.. .. ...I
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                                                                                .I I -... : 1 : :: : I... I........,... .....--
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                                    11.I  - 1 . 1. .... . - -I. I  ''... ....- .... - . - . - . . - I I _._ ........-. -... 1- _-.
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                                   I I. - - - .1- .. ''.... I.... .. .... - -  . . - -,- ,I  .-I. .-
                                                      _1,,., , 1.1.1-1 1. I I.. I....  .I. .1 . _1 - 1. _-..._....-..
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                                       ***** maxiffiurnS bbak Depth(Ft) :  9 Q: * * * * *::: :- -.1.1 -- I-.-"......-V 1.
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                                                                                    I-.  .... ... ....-I I... 1. . -... .-
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                                    .... I ... . ...--. . - I.... .-.1 ' -.,. ". '111.1- - .-1. I . . . --.. . I  - : :  " '..-..---  -   -  I '."..,..- --1 I
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                                                                                   I - -11. ''. .. - . . ... . __  .- .. ... .. I - ........
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                                    .- ... - .-...........- .. ....-......  ,  1-1- - --.- I -.11. - -....... .- . --  ...- 1-11 I. I-I -1. I I-- - I I---- .
                                                                                     ... _.. . ..''...... .. -... 1-1. '_.... . ._..... -...-.. ..
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                                     I . I I....., I. - - I -11 .1 -- 11 1. I I.. I-.  -- ...
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                                   I, ,... ... I.., ,..., ,. . - . .. 1-1.1-F -- --.1,1-1 ... ------!:: .1 ..., . -.....-I.
                                                                                       ....  I. ... .  1.11. I .- Fl. . ,  - I   ,..... I. .1, , , ,., -1-1-1
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                                                                                      . .. .... --.....- .... , . _.. ... ... . . ... ,  . ''...,
                                                                                    ...    II 1.III -.1-II -  - I                                               1. ....  . Increase jn Runoff Deoth(lh)-.' 0.80--.- 1V  1 I - 
                                                                                   . .. - ... - 1. ..  . ,. ,. ,. ,- .,  II I,... .1.1.1
                                    .II...   , . ..., .... .. . .. .. ... .. 1--I. .  -11-11 --
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                                         ., -.... -1-1- .... - ... .. - I - - 1. . I-- ... ..... I ..''
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                                   ... 1. I-. . . .... 11 . - .  _-.1. ''. ..., 1.--- -1 -.1.1'..--    1.11 -... - _- ... .. .1..
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                                                                                                  . 11 ... I----  . . 11 -  ..  I -111.11,.
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                                                                                    .- I ... . .11 . .... . .1 - -I :1.
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                                      . - -  --...I--.. 1. ........... -.1. II - --- ...I
                                       .. . .1 ., . .I - I. ..... .1 ... . . ..... . - .1 -  , ,,I.-1, ,1. ..  -. -
                                    ,-.1 II 1...I11- . I -.....I....-   I.I..I........ .- I1.11.I. I -11.1.
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                                                                                                             I. I-.1- -..- -- _.
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                                    .,I...I1. 1. . ..1 1. I... . _I.  I, , .1 .... -..... _-111 I I.I. - I ,I
                                     .... I. .1. -....,,. I '' -111 1. ..  ''..., .. 1. - I- II- .. 1.
                                        ... -. . I 1. .- 1. -. -.. ....1 . ''.. , I... 1. , 11
                                    I. I. .. 1. 11 11 . ,. 11 .... .. ....I . ...II --. 1. .   . .....
                                    .......I . 1. --- -. ..1 I .1, .-I...... ...''. I 1: 1: ::I...
                                   -. . I.. . .... ..I111, .... 1.11-I -.1 I. I.  . .1 1. 1.  I , -..
                                    ....I I- .1 I . - .--. -- . .... . .. . -, .1. I .11,I..
                                   .... .. .1 ... ''.. . . .11-- .11  .. . I. I... - II--.11 1..I_- . -...
                                        -. . .-.. ..., .. I..  -I - .I. . I -I, ,... I ''.. .. , . .. .   -
                                    .1 _  . 1--..-... ,... ..... .. ...  I.. I.  - -1. -- .... I I 1-1-.. . I.
                                        I-1.11. .  I- ''. ... I -  .... - -- ., -..  - - - .. ,I.I..
                                         .-.1. ... I . III I I .. . . II I .1  I,
                                    -''I.....I'll., ...-.-. -- 1.  . II-..
                                    ,II I11.II.II-- .. -. - -1. . .- 11  I -- - . 1.-...  I .
                                                                                  . .. 1.  ,.. ... .  I .  - I 1-1. ...
                                   I-I  . I -I - -. .1  . .   11, .... I-.  .. .1
                                                                                    -- I . .1 .1.1 ..II--
                                                                                                                     ,.  ...,....-.-
                                                                               -'' . ., - ........I-
                                                                             .... .  I.1. I.. III. I : ::
                                    Amount: of Rainfall(In)- '1.00: : .  . I. II1.1-11- ---- :. .1 . -
                                                                                                                                  II-.. . .. I ..., I. ".. ....I:: ::..I.. ... 11
                                                                                    .I I. I... 11 .1 I.I.I III I. .
                                                                                 -,... ., .1III   ..."I  . I  ...-
                                        I. ..... - . I-. I-''I''..", .1 . . .I --. .. . .-
                                                                                        ..., ., . .. .... ...I ''..I.... II ...
                                              11 . I. I. -.,I,. I 1. .1.1. 1.1.11 ...   I... I-1.. -
                                             I , .''.  I. . .  I1.,... ... - ..
                                      I . . I I I  . . ... - .1-I II -I... --- . .-I. I.II..
                                    .... - I. - . I. .-.1 . - -11 . I.... ... . I.  . 1 I. -. I. ..... .
                                    . .. -I- I II. . . I -,I . .                                                                                         . .' '  - . I.    ,         -   ... ... .  . .. . .  . . -  . I. I .  ...   I  .  . I 1 .I.   ......-, I
                                             .1...  I.-  - I 1.I,  ''.., I... ,  -I..1..
                                                          1 ...II .1 I -.1 .I.-.. 11 1... --
                                        ....I.II III. I. , I,.. ..1 I. ... .  -  :: :::   -,.- -I
                                    I. ,.I- .-.. - - . - I- .. ..  -.. I - 1.  II I .. -....
                                    .I I I.. . -... -. .. ...I111.1 - ...I.I.
                                                  I I ,. 1 .I..-I ..  ....1.
                                                                          II.1 I. - 1. ..
                                                                                                                           II I.I.11 .,
                                                                .,
                                                                                                  1. I -I--- .. I I... - _::_---:::
                                    Aggr. Subbase ReservoirTilling Time(Hrs). 2 , '.I.. - -: ..1.  , -1. I
                                                                                            .       :I11         I..     .I, :                     ,.  . .''  -   11, .-I
                                                                                                       .II.II.. . .
                                                                                                             .1  .. .- -II
                                                                                III.. . .... .1-
                                                  I. III 11.I .I ....  II., I..1, .-I
                                     . I II.. .1.I.I I .III..  - - I.. .-
                                    . I.-- I ... I ..1... 4-. I. I.,   I -'. I  I------ I--... 11- ... - -::::I-I .II
                                     I.1....  I.. -... .  I 1.1 I .,.,.... 1.I
                                    .... ''  I.11 I,. .. I 1. . ...... .....  .. .. I-_-.-
                                                   . I.... 1, -.1  -.1 . .... . ..I I ::. ....
                                           .I.I .- 11 I ,.1 ,. , -1. 11 .1... I,.
                                                 II -I .....  . I 1. 1, ,.. ....II. .... ..-
                                                    ... - I. -... -.1 I  -.1  .1. -  -. 11 ..,-. -
                                                              . . II I_ 11  - 1.  I... . . ... . I.                                                                      .1- .... " ''    - ,I.''  ,    .  .. .  . ..  1 1 . . .
                                    I.. . ..- . .. .. I--  - ,.  .I.1II 1.-I-- 11 ... 1-1. -
                                                                                                                               1. . -11 . - - I .....   II .. I 1-1..II-  ..-
                                               III.II...,.... II---
                                                                                                                          I I, ....-
                                                                                                              I.-11 I I-- ,I I.....
                                    :***** Porous Pavement' Area(Sq Ft)-! 3570 ***** I1. ...... '.... . I.I11, I I 1. 1. I
                                                                                                                -,.I.I-
                                     II - .
                                    I..I_..I.I.I. , I:. I..I.. . -  .. 1.
                                     I..-I.. ... .. -  I.1.
                                      1.-.. ..I.  . -.I. . - - I.I-.I...I--
                                     1. --I. I - , -.....-- - I-,III.1 .I1.I....
                                                                                                    I.- 1. ... ...
                                    I..II.. ..-I. I-
                                    ,.I ....- - -I III.. .. 11 I  I.-.
                                       -   .11       .11 -I             I   -   -  -           -    ..    .   --  . I   .  . I.          . I ,         ,       , .   .    -      . .  . .  -  --    .  .    I -I I I     ...    . I. -  1.
                                     I1. ...I I...  _-- .- -- ...-. -1. - :::::: ::
                                      .. .,I ,- I I.  .. - -. .II-I . .. I .. I .1 -I   - ,''.. . .I 111
                                    1.I.  ..1.-11 ..I.... .... .-  I-,
                                                             .. 1. .1 . .1.-. I  I..... 11 ...I,... ,.. II, -1.
                                         II, . ''I.-I .1 - I...1
                                       I. I.II II...1.I
I .I


1
                                                                                 115


1


1








    I                               ~~~~~~~DESIGN OF POROUS PAVEMENT FOR: FIRST FLUSH~:



H                    ~~~~Project: 10.4 DESIGN OF POROUS PAVEMENT                                      v

                          *---ï¿½> Feasibility: Input Parameters ï¿½<<--

                        InfilttinatJnr)   0.58.

U                    ~~~~~~~Max. Allowable! Storag~e~ Time: of AggeaeSbaeHs:7

*                     Vo~~~~~~~~~~~~~~~~id Ratio Aggregate Subbase:0.

                       ADepith to ~Seasonal: High 'Groundwater 'Table(Ft .0

I                    ~~~~~~~~~~~~~~Mn.~ Dist.jrom Agg~r. Sbase to Groundwater,.Table(Ft).:,:2.0

                             ***    aximum: SbaseethF) 5. 

                          ->>>;;! DesigjnO nut Parameters ï¿½<

                       Increase in ,Runoff Depth(Ln). 0~.50       ... ....... ....
                                      Contributing~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ..ing  .r S  .....4356
              U        Depth of Aggregate Subbase(Ft): 2.0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ......

                        D t Porosfaemn  AgregaeSba(SqFt):!   2269 ....






















         I                                              ~~~~~~~~~~~~~~116






                                                                                       T FIGURE 20




                                                                          P





                                  6"


                               Dr d, D
                             DT    dP            VR


                                 / D,           f .






                     f  =  Infiltration Rate(ln/Hr)

                     Ts =  Max. Allowable Storage Time of Aggregate Subbase(Hrs)

                     VR =  Void Ratio Aggregate Subbase

                     DT =  Depth to Seasonal High Groundwater Table(Ft)
                     DM =  Min Dist from Aggr. Subbase to Groundwater Table(Ft)

                     Ac =  Contributing Drainage Area(Sq Ft)

                     AQ =  Increase in Runoff Depth(ln)

                     dp =  Depth of Aggregate Subbase(Ft)

                     P  =  Amount of Rainfoll(ln)

                     T  =  Aggr. Subbase Reservoir Filling Time(Hrs)





             For First Flush Design:
                                 AQ A AQ A
          Area of Pavement =    12  C                    Area of Pavement =
                                 VR dp                                            d  
                                                                           VRV  d,   P +  f T
                                                                                VR  12   12





                                                    | POROUS PAVEMENT SCHEMATIC


                                                                        REVS'D:            DRAWN:  K.F.F.
                                                                        CHK'D:              CHK'D:   V.E.M.
                                                                        DATE:               DATE:  OCT. 1990

Smnth Demer Normam                                                             REVS'D:             SCALE:  N.T.S.
Engineers - Planner - Surveyors - Landscape Architects             A  S           'ID:
  Central Park  S.ix Manhotton Squre Suite 102                 HAMTNo                    CHROD:
         . Hmpton. rno a 23666                    TON   ROADS              DATE:                 PAGE  117
 (so4)e65-9gso (804)627-6900 Fo= (604)865-1533  PLANNING DISTRICT COMMISSION   DATE:







10.5   Maintenance Requirements

       The surface of porous asphalt pavement must be cleaned regularly to avoid it becoming

clogged by fine material. This cleaning is best accomplished through use of a vacuum cleaning

street sweeper. Outside of regular cleaning, porous pavement requires no more maintenance than

conventional pavement. In times of heavy snowfall it must be recognized that application of

abrasive material should be closely monitored to avoid clogging problems once the snow and ice

has melted. No method of maintenance has been satisfactory on fully clogged pavements, and only

a superficially clogged section showing a water penetration of 0.38 inches per second can be

restored to normal operation. The best method for cleaning is brush and vacuum sweeping followed

by high pressure water washing of the pavement. Vacuum cleaning alone, once the pavement is

clogged, has been found to be ineffective. The oils in the asphalt bind dirt, and only an abrading

and washing technique can be effective in the removal of such dirt. Clogging to a depth of 0.5 inch

is sufficient to prevent water penetration.

10.6   Life Expectancy

        Very limited data is presently available to assess the useful life expectancy of porous

pavement as a BMP. Major elements in the longevity of porous pavements are proper construction,

inspection, and routine maintenance. The porous pavement surface must be vacuum swept routinely

to keep the asphalt pores open. Life expectancy of six (6) months to two (2) years of proper

functioning of porous pavement as a BMP is a good estimate.

10.7   Cost

        Porous pavement costs should be considered as incremental costs or extra costs, incurred

over and above the cost of installing a conventional parking lot. Extra costs are associated with the

additional depth of the reservoir base course, porous asphalt, filter fabric and sediment and erosion

control. A typical porous pavement of 3000 sq. ft. of surface area with 1.5 feet of aggregate


                                          i118







subbase could cost an extra $5,000-6,000 over a conventional pavement. Some savings may be

realized in the reduced cost of the conventional storm drainage system.

10.8   Construction Methods and Specifications

       (Adapted from the Construction Specifications of the City of Rockville, MD.)

        10.8.1 Stabilization

               To preclude premature clogging and/or failure of this practice, porous asphalt

       paving structures should not be placed into service until all of the surface drainage areas

        contributing to the pavement have been effectively stabilized in accordance with Virginia

        Standards and Specifications for Soil Erosion and Sediment Control.

        10.8.2 Subrade Preparation

               Alter and refine the grades as necessary to bring subgrade to required grades and

        sections as shown in the drawings.

               The type of equipment used in subgrade preparation construction shall not cause

        undue subgrade compaction. (Use tracked equipment or oversized rubber tire equipment -

         DO NOT use standard rubber tired equipment.) Traffic over subgrade should be kept at

        a minimum. Where fill is required, it shall be compacted to a density equal to undisturbed

        subgrade, and inherent soft spots corrected.

        10.8.3 Aggregate Base Course

                All stone used shall be clean, washed, crushed stone, meeting Virginia Department

        of Transportation (VDOT) specifications.

                Aggregate shall be of two sizes: the stone reservoir base course shall be to the

        depths noted on the drawings (maximum three-inch (3"), minimum 1.5 inch, and a two-

        inch (2") deep top course of 1/2" aggregate).




                                           119







       The stone reservoir base course shall be laid over the bottom filter course to the

depths shown in the drawings, in lifts to lay naturally compacted. The stone reservoir base

course shall be compacted lightly. Keep the stone reservoir base course clean from debris

and sediment.

10.8.4 Porous Asphalt Surface Course

       The surface course shall be laid directly over the 1/2" aggregate base course and

shall be laid in one lift.

        The laying temperature shall be between 230ï¿½ and 260ï¿½, with minimum  air

temperature of 500F, to assure that the surface does not cool prior to compaction.

        Compaction of the surface course shall be done while the surface is cool enough

to resist a 10-ton roller. One (1) or two (2) passes by the roller is all that is required for

proper compaction. More rolling could cause a reduction in the surface course porosity.

        The mixing plant shall certify the aggregate mix and abrasion loss factor and the

asphalt content in the mix. The asphaltic mix shall be tested for its resistance to stripping

by water use according to ASTM D 1664. If the estimated coating area is not above 95

percent, anti-stripping agents shall be added to the asphalt.

        Transporting of the mix to the site shall be in clean vehicles with smooth dump

beds that have been sprayed with a non-petroleum release agent. The mix shall be covered

during transportation to control cooling. It should be pointed out that the mix is required

to be covered during transportation by Virginia Law.

        The mix of asphalt shall be 5.5 to six (6) percent of weight of dry aggregate.

        Asphalt grade shall meet AASHTO Specification M-20 for 85 to 100 penetration

 road asphalt as a binder in the northem United States, 65 to 80 in the middle states

 (Virginia), and 50 to 65 in the South.


                                   120







                    Aggregate grading shall be as specified in Table 15.


               Table 15 - Porous (Open-Graded) Asphalt Concrete Formulation

                                                                   Probable Particle Data

                                                                                        No. in
                                                                                        100g of
                                              Volume,       Width,                     Asphalt
  Material         Screen      Weight, %         %           MM         Weight, g      Concrete

 Aggregate      Through 1/2        2.8          2.2          10.7         1.667           1.7

                Through 3/8       59.6         46.3          8.0          .697           85.5

                 Through #4        17.0         15.3          4.0          .087          195.4
 Sub-total Coarse Aggregate        79.4         61.8                                     282.6

                 Through #8        2.8          2.2           2.0          .0109         255.6

                Through #16        10.4         8.0           1.0        .00136         7647.0

                Through 200        1.9          1.5          .06         .000294        6462.0
   Asphalt                          5.5         10.5

     Air                            0           16.0

                                   100.0        100.0

Source: City of Rockville, Maryland (1982)


             10.8.5 Protection

                     After final rolling, no vehicular traffic of any kind should be permitted on the

             pavement until cooling and hardening has taken place, and in no case less than six (6) hours

             (preferably a day or two).

             10.8.6 Workmanship

                     Work shall be done expertly throughout and without staining or damage to other

             permanent work. Make transition between existing and new paving work neat and flush.

             Finished paving shall be even, without pockets, and graded to elevations shown. Iron

             smoothly to grade, all minor surface projections and edges adjoining other materials.

                                               121







          GRID/MODULAR PAVEMENT

          11.1   Description

                 Grid/Modular pavement as a BMP is similar to other infiltration practices.  Instead of a

          conventional pavement, a pervious pavement consisting of a grid made of concrete, clay bricks, or

          granite sets can be constructed. Void areas of the grid pavement can be filled with pervious

          material like sand, gravel, or sod.

                 There are three categories of grid paving material. They are based on their surface

          configurations, and are listed in Table 16. Representative grid pavements are shown on Figure 21.


                                  Table 16 - Types of Grid Pavements

                 Type                          Configuration                     Brand Names

             Lattice Pavers                    Flat Grid-like            Unigreen, Turfstone, Grasstone
           Castellated Pavers                Raised Battlements             Monoslab, checkerblock
        Poured-in-place Pavers                 Flat grid-like                      Grasscrete
I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

                 A schematic of Grid/Modular pavement is shown on Figure 22. Grid/Modular pavement

          detail is shown on Figure 23.

          11.2   Applicability

                 Grid/Modular pavements should be used for areas with light traffic and less frequently

          travelled parking lots. Another use can be for walkways in recreational areas. All driveways in

          residential areas, auxiliary parking, emergency fire lanes etc. can be good applications of

          grid/modular pavements.

          11.3   Design Criteria

                  The design criteria for grid/modular pavement is similar to the infiltration trench criteria.

          It is included here for ease of use.



                                                   122







11.3.1  Soil Permeability

       Soil textural classes with infiltration rates greater than or equal to 0.27 inches per

hour should be used for the installation of grid/modular pavements. This infiltration rate

is associated with soil textural groups of sand, loamy sand, sandy loam, loam, and silt loam.

The infiltration rate of the underlying soil and the depth of the groundwater table are the

major limiting factors in the selection and feasibility of the grid/modular pavement as a

BMP.

11.3.2  Denth of Subbase

        The final infiltration rate of the soil below the infiltration trench determines the

maximum allowable subbase depth. The surface area of the grid/modular pavement can

be minimized by making the subbase as deep as feasible. The subbase of the grid/modular

pavement can also be made shallow and broad. The increased surface area of the bottom

of the subbase increases exfiltration rates and provides more area for soil filtering of

pollutants.   A  larger subbase bottom' also helps in reducing clogging by providing

exfiltration over a wide area.

11.3.3  Groundwater Table

        The seasonal high groundwater table should be located at least two (2) to four (4)

feet below the bottom of the subbase of the grid/modular pavement. The soil permeability

(infiltration rate) and the groundwater table are the two parameters which determine the

maximum allowable depth of the grid/modular pavement.

 11.3.4 Proximity To Wells and Foundations

        Grid/Modular pavements should be located at least 100 feet upgradient from any

 drinking water supply well to minimize the possibility of groundwater contamination. Also




                                   123







the grid/modular pavements should be located at least ten (10) feet downgradient and 100

feet upgradient from building foundations.

11.3.5 Design Storm

       Grid/Modular pavements can be designed for a specific storm or for the first flush

runoff volume. If for first flush, the subbase storage volume can be sized based on 0.5

inches of runoff per impervious acre in the contributing site area.

       Grid Modular pavements are normally designed for water quality. As such, a

significant portion of the runoff volume (storms producing more than 0.5 inches of runoff)

will bypass the grid/modular pavement and is not infiltrated.

11.3.6  Storage Time/Maximum Draining Time

       All grid/modular pavements should be designed to drain within a maximum time

of three (3) days (72 hours), or a minimum time of two (2) days (48 hours). These values

are derived from literature. The Virginia Stormwater Management Regulations recommend

two (2) days (48 hours).

11.3.7 Stone Aggregate

       The stone aggregate which fills the grid/modular pavement forms the reservoir

through which the storm runoff passes and is filtered. The aggregate material should be

clean, washed stone. Wash run gravel is preferred. The City of Virginia Beach

recommends using James River stone as aggregate.  The clean washed stone aggregate

should have a maximum diameter of three (3) inches and a minimum diameter of one (1)

inch. Void spaces for the stone aggregate are normally within the range of 30 to 40

percent. A table showing open graded coarse aggregates is included in the Appendix.







                                  124







11.3.8 Runoff Filtering

       It is important to prevent any floatable material, settleable solids, grease, and oil

from entering the grid/modular pavement. Runoff filtering devices such as vegetative filter

strips (minimum of 20 feet) can be used in front of the grid/modular pavement to prevent

objectionable materials from entering the subbase.

11.3.9 Overflow Recuirements

       In all cases, the overflow path of storm runoff exceeding the capacity of the

subbase of the grid/modular pavement should be evaluated and accommodated.  The

grid/modular pavements are designed to treat the first flush volume of runoff and control

small drainage areas.






























                                  125






                                                                                               FIGURE 21




    I~~~~~~~~~Z










                                                                     Tu rfstone
                                                                     (lattice)










                                                                          Monoslab
                                                                         (castellated)













                                                                    Grasscrete
                                                                    (poured-in-place)





                                                          REPRESENTATIVE GRID PAVEMENTS


                                                                            R EVS'D:             DRAWN:   K.F.F.
                                                                            CHK'D:               CHK'D:   v.p.m.
                                                                            DATE:                DATE:  6-cT. 1990
                       &nrffi Den-ff Normam                                     REVS'D: ~~~~~~~~SCALE:  N.T.S.
E.9insam - Plaonnm - S.-yaors - Landscape Arohitacter                      -            
    C trlP.k Si. Monhattan Squa* o im 0   HAMPTON   ROAD  S                       CHK'D: PAE  2
          Hampton, V'-,ginio 23666                                                DATE:                  P G         2
(80:)855-9610 CB04)627-690DFo (S04)865-1533  PLANNING DISTRICT COMMSSI O N~--                    -      






                       11.4   Design Example
                                        Design modular pavement for the driveway of a single-family residential lot. The
                               driveway size is 24 feet x 40 feet. Associated runoff for a 1.5-inch storm is 0.6 inch.
                               Underlying soil is border-line sandy loam with an infiltration rate of 0.85 inches per hour.
                               Depth to groundwater is three (3) feet. Pavement subbase depth should be one (1) foot.


I -
                                     ..  ......DESIGN OF GRID  MODULAR PAVEMENT................

     IProject: 11.4DiESIGN EXAMPLE. FOR.A .DRIVEA.



                              I~nfltrationi Rate(InIr): 01.85 ï¿½- I)- :)ï¿½.ï¿½ï¿½ï¿½:ï¿½.........

       I           - ~~~~~Mximum  All~owal  Soage Tie of  avement(Hrs): 48
                              Vid Ratio Subbase Medi~um:l~            i-ji-l'i.-: 0.~
U                          D~epth to Seasona~l High Groundwater Tabeft

       I~ ~ ~       ~ ~~m D.ist. from Subbase b~ottom to G'ro~undwate~rTable(F3:- '2.~ï¿½~ï¿½':ï¿½    ..ï¿½':e
                              ***~~ Maxim um~ Subbase DepthPFt):~ ~ :f' 1.0                  _-
                   I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. .... Deig ....... Parmeer    ....  ...-. .
               II:         ~~:l:~ï¿½ï¿½1~L~ï¿½:~~:j_: j~li~ij~~~~_~~_:::j~_.. ...... ... ..... ..... . ... .. ...lli-ii,_ 
                                                                      :-:i~~~~~~~~~~~~~~~~~~i~~. ..........-... . ....













I
                                         :-::~~~~~~~~~~~~~~. ...... ....
                                                                            ...   .......  . .... .. . ....~-  ili:-jiii:_j::ill-~i-I --:- ....-
                                              -i-***** Pavement AreB(Sq Pt):~~~. ..  .... .. 2  "4::: ..... .... .. .....-l::lili~lij jli
















                 U                                            ~~~~~~~~~~~~~127
                   I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...  .......... ...... .
                        I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.... ....  ..






                      U  -           ----~~~~~..-                        _ _ -~~~~~IFIGURE                               221





                    I                                                                          ~~~~~~~~~~~~~~~~~~~~~~~~~P









 I~~~~~~D  ds                        V











                               f  =  Infiltration Rate(In/Hr)

                                 Ts=  Maximum Allowable Storage Time of Pavement(Hrs)

                               VR =  Void Ratio Subbase Medium
                               DT =  Depth to Seasonal High Groundwater Table(Ft)
     I~ ~~ ~~~~~D = Min Dist from Subbase bottom to Groundwater Table(Ft)
                               Ac =  Contributing Drainage Area(Sq Ft)

                                 AO=  Increase in Runoff Depth(ln)
                                 ds=  Depth of Subbase(Ft)






               I~~~~~~~~~~~~~~~~~~~~~~~~A Ac
                                               Area of Pavement = 1
                                                                        VR ds








             I                                                    I ~~~~~~~~~~~~~~~~~GRID/MODULAR PAVEMENT SCHEMATICI


                                                                                    REVS'D:____         DRAWN: K.F.F
                                                                                    CHK'D:  C___        CHK'D   V.P.M
                                                                                    DATE: ____ _ DATE: OCT. 1990
         Smrthi Denier Normamp                                                            REV         _   SCALE: N.TS.
         Enginee.s Pa-s - nn   Surneynrs - Landscape Architects                           C H K'D
*          ~~~C~nno Pc~k H Sxn46h27 a 69010 So.1t 1802)s1  HAMPTON ROADS                  CHDPA
          (soB6-91 04)67600Fx (0)6513               PLANNING DISTRICT COMMISSION          DAE_______






                                                                                        | FIGURE 23
                                                                 SOIL
                                                        / r      4" BLOCKS

                    nFn n-f 1n' -2" SAND
                                                                 EXISTING SOIL






                             AUTO  DRIVEWAY



                                                                 4" BLOCKS
                               %,/ICICICI~~ %,  ,.1-2" SAND


                                                                 CRUSHED STONE 4" TO 6"

                   I  g                              S   r          EXISTING SOIL




                             TRUCK  DRIVEWAY


                                                                  2" SOIL
                                                                 4" BLOCKS
                                                                  1"-2" SAND
                    ?I   [1    I-1    I I "".. ::: ::   .:...v....::::......:...... :...::.:...........  I..........: ...:

                               *t~~~~~~ 2     - t 2CRUSHED STONE 4" TO 6"

                                                                  EXISTING SOIL


                    yX/.,\ v\//./\\\/ /./\\//./\/ //x  x  <,.x  x. x   . //,.-/\< \

                            ROADS UNDER LAWN


                                                        MODULAR PAVEMENT DETAIL

                                                                         REVS'D:           DRAWN:  R.E.C.
                                                               (J}  /   >   CHK'D:           CHK'D:   V.P.M.

   Smrith Demer Normam                                                       REVS'D:            SCALE:   N.T.S.
   Engineer - Plonnenr - Surveyors - Landscape Archittsa  L ATa  iA          CHK'D:
     Centrl Park  Si. Manhottan Squre Suite 102  HAM T        N              1RO
            Hampton. Viriqnio 23686                                         DATE:                PAGE   129
    (804)865-9610 (804)627-6900 Fox (604)665-1533  PLANNING DISTRICT COMMISSION  DATE: r__ 
I







                 11.5    Maintenance Requirements

                        Grid/Modular pavements, if constructed properly, should have minimal maintenance

                 requirements. Routine maintenance will involve regular mowing of the grass and replacing or

                 adjusting the unequal settlement of the grid pavement. If grass is used in the void areas, the

                fertilizer to be used should not cause deterioration of the paving material.

                 11.6   Life Expectancy

                        Grid/Modular pavement may have a longevity from six (6) months to two (2) years. Proper

I               ~~~~~construction, inspection, and maintenance are the key elements enhancing life expectancy.

                 11.7    Cost

                         The cost of installing a Grid/Modular pavement may range from three (3) to seven (7)

I               ~~~~~dollars per square foot. This cost will be in addition to the installation of an infiltration trench if

                 needed. No filter fabric is required for the top and side portion of the aggregate reservoir subbase

                 of this BMIP.

                 11.8    Construction Specifications

                         Manufacturers of brand name grid pavers have standard specifications for their product.

I               ~~~~~Those  specifications should be followed  strictly for construction of the pavement.   The

                 specifications outlined under infiltration trench also apply for this BMP. They are included here

                 for ease of use.

  1                      ~~~~~~11.8.1 Timing

                                 Grid/Modular pavement should not be constructed or placed in service until all of

                         the contributing drainage area has been stabilized and approved by the responsible

  *                      ~~~~~~~inspector.







          I                                                ~~~~~~~~~~~~~~130







11.8.2 Subbase Preparation

       Excavate the subbase to the design dimensions. Excavated materials should be

placed away from the excavated sides of the subbase to enhance wall stability. Large tree

roots must be trimmed flush with the subbase sides. The side walls of the subbase should

be roughened where sheared and scaled by heavy equipment

11.8.3 Stone Agregate Placement and Comnpaction

       The stone aggregate should be placed in lifts and compacted using plate

compactors.   As a rule of thumb, a maximum loose lift thickness of 12 inches is

recommended. The compaction process ensures conformity to the excavation sides, thereby

reducing the potential for soil piping and settlement problems.

11.8.4  Contamination

       Care shall be exercised to prevent natural or fill soils from intermixing with the

stone aggregate. All contaminated stone aggregate must be removed and replaced with

uncontaminated stone aggregate.

11.8.5 Unstable Excavation Sides

        Vertically excavated walls may be difficult to maintain in areas where the soil

moisture is high or where soft cohesive or cohesionless soils predominate. These conditions

may require laying back of the side slopes to maintain stability; trapezoidal rather than

rectangular cross sections may result.

11.8.6 Vegetative Buffer

        A vegetative buffer of at least 20 feet wide (wider if possible) should be used to

intercept surface runoff from all impervious areas.







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                              11.8.7 Traffic Control
                                     Heavy equipment and traffic shall be restricted from travelling over the infiltration
I                               areas to minimize compaction of the soil.
I
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         12   GRIT-OIL SEPARATOR

                 12.1 Description

  I                      ~~~~~~~Grit-oil separators are considered practical to remove hydrocarbons, coarse sediments, and

                 grit before they are conveyed to a storm drain system. Pollutants tied to coarse sediments may also

                 be removed. A typical grit-oil separator consists of three chambers. The first chamber receives the

                 storm runoff through a storm drain or the opening of a curb inlet. Any grit or sediment is trapped

                 in this chamber. Floating material like bottles, leaves, and other containers are also trapped in the

                 first chamber. The first chamber is connected to the second chamber through a minimum of two

                 orifices which are screened by trash racks to prevent clogging of the orifices. The orifices are

                 located at a minimum of 18 inches above the floor of the structure. The second and third chambers

I                ~~~~~are connected by an inverted pipe, and the runoff passes upward fromn the bottom opening of the

                 pipe. The first two chambers maintain a permanent pool of water. The third chamber connects to

                 the storm drain system or other infiltration BMIP.

                          A Grit-oil separator schematic is shown'on Figure 24. Plan view of a Grit-oil separator is

                  shown on Figure 25 and details are shown on Figure 26 and Figure 27.

                  12.2    Applicability

  3                       ~~~~~~~~Grit-oil separators are typically installed in parking lots or commercial sites of one (1) acre

                  or less. This BMIP is suitable for sites where there is vehicular traffic and a good chance of oil and

I                ~~~~~grease being washed off by the storm runoff.  Grit-oil separators are most suitable for sites such

                  as convenience stores, gas stations, etc.

                  12.3    Design Criteria

  I                       ~~~~~~~Grit-oil separators are sized to provide a permanent pool of water or wet storage of at least

3                ~~~~~4.5 feet in depth. This depth consists of a minimum of 1.5 feet from the bottom of the structure

                  to settle the grit and sediments and a minimum of three (3) feet above the pool to provide wet



                                                             133







storage for oil, sediments, and objectionable floating material. The basin is sized to provide 200

cubic feet of pollutant storage per contributing acre of the site.

        The length of the first chamber should be a minimum of six (6) feet. The second and third

chambers should be at least four (4) feet long. The width of the structure should be a minimum of

2.5 feet. Longer lengths and greater widths can be used to reduce the height of the structure.

        The total height of the structure will be determined by the head required to pass the

developed condition ten-year discharge through the inverted drawdown pipe connecting the second

and third chambers, if the grit-oil separator is connected to the storm drain system (on-line). For

the curb inlet opening configuration, the head required to pass the storm runoff through the inverted

drawdown pipe can be for the first flush discharge only.

        A minimum of 1.5 feet of freeboard should be provided as a part of the total height of the

structure.

        The inverted drawdown pipes should be at least six (6) inches in diameter and there shall

be at least two in number.  Drawdown pipes larger than six (6) inches in diameter and more than

two (2) in number can be used to connect the second and third chamber to reduce the total height

of the grit-oil separator.

        Access to each chamber for regular clean-outs and inspection should be provided through

separate manhole covers. Each chamber should have steps for easy accessibility.

        The walls and bottom floor of the structure should be constructed of reinforced concrete and

should be structurally sound. If constructed on wet soil or where the groundwater table is high, the

structures should be checked for floatation.









                                           134





I

1 12.4 Design Examples

                                         A commercial site, 13,000 square feet in size is entirely paved. Design a grit-oil separator

I with three (3) chambers to control the runoff from the site. The storm runoff being transported to

I the BW is one (1) cubic feet per second (CFS). I
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                                                                                                     .. 1. -. ....111.-.. -.I. -I.
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                                          _ I --.. . - I_....-.. I - -_I. . .11 .11.1 .... ''...., . I.. I., . .. ... -... 11 . .... I''...
                                         ,I ,.- -__ I--- '' . 11, .I - - -  -11.11 . ... -- - .. -  - .. - . . I.-I -1 . 1. 11 I. .. - I ... ... .''... , _. , .. _ - -1 ... .. ... .,.  I''  . " , I  -. -1.1 .....- ...
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                                          1.-....,  -.. I ............... ''. , " I_, .. - "". -  .-- 11.1I. _ . I _ ,.. I ''..  __ .... . .'' I..  .    - II.. II I -.1. - I. - -I- . 11-1.1--  I .1 I I .I..-.--..
                                        -.. - 1. ... .I.. ....  I I.. ....., .1. _ .- ..... ..''.............- - .... .... ..... . .... - .I. , _ _...... .
                                       . ...._- ... -1 .... . . . ....
                                       .. . . 1. , .8 1 I..  I.. .I.. .....  I I.-I.I.-I-I... -'.........I...'',.... ..".1, -".1,...I....'-.... .......- ".."''....
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                                                                                                   .. .-11. I, _ . I. I. I.
                                                                            . .. 1, ..."".-  -...... .-  -...... .  I.... -.....'..... .- ---... - .. I.. .....
                                                                                                                .._ - 11 I..  ..  I ...
                                       .I -1 11 .11 . - I---.
                                        Npth of Oll: tora& (Ft) 4.00:iiiii --.-F-,-. --,I. -1-I.... -,  1. ,I....
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                                                                                          _ ...''.. - . - -  --.- I- --.... .-
                                                                                    ... I. -...I.. .. .... . .-... . ... ...... .. - .- I... III
                                       ..... ...'''., .. _I I I -.-.. _...... - I - . . _ __ ... -.... _1 ..-  ......... I...- I--- ...II-I I '. ........ .....''.-  .... . ...I.... - - - .......-. 1.
                                       .. .... ... .....II .I -..  -...'' ...... ...  I ... - . .... . ..-... I . .-111  --.I...--- _. -_.
                                        ---....... I-- - -..   -.111 I.. .... . . .. I..''., . ..... - ._ .-.. .I---. _. . . .... ... .---- . . ., _ -11.1... 1. I.. I - - 1. I------ ........- .... ...- I-  ... .1  .._ . .1.
                                             --_.. '' -_ 1. I.  - _ ._ .. .. ....   ..  -_  -I-  I. I .-
                                       .. I---. .I. I.. . II ...I. -.11 '' , -.I. .. -.1 I II1.11 . . I..I II. . . --  .
                                        I. I.  .11 - I  1.1 .. . ... - . -.... 1. -  11 . ..... ...... ,...-..._ -...... ... I., - I .
                                        .._ ._  ... ...I _ ..... I. .. . .. .......-.-- .. . ., ,I ..
                                                                                                 I. --........... . .- -. .  I -1 I. - .... .... I I  , I. .... ... .. -.. 1, . ...,  ''... .. , F"',
                                                  .. I - 1. .1I  .-  .....-.. ..... .1. ....I. 1. ... - 1. -...- '' -- I ,.....,
                                          .. .... ..... - --....I. - - .. . I.. '. ''. .- _,_ .. .. . .... .1.. ... I.. .... I.. .. .1 . .. .... ..
                                        -.... ,.  ,.  I .....I . --.. I -..... ..... .. .... .11 - .. . .- ..._ ......-., .. . . ., -1 .
                                        I -I_..I I .  .- . .  ,. ,. ......  ... .I - .. I I - I11 ''.. I I.
                                       -II .1.I ...I.- : :: !:::: : :.
                            .                                                                                   ..  I  -I
                                              I .  ........... I , .... . . I I ..:: - , 1,
                                       :: eight to Pass Flow through Drawdown PiMl7t).'O.28  ::X ::: . I -11 --  I. - I  I
                                                                                                              ,                ::: : :  - I-- II    -- I--.... 1.
                                                        II-..I - I 1. .. ....... - I -1. I I-I - -.
                                                                                                                                         ....I....  . '.
                                                                                                                            . -  . 11 . I. -I .1
                                         . I. I.I I...I.... . ... I ,.......-
                                        -- I. . I.1  .. - II ..1 I....... II . .  II--...-. -..
                                       .....  .. I  -1. . 11I.1.   . I -_  . -   -., -11 ....., _ .. ..
                                        I... I. I _ .--. -11 I .I'll - I I I I-I .. ,.I.. ...  . I
                                           -- ..., ......II.I... ". -  -- _  . I--- -- - I.., I, -_ _I. I 'I, ..I..
                                        ..... . . .. .., .. . ''''I-.- 11 -1.. . ... I I. - I  II -11...
                                       .....I..I. ..I I . .. - - -..... - , ... _  ...I.. - . -I, . I. ....,--
                                        ... I.,I... ,  -111III .. - I. ..... ......- I. . - -I . II .1-..
                                        -I. ..I.. - 1. II .I.. - -- . . . .. ..... .. 1. . _ __-. ...  I .. -, 1. I.. I. ....
                                        .,.  .  ... ,II-, , _ . I -  I I. .  - .1 .11 I I.1
                                        -II I  ...'' . I .1 .. .I II. , --,.11I,. - 11 .1
                                        . .11 .. ..,I-...I..... I I.... .. .. .- I.. I. .....,.. I  ...
                                       11 I . I .... ... I . I. ... 1-1 ..,.,.I, .. ,,.., . _I 
                                        .. 1_1 . ...-,I II I .. - .  I1.-. I.. I .. ....  - 1-111.11
                                                                                .          . I                 .            .   .   .   I       . . .  -   ,  ..I        .    .I.  _  . .. .. ...
                                                                                                                    ..1 -.1 .I..
                                       .11 -I'..... -  ., 1. -1
                                                                                                                     .11I.... . I -.. -.
                                                                                                                    I ,           ,    I ,.. .,        .  -      ,  .1.  . . . . .  .. I --  -
                                           First Chamber(L x W x H): (6.00 "  x 2-50' k  7.28') . I. I .''......, . . I.1
                                                   .:I.... , .'',I, ,. ,
                                                                                                                        .'',.I.. ... I.1. .1 ..I . 1- - .- -... 11  ..
                                                                                                                  . ___..... . _  -1
                                            III I. ...I.I I.I I...I. . I... I .. . .. ... . - -.-- I .I - .-
                                                                                             .1,_.-.. . ... .. . . , ''. __ .. .1. ... ,I,, .1.
                                        I. .I ., -'...11 ... 1. . . .I .1 . 1''.. .I I...I 11,. . .. .
                                       I.III1,. .'', ..1. I ''.. '. -II 1. I-  ., I .1.
                                        ...I.. ...-  :: :: :: :::::. . ... 1,   I.,. .. . .I..'- --I-.I -... I ... - -.. - .I.
                                       ..''..,I.-I I. . ..  , . . 
                                        -... II  .... . ...'', I .1 .-_1-111 .......... .1 ...I.11.1.1 .... .1. . ..  II-
                                       ''.. .1 I I ..-...... ...  I------ .. ..... .. ... . _ .. . 1. ,._1II II
                                           .1, I . .. II ... I.. ,  I ....... - . . I . -11 1. .... . ..... .1 . .1 I 1. ., .. ..   I------ ... - I. . ... I... 1, . . II.- .
                                        .....I_- - ...11I, '' ...... - -.I.,... 1. I 1.I... .. . ..- I..I ., - , .. I . ,., . . I.. . 
                                        -- 11.1 -1 ... - --- ..., I .I  I ... 1- , I. ...- .... 1. I -''I  . I- I:::::-
                                                                                                                                II1..11 .1 --  I.. I...
                                                                                                                       .1.-.. I- I. .. 1.
                                                                                                                        ,I. ,I ,-.. , ..,
                                                                                                       II.. . I . I
                                                                                                                        .-.1.I-I I...
                                       Second Chamber(LxWxH)-(4.00'x::2.50',7.28') I.-I..'''. _- I.,I......
                                                                ... . I. I.11II..  II - .I..I
                                                        I.., ... . I.I., .- .1I,..1 . .
                                       1. . I11.-I.I.-- .. I... . . 11 - II .. .. I.. .1.11
                                       .......  .-  I ,: :. -. :I, -1 I.11 ::: ::: -.: . --- .. ... -11,,.I ,.1 ..
                                                                         .. I.--.. 1. .- . .1 .. - - II--- I. ... - 1. 1. ..... . I.1. .1I. -.1 1.
                                            _. - ... ..-I. . ... ... .. . ..... .. .. .... .-_ I., I. , .,  _ ..... -... .I ,I---_ - 1,
                                             --....... I . .... .1  -  I .. . I .. . - I - 11 I I - .  . .. .I ...-
                                       -. ,.. II. 11I. 1.... I  I.. I  .....- . 1. - ..,-. I '' ,..
                                            .II . ., . ..1.  I.I . .. ... ... ...  I .... - I. .. -..  II..I. --
                                            -I ... - 11 11 .I.,I.. - I I - .1 I ,.,I. ... ....11  _.1. ,. . . I
                                        -I. .I.. .1 . - I ....,
                                                         I.-- .. - .. . .. 1.-',,
                                       ... -II .1 ... I.. I... I ..I-- I  .I.. 11--I11 .. --
                                        _.. -..  11II.. -. .. .. . -I, .I... - .1 .1, .I -.
                                       -. I. ., . .-I - .-I....I-. . .I  I.II I. .. ..
                                                                                                                                         ..I,_... I ... I. , I.
                                                                                                                                __ II.II-I ...
                                                                                                                                   . .1. -.1 I...
                                                                                                                                   .II
                                                                                                                              1 .I. -1 11  , ._.-II'll,
                                       .'W x H): (4.001 2.50T i 71)::: i i   :  : F . . .11.1-1 - I-I..
                                        'Third Chamber(L x'x1 .281. ,.,. -..... -...-.. -. .II
                                                                                                     I1.-.1. - -1. .1I - ..1.1.  ..., 1,
                                        .IIII II.-. . ....I.- .. .1 I.I  I-II.. .1-
                                         11III.III. 1, 1. ,''...I,. I.,I1..-..I.... ..
                                          .,., .I...,  1. 11  II--- ....II- .I -. .1I
                                        I ...I-. I... I..I,,.I. . ... .,,_-
                                        11,-..  . .I .-I1. - .I-.-II.. ...I
                                            ...I . .I.-  I- ....1-.I.11 I.. II I1. .-- .. .I- .. . - 1.  1, - --,,  I., .1,
                                                 .  I . .- - - .. I. II1.-II1. ... . I.  .. - , .I
                                        .. ..., .I 11 ,I-.... '' ..... ,.I...  I  I -..I1. ... .11 I . -I....
                                                  -. I 1. ..II. ..II .. -.  .- .. 11 .  , ,,. . .,
                                              _... . . I.,I.. II...  -  .I. .11  . . -1II 11 .1
                                                 .. I-.III . I. .I 11  .1.''., ..I .-,I.-.II- . I .. I II .I

I 


1 135


1


1






                                                                                       L  FIGURE 24
                                 j-L                    o4'-0"        4'-- 



                                                                   d
                                   W                       N





                                                 TOP  VIEW



                                          L








                           L              = Length of Ch4-0mber(Ft)










                          W  =  Width of Chomber(Ft)
                                 HF =  Free-Board(Ft)HF


















                          He =  Overall  Height of Chamber(Ft)
                                          Hc



















                          LDo =  Length of Chomberge(Ft)












         V  ,        c  p= 200 Ac    L                         = ( -       HC= 1.5 + Do + Hp + HF
                                   L W                  d-) N
                                                      12


                                                       GRIT-OIL SEPARATOR SCHEMATIC


                                                                       REVS'D:            DRAWN:  K.FF.
                         ~( (,{ J' WJ~      ~  '-          , _/ .       ~CHK'D:                CHK'D:   V.P.M
                                              ...,./-DATE:                                   DATE: OCT. 1990

Smith Demer Normam                                                            REVS'D SCALE: N.T.S.
Engineen - Plnns - Surveyor - Londconpe A rchitecn  T A  ONm IROADm              K'D: ArA 
  Central Park  Six Manhattan Square Suite 102  H     CHROD
         Hom.,to, ga 235,6o] DATE: PAGE t                        36
 (804)65-V610 (804)627-600 FAc (804)s5-15303 PLANNING  DISTRICT COMMISSION                    H





                                                                                    FIGURE 25

















                                  Li L90)               !   1  (4'-V') II L~  4'j )11

                                PROVIDE STANDARD
                                LOCKABLE MH & STEPS
                                (TYPICAL)
                                      /  j        .     .      6" DIA. ORIFICES


IO 



                                TRASH RACK-
                                               (VARIABLE)






                                             PLAN VIEW











                                                   IGRIT-OIL SEPARATOR DETAIL

                                                                    REVS'D:          DRAWN:  R.E.C.
                                                           ~~~~ ~~CHK'D:               CHK'D:  V.P.mb
                                                                    DATE:            DATE:  OCT. 1990
      Smfff Demer Normam                                                                       SCALE: N.T.s.
      Engineer  - Planner  - Suryor.  - Landscape Archittacts                CHK'D:
        Centrl Par .,Six Manhattan Square Suite 102  H A M     PSPAGE  137
               Harpton. Virgini  23666DAE
       (804)865-9610 (804)627-6900 Fax: (804)B65-1533  PLANNING DISTRICT COMMISSION  DATE:
I~~~~~~~~Hmln rii 56






                                           Li I~~~~~~~~~FIGURE  261

                          cU-
                                    H-u0~~~~~~~

                                          -Jz~~~~~~~~~~~~




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                                m      ~~~~FIRST          0                   <        < 

                                        2'MAX-. TYR    0- cco  Z.  OO<L              C ccso
                                    0~~~~ogo m uc                                   a.mu





                                                  HEAD TO
                                   6"MIN:  10" ~PASS TEN (1 0) '-"-MI-N.' i'-6`
                         04   6MN.      z-'O
                                       IFREE YEAR DISCHARGE 
                                        BOAR~   TR   ~EV 0-
                                                               PIFE      5~ ~ ~~~~~~~~Li   i


                              _ _ _ _ _                                  L oO~~~~0cc

                                                           0.<~~~~~~

                                             a. J             ('-0  2'O  r)N

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                                     Li~~~~~~~~~~~L                       2L~>  

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                             LLJ






                                                LLI~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                          Th>.  
                                        C) ~ ~ ~ ~ ~ ~ ~ ~ ~~~ESD____                           RW       E
                                        m~~~~~~~~~~~CIO 0___  CH'DVP
    *                  -                                                     DATE ________   DATE OCT 199
                          kiLSAE    T
     -~~~~~~~~~~ SruhurLoriumr                                 --___
             Er  ~ea Pane: Suvyrs-Lndcp  Aciecsr'',r
               Ce       Pi, Se   anatn   ecr  ute12                                  __<___

                                                                   GRIT-OIL EPARATC) PEAGE13


                          (804)885 66 0 (804)827 6900 Fat (804)865-1533  PL~~~~~~ANNN ITRITCOMSSO  DATE: OCT. 1990__
I~~~~~mfiDnf ora                                                               ESD   CL:NTS






                                                                                  FIGURE 271

                                        ALUMINIZED CMP, HALF
                                        ROUND, WELDED TO
                                        PLATE, BOLTED TO WALL      END PLATES WELDED
                                                                   TO BOTH SIDES OF
   TRASH RACK              HALF INCH  ,-MIN.6 INCH                        TRASH RACK.
                          HOLES 4"0.C        ORIFICES
                                      \  d  .   \   f / j   __   \   FIRST CHAMBER
     \ /
                            .    :O ï¿½ ï¿½0ï¿½ ï¿½ ï¿½ ï¿½ pï¿½ ï¿½ ï¿½ ï¿½ -\   TRASH RACK
                          000000  0        0000
    / .                     oooooQooojooooo                      *.                    . v
                           0oo o        ooo o o o        o i   

             ~ .,         o o o o o o o  o'o o o o o.,                                    v
                          0000000000000
                          0000000000000
                          0000000000000 

                                                                  MIN. 6" ORIFICES
                                                             o  o  FORMED AROUND
                                   I/i~~   I   I         ,, , SCH.40 PVC SLEEVES
                           1_61    2' MAX.      I                   (MIN. 2 REQ'D.)
       SEE DETAIL BELOW        VARIES 2'-2" MIN.                           SECOND CHAMBER


         SIDE VIEW         FRONT OF TRASH  RACK                           PLAN VIEW



             i ï¿½.:."   -ALUM. ANGLE
          ' i]--l    I .AS RECOMMENDED             90' ELBOW                        WALL PIPE
                       BY MANUFACTURER


                                                $  PERM. 
        S.S. 1/2" DIA.                             o W.S.               
        EXPANSION BOLTS                                  7             I
        WITH WASHERS
                                                      I      f


                                                        .____~~ . ,  v .      CONC. WALL




                                               SECOND CHAMBER            ï¿½      THIRD CHAMBER
WELDED SEAM- 





                 DRAWDOWN  PIPE                                 DRAWDOWN  PIPE
                (ALUMINIZED CMP)                                  (CAST IRON)



                                                IGRIT-OIL SEPARATOR DETAIL


                                                                  REVS'D:          DRAWN:  R.E.C.
                                                     , (,/2~  f    -  CHK'D:        CHK'D:  V.P.M.
                                                                    DDATE    :     DATE:  OCT. 1990

Smth Demer Normam                                                     REVS'D:           SCALE:  N.T.S.
Engineer - Plonner - Surveyors - Landscape Architects  T              CHK'D: R A..
  Ccetrol Pak  Six Manhattan Square Suite 102  1J. LHL.VJL  ROADS
         88-Ham8opton V)rqi6io o23r6c6X6<so4)-  PNDATE:                                  PAGE   139
 (804)865-9650 (804)627-6900 Fax: (804)865-1533  PLANNING DISTRICT COMMISSION    DATE: ________







12.5    Maintenance Requirements

       Grit-oil separators function properly only if they are cleaned regularly, at least twice a year.

Oil soaked grit and sediments, and oily sheen slurry in the first and second chamber should be

pumped out and properly disposed. Material should be dried and tested for toxicity according to

Southeastern Public Service Authority (SPSA) of Virginia approved procedures. The dry material,

if non-toxic, can be disposed of at the regional landfill with prior approval from SPSA.

12.6   Life Expectancy

        Grit-oil separators have not been in use for a long enough time to accurately estimate their

life expectancy. If routinely cleaned and maintained, they can function properly like any storm

drain manhole and may last 20 years. More monitoring data needs to be gathered before making

an accurate estimate of their longevity.

12.7   Cost

        There are significant costs associated with this BMP. They can range from $5,000 -

$20,000 for each installation depending on the size of the structure. Pre-cast versions of the

concrete chambers may lower the total construction cost.

12.8    Construction Specifications

        The materials used for installing a grit-oil separator are associated with normal storm drain

construction. Typical storm drainage specifications can be easily utilized for this BMP. It is

recommended to refer to local jurisdictions' storm drain design manuals for these specifications.













                                           140







         13   WATER QUALITY INLET

                  13.1 Description

                          The water quality inlet is a smaller version of a grit-oil separator and functions in a similar

I                ~~~~~fashion.  Instead of a three chamber design, a water quality inlet can consist of only one or two

                 chambers. The first and second chambers are connected through the inverted drawdown pipe. The

                  outflow from the second chamber flows to the storm drain system or other infiltration BMIP.

                          A water quality inlet schematic is shown on Figure 28. Details for the water quality inlet

I                ~~~~~are shown on Figure 29 and Figure 30.

                  13.2    Applicability

                          Water quality inlets should be used for small commercial sites or as pretreatment facilities

I                ~~~~for other infiltration BM~s.  The outflow from the inverted drawdown pipe can be directly

                  transported to an infiltration facility, thus eliminating the second chamber.

                  13.3    Design Criteria

                          Water quality inlets are sized to provide a permanent pool of water or wet storage of at least

                  4.5 feet in depth. This depth consists of a minimum of 1.5 feet from the bottom of the structure

I                ~~~~~to settle the grit and sediments, and a minimum of three (3) feet above the pool to provide wet

                  storage for oil, sediments, and objectionable floating material. The basin is sized to provide 200

                  cubic feet of pollutant storage per contributing acre of the site.

  I                       ~~~~~~~The design criteria listed under grit-oil separator are also applicable to the water quality

                  inlet. Water Quality Inlets are sized to provide a permanent pool of water or wet storage of at least

                  4.5 feet in depth. This depth consists of a minimum of 1.5 feet from the bottom of the structure

                  to settle the grit and sediments and a minimum of three (3) feet above the pool to provide wet

                  storage for oil, sediments, and objectionable floating material. The basin is sized to provide 200

                  cubic feet of pollutant storage per contributing acre of the site.


           U                                                  ~~~~~~~~~~~~~~~141







  3                     ~~~~~~~Instead of three chambers, a water quality inlet may have one or two chambers. The length

                 of the first chamber should be a minimum of six (6) feet. The second chamber should be at least

I               ~~~~~four (4) feet long. The width of the structure should be a minimum of 2.5 feet. Longer lengths and

                 greater widths can be used to reduce the height of the structure.

                         The total height of the structure will be determined by the head required to pass the

                 developed condition ten-year discharge through the inverted drawdown pipe connecting the first and

                 second chambers, if the water quality inlet is connected to the stormn drain system (on-line). For

H               ~~~~~the curb inlet opening configuration, the head required to pass the storm runoff through the inverted

                 drawdown pipe can be for the first flush discharge only.

                          A minimum of 1.5 feet of freeboard should be provided as a part of the total height of the

I               ~~~~~~structure.

                          The inverted drawdown pipes should be at least six (6) inches in diameter and there should

                 be at least two in number. Drawdown pipes larger than six (6) inches and more than two (2) in

3               ~~~~~number can be used to connect the first and second chamber to reduce the total height of the water

                  quality inlet.

                          Access to each chamber for regular clean-outs and inspection should be provided through

3               ~~~~~separate manhole covers. Each chamber should have steps for easy accessibility.

                          The walls and bottom floor of the structure should be constructed of reinforced concrete and

I                ~~~~~should be structurally sound. If constructed on wet soil or where the groundwater table is high, the

3                ~~~~~structures should be checked for floatation.











           3                                                 ~~~~~~~~~~~~~~142






                  13.4   Design Example

                           A commercial site, 13,000 square feet in size is all paved. Design a water quality inlet with

I               ~~~~~two (2) chambers to control the runoff from the site. The storm runoff being transported to the

                  BMP is one (1) cubic feet per second (CES).



      I           .~~~~~~~~~~~~~~~~~~~~~~~~~-~DESIGN: OF-ATERM QM IY INLET
                                                           ... ..... Ipt  aamtr  ......



                                 Length~~~~~~~~~~~~~~~~~::: Ch!bX(:) 6.0





     I~~~~~~~~~~~~~~~                             ~~~~~~~ --ï¿½  OuTpu Values  MERC

                         D~rbept.45 ofOlSIorgeN(OR 1.000-


                                       First Chamber(L x W  x H): (6.00' x 2.50' x 7.28')~~~~~~~~A 00 



                 U                                            143~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   I~~~~~~~~~~~~~~~~~~~~~~t60
     I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~AX ...... ...






                                                                                                   FIGURE 28



           IL                          4'- "                                      L                4_ On



             HF
                                                                              # OF                d
      (i ' '                                    ! /               --DRAWDOWN                                _
    i                  HI a     _I                                        W PIPES                            --




             1il'-6"                   " .



                       SIDE  VIEW                                                 TOP  VIEW





                               Ac =  Contributing Drainage Areo(Acres)
                                L  =  Length of Chomber(Ft)

                                W  =  Width of Chamber(Ft)

                               HF =  Free-Board(Ft)
                                Q  =  Contributing Flow From Impervious Area(cfs)

                                d  =  Diameter of Drawdown Pipe(ln)

                                N  =  # of Drawdown Pipes
                               Hp =  Height to Pass Flow through Drawdown Pipe(Ft)

                               Hc =  Overall Height of Chamber(Ft)
                               Do =  Depth of Oil Storage(Ft)
                                V =  Volume of Oil Storage(Cu Ft)





                                            V                   0.07 Q2
              V =  200 Ac            Do    L W           Hp=    d      2          Hc= 1.5 + Do + Hp + HF
                                                               ( ) N





                                                          I  WATER QUALITY INLET SCHEMATIC


                                                                                REVS'D:             DRAWN:   K.F.F.
                                                                               CHK'D:              CHK'D:   V.P.M
                                                        V  ~'  --~  j~'  ~,    DATE:                DATE:  OCT. 1990

Smith Demer Normam                                                               REVS'D:             SCALE:  N.T.S.
Enginer  Pnner - Survyors - Lndcpe Architect  HAMPTON   ROADS                    CHKD:                 PAGE   144
  Can to Pork  Six Monhotton Squor. Suite 102               HA                    M PTON ROADS
          Hempton. Virginia 23566 DATE: PAGE 144
 (804)865-96,0 (804)627-6900 Fo.: (804)865-1533  PLANNING DISTRICT COMMISSION    DATE:

               I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~






                                                                                       | FIGURE 29


















                                      L1  (6'-0")              I   L2 (4'-0"

                                    PROVIDE STANDARD
                                    LOCKABLE MH & STEPS
                                  /    (TYPICAL)









                                               (VARIABLE)






                                             PLAN  VIEW













                                                     WATER QUALITY INLET DETAIL


                                                                        REVS'D:             DRAWN:   R.E.C.
                                                                        CHK'D:              CHK'D:   V.P.M.
                                                                        DATE:               DATE:  OCT. 1990

Smith Demer Normam                                                                      _r SCALE: N.T.S.
Engineers - Pl.nn.ers- Suyaror- Londscpe .Architects:, HAMPTO N   ROADS        CHK'D:
  (sntmo4  P-o   slo.hatt-oo ,,,u Su-its 102                         DSHCPAGE   145MON
 (SO4)865-96  (80m4o)627-69900 FaV(804)865-1533  PLANNING DISTRICT COMMISSION   DATE:






                                                                                     jFIGURE 30








*~~~~~~~~~~~~~~ 0

                               0~~~~~~~~~~~~~~~~~~0wU



                                               W,  <                                U-  -

                                                          0 z       [if       >0
                                                 0i~~~~uZL    cr~-               00
                                 2'MAX. TYP.     a,-~      0    )0 < U   -    ofj 

                                           00~~~~~)0<    ctu / c~

                                        co< Lud- 
                                          n,0 L  <     LLJ 
                                   Li   b LJ U<                                    m
                           ~~~~~~~~~~CD00
                         Lu-~~~~V    Lu   0~
                        _E         _~j  L'iOf )0   H
                         I-~~~~~~ ry                LuJ  ZyC

                          LLJ                                          Lu            0 L  
                            Lu~             ~~~~~ <DOJ   LuJ
                          ~~~~~~~~0II ' ~0h-K                           LuJ

                                       U') ~ ~     'H             /  Lu Z ry
                    6" MIN                       5 AS10/-~


                              LLJ~~~~~~~~~~~~L


           ~~~~~~~~~~~~~LI



                                                     -0 a

                                                       an <
        ~~~~~~~~~~~~~~~~~~~~~~~~~~i

                                                      WAECULT)ILTDTI






                                                         W~~~~ATE: OUALITYINLE   DAE   OTAIL9



   'Smr-Thi Denier Normam                                                      RESDSAENTS
    Eng no.    - Pla-nne-  Surveyor. -  Landacape  Architect.  - _CHKR V'D:_ _ _ _
       Ctrlpak  kHpV2tn qu~  Suite 102    HAMPTONJ1  ROADS   PAGDE 146__
             (80)68    (84)27 800Foe(84)85-133 PLANNING DISTRICT COMMISSION   DAE________I







13.5 Maintenance Requirements

        Water quality inlets function properly only if they are cleaned regularly, at least twice a

year. Oil soaked grit and sediments, and oily sheen slurry in the first and second chamber should

be pumped out and properly disposed. Material should be dried and tested for toxicity according

to Southeastern Public Service Authority (SPSA) of Virginia approved procedures. The dry

material, if non-toxic, can be disposed of at the regional landfill with prior approval from SPSA.

13.6   Life Expectancy

        Water quality inlets have not been in use for a long enough time to accurately estimate their

life expectancy. If routinely cleaned and maintained, they can function properly like any storm

drain manhole and may last 20 years. More monitoring data needs to be gathered before making

an accurate estimate of their longevity.

13.7    Cost

        Since a water quality inlet is smaller than a grit-oil separator, the cost will be slightly less.

A typical two chamber structure may range from'$3,000 to $8,000. The installation cost of another

BMP if used in combination, is an additional cost.

13.8    Construction Specifications

        The materials used for installing a water quality inlet are associated with normal storm drain

construction.   Typical drainage specifications can be easily utilized for this BMP.   It is

recommended to refer to local jurisdictions' storm drain design manuals for these specifications.













                                           147







14   BMP COMBINATIONS

       All BMPs are designed to receive the first flush of runoff volume during a storm. The remaining

runoff volume is not treated by the BMP, and is conveyed to the storm drain system or a detention or

retention pond downstream. The separation of the first flush and the remaining runoff can be accomplished

by constructing a weir. Height of the weir can be designed to divert the first flush to a BMP and the

overflow to a storm drain or downstream SWM facility. This concept is shown on Figures 31 and 32.

       Exfiltration of the runoff volume can be increased by constructing an infiltration trench under a

grassed swale. Figure 33 illustrates the concept. Porous pavement can be combined with a surface

infiltration trench and grass filter strip. Runoff can be evenly distributed to the infiltration trench by placing

slotted curbs at the end of the porous pavement and by constructing a berm of Austin triangle at the end

of the grass filter strip. A schematic of this combination is shown on Figure 34. Figure 35 shows another

possible combination of grassed swale and infiltration trench. For design criteria and other pertinent

information, refer to individual chapters on each BMP.

        Commercial/imdustrial parking lots generate significant loads of grit and oil. If infiltration practices

are used to treat surface runoff, they will be rapidly clogged. Surface runoff needs to be pretreated to

remove all objectionable material before it enters the infiltration practices. Figures 36 to 38 show possible

combinations of grit/oil separators and water quality inlets for pretreating the runoff. Individual chapters

on these BMPs describe the design criteria and other pertinent information.















                                                  148






                                                                               FIGURE 31























                                                                                 FLOW TO BEST

                                                                                  PRACTICE











                              OVERFLO




















                                               CONCRETE DIVERSION DETAIL


                                                                REVS'D:           DRAWN:  R.E.C.
                                                ï¿½r.             ~~~~~~~CHK'D:      CHK'D:  v.p.m.
                                                                DATE:             DATE:  OCT. 1990
                    Smitt~~~~~~~Demer~~~~~~~ormam ~~~~~~SCALE:  , N.T.S.
Ii ..- Pl-- -   S-.ys- L..d . . .   P . A,.hit..t.                   CHK'D:
 Ce'tral Park Si. Mohtto, Sq.,., Suits 102                         HMTNRASDE:PAGE   149
 (BO4 95-610 (804)627-6900 Fax: (804)865-1533  PLANNING DISTRICT COMMISSION  AE
                              Hompton. V~~~~~~~~~~~rginio 23666~~~~~~~~~~





                                                                                             | FIGURE 32














                                                                                                FLOW TO BEST
                                                                                                MANAGEMENT
                                                                                                PRACTICE

OVERFLOW





                          m00I                          -











      STORMWATER RUNOFF












                                                               DIVERSION BOX DETAIL

                                                                             REVS'D:  DRAWN:  R.E.C.
                                                                             CHK'D:   CHK'D:   v.p.M.
                                                      L. ] ,     DATE:    DATE: OCT. 1990
Smith Demer Normam                     RVSDSCALE: N.T.S.
Engineers - Planners - Surveyor.. - Landscape Archite  HAMPTON   ROADS  CHK'D:
  Central Park   Six Manhattan SQuare Suits 102  HA MPTON
          Cen mpton. Virginia 23566 DATE:             PAGE 150
 (804)865-9610  (804)627-6900 Fo. (804)855-1533  PLANNING  DISTRICT COMMISSION  DATE:
                 I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 






                                                                               | FIGURE  331








            Cl           .      .     .. /-- RAILROAD TIE
            DIRECTION OF FLOW          CHECK-DAM
                                                                 IOBSERVATION WELL

                                                        /   GRASSED  SWALE
               .... . ,,'.z/9 /  \/.'? I Z/y '../ 1l/N y1/ tl/ I///' /I/ tlII/ tl/tl/ i/III/II/I  11t,111 ftl II III II III III III if/ III III I /II t/I


                                  STONE  //                   I 
                                         .  /"         UNDERGRO)JND TRENCH
                                         I//                  I I                 r/>/





                                            SIDE VIEW






             A   SIDE-SLOPES          RAILROAD TIE CHECK-DAM
            /t        ' ,   ,__,, /                               F--OBSERVATION WELL

        Y/ ' Y  Y/',Y Y  Y  Y/ Y  Y


          /GRASSED SWALE                              UNDERGROUND TRENCH








                                            TOP VIEW





                                                    UNDER-THE-SWALE TRENCH 


                                                                 REVS'D:         DRAWN:  R.E.C.
                                                                 CHK'D:          CHK'D:  v.P.M.
                                              X/ ~f'   --,* /'  ',  DATE:         DATE:  OCT. 1990

Smi     Demer Normamnn                                            REVS'D          SCALE:  N.T.S.
Enginere - Planners - Sur.eyorm - Landscape Architee.  AA         REVS CHKD:
 Centrot Park  Six Maonhattan Square Suite 102 HAMPTON ROADS
        Hptngo.  .rgii.o 2,6                                   DATE: PAGE i
(804)865-9610 (804)627-6900 Fao: (04)8s65-15.  PLANNING DISTRICT COMMISSION    DATE:





                                                                                             FIUE 3 4
                                 BERM                                  SLOTTED CURBS          FIGUR

   IJ 

                           JI  \Ir~~~~~~~ I~


                          '4'  *  4,  '4,

                        4-  4,  .4G R A S 4,  14,  4
                               FILE R


                               FILTER    14                              POROUS PAVEMENT
                                TR~ I         I  I





                  I  4'   '4' 1, IV       '4    '4'





                          AUSTIN TRIANGLE              _     _
                          OR BERM                    TOP  VIEW
                                  O ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~PRU








                               GRASS FILTER   STRIP                                   POROUS
  I ~      ~       r           /                  -1                  \N.%      OU    PAVEMENT








                                                  -  KN TRENCH     1  -' '~~~~~~                PERFORATED
                                                    STRE N CH    ""-PIPE


                                    ~~~~~~~~~GRVL-  - r~                           - --' ~
                   AUSTI- -N NLTE


                             TRIANGLE~~~~~~~~~~~~~~~~ "SND/,                                  CURB




      *                          -, ~~~~~~~~PIPE FRAME&tKAYYI>NN/
                  N N        -      COVERED WITH
                                   MESH AND--
                                   FILTER FAB3RIC _
                                                   SIDE VIEW

         U                                        ~~~~~~FILTER STRIP PORO)US PAVEMENTI

                                                                         REVS'D:____   DRAWN: R.E.QC
                                                                         CHKD:            CHK'D: -CHKD    VP
                                                                          DATE:  ______     DATE :  ocTr   1990
                            Smrffi  Demer Normam                             REVS'D: ~~~~~~~~~~SCALE: N.T.S
       Engineers - Planners - Surveyarm - Landscape Architects  'Ct T
*         ~~~~Central Park  Si. Manhattan Squore Suite 102  HAMPTO~JN ROADS     CHK'D: ___
                Ham pton, V'irginia 23566                     DAEIAG                                                       5
        (804)865-9610 1604)627-6900 I`=r (804)865-1533  PLANNING DISTRICT COMMISSION DAE  ______
         I~~~~~~~~~~~~~~~~~~~~~~~~ï¿½






                                                                                            FIGURE 35



                  If'\                                                      I
                            i-



                          II
















































            I    r                   -    ,'\       CHK'D:      CHK'D: V.----P-.M.



           Hcmpton, Vigno 2B"A                                      c
         (B04)865-9610  (0)2-3FPLANNING DIS SION
                                        0
                             > ZZZ
                                        0


                                        IJ












I~~~~~~~~~~~~1






                                                                       I  FIGURE 36







                                                        / OBSERVATION WELL




                             -_  DRAACOWN                   I            OVERFLOW PIPE
           iI -           - ....... PIPE  TO STORM 
       PERFORATED              /                                             SYSTEM 
       TRASH RACK / -]  I I 











                            A I          I,
                                                UNDERGROUND TRENCH

                                       SIDE VIEW




                                                          -OBSERVATION WELL


 | II ie1              II P   in ,- ,.=,o - Landscopm Arcgggugg11'iat  CHKBB88'D1 l'  I

















 / IUNDERGROUND TRENCH WITH  RIT/OL SE PARATOR I



                                         Ho-o -o 2 3f6O VDATE:R DATE:  CT 1990

0i45i-91I              -/04)676                                    REVS'D:           CAWNTS
Engineer - Pioneers- Surveyor - Londscope Achitts                  CHK'D:TCKD VPM
Egters-Ptors Macvnehattson--$q .... $uste t02  HAMPTON  ROADS      CHKD:              PAG
       (~04)65-9610(4)769  F(804)865-15  PLANNING DISTRICT COMMISSION    DATE: __.o





                  U                                                            ~~~~~~~~~~~~~~~FIGURE 37j


             H                                                 ~~~~~~~~~~~~~~~~~EMBANKMENT

          I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..... ....... ...
                         U-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...........
                           L-z~~~~~~~~~~~~~~~~~~~......
                                         PIPE~~~~~~~~~~~~~~.................
                                        DRAWDOWN~~~~~~~~~~~................

                                          DRAWDOWNA~ ~ ~~~I~LTATO.::
                                                                            ...   BSI WT..::. .........

                                                                                       NOOUTLET""""K


                                                       LEVEL SPREADER  ~~~~~~~~. ..........
          I~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~...................
           I                                TOP VIEW~~~~~~~~~~~~~~~~~~~~~~.................
~~~~~~~~~~~~~~~~~WRQAITYILT........................
           * ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~....... ..............
                                             DRAWDOWN~~~~~~~~~~~~~~~~~........ ....
                                      PIPE~~~~~~~~~~~~~~~~. . . . . . .  ILTRATION.......
          I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~........ BASIN .........
          I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~................. ....\~.......
                                       WATER~~~~~~~~~~~~~~~.. .......... .INLET.......
             I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....... 
                                                              SIDE~~~~~~~~....... ..........W..
         I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~....   ....  .........
                                                                ... ....IN   ....RAIO    ...ASI... . ...j.
                                                                      ......V'D:____  ................ I......E....
            I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...... ..........




      *                           HAM~~~~~~~~~TP VIEW


                     Central~~~~~~ ~~~~~~~~~~~~~~~~~~~........ ....   ... .ahta  .gar   ....t2________

~~~~~~~~~PIP






                                                                                   FIGURE 38








                                                                    ,,OBSERVATION WELL




            Z1 WEIR\ --]                                            | 0           OVERFLOW PIPE
                   DRAWDOWN                                         i            TO STORM
                   PIPE                             _i                            SYSTEM
  DIVERSION                                                         ,,                        /
  BOX                                                               I I




                   WATER QUALITY INLET      


                                                           UNDERGROUND TRENCH


                                             SIDE VIEW


              DIVERSION BOX
  t      /         DRAWDOWN                              ï¿½OBSERVATION WELL
                   PIPE
                                              I I/ I  I            o 0

                          ____         l  /~I
                                                           UNDERGROUND TRENCH
                                                                                 OVERFLOW PIPE
                   WATER QUALITY INLET                                           TO STORM
                                                                                 SYSTEM
   OVERFLOW

                                              TOP VIEW










                                                     OFF-LINE TRENCH SYSTEM


                                                                   REVS'D:          DRAWN   RE.C.
                                                                   CHK'D:           CHK'D:  V-P.M.
                                                                   DDATE:           DATE:  OCT. 1990
Smrith Demer Normann                                               REVS'D:          SCALE:  N.T.S.
Enginears - Planner - Surveyor -- Landscape kArchitOct   rFl-  T   R V A  JCHK'D:
 Crntoc IPck Si. Mahota Sq..n Suite 102 HAMP                   TON RADS,
        (0C Han9   Pon, H gia o 236665 -  PLANNING   ROA      D    ATE:              PAGE   156
(804)865-610 (s04)627-6900 o.: (804)865-1533  PLANNING DISTRICT COMMISSION    DATE: '_ 













                                                            TABLE 11-5
                                       SIZES OF COARSE AGGREATES - Open Graded

                       Amounts Finer Than Each Laboratory Sieve (Square Openings*), Percentage by Weight
 Va.
 Size
 No.       4        31/2      3       21/2       2       11/       1        J/        1/2      3I/H    No. 4  No. 8  No. 16 No. 50 No. 100

  I    Min. 100   95ï¿½5              43ï¿½17              Max. 15            Max. 5
  2                        Min. 100   95ï¿½5              43ï¿½17            Max. 15   Max. 5                               >
  3                                          Min. 100  63ï¿½ 17             Max. 20   Max. 5                                                              1
 357                                         Min. 100            60ï¿½20              20ï¿½ 10            Max. 5                                            z
                                                                                                                                            Z
  5                                                    Min. 100  95+5    58ï¿½17   Max. 15   Max. 5                           _
  56                                                   Min. 100  95+5    58+17   25ï¿½10   Max. 15  Max.5                                                  X
  57                                                   Min. 100   95ï¿½5              43ï¿½17             Max. 7  Max. 3
  68                                                             Min. 100  95+5              48ï¿½17  Max. 20 Max. 8  Max. 5
  7                                                                      Min. 100   95+5   57ï¿½17  Max. 15 Max. 5
  78                                                                      Min. 100   95ï¿½5   60 ï¿½20  Max. 20 Max. 8  Max. 5
  8                                                                                Min. 100   92ï¿½8   25ï¿½15  Max. 8  Max. 5
  9                                                                                         Min. 100  92ï¿½8  25ï¿½15  Max. 10  Max. 5
  10                                                                                         Min. 100  9248                               20ï¿½10
*In inches. except where otherwise indicated. Numbered sieves are those of the U.S. Standard Sieve Series.



 Source: Virginia Department of Transportation - Road & Bridge Specifications
               January 1987






                                                      Table 1.66b
                       SEEDING MIXTURES, RATES AND DATES: SOUTHERN PIEDMONT AND COASTAL PLAIN

                                                                                   RAT-S               )ATES
      SITE
   CONDITIONS                    SEEDING MIXTURES                                          PER     3/1    4/15   8/1
J<                                                                               PER      1000       to    to      to
                                                                               ArCF      ft 2    4/1   RAl   10/15
      HIGH
   MAINTENANCE   1.  Tall fescue -------------------------   90%                250         6
      LAWNS          Kentucky bluegrass-------------------  10%                 lbs        lbs      X     no       X

                  2.  Tall fescue-------------------------- 50%
                      Ladino clover------------------------  10%
      LOW            Red clover---------------------------  10%
   MAINTENANCE        Korean Lespedeza---------------------  15%                 80         2             (a,b)
     GENERAL         Annual ryegrass---------------------- 15%                  lbs        lbs      X       X      X
      USE
                  3.  Tall fescue-------------------------- 50%                                                           m
                      Sericea lespedeza-------------------- 30%                  70        1ï¿½              (a)            z
                      Annual ryegrass---------------------- 20%                 lbs        lbs      X       X      X a
                                                                                                                        X
   DROUGHTY      4.  Tall fescue-       ---------------- 50%
    AREAS,            Sericea lespedeza-------------------- 20%
    SANDY             Korean lespedeza-------------15%                           80         2             (a,b)
    SOILS             Annual ryegrass --------------15%                         lbs        lbs      X       X      X

                  5.  Tall fescue-----------------           65%
    POORLY            Korean lespedeza--------------------- 20%
    DRAINED          Annual ryegrass---------------------- 10%                   80         2             (a,b)
     AREAS            Redtop                                  5%                lbs        lbs      X       X      X



   a. After May 1, use 10 lb/A german millet or 2 lb/A weeping lovegrass in place of annual ryegrass.
   b. After May 1, Korean lespedeza will not reseed itself. You may increase the amount of other legumes
      accordingly.












                                                                    Table 1.66c
                                   CHARACTERISTICS OF GRASSES APPROPRIATE FOR EROSION CONTROL

o                                                           nldTN~AWF TMl rDuirr''


  COMN         NAEi Ln                                  ~jcnLUL

      (BOTANICAL NAME)      C     D  '.*.         j C .!      - .4-                      REQUIREMENTS                         REMARKS 
                                     -~~~~~~~~~  I  CD   -C C D    LIO0         0



    KENTUCKY BLUEGRASS                              6.0-                           )rable moisture, and        Suitable for fine turf.  Poor          lany
     (Poa nratense)           P  C   X        10-28 7.0        X   X   X        X  liberal phosphorus.          drought and heat tolerance.    v        arieties
                                                                                                          Best when used with bluegrass, as lanhattan
    PERENNIAL RYEGRASS                              5.5-                                                        20% or less of mixture.  Quick        Citation
    (Lolium perenne)        SP  C        X    5-14 7.5        X   X   X        X  Similar to blueqrass.   gjermination.                               Pennfine
                                                                                                           Include in fine turf. 5hade and
    RED FESCUE                                      4.5-                           Do not fertilize heavily  drought tolerant.   Persists best  Rennilawn
    (Festucca rubra)         P  C   X         7-21 6.5  X   X   X   X              -ith nitrogen.              in cool environments.                  Jamestown
    REED CANARYGRASS                                5.0-                           Do not mow closely or        Tall, coarse;ddapted to wet soils,
    (Phalaris arundinacea  P  C   X           5-21 7.5  X   X   X   X   X   X  3ften                           waterways, muck and peat soils.           lo
    TALL FESCUE                                     5.5-                            low often to prevent        Widely adapted. Tolerates drought,Kentucky
    (festuca arundinacea)  P  C          X    5-14 8.0        X   X   X            Dunchiness.         infertility, moderate shade.                     31
    GERMAN MILLET                                   4.5-                                                        Warm season temporary or corn-        lo Rlamed
    (Setaria italica)        A  1.   A'    4-14 7.0  X   X   X   X            D     o not use in fine turf.  panion rirass.                           Varieties
                                                                                                          Cool season temporary or compan-
                                                                                                          ion grass. Cannot tolerate temp-
    ANNUAL RYEGRASS                                 5.5-                                                        erature extremes or drought.          qo Named
    (Lolium multiflorurn)   A  C         X    5-14 7.5        X   X   X         X  Jo not use in fine turf.  Somewhat shade tolerant.                 Varieties
    OATS                                            5.5                                                         Cool season temporary or compan-
    (Avena sativa)           A  C        X    5-10 7.0        X   X        D        o not use in fine turf.  ion grass.  Use spring oats.             -anq
                                                                                                          Cool season companion grass.Adapt-
    REOTOP                                          4.0-                                                        ed to very acid, infertile soils.  4o Named
    (Aqrostis alba)   P i  C   X              5-10 7.5  X   X   X   X   X   X  Jo not use in fine turf.  Can be used an smooth,steep slopesiarieties
                                                                                                          Cool season temporary or companion
    RYE                                             5                                                           grass, best used in late fall
    (Secale cereale)         A  C        X    4-7  7.0  X   X   X   X              )o not use in fine- turf.  seedinos.                               Abruzzi
                                                                                                          Warm season temporary or companion
    WEEPING LOVEGRASS                               4.5-                           blow yearly to encourage   grass. Tolerates acid,infertile   qo Named
                        ~   D  W        X    5-14 RIn  X   X   X   X              nersistence.                 soils. steeo.droua~htv slopes.         Varieties
    A-annual            -
    P-perennial                                                                W-warm-season plant, grows in surmmer
   ~P-qhnrt-livpd aorpnnial- lasts 3-4 years                                   C-cool-season plant. orows in sorinq and fall












                                                                      Table 1.66d
                                    CHARACTERISTICS OF LEGUMES APPROPRIATE FOR EROSION CONTROL


                                  0  ~      I DRAINAGE TOLERANCE


                                              a-. - dZ   _j cc ï¿½  ï¿½-
(5                                                                                                      S        aninch    all  lN                     m 




                                    VNeeds high phosphorus and                                      tap roots.  Useful with tall fescue in           Chesapeake
    COMMON                                                                                             low-maintenance stands.  Will reseed






   (Trifolium pratense)  sP   C   7-21 7.0          X   X   X    - frequently.                          itself.                                         Pennscott
                                                                  Needs high lime or tanicalreous   18-4" tal. Has spreading root stARoc.





                                                                                                   Prostrate, spreadingle plants, 10-18 inchesft. tall; long.
  FLATPEA                                                           Needs hi gh phosphorus and         tap roots.  Useful with tall fescue in          Chesapeake
   RED CLOVER                            6.0-                        potassium.  Do not mow             low-maintenance stands.  Will reseed             Kenland
   (Trifolium pratense)  sP   C   7-21 7.0          X   X   X        frequently.                        itself.                                         Pennscott
                                                                                                                                                 Tillman

  WHITE CLOVER                          6.0-                        Needs favorable moisture,          Prostrate plants spread by stolons.              ComWi reseeon
   (Trifolium repens)      P   C   7-21 7.5         X   X            high fertility, high pH.           Cannot persist with tall plants.                White Dutch
                                                                  Needs high lime or calcareous   18-24d tall.  Has spreading root stoclrant s.
                                                                  soil, high phosphorus.  Will    Tolerates acid to pH 5.0 when soil has
   (seca)0-n                                                         Will not persist under frequent    h igh lime content.  Deep rooted, somewhat    Chemung
  CROWN VETCH                           5.5                         mowing.  Will not tolerate         shade tolerant.   Useful on steep slopes         Penngift
   (Coronilla varia)       P   C  14-21 8.3   X   X   X              wet soil.                          and rocky areas.                                Emerald
                                                                                                   Prostrate, spreading plants 2-3 ft. tall.
                                                                                                   Adapted to drought, low fertility,
  FLATPEA                                                            Needs lime and high               partial  shade, cold winters.  Chokes out
   (Lathyrus                             5.0-                        phosphorus.  Do not mow            woody vegetation.                               Lathco
    silvestrus)            P   C  14-28 7.0   X   X   X   X   X   closely.
  ANNUAL LESPEDEZAS                                                                                    Companion legume for warm seasons.  Acid
   (Lespedeza striata,                   5.0-                                                           tolerant.  Short tap roots.  Will reseed
    L. stipulacea)         A   W   5-14 7.0   X   X   X   X   X   Do not mow closely.   iShrtta                                            Kobe
                                                                                                   Very deep rooted.  Drought tolerant.
   SERICEA LESPEDEZA                                                                                    Useful on infertile slopes.  Does not            Serala
   (Lespedeza cuneata )                  5.0-                        Will not persist under             persist in coastal plain.                        Interstate
                          P   W   7-28 7.0   X   X   X   X          frequent mowing.
   P-perennial                                                              W-warm season plant, grows in sumner
   A-annual                                                                 C-cool season plant, grows in spring and fall                                             -
                                                     NOTE:  Seed of all legumes must be inoculated with the correct strain of bacteria.                            C1



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     I ~~~~~~~~APPENDIX III
I GUIDANCE CALCULATION
    I ~~~PROCEDURE

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          SOURCE: CHESAPEAKE SAY LOCAL ASSISTANCE DEPARTMENT.
I
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INTRODUCTION

      This procedure is designed to help applicants determine compliance with a locality's
Chesapeake Bay Preservation Act program. This procedure does not supplant any informa-
tion or requirement of other stormwater management programs, namely any local initiative
adopted pursuant to either the Erosion and Sediment Control (ESC) Law [ï¿½ 10.1-560, et. seq.]
or the Stormwater Management (SWM) Law [ï¿½ 10.1-603.1, et. seq.]. While all three programs
are intended to protect water resources from further degradation, each requires separate
engineering analysis. In general, these programs require calculations as follows:

ï¿½     a CBPA program: stormwater quality

ï¿½     a SWM program: stormwater quantity and quality

ï¿½     an ESC program: two-year design storm runoff volumes and velocities

Many localities may combine all aspects into one, comprehensive program. This calculation
procedure would then be just one aspect of that program and a development proposal's
submittal.


                   Determine if the site is in a Chesapeake Bay Preservation Area.


      The Regulations' require localities to designate Chesapeake Bay Preservation Areas
(CBPAs). Guidelines for local designation are contained in Chapters II and mII of the Local
Assistance Manual and Part 11 of the Regulations. CBPAs consist of two different classifica-
tions: Resource Protection Areas (RPAs) and Resource Management Areas (RMAs). The
stormwater management criteria apply equally to both RPAs and RMAs.

      While localities have flexibility to determine their own CBPAs, those areas will
generally include the following land features:

In RPAs:    tidal wetlands, nontidal wetlands contiguous to tidal wetlands, tidal shores,
             tributary streams, a buffer area (of not less than 100 feet), and other lands as
             designated by the locality;


In RMAs:    floodplains, highly erodible soils, highly permeable soils, nontidal wetlands not
             in the RPA, and other land as designated by the locality.




                                         162






I                                                            A        A ItllU~l~~e[*r~ET*~4~*~lr~l~lU1- m*  Illllaw

               Determine from the locality's designation maps and criteria if the site is subject to this
        procedure. Localities may require the entire site to comply with the Regulations even if only
        a portion of the site is in a CBPA. Determine the locality's requirement on total site compliance.



        | STEP TWO: [    Determine if the site is classified as new development or
                             redevelopment.


        The Regulations provide the following definitions:

        Development means the construction, or substantial alteration of residential, commercial, industrial,
        institutional, recreational, transportation, or utility facilities or structures.

        Redevelopment means the process of developing land that is or has been previously developed.


               Check with the locality to see if further clarification is provided concerning redevelop-
        ment.


          NOTE:      Any site in an Intensely Developed Area is automatically classified as redevel-
                      opment, regardless of the site's present or previous condition.
                      [ï¿½ 3.4 of the Regulationsl

               For development, the post-development nonpoint source pollution runoff load cannot
        exceed the pre-development load based on "average land cover conditions." This standard can
        be referred to as a "no net increase" standard. STEP THREE will further discuss "average land
        cover conditions."

               For redevelopment sites not served by BMPs, the post-development non-point source
        pollution runoff load must be 90 percent or less of the pre-development load for that site. This
        standard can be referred to as a "10 percent reduction" standard. Redevelopment criteria are
        not based on average land cover conditions.

               For redevelopment sites with BMPs, the following provision(s) must be satisfied to
        constitute "being served by water quality best management practices":

               (1)    In general, runoff pollution loads must have been calculated and the BMP
                      selected for the expressed purpose of controlling NPS pollution. However, if
                      existing facilities can be shown to achieve the current standard of NPS pollution
                      control, local authorities may consider the site as being served by water quality
                      BMPs.

                                                       163









      (2)   If BMPs are structural, facilities must currently be in good working order, per-
             forming at the design levels of service. The local authority may require a review
             of both the original structural design and maintenance plans to verify this pro-
             vision. A new maintenance agreement miy be required to ensure consistency
             with the locality's SWM requirements.



 I STEPTHREE.-I  Determine the relative pre-development pollutant load of the Keystone
                    Pollutant (LP.).

      The Keystone Pollutant for Tidewater Virginia is total phosphorous. The selection of
total phosphorous as the keystone pollutant is discussed in Attachment A. For the remainder
of this procedure, "pollutant" or "pollutant loading(s)" will mean total phosphorous.

      Following development or redevelopment, impervious cover is the key determinant in
the levels of pollutant export. Up to 90 percent of the atmospheric pollutants deposited on
impervious surfaces are delivered to receiving waters.2 So, for STEPS THREE and FOUR, the
site designer need only determine the amount of total area subject to these criteria and the
proposed amount of impervious cover (or equivalent). Guidance on determining equivalents
is given in Attachment B. Worksheets A and B will help with these next two steps.

      The zoning classification or proposed density of a site will allow applicants to estimate
impervious cover. Compliance and final engineering calculations, however, should be based
on impervious cover shown on the final site plan. Even so, localities and applicants are
encouraged to "err" conservatively, as properties tend to become more impervious with time,
e.g. the expansion of a structure, paving a driveway, adding more parking spaces.  A
conservative estimate indicates more rather than less, impervious cover. Localities may wish
to set a minimum for particular land uses but require the determination of proposed impervi-
ous cover and use the higher number. Representative land use categories and associated
pollutant exports are shown in Table 1.


FOR DEVELOPMENT:

Average Land Cover Conditions (Iwaterihe)

      Just as a locality must designate CBPAs, a locality must also establish baseloads for
watersheds within its jurisdiction. Once set, the baseload will not change unless technology
provides a more precise answer. Watershed delineations serve as the baseline for a calculation
procedure and do not constitute an additional regulatory step. The two options available to
localities are:


                                         164









      1.    A locality will designate watersheds within its jurisdiction and calculate the
             average total phosphorus loading and equivalent impervious cover for each
             individual watershed, or

      2.    A locality will declare its entire jurisdiction as part of Virginia's Chesapeake Bay
             watershed with an average total phosphorus loading (FvA) of 0.45 pounds/acre/
             year and an equivalent impervious cover (IVA) of 16 percent.


      Some localities may begin with OPION Two while they gather the necessary data for
OPION ONE. Guidance on how a locality should calculate individual watershed loads is
provided in Attachment B. Discussion of the default loadings is in Attachment C.


      With I       L  can be calculated using the Simple Method.3 The derivation of the
Simple Method can be found in Appendix A of Controlling Urban Runoff:A Practical Manual
for Planning and Designing Urban BMPs, published by the Metropolitan Washington Council
of Governments.

     I L, = P x Pi x [0.05 + 0.009(wc ,ha)] x C x A x 2.72 / 12|
where:

Lpr = relative pre-development total phosphorus load (in lbs/yr)
P =   average annual rainfall depth (in inches)
      = 40 inches for Northern Virginia area
      = 43 inches for Richmond Metropolitan area
      = 45 inches for Hampton Roadsarea
P. =   unitless correction factor for storm with no runoff = 0.9
Iwat =hed equivalent impervious cover for watershed,
      or "average land cover conditions" (percent expressed in whole numbers)
C =   flow-weighted mean pollutant concentration (in mg/l)
       = 0.26 mg/l when Iwarhe, < 20
       = 1.06 mg/l when 'Iwa d 2 20
A =   applicable area of site (in ac)

  NOTE:    12 and 2.72 are conversion factors


FOR REDEVELOPMENT:

Pre-development loads for redevelopment sites are not based on average land cover condi-
tions. Instead, pre-development loads are based on the site conditions at the time of plan
submittal. Therefore, determine existing impervious cover or equivalent.

                                            165










With I.p,,, L. can be calculated using the Simple Method.

       Lp = P x Pi x [0.05 + 0.009(I ip,)] x C x A x 2.72 / 121
where:

L  = relative pre-development total phosphorus load (in lbs)
P =   average annual rainfall depth (in inches)
      = 40 inches for Northern Virginia area
      = 43 inches for Richmond Metropolitan area
      = 45 inches for Hampton Roads area
P. =   unitless correction factor for storm with no runoff = 0.9
I,,e(pre, = equivalent pre-development impervious cover of the site
       (percent expressed in whole numbers)
C =   flow-weighted mean pollutant concentration (in mg/l)
      =0.26 mg/1 when site(p) < 20
      = 1.06 mg/l when Isite() > 20
A =   applicable area of site (in ac)

 NOTE:      12 and 2.72 are conversion numbers


[STEP FOUR. I     Determine the relative post-development pollutant load (L a).

      Just as with STEP THREE, the designer needs to know the post-development impervi-
ous cover (or equivalent). For both new development and redevelopment, post-development
loadings are site-specific.


FOR NEW DEVELOPMENT

Again, the Simple Method is used.

     I| Lt = P x Pi x [0.05 + 0.0O09(I,(t,)]1 x C x A x 2.72 / 12

where:

L   =t  relative post-development total phosphorus load (in lbs)
P =   average annual rainfall depth (in inches)
      = 40 inches for Northern Virginia area
      =43 inches for Richmond Metropolitan area
      =45 inches for Hampton Roads area
P. =   unitless correction factor for storms with no runoff = 0.9

                                        166









 I,    = equivalent post-development impervious cover
       (percent in whole numbers)
C = flow-weighted mean pollutant concentration (in mg/1)
      * For OPrION ONE: LOCALLY DESIGNATED WATERSHEDS
      = 0.26 mg/i when ILep < 20
      = 1.06 mg/l when Ia o > 20
      * FOR OPTION Two:  A. CHESAPEAKE BAY DEFAULT
      =0.26 mg/l for all Ib,
A   =   applicable area of site (in ac)

 NOTE:    12 and 2.72 are conversion factors

FOR REDEVELOPMENT:

Again, the Simple Method is used.

       L-t =P x Pj x [0.05 + 0.009(I0,P,)] x C x A x 2.72 / 12

where:

L   t = relative post-development total phosphorus load (in lbs)
P = average annual rainfall depth (in inches)
      = 40 inches for Northern Virginia area
      = 43 inches for Richmond Metropolitan area
      = 45 inches for Hampton Roads area
Pj = unitless correction factor for storms with no runoff = 0.9
Iste(  = equivalent post-development impervious cover
       (percent in whole numbers) 
C = flow-weighted mean pollutant concentration (in mg/1)
      = 0.26 mg/l when Itelpo < 20
      = 1.06 mg/l when I      ï¿½> 20
A = applicable area of site (in ac)

 NOTE:      12 and 2.72 are conversion factors


|STEP FIVE:        I Determine the relative removal requirements (RR).

Remember from STEP TWO, the performance standards are different.

FOR DEVELOPMENT:

                                RR = Lp3t - Lp

                                          167





     ~~~~~~~~~~~~~~~~I W



FOR REDEVELOPMENT:

                                 RR = LL  -  0.9(Lp)


If the calculatednumberisless thanorequal tozero, STOP. Note thatin watersheds usingthe
Tidewater weighted average, FVA = 0.45 Ibs/ac/yr, new single-family home parcels one acre
or greater do not require BMPs.

      If no BMPS are required, the applicant need only submit documentation to support his
or her findings. If such findings are found correct by local officials, the applicant has then
satisfied the stormwater management criteria. The state Stormwater Management Law and
the Erosion and Sediment Control Law also deal with other water resource related provisions,
such as quantity-related requirements.

      If removal efficiencies are required, continue on with STEP SIX.



      |~STEP SI    Identify BMP options for the site.


      Best Management Practices (BMPs) can be used to remove pollutants. BMPs are not
always structural. For instance, trash removal can drastically reduce the amount of solid
wastes that reach our streams. However, for the purpose of this discussion BMPs will mean
any structural or mechanical device capable of preventing or reducing the amount of pollution
from nonpoint sources.

      The use of certain BMPs may be limited on some sites by soils, topography, area and
other physical characteristics. Most BMPs can only be applied under restricted site conditions.
Improperly sited, a BMP cannot perform as designed and may become a chronic maintenance
problem. A poorly maintained BMP may even contribute pollutants, e.g. an eroding pond
embankment sends sediment into the receiving stream.

      BMPs and their associated pollutant removal efficiencies are shown in Table 2. This list
is by no means a complete listing of available BMPs, nor does appearance on this list indicate
appropriateness for a given situation.







                                        168





              ~~~~~~~~~~~IMI



*    ~SE SEEN Determine if feasible BMP options can meet the pollutant
    m                   ~~~~~removal requirement.

         Ifruof from           t    h niesite passes through the BMthe applicant need only select a
                 BUTwih n ffcinc rtin eua t o geaerthanthefienyrqrd[a determined
 inSE IV] If, as is usually the case, only portions of the site are covered by Bui~s, a
        weigtedsummation must be made.

 I         ~~~Localities may allow pollutant reduction credits for serving off-site areas which drain
     through BMPs on the subject site. However, while applicants might claim pollutant reduction
 Ecredits for serving off-site areas, applicants MAY NO! claim credit for one or more off-site
     BMI~s serving their property (even if, in fact, they do). Neither the Act nor the Regulations
     allow for such an off-set program.

I         ~~~Worksheet C will help with this step of the procedure.

            If no combination of BM~s can meet the required standard, the applicant must consider
     a different site design. Increasing the proportion of site area covered with vegetation is one of
 Ithe best ways of lowering the required removal efficiencies. A different site layout may make
     a more appropriate BMP possible; for example, placing structures on 'tight" soils may leave
     more permeable soil for infiltration areas.











          I~~~~~~~~~~~~~~~~~~~~0 










ENDNOTES

 Chesapeake Bay Local Assistance Board, Final Regulations: VR 173-02-01 Chesapeake Bay
Preservation Area Designation and Management Regulations. September 1989.

2 Thomas R. Schueler, Controlling Urban Runoff: A Practical Manual for Planning and
Designing Urban BMPs (Washington, D.C.: Metropolitan Washington Council of Govern-
ment, Department of Environmental Programs, 1987), 1.4.

3 Ibid, 1.9-1.13.





































                                       170










ANNUAL STORM PHOSPHOROUS EXPORT                                                                     TABLE 1

                                    For Existing Urban Land Uses
                                          (in pounds/acre/year)

                                                                    ANNUAL RAINFALL
                                  IMPERVIOUS                                (in)
                                      COVER
       LAND USES                       (%)              40      41       42     43      44      45


                                         0              0.11    0.1     0.11':0.11    0.12    0.12
       5.0 acre residential lots        5              0.20   :0.2: 0.21    0.22    0.22  ':0.23i-
       2.0 acre residential lots        10             030  0:ii.303:  031  0l.:032:   0.33  i.:0;330.
       1.0 acre residential lots        15             0.39    0.401 0  .4 1    0.42    0.43    ::.44
                                        16             0.41   i.i:0.421i: 0.43  :i-044 0.4 5  i:iiO0.46ii
                                        17             OA3   iA;,4oi  0.45  :i::i4A6- 0.47  .48
                                        18             0.4OA5     -0 .46" OA7  i :'iOA.8-:i 0.49  :-i051ti:
                                        19             0.47   .0.4A8i  0.49  :i0.-50i.  0.52   -0;.533i
       050 acre residential lots        20             2.03      ::08    2.13    2.18:  2.23    2.28
       033 acre residential lots        25             2.42  i.::2.481i-  2.54  - :2.61:.  2.67  :2.72;
       0.25 acre residential lots       30             2.82   .:-289ii 2.96  .i -:3.-i:;  3.10    3.17:
                                     - 35               3.22    330:1. 3.38    3.46    3.54 :!3.62:
       Townhouses                       40             3.61   ;'3.70:    3     :3.8     3.97  :i:i-:84.06 3.-97
                                        45             4.01                .411    4.21    431    4.41    4.51
                                        -50           44.41     .52    4     .63  74    4.85  i4.96;
       Garden Apartments                55             4.80    492    5.04 5.16ii 5.28            40i
                                        60             5.20  ':':5.33-  5.46     59    5.72    5.85
                                        65              5.60  ?5.74, 5.88  !::6.02.!   6.16  :6.3.0
       Light                           70              5.99 :614    6.29         . 6     .59    6.;74:
       Commercial/Industrial            75             6.39    6.55:    6.71    6.87    7.03    79
                                        80             6.79  :696    7.13    7.29    7.46    7.63
       Heavy                            85             7.98   :8.17'ii    8.37     '8 i7 8 .77    8.97
       Commercial/Industrial        L90                758  7ii,.i7i:.  7.96  .:-':8.15-:  8.34  :.-:853:
                                        95             7.98    8.17:   8.37  i.8.57:   8.77  ii-8.97
       Asphalt/Pavement                100             8.37    8.58;; 8.79    9.000   9.21  :9.42:


                                      For Non-Urban Land Uses
                                          (in pounds/acre/year)

                                                    SILT LOAM   LOAM   SANDY LOAM
            LAND USE                                  SOILS          SOILS          SOILS

            Conventional Tillage
            Cropland                                   3.71           2.42           0.83
                                 - -  - i. ;:: I.    : :  :iR E  T ........................ i  i) :  i    : - : T.E  :.       
            Conservation Tillage                                                     0ii5-ii::i2


            Pasture Land                                0.91          0.59           0.20

           Forest: :Land0.;:- --9-:,: 0                       0.19.-2    :    0.04:

                                                      171









STRUCTURAL BMPs FOR CHESAPEAKE BAY PRESERVATION AREAS                                 TABLE 2


                                                                     Average
                                                                     Total P
                                                                    Removal
                    Acceptable BMP                                 Efficiency

              A.    Extended Detention

                     (1) Design 2 (6-12):                             20%

                     (2) Design 3 (24 hours):                         30%

                     (3) Design 4 (shallow marsh):                    50%


              B.    Wet Pond

                     (1) Design 5 (0.5 in/imp.ac):                    35%

                     (2) Design 6 (2.5 Vr):                         40-45%

                     (3) Design 7 (4.0 V):                            50%


              C.    Infiltration

                     (1) Design 8 (0.5 in/imp. ac):                   50%

                     (2) Design 9 (1.0 in/imp. ac):                   65%

                     (3) Design 10 (2-year storm):                    70%


              D.    Grassed Swale

                     (1) Design 15 (check dams):                     10-20%



These designs are taken from Metropolitan Washington Council of Governments, Controlling Urban Runoff:
A Practical Manual for Planning and Designing Urban BMPs, ,1987

Effeciency ratings are taken from John P. Hartigan, P.E., Three Step Process for Evaluating Compliance with
BMP Requirements for Chesapeake Bay Preservation Areas, 1990

                                           172






I


    WORKSHEET A: NEW DEVELOPMENT OmN ONE: LOCa.Y DESIGNA'ED WmRsms


         Compile site-specific data and determine site imperviousness (Ij,).

                                  POST-DEVELOPMENT
            A*                         =             acres
               I:**   structures      =             acres
                   parking lot        =             acres
                   roadway            =             acres
                   other              =             acres
                                      ____=  acres
                                      ____=  acres

                   total I,           =             acres

            I, = (total I/A) X 100    =             (percent expressed in whole numbers)

            *Although the area subject to regulations may be only the area actually in a CBPA, some localities
              may require all of the site to comply with criteria.
             I, represents the actual amount of impervious area.

            Determine the average land cover conditions (IWathed).

           Use I,,.,.h as determined by the locality. If I,,,,~,m < 20, use C   = 026mg/1. If I,,a> 20, use C,, =
           1.08 mg/i.



    I3     Determine need to continue.

           I,    =               %  (from Step 1)
           Iw,    - =            %  (from Step 2)

           If I, < Iw tead STOP and submit analysis to this point.
           If IS > Iw      CONTNUE.

         Set constants.

           Pi     = unitless rainfall correction factor    P     = annual rainfall depth in inches
                   = 0.9 for all of Tidewater Virginia           = 40 inches for Northern Virginia area
                                                             = 43 inches for Richmond Metropolian area
           C,    = flow weighted mean concentration              = 45 inches for Hampton Roads area
                     of total phosphorus
                   = 0.26 mg/ I for I, < 20
                   = 1.08 mg/l for I, > 20.


            12 and 2.72 are used in the equation as unit conversion factors.



                                                     173










WORKSHEET A: NEW DEVELOPMENT OpnoN ONE: LocAY xDErrCNAw  WATERSHEDS

      Calculate the pre-development load (LM).

      Lp.r   = P X PiX [0.05 + (0.009 X I'w,,)J X Cpf XAX 2.72 /12

                     -  0.9 X [0.05 + (0.009  XJX      x      X 2.72 / 12

             1  ________pounds per year



U      Calculate the post-development load (LP,,).

       LP.-   = P X Pj X [0.05 + (0.009 X I5w)1] X Cp, XAX 2.72/12

             -      X 0.9 X [0.05 + (0.009  X  I  x   x       X 2.72 /12

             1           pounds per year


       Calculate the pollutant removal requirement (RR).

       RR    =L.  -LIP



                         pounds per year

       To determine the overall BMP efficiency required (%RR) when selecting BMP options:

       %RR   =RR/ L"X 100

             I  - (   /9X100



















                                          174










WORKSHEET A: NEW DEVELOPMENT   ORoN Two: VA. CHESmAE BAY DEFAuLT


     Compile site-specific data and determine site imperviousness (I.Rd).

                               POST-DEVELOPMENT
       A*            -                          acres
       I.:**   structures         =             acres
               parking lot        =       .     acres
               roadway            =       .     acres
               other              =             acres
                                  ____=  acres
                                  --.___=  acres

               total Ia           =             acres

       I,,. = (total I/A) X 100   =             (percent expressed in whole numbers)

       *Although the area subject to regulations may be only the area actually in a CBPA, some localities
         may require all of the site to comply with criteria.
        I, represents the actual amount of impervious area.

       Determine the average land cover conditions (Iwateshed).

       Use I,<,,h =    IVA=16 bec aus e F,,.. = OA5 lbs/ac/yr for Virginia's Chesapeake Bay Watershed. Use
       Cp,, = 0.26 mg/1.



[[   Determine need to continue.

       I.,,    =             %  (from Step 1)
       w..d =        16     %  (from Step 2)

       If I., < Ite . a,d STOP and submit analysis to this point.
       If Ite > I ,erhed' CONTINUE.

    Set constants.

       PJ      = unitless rainfall correction factor   P     = annual rainfall depth in inches
               = 0.9 for all of Tidewater Virginia           = 40 inches for Northern Virginia area
                                                           = 43 inches for Richmond Metropolian area
       C       = flow weighted mean concentration            = 45 inches for Hampton Roads area
                 of total phosphorus
               = 0.26 mg/l for all I,,,,

       12 and 2.72 are used in the equation as unit conversion factors.






                                              175









WORKSHEET A: NEW DEVELOPMENT                                  OProN Two: VA. CHEsAPEAKE BAY DEFA ur

[;N   ~ Calculate the pre-development load (Lp,.).

       3p1    = P X Pj X [0.05 + (0.009 X Iw,,d)] X Cp, X A X 2.72 / 12

              =       X 0.9 x [0.05 + (0.009 X   )] X 0.26X     X 2.72 / 12

              =            pounds per year


[I     Calculate the post-development load (Lpo,).

       Lpoe   = P X Pj X [0.05 + (0.009 X I,,)1] X C X A X 2.72 /12

               =      X 0.9 x [0.05 + (0.009 X   )] x 0.26 X     x 2.72 /12

               =            pounds per year


37 1    Calculate the pollutant removal requirement (RR).

       RR     = Lpo. - Lp,



               =            pounds per year

       To determine the overall BMP efficiency required (%RR) when selecting BMP options:

       %RR   = RR / Lp X 100

               = (      /      )X100


















                                                  176









         WORKSHEET B : REDEVELOPMENT

U    ~~~~      Compile site-specific data.
                                           PRE-DEVELOPMENT                 POST-DEVELOPMENT
                           A* ~ ~ ~ ~~            _____ acres                  -  _____acres
 I                   Ta:~~~~~~1    structures   =  _____acres                   -  _____acres
                            parking lot       =____acres                       -____acres
                            roadway           =____acres                       -  ____acres
                            other              -    _____acres                 -    _____acres
           U                                      ________ ~~~~~~~~~~~~~~~~acres   ____acres

   I                        ~~~~~~~~~~total I~acres                                ______acres

                     I = (total IaA) X 100     =  _____percent expressed  =  _____percent expressed
                        1   =0.05 +I (0.009 X I)            in whole numbers                in whole numbers
                                                 -   _________unitless        - _  ____   unitless
                     C:I    1>20 =1.08 mg/I
                             < 20 =0.26 mg/i       ____  mg/I l___m/
               *Although the area subject to regulations may be only the area actually in a CBPA, some localities
                may require all of the site to comply with criteria.

              Set constants.
                     = unitless rainfall correction factor   P  = annual rainfall depth in inches
I~ ~ ~ ~~~~P =          0.9 for all of Tidewater Virginia         = 40 inches for Northern Virginia area
                                                               = 43 inches for Richmond Metropolitan area
                                                               = 45 inches for Hampton Roads area
 1                 ~~~~~~12 and 2.72 are used in the equation as unit conversion factors.

               Calculate the pre-development load (L pr).
I~~~~~IP   = P XP. XRcm)X CpX A X2.72 /12

 3                   ~~~~~~~~= -XO.9X-X-XX2.72 /12

                         - _______pounds per year

I              ~~~~Calculate the post-development load (LP,,)'

                  L  =0  P XPi.XRvm X Cp X A X2.72 /12

                         -    0O.9 X     X _X            X 2.72 /12

                                  pounds per year

              Calculate the pollutant removal requirement (RR).

I              ~~~ ~~~RR  =LP9- (0.9 X L)                   %RR   =(RR/L0) X 100

                           ______-(0.9X                          = (/             ) )X IO

                                  pounds per year                -  ___
                                                    1 77










WORKSHEET C: COMPLIANCE


         Select BMP options using screening tools and list them below. Then calculate the load
        removed for each option. DO NOT LIST BMPs IN SERIES HERE.
                                                Fraction of
                            Removal           CBPA Drainage
                            wit  Removal       Area Served                              Load
           Selected        Efficiency   X       (expressed in    X     L        =    Removed
            Option           (%/100)            decimal form)         (Ibstyr)         (lbs/yr)











  2a    Estimate parameters for non-CBPA drainage areas on the project site (if the locality
         does not require complete compliance for the whole site). If the locality requires
         compliance for the whole site, omit this step.

         A (on site, non-CBPA)          =            acres
         I:     structures        -                  acres
                parking lot            =            acres
                roadway                =            acres
                other                  =            acres
                                       =     -- _________acres
                                       *= -__       acres

                total I                =            acres

         I = (total I./A) X 100         =            %
         Rv = 0.05 + (0.009 X I)

         C:     I > 20 = 1.08 mg/l      =            mg/
                I < 20 = 0.26 mg/l


         When using VIRGINIA CHESAPEAKE BAY DEFAULT (F. = 0.45 Ibs/ac/yr), C=0.26 mg/1 for all I,,

         Calculate post-development load for on-site non-CBPAs.


         Lpo,)         = P X P X R XCXAX2.72/12

                        =       XO 0.9X      X       X       X2.72/12

                        =           pounds per year
                                                                                         Revised 7/90

                                               178











       Determine loadings for off-site areas if the locality allows this option.
       Iwua,,    = from locality OR    ,,= IA = 16

       If I.,d < 20, use C,, = 0.26 mg/l.
       If IWabLhd > 20, use C.. = 1.08 mg/l.
       If IW., = IVA use C.h, = 0.26mg/l.


               = P X Pj X [0.05 + (0.009 X I,,,)]X C,, X Affi, X 2.72 / 12

               =       X 0.9 X [0.05 + (0.009 X     )] X      X       X 2.72 / 12

               =            pounds per year

[I     Total non-CBPA pollutant loading.

       Step 3     +   Step 4   =   total non-CBPA loading

                  +            =             pounds per year


[]     Calculate credits if the locality allows this option.


                           Removal                                    Load
          Selected         Efficiency    X         L=    Removed
           Option           (%/100)              (lbs/yr)            (Ibs/yr)










I]      Calculate overall compliance.

       Step 1     +    Step 5  =   total load removed

                   +            =             pounds per year

        If total load removed > removal requirement, criteria are satisfied.

                   >





                                                                                          Revised 7/90

                                                  179








ATTACHMENT A

       Many different pollutants can be identified in our streams and water bodies. The
Regulations merely require the control of "nonpoint source (nps) pollution." The Model
Ordinance defines NPS as pollution consisting of constituents such as sediment, nutrients, and
organic and toxic substances from diffuse sources.  Trying to deal with all the possible
pollutants would make any calculation procedure complicated and expensive. To simplify the
calculations needed, a "keystone" pollutant can be selected. A keystone pollutant shares the
general characteristics of most other pollutants. By removing the keystone pollutant, other im-
portant pollutants will be simultaneously removed. Chapter 2 of A Framework for Evaluating
Compliance with the 10% Rule' reviews each of the major pollutants found in urban runoff for
their suitability as the keystone pollutant, based on the following three criteria:

1.    The pollutant must have a well-defined adverse impact on the Chesapeake Bay.

2.    The pollutant should exist in a "composite" form, i.e. in a roughly equal split between
      particulate and soluble phases.

3.    Enough research data must be available to provide a reasonable basis for estimating
      how keystone pollutant loads change in response to development and to current
      stormwater control measures.

      The only urban pollutants that appear to meet all three criteria for suitability as a
keystone pollutant are: total phosphorus, total nitrogen and zinc (Table 3). Of these three, total
phosphorus exists in the most equivalent proportions of soluble and particulate forms (40 / 60).
Total nitrogen and zinc are less proportionate, at 20/80 and 25/75, respectively.



             TABLE 3

                                   Well-Defined        Composite  Adequate
              Pollutant          Impacts on the Bay?     Form?        Data?

             Sediment                 yes                no            no
             Total Phosphorous        yes                yes           yes
             Total Nitrogen :.       1 - yes              yes I  yes
             Coliform Bacteria        yesn
             BOD/COD                  yes                yes           no
             Oil/Grease               yes                no            no
             Zinc                     yes                yes           yes
              Lead                    yes !:noii             yes  :    '  '
             Toxics                   no                 no            no


                                         180









      By removing total phosphorus, an equal or greater level of removal for most other urban
pollutants is simultaneously obtained. An equal or higher level or removal is possible for
nearly every other pollutant, except total nitrogen. Total nitrogen is primarily found in soluble
form, which is much more difficult to remove with current techniques. Nevertheless, by
removing phosphorus, a reasonable degree of nitrogen is still removed as well.

      Based on this review, total phosphorus was selected as the best candidate for the
keystone pollutant in Tidewater Virginia. In doing so, Virginia will target the same pollutant
as Maryland, preserving some consistency in our multi-state Bay preservation effort.


ENDNOTE:

S $chueler, Thomas R. and Matthew R. Bley, A Framework for Evaluating Compliance with
the Chesapeake Bay Critical Area (Washington, D.C.: Maryland Critical Area Commission
and Maryland Department of the Environment, 1987).








        ATTACHMENT B

 I           ~~~The Regulations require new development stormwater management criteria be based
        on "average land cover conditions." Watershed designations serve as the baseline for a
        calculation procedure and do not constitute an additional regulatory step. Localities will have
        two options:
        1.    A locality will designate watersheds within its jurisdiction and calculate the average
        phosphorus loading and impervious cover for each individual watershed, or

        2.    A locality will declare its entire watershed as part of Virginia's Chesapeake Bay
        watershed with an average phosphorus loading of 0.45 pounds/acre/year and impervious
*      ~~cover of 16 percent.

        A locality may begin with Option Two while they gather the necessary data for Option One.
        Figure I shows how Fairfax County could break up its watersheds. This discussion revolves
        around Option One. Option Two is discussed in Attachment C.
        To determine average land cover conditions within a watershed, the locality must follow a
        three-step procedure:
        1.    Evaluate individual watersheds. We recommend a minimum watershed area of 100
               acres. Localities may wish however, to use watershed delineations used for other
               aspects of its work, e.g. a sanitary sewer master plan.

        2.    Know existing land use data. The Regulations are based on present land uses, not
              proposed land uses. A comprehensive plan is more future oriented than a zoning map.
               Still, a zoning map does not always indicate present use. A locality may also be able to
               use current aerial photographs. Data may be cross-referenced with Commissioner of
 3           ~~~Revenue information.

        3.    Compute a weighted average of impervious cover (or its equivalent). The Simple
              Method (and the nonpoint source pollution load) is highly dependent on the percent of
              impervious cover. Some land uses contribute nonpoint source pollution but do not
              have "impervious covers," e.g. forest and agriculture lands. Therefore, conversions, or
              equivalents, must be determined. Use Table I to find equivalent loading/impervious
              factors for non-urban uses. Localities may use other documented loading factors,
              especially if found to be more appropriate to that locality, as long as the factors are used
               consistently.
              Weighted averages are frequently computed for quantity related analyses and this
 I          ~ ~~process is identical. Figure 2 shows how average land cover conditions might be
               calculated for a 100-acre watershed.


                                                 182











  IFASMLEFAAX COUTYn WATERSHEDS                                                                              FicupRE I






                             SUGARLANO      NICHOLS
                               UNRUN    POND

                                                    RANCH BULL NECK RUN
                                         K                        ~~~~~~~~SCOTTSRU RUN

                                ~~ o~~~e ~~~~.. ~~~~                           TURKEY RUN
                    I twnatloni
                                  A;,P~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0
                            HORSEPEN         DIFFICULT RUN


    I ~ ~~~~~~U RUN CREEKf


                                                               FaWsChafd,        ARLINGTON
            .'ULLt fFdoiCUT

              RUN




             LITTLE ROCKY RUNHAE
            .JOHNNY MOORE CREK           WL          POHICK CREE

                OLD MILL BRANICHU








                       Watershed Boundary             KNCHG ON


             BULL RUN Watershed Name




       Source. County of Fairfax, 1987 Annual Report on the Environment (Fairfax, Va.: Environmental Quality
I     ~ ~Advisory Council and Office of Comprehensive Planning, 1987), p. 16










                                                       183









CALCULATING AVERAGE LAND COVER CONDITIoN                                           FIGURE 2


100 acre Watershed
Wooded          = 20 acres
                                       /    t:L:iE !D. L    odec ,
                                               Low-density
Low-density
Residential    = 20 ac      res                                    .
(1-acre lots)

Agriculture
 Pasture       = 30 acres
 Conservation
       tillage  =  15 acres      Wooded            Agricultural
  Conventional
      tillage   =  15 acres

Total acreage     100 acres





Land Use            Loading: *          # of Acres      Weighted Load:
                    lbs/acre/year                           lbs/year

Wooded                 0.12                20                  2.4
1-acre lots            0.42                20                  8.4
Pasture                0.59                30                 17.7
Conventional           2.42                15                 36.3
Conservation           1.52                15                 22.8

                                           100                87.6

* Phosphorous; based on rainfall of P=43 inches/year and loam soils.

     = Sum of weighted loadings
           total acreage

  = 0.12(20) +0.42(20) + 0.59(30) + 2.42(15) + 1.52(15) = 88 lbs per year = 0.88 lbs per acre per year
             20 + 20 + 30 + 15 + 15                  100 acres

                  Equivalent Impervious Cover  =  Iwatshe        =     19

                                          184








ATTACHMENT C

      Not all localities will have the ability to designate individual watersheds and compute
an average watershed baseload. For that reason, the department has determined a default load
for Tidewater Virginia.

Following the procedure outlined in Attachment B:

1.    Designate watershed.

      The department chose the entire Virginia portion of the Chesapeake Bay watershed -
      not just Tidewater Virginia (as defined by the Chesapeake Bay Preservation Act). The
      department encourages multi-jurisdictional cooperation among localities to designate
      large-scale watersheds as well.

2.    Evaluate existing land use data.

      Existing land use data is given in Virginia's Chesapeake Bay Initiatives:First Annual
      Progress Report (September 1985) produced by the Virginia Council on the Environ-
      ment. This breakdown is shown in Figure 3.

3.    Compute a weighted average of impervious cover (or its equivalent).

      Because urban areas are most likely to adopt Option One, urban areas are excluded from
      the weighted average. In addition, loading rates for "urban" areas are highly variable.

  FVA = relative total phosphorus load for Virginia's Chesapeake Bay watershed

  Foo = relative total phosphorus load for any land use (X)

  FVA = %FOR(FFOR) + %PAST(FpAsr) + %CST(Fc9r) + %CVT(Fcvr)

      = 0.66(0.12) + 0.21(0.59) + 0.07(1.52) + 0.06(2.42)

      = 0.45 lbs/ac/yr


      Use Table 1 to determine the equivalent impervious cover. The average loading, FVA =
      0.45 lbs/ac/yr, falls between impervious covers of 16 to 18 percents. Because of the
      differing annual rainfall across the state, the department has choosen the most conser-
      vative value of 16.

         FVA = 0.45 lb/ac/yr <=> I'VA = 16%



                                             185










Therefore, the default load for Virginia's Chesapeake Bay watershed is 0.45 lb/aclyr with an
equivalent impervious cover of 16 percent. Localities are encouraged, but not required, to
customize this aspect of the procedure, even if computing individual watersheds is not
feasible. The Town of Herndon might use 'VA = 18, Caroline County might use 'VA = 17 and Isle
of Wight County would retain 'VA = 16.



VIRGINIA LAND USE DATA                                                               FIGURE 3



              itotal                   i FORi. PAST :CI                            CVT
              :area-i-rea ?areaI  %            % Narea  %  area    %   area        area::
River Basin    (sq.mi.) URB  (sq.mi.) FOR:(sq.mi.) PAST   (sq.rmn) CST (sq.mi.) CVT (sqrni.)

Potomac       14670-. 7           :     :i02:7::'i--: 56    ::8215ii: ..i:. 26   :i:3814-i: 7   1027 i    4    587
Rappahannock  ::3    1       26     4  1684i   20             68   :210        7    184:
York         -i2980i i 0.2  iii.  70 :2090i.:  13 .i388;  10.1  302            6.7 -::200
James          0495   3    315::  73 :.7661 :1::i  4.30 14 1469 420
Eastern Shore   --i10:00  1.5    15iii-.::i-i   50    0  i-805    i: 85:::.i 9  0  31 :310
Total (w/urban) 31781!i:ii .5  i138i  63 20150:::!i  20    6286  7   2259 : 5 :1701
Total (w/o urban):30398i  n/a iin:/a:ii:l 66 20150:~:ii  21    6286  7   2259  6   1701--

URB = urban land uses
FOR = forest cover
PAST = pasture land
CST = conservation till acreage
CVT = conventional till acreage

Source: Commonwealth of Virginia, Council on the Environment, Virginia's Chesapeake Bay
        Initiatives: First Annual Report (Richmond, Va.: Council on the Environment, 1985).

















                                           186








    BEST MANAGEMENT PRACTICES
      DESIGN GUIDANCE MANUAL
            FOR HAMPTON ROADS




     De    . ~,  \
I~~ï¿½  



                  '.. r-f


    I  . ï¿½ . .. . ....
I~~~~~~b -I. .'  ..         ,.Z4


                                 '4 -3(
    SUMMARY OF HELP SCREENS
    BMP MASTER, VERSION 1.3



          HAMPTON ROADS PLANNING DISTRICT COMMISSION
                   DECEMBER 1991








                                DISTRIBUTION

Distribution of this report and the accompanying software is governed by the
provisions of the December 7, 1991 Letter Agreement between the Hampton Roads
Planning District Commission (HRPDC), Chesapeake Bay Local Assistance Department
(CBLAD) and Smith Demer Normann, Ltd. (SDN). Specifically:

     "HRPDC and CBLAD shall have the full, complete and perpetual right to
     reproduce and distribute the Manual and Software within Virginia to
     political subdivisions subject to or interested in the Chesapeake Bay
     Preservation Act. HRPDC, CBLAD and such political subdivisions shall
     have unrestricted use of the Manual and Software for purposes of
     implementing the Chesapeake Bay Preservation Act, related laws andlor
     regulations and other governmental purposes; provided, however, that
     the rights of all nongovernmental agents and contractors to use the
     Manual and Software shall be limited to services performed for or on
     behalf of HRPDC, CBLAD and/or such political subdivisions. HRPDC and
     CBLAD shall not distribute the Software outside of the Commonwealth
     of Virginia or to any person, entity or political subdivision other than the
     foregoing. HRPDC and CBLAD shall contractually restrict their
     distributees from making any further copy of, distribution of, or
     otherwise making use of, the Software inconsistent with this paragraph.
     Any diskettes distributed hereunder will have an appropriate label
     permanently affixed to reflect the foregoing restrictions.

     "SDN will have the full, complete and perpetual right to distribute the
     Software outside of the Commonwealth of Virginia, provided that the
     programs contained therein shall be modified such that they are
     inapplicable to the Chesapeake Bay Preservation Act requirements in
     effect in Virginia.  SDN may only use the Manual and Software in
     Virginia as a contractor for HRPDC, CBLAD or a political subdivision to
     whom the Manual and Software have been distributed pursuant to
     Paragraph 5 above and for the purpose for which the Manual and
     Software were so distributed. The foregoing notwithstanding, SDN shall
     have the right to conduct research and development of the Manual and
     Software for marketing and distribution outside of the Commonwealth
     of Virginia. Notwithstanding anything herein contained to the contrary,
     SDN shall not be precluded from using its engineering and programming
     expertise, technical know-how, formulations and designs in the
     performance of best management practices services for other clients and
     customers as long as such services do not involve a prohibited use of the
     Software and Manual. SDN shall not distribute the Software in Virginia
     or take any other action which could reasonably be foreseen as adversely
     impacting the efficacy of the Software for the implementation of the
     Chesapeake Bay Preservation Act and related purposes.  SDN shall
     contractually restrict its distributees from making any further copy of,








 I           ~~~distribution of, or otherwise making use of, the Software inconsistent
              with this paragraph. Any diskettes distributed hereunder will have an
              appropriate label permanently affixed to reflect the foregoing
              restrictions. 


U       ~~The Hampton Roads Planning District Commission and the Chesapeake Bay Local
         Assistance Department request the cooperation of recipients of this Manual in
3       ~~complying with these restrictions.
















3                           ~~~~SUMMARY OF HELP SCREENS

                             BMP MASTER, VERSION 1.3






                                     December 10 , 1991


   3                                     ~~~~~~~~~~~~Prepared For:

                            Hampton Roads Planning District
  3                                 ~~~~~~~~~Chesapeake, Virginia




                                        Prepared by:












                                    SDN Water Resources
                                        Central Park
                               Six Manhattan Square, Suite 102
                                  Hampton, Virginia 23666
                                        (804) 865-9610







*B 1,General Info
                    General Info

Information regarding any questions, comments, or
updates for this program should be addressed to:

                 Smith Demer Normann
          Six Manhattan Square, Suite 102
              Hampton, Virginia 23666
          (804)865-9610, Fax (804)865-1533

 If you like this program and are interested in what
Smith Demer Normann can do for you, please call us.
SEE ALSO: ^AUTHORS' NOTE^ ^DISCLAIMERA
*E

*B 2,AUTHORS' NOTE
                   Authors' Note
 ~----.................--................--.........- -
BMP MASTER, VERSION 1.3 IS FOR THE EXCLUSIVE USE OF
THE HAMPTON ROADS PLANNING DISTRICT COMMISSION.
THIS SOFTWARE PROVIDES HELPFUL, PRELIMINARY DESIGN
GUIDELINES FOR SEVERAL BEST MANAGEMENT PRACTICES
 (BMPs). PROPER SELECTION, DESIGN, AND SITING OF
 THE BMPs ARE THE SOLE RESPONSIBILITY OF THE USER.
 THE USER SHOULD BE WELL VERSED IN ENGINEERING
PRINCIPLES IN ORDER TO PROPERLY DESIGN BMPs.


 SEE ALSO: ^General Info^ ^DISCLAIMER^
*E

*B 3,DISCLAIMER
                     Disclaimer
 --~~--------.................................~-.......
 ALTHOUGH THIS PROGRAM HAS BEEN TESTED BY ITS
 DEVELOPER, NO WARRANTY, EXPRESSED, OR IMPLIED, IS
 MADE BY THE DEVELOPER AS TO THE ACCURACY AND
 FUNCTIONING OF THE PROGRAM AND RELATED PROGRAM
 MATERIAL NOR SHALL THE FACT OF DISTRIBUTION
 CONSTITUTE ANY SUCH WARRANTY, AND NO RESPONSIBILITY
 IS ASSUMED BY THE DEVELOPER IN CONNECTION
 THEREWITH.


                   < Page Down >
*p
                 Disclaimer(Con't.)

 SMITH DEMER NORMANN SHALL IN NO EVENT BE LIABLE FOR
 DIRECT, INDIRECT, SPECIAL, INCIDENTAL, CONTINGENT,
 OR CONSEQUENTIAL DAMAGES RESULTING FROM ANY DEFECT
 IN THE SOFTWARE OR ITS DOCUMENTATION.

 YOU MAY NOT COPY THIS SOFTWARE, MAKE ALTERATIONS OR
 MODIFICATIONS TO THE SOFTWARE OR ATTEMPT TO
 DISCOVER THE SOURCE CODE OF THIS SOFTWARE.  THIS
 SOFTWARE MAY NOT BE SUBLICENSED, RENTED, OR LEASED.

 SEE ALSO: ^General Info^  ^AUTHORS' NOTE^
*E

*B 10,Site Criteria
          BMP Selection from Site Criteria

 This function will allow a general selection of
 feasible BMPs based on input site criteria. The






selection process is for site planning purposes to
determine which BMPs could be designed in the BMP
Design module. The site criteria used include site
area, ground water table, land slope, soil type,
and proximity to drinking water wells. A report
can be produced giving BMP site restrictions.

SEE ALSO:
   ^General Info^ ^BMP Design" ^Removal Efficiency"
*E

*B 11,Removal Efficiency
      BMP Selection from Removal Efficiency

This function will allow a general selection of
 feasible BMPs based on removal efficiency for a
calculated pollutant loading increase. The
selection process is used with the BMP Selection
 from Site Criteria to determine which BMPs could be
designed in the BMP Design module. A report can be
generated giving valid BMPs and the maximum removal
 efficiency ranges.

 SEE ALSO:
   ^General Info^  ^BMP Design^  ^Site Criteria"
*E

*B 12,BMP Design
                     BMP Design

 This function will allow the design of a BMP that
 is selected from a menu of feasible choices. The
 valid BMPs are made selectable by executing the
 BMP Selection from Site Criteria and also Removal
 Efficiency first. However, you can still choose
 to design a BMP that is not technically feasible
 from the list.


 SEE ALSO:
   ^General Info^  ^Site CriteriaA  ^Removal EfficiencyA
*E

*B 13,Buffer Equivalency
                 Buffer Equivalency

 In the Chesapeake Bay Preservation Act (CBPA), a
 100 foot buffer is deemed to achieve 40% reduction
 of nutrients.  This preliminary procedure
 determines the area needed for a BMP to provide
 reduction of pollutants from a new development
 site. The BMP thus determined can be installed in
 the buffer area or elsewhere on the site.  The
 extent of the reduction of buffer area is the
 individual jurisdiction's decision.

                   < Page Down >
*p
 For example, the buffer area might be reduced from
 100 feet to 80 feet, with a BMP.









SEE ALSO:
   ^General Info^ ^Efficiency of BMPs^
*E

*B 15,Edit Keys
                     Edit Keys

Cursor Keys- Move cursor direction of cursor key
Ctrl-Left  - Move cursor word left
Ctrl-Right - Move cursor word right
Home       - Move cursor to beginning of field
End        - Move cursor to end of field
Ctrl-Home  - Move cursor to first field
Ctrl-End   - Move cursor to last field
Tab        - Next field   Shift-Tab   - prev Field
Enter      - Next field   Ctrl-Enter  - Done

                        <PgDn>
*p
                  More Edit Keys

 Insert     - Insert toggle
Delete     - Delete character at cursor
Backspace  - Delete character left

Ctrl-Bckspc- Delete word left
Ctrl-T     - Delete word right

Ctrl-U     - Delete to end of field
Ctrl-R     - Restore field with previous data
Ctrl-Y     - Delete to end of form or last field
*E

*B 20,Area Served
                    Area Served

The area to be served by the BMP in Acres. The
area cannot be zero.







 SEE ALSO:
   AGeneral Info^  ^Edit KeysA
*E

*B 21,Ground Water
                 Ground Water Table

 The depth in feet of the seasonal high ground water
table for the site.  The value cannot be zero.







 SEE ALSO:
   ^General Info^ ^Edit Keys^
*E






*B 22,Slope
                 General Land Slope

The general slope of the land for the site, given
in percent. Example: 3.0%







 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 23,Well Proximity
                  Proximity to Wells

 This is a Y or N question to determine if there are
 drinking water wells within 100' downslope of the
 BMP.






 SEE ALSO:
   ^General Info^  ^Edit KeysA
*E

*B 24,Project Name
                     Project Name

 This is your project title or name. This can be
 used for a variety of purposes. Some possible uses
 might be: job number, site name, alternative name.






 SEE ALSO:
   ^General InfoA  ^Edit Keys^
*E

*B 25,Soil Type
                      Soil Type

 This is the soil type for the site. The soil is
 selected from a pop-up menu by using the arrow keys
 or the first letter of the soil category name, and
 the ENTER key. The soil types carry with them
 default infiltration values that are used later in
 the design process.



 SEE ALSO:
   ^General Info^ ^Soil Table^
*E

*B 26,Soil Table
 Soil Category Name   Inf Rate(In/Hr) Wtr Cap(In/In)




                                      4







 Sand                       8.27            0.35
 Loamy Sand                 2.41            0.31
 Sandy Loam                 1.02            0.25
 Loam                       0.52            0.19
 Silt Loam                  0.27            0.17
 Sandy Clay Loam            0.17            0.14
 Clay Loam                  0.09            0.14
 Silty Clay Loam            0.06            0.11
 Sandy Clay                 0.05            0.09
 Silty Clay                 0.04            0.09
 Clay                       0.02            0.08
*E

*B 30,Site Area
                      Site Area

This value can be either the physical area of the
 site in acres or the area of the site served by one
particular BMP. For example, if 50% of a 20 acre
 site drains to a BMP, you could input either:

            20 acres, 50% served
        or 10 acres, 100% served.

 IMPORTANT NOTE: If the area served is less than
 100%, you MUST compute the impervious cover for
                    < Page Down >
*p
                  Site Area (Con't.)

 just that portion of the site being served by the
 BMP. In order to determine the overall removal
 requirement of the ENTIRE site, the loads from all
 the separate watersheds must be added together.





 SEE ALSO:
   ^General Info' ^Edit Keys' ^ImpCvrPost^ ^PctAreaServed^
*E

*B 31,ImpCvrWtrshed
                Impervious Cover Pre

 If the site is a new development, then this value
 represents the % of impervious cover for the
 watershed that the site is a part of. Defaults:
               16% -- Tidewater Area
               53% -- City of Norfolk
 If the site is being redeveloped, then this value
 represents the pre-redevelopment imperviousness of
 the site.

 SEE ALSO:
   ^General InfoA  'Edit Keys^
*E

*B 32,ImpCvrPost
               Impervious Cover Post

 This value is the percent impervious cover for the
 area listed in Site Area based on post-development
 conditions due to pavement, concrete, structures,
 etc. REMEMBER, when the percent area served is
 less than 100, this percent impervious cover must







represent the impervious cover in the sub-watershed
actually being served by a selected BMP. The value
in most cases will be greater than the Impervious
Cover Pre.
SEE ALSO:
   ^General Info^ ^Edit Keys^ ^Site Area^ ^PctAreaServed^
*E

*B 33,PctAreaServed
                 Percent Area Served

This value can be either the physical area of the
 site in acres or the area of the site served by one
particular BMP. For example, if 50% of a 20 acre
 site drains to a BMP, you could input either:

            20 acres, 50% served
        or 10 acres, 100% served.

 IMPORTANT NOTE: If the area served is less than
 100%, you MUST compute the impervious cover for
                    < Page Down >
*up
             Percent Area Served (Con't.)

 just that portion of the site being served by the
 BMP. In order to determine the overall removal
 requirement of the ENTIRE site, the loads from all
 the separate watersheds must be added together.





 SEE ALSO:
   ^General Info^ ^Edit Keys^ ^ImpCvrPost^ ^Site Area^
*E

*B 34,Redevelopment
                    Redevelopment

 This is a Y or N question asking if the site is a
 redevelopment. Answering N implies the site is a
 new development.






 SEE ALSO:
   ^General Info^  'Edit Keys^
*E

*B 35,Rainfall
               Average Annual Rainfall

 This is the annual rainfall depth in inches. It is
 45 inches for the Hampton Roads area.







 SEE ALSO:







   ^General Info^ ^Edit Keys"
*E

*B 40,OutFileName
                 Output File Name

This is the pathname or device name specified for
output used by reports.  Typical uses are:

  PRN             -- The standard printer.
  EXAMPLE.RPT     -- An example file for reporting.
  C:\BMP\TEST.OUT -- A full pathname to a file.



SEE ALSO:
   ^General Info^ ^Edit Keys"
*E

*B 100,Biofiltration
                    Biofiltration
 ~-----.....--.................----- -..~ -...........
 Biofiltration as a BMP is similar to swale and
 filter strip BMPs. Surface runoff can be treated
by biofiltration to remove urban pollutants. The
 runoff receives treatment through interaction with
vegetation and the soil surface. For a swale, the
 design depth of flow should be at least two (2)
 inches less than the winter vegetation height.
 Emergent wetlands plants can also be used to
 provide water quality benefits.

 SEE ALSO: ^General Info^ ^Efficiency of BMPs^
*E

*B 101,Dry Well
                      Dry Well

 The dry well is a variation of the infiltration
 trench and is designed exclusively for runoff from
 rooftops. Roof leaders are extended to a stone
 aggregate filled trench located a minimum of 10
 feet from the building foundation.



 SEE ALSO: ^General Info^
   ^Infiltration Trench^ ^Efficiency of BMPs^
                   < Page Down >
*p
                    General Note

 The use of infiltration BMPs like Infiltration
 Trench, Dry Well, Porous Pavement, Infiltration
 Basin, Underground Storage, and Grid / Modular
 Pavement on fill material is not recommended. Fill
 areas are susceptible to slope failure due to fill
 material becoming saturated when infiltration
 practices are used.




*E

*B 102,Infiltration Trench
                 Infiltration Trench






 ---------------------------------------------------
An infiltration trench is typically 3-8 feet deep
and filled with stone aggregate to create an
underground reservoir. Runoff can either drain from
the reservoir into the underlying soil
 (exfiltration) or be collected by underdrains and
directed to an outflow.

SEE ALSO:
   'General Info^ 'Infiltration Basin'
   "Dry Well^   'Efficiency of BMPs^
                  < Page Down >
*p
                    General Note

Typically, infiltration trenches can only
accommodate limited quantities of runoff and are
used for sites of less than 10 acres in size.
The use of infiltration BMPs like Infiltration
Trench, Dry Well, Porous Pavement, Infiltration
Basin, Underground Storage, and Grid / Modular
Pavement on fill material is not recommended. Fill
areas are susceptible to slope failure due to fill
material becoming saturated when infiltration
practices are used.

*E

*B 103, Infiltration Basin
                 Infiltration Basin

Whereas infiltration trenches serve small sites, an
 infiltration basin can serve drainage areas up to
 50 acres. They are designed to promote exfiltration
through the underlying material. They should be
vegetated and often include devices which prevent
 course sediment from entering the basin as well as
emergency spillways for extreme storm events.

 SEE ALSO: ^General Info'
   ^Infiltration Trench"  "Efficiency of BMPs^
                   < Page Down >
*p
                    General Note

 The use of infiltration BMPs like Infiltration
 Trench, Dry Well, Porous Pavement, Infiltration
 Basin, Underground Storage, and Grid / Modular
 Pavement on fill material is not recommended. Fill
 areas are susceptible to slope failure due to fill
material becoming saturated when infiltration
practices are used.



*E

*B 104,Grassed Swale
               Grass Swale(w/Chk Dam)

 Grassed swales are typically used in low density
 areas as an alternative to curb and gutter drainage
 systems. The pollutants are filtered out by the
 grass and subsoil.  Check dams may be used to
 temporarily pond runoff, allowing infiltration over
 a period of time. They cannot, however, accommodate







major runoff events and usually lead to other
downstream BMPs.
SEE ALSO: 'General Info^
   'Filter StripA  'Efficiency of BMPs^
*E

*B 105,Filter Strip
                    Filter Strip

Also known as buffer zones, filter strips are
 similar to grassed swales except that they are
wider. They should be at least 20' wide and not
used on slopes greater than 15%. The filter strips
are usually forested and accept evenly distributed
sheet flow.  They can not accept channelized flow
and function effectively. There are secondary
benefits including aesthetics, wildlife habitat,
and noise screening.
SEE ALSO: ^General Info^
   'Grassed Swale^  AEfficiency of BMPs^
*E

*B 106,Porous               Pavement
                   Porous Pavement

Porous pavement detains and minimizes the effects
of runoff containing traffic generated pollutants.
 It provides for removal by infiltration and
bacterial action. It has a number of shortcomings
which generally confine it to low volume traffic
areas such as parking lots.  It consists of a
graded aggregate cemented by asphalt cement, with
numerous voids providing a high permeability.
SEE ALSO: ^General Info'
   'Grid/Modular Pavement^ 'Efficiency of BMPs^
                   < Page Down >
*p
                    General Note
 ---------------------------------------------------.
The use of infiltration BMPs like Infiltration
Trench, Dry Well, Porous Pavement, Infiltration
Basin, Underground Storage, and Grid / Modular
Pavement on fill material is not recommended. Fill
areas are susceptible to slope failure due to fill
material becoming saturated when infiltration
practices are used.




*E

*B 107,Underground Storage
              Underground Storage Trench

Underground storage trench is designed to remove
 sediments and hydrocarbons from parking lots and
commercial sites where there is not enough space
 for infiltration systems. Underground storage
trench, as a BMP should ONLY be installed when
 other BMPs are not feasible. It functions like an
 infiltration trench but can accept concentrated
 runoff. Unlike a surface trench, underground
 storage trench can be installed under the pavement
 of a parking lot. It is important and recommended
                   < Page Down >




 I                                     ~~~~~~~~~~~~~~~~~9






*p
         Underground Storage Trench<Con't.>         2

to pretreat the concentrated runoff before it
enters the underground storage trench. The
pretreatment can be accomplished by installing a
water quality inlet upstream of the underground
storage trench. If installed under the pavement of
a parking lot, the pavement should be properly
designed for the appropriate loadings.
  While this BMP is not visible and can be
aesthetically pleasing, maintenance and
 replacement costs can be prohibitive and costly.
                  < Page Down >
*p
          Underground Storage Trench<Con't.>         3

 Proper engineering judgement should be exercised in
 selection, design, and siting of this BMP.






 SEE ALSO: ^General Info^
   ^Infiltration TrenchA ^Efficiency of BMPs^
                   < Page Down >
*up
                    General Note

 The use of infiltration BMPs like Infiltration
 Trench, Dry Well, Porous Pavement, Infiltration
 Basin, Underground Storage, and Grid / Modular
 Pavement on fill material is not recommended. Fill
 areas are susceptible to slope failure due to fill
 material becoming saturated when infiltration
 practices are used.




*E

*B 108,Grid/Modular Pavement
                Grid/Modular Pavement

 Using the same concept as porous pavement, this
 type of pervious pavement consists of a grid made
 of concrete, clay bricks, or granite sets. The
 void areas of the grid are filled with a pervious
 material such as sod, gravel, or sand.



 SEE ALSO: ^General Info^
   ^Porous Pavement^ ^Efficiency of BMPs^
                   < Page Down >
*p
                    General Note
 -- -------------------------------------------------...
 The use of infiltration BMPs like Infiltration
 Trench, Dry Well, Porous Pavement, Infiltration
 Basin, Underground Storage, and Grid / Modular
 Pavement on fill material is not recommended.  Fill
 areas are susceptible to slope failure due to fill
 material becoming saturated when infiltration



                                       10







practices are used.




*E

*B 109,Grit-Oil Separator
                 Grit-Oil Separator
 .- - - - ----.....  ---...---...--.......--......--.......
Used to meet some of the water quality requirements
where oil and grit deposits are likely such as in
parking lot areas and commercial sites. A typical
grit-oil separator consists of three chambers. The
 first two chambers maintain a permanent pool of
water. The third chamber connects to the storm
drain system or other infiltration BMP.


 SEE ALSO:
   ^General Info^ ^Efficiency of BMPs'
*E

*B 110,Water Quality Inlet
                 Water Quality Inlet

Water quality inlets are typically used to serve
parking lots one acre or less in size.  A typical
water quality inlet consists of one or two
 chambers. The water quality inlet is a smaller
version of a grit-oil separator and functions in a
 similar fashion.



 SEE ALSO:
   AGeneral Info^  ^Efficiency of BMPs'
*E

*B 111,Detention Pond
                   Detention Pond

Not available in this release, part of Phase II.








 SEE ALSO:
   ^General InfoA  ^Efficiency of BMPs^
*E

*B 112,Retention Pond
                   Retention Pond

Not available in this release, part of Phase II.







SEE ALSO:
   ^General Info^  ^Efficiency of BMPs^
*E

*B 113,Extended Ret/Det Pond
                Extended Ret/Det Pond

Not available in this release, part of Phase II.







 SEE ALSO:
   ^General Info^  ^Efficiency of BMPs^
*E

*B 114,DP/RP (Wetland Bottom)
               DP/RP (Wetland Bottom)

 Not available in this release, part of Phase II.








 SEE ALSO:
   ^General Info^ ^Efficiency of BMPs'
*E

*B 200,Infiltration Rate
                  Infiltration Rate

 The infiltration rate can be pre-selected by first
 executing the BMP Selection from Site Criteria.
 Choose the appropriate Site soil type. A different
 value can also be input, based on information from
 County Soil Survey or site soil survey.




 SEE ALSO:
   ^General Info^  ^Soil Table^ ^Edit Keys'
*E

*B 201,Max. Storage time
       Maximum Allowable Ponding/Storage Time

 The maximum allowable ponding/storage time is the
 time for which a BMP is designed to completely
 drain. 72 hours is the maximum (and default)
 value, but other values can also be input.





 SEE ALSO:
   ^General Info^ AEdit Keys^
*E





                                      12







*B 202,Void Ratio
                     Void Ratio

The void ratio is the ratio of voids to the volume
of the solids. 0.4 is the common void ratio and is
the default value. The value cannot be zero nor
 can it be greater than 1.





 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 203,Min. Vert. Dist.
   Minimum Distance From BMP Bottom to Ground Water

 The minimum distance from the BMP bottom to the
 ground water table is defaulted at a recommended
 minimum of 2.0 feet. However, the user can design
 with any value greater than 2.0. The Virginia
 Stormwater Management Regulations require that the
 invert of the infiltration BMPs should be four (4)
 feet above the seasonal high groundwater table.


 SEE ALSO:
   ^General InfoA ^Edit Keys^
*E

*B 204,Runoff Depth Increase
              Increase in Runoff Depth

 The increase in runoff depth is the change in
 runoff (increase) that will result from site
 development/improvement. The designer will have to
 compute this using currently accepted hydrologic
 methods and convert the runoff to depth.

 For Dry Well, increase in runoff depth is from the
 Rooftop area.

 SEE ALSO:
   ^General Info^ ^Edit Keys^
*E

*B 205,Contributing Drainage Area
             Contributing Drainage Area

 This is the total site area (in square feet) that
 contributes runoff to the BMP. This value must be
 greater than zero.

 For Dry Well, this area is the Rooftop area.




 SEE ALSO:
   ^General Info^ ^Edit Keys^
*E

*B 206,BMP Depth
                      BMP Depth




                                      13






This is the depth to which the BMP is to be
designed. The maximum value is determined in the
feasibility window encountered immediately prior to
this design window and is automatically defaulted
ahead.




SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 207,Rainfall Amount
                 Amount of Rainfall

This is the depth of rainfall associated with the
design storm. Some examples are:

               2-Year storm=3.0 inches
              10-Year storm=5.0 inches




 SEE ALSO:
   AGeneral Info'  AEdit Keys"
*E

*B 208,BMP Filling Time
                  BMP Filling Time

 This is the time in hours for the BMP to fill. The
 default value for Dry Well is one(l) hour. For
 other infiltration BMPs, the default value is
 two(2) hours. The designer is responsible for
 determining this time based on accepted hydrologic
 time of concentration methods.



 SEE ALSO:
   AGeneral Info"  "Edit Keys'
*E

*B 209,Basin Side Slopes
                  Basin Side Slopes

 The desirable side slopes (horizontal to vertical
 ratio) for the infiltration basin. This represents
 the side slopes for the entire basin. Consideration
 of variable side slopes is not possible in this
 program.




 SEE ALSO:
   AGeneral Info'  AEdit Keys^
*E

*B 210,Basin Top Width
                  Basin Top Width

 The desirable top width of thp infiltration basin.






                                      14













SEE ALSO:
   AGeneral InfoA  ^Edit Keys'
*E

*B 211, Depth    of  Subbase  Aggregate
              Depth of Subbase Aggregate

The design depth of stone aggregate reservior.








SEE ALSO:
   'General Info^ AEdit Keys'
*E

*B 212,Runoff Depth from Well Area
        Runoff Depth from Area Over Dry Well

The depth of stormwater runoff contributed only by
that area over the dry well.







SEE ALSO:
   'General Info' "Edit Keys'


*B 213,Depth of Soil Over Well
         Depth of Soil Overlying Dry Well

The depth of the cover mate- Ul over the BMP. In
the case of the Dry Well t:  represents the soil
cover and typically is one Loot. For the
Underground Storage BMP, this cover could possibly
be pavement, soil, or concrete.




SEE ALSO:
   ^"General Info^  'Edit Keys^
*E

*B 214,Water Capacity of Soil
     Water Capacity of Soil Overlying Dry Well

The effective water capacity of a soil is the
 fraction of the void spaces available for water
 storage, measured in terms of inches per inch.







                                      15









SEE ALSO:
   ^General Info^  ^Soil Table^  ^Edit Keys^
*E

*B 215,Discharge Rate
       Discharge Rate (Cubic Feet per Second)

The discharge for which the Biofiltration is being
designed. The discharge could be the discharge
 associated with one-inch(l") rainfall or two(2)
year rainfall.





 SEE ALSO:
   ^General Info^  AEdit Keys^
*E

*B 216,Manning's 'n' Value
      Manning's Coefficient of Vegetation Cover

 The roughness coefficient used with the Manning's
 equation. Typical values for vegetation are:

   Dense grass up to 6" tall               -- 0.07
   Dense grass 6" - 12" tall               -- 0.10
   Dense grass    > 12" tall               -- 0.20
   Vegetation with coarser stems
   (wetland plants, woody plants, etc.) -- 0.07

 SEE ALSO:
   ^General Info^  AEdit Keys^
*E

*B 217,Depth of Flow
           Depth of Flow in Parabolic Swale

 The depth of the flow for which the Biofiltration
 swale is being designed. It is recommended to use
 the design depth of flow at least two(2) inches
 less than the winter vegetation height.





 SEE ALSO:
   AGeneral InfoA  AEdit KeysA
*E

*B 218,Longitudinal Slope
                 Longitudinal Slope

 The ground slope along the water flow path in the
 Biofiltration swale or Grass swale with check dams.







 SEE ALSO:



                                      16







   ^General Info^ ^Edit Keys^
*E

*B 219, Length       of  Swale
                   Length of Swale

 The longitudinal length of the Biofiltration swale.
 Typically the length is 200', but site constraints
may require a shorter length to be used. In such
 cases, use a length less than 200', for example
 150'.




 SEE ALSO:
   "General Info^ ^Edit Keys^
*E

*B 220,Bottom Width Check Dam
              Bottom Width of Check Dam

 The bottom width in feet of check dam.







 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 221,Side Slope
      I-    -     -     -     -     -    --Side Slope

 The ratio of the swale side slope. A typical value
 might be 3 or 4 to signify 3:1 or 4:1 respectively.







 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 222,Total Hydraulic Length
           Total Hydraulic Length of Swale

 The total hydraulic length of swale in feet can be
 input by the user or calculated internally. Simply
 input the known value if known or input a 0 in the
 field and process the screen with Ctrl-Enter.
 Another window will pop up allowing the input of
 parameters that will be used to calculate the
 hydraulic length to be used.


 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 223,Max. Ponding time



                                       17







          Maximum Allowable Ponding Time

The maximum allowable ponding time is the time for
which a Grassed Swale is designed to completely
drain. 24 hours is the maximum (and default) value,
but other values can also be input.





SEE ALSO:
   ^General Info' ^Edit Keys^
*E

*B 300,Contributing Impervious Area
        Contributing Impervious Drainage Area

This is the impervious area in acres which is
contributing runoff to the grit-oil separator.
Roof surface area can be neglected.






 SEE ALSO:
   ^General Info^ ^Edit Keys'
*E

*B 301,Length of Chamber
                 Length of Chamber
 ......------------...................---- ---- ----~- ~--
 This is the length of the first chamber in a grit-
 oil separator or a water quality inlet. The
 minimum recommended length is six (6) feet. A
 higher value can be input.





 SEE ALSO:
   ^General Info' ^Edit Keys'
*E

*B 302,Width of Chamber
                  Width of Chamber
 --~--......~-.......................-------..........
 This is the width of the first chamber in a grit-
 oil separator or a water quality inlet. The
 minimum recommended width is 2'-6". A higher value
 can be input.





 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 303,Contributing Flow from Impervious Area
       Contributing Flow from Impervious Area

 This value is the runoff generated from the



                                      18






contributing impervious area. The runoff in cubic
feet per second (cfs) is the "first flush" flow for
the curb inlet opening or 10-year storm entering
the grit-oil separator from the storm drain system.




SEE ALSO:
   ^General Info'  'Edit Keys'
*E

*B 304, Diameter  of Drawdo wn  Pipe
            Diameter of Drawdown Pipe

This value is the diameter of the drawdown pipe in
 inches. This pipe in a grit-oil separator connects
the second and third chambers. In a water quality
 inlet this is the outflow from the main chamber.
The minimum diameter recommended is six (6) inches.
A larger size drawdown pipe can be used. The pipes
 can be cast iron or aluminized corrugated metal
pipe.

 SEE ALSO:
   'General Info' ^Edit Keys^
*E

*B 305,Number of Drawdown Pipes
              Number of Drawdown Pipes

 This value is the number of drawdown pipes
 connecting the second and third chambers in a grit-
 oil separator or from the main chamber of a water
 quality inlet. The minimum recommended number of
 drawdown pipes is two (2).




 SEE ALSO:
   ^General Info' 'Edit Keys'
*E

*B 306,Free-Board Value
                  Free-Board Value

 The minimum recommended free-board value is 1'-6".








 SEE ALSO:
   'General Info' 'Edit Keys^
*E

*B 320,Efficiency of BMPs
                Efficiency of BMPs(%)

     (40-80) BioFiltration
     (50-70) Dry Well
     (50-70) Infiltration Trench
     (50-70) Infiltration Basin




                                      19







     (10-20) Grass Swales(w/ chk dams)
     (20-50) Filter Strips
     (50-70) Porous Pavement
     (50-70) Underground Storage
     (50-70) Grid/Modular Pavement
     (10-25) Grit-Oil Separator
     (10-25) Water Quality Inlet < Page Down >
*p
           Efficiency of BMPs(%) <Con't.>

     (20-50) Detention Ponds
     (35-65) Retention Ponds
     (25-60) Extended Det/Ret Ponds
     (40-75) Det/Ret w/ Wtlnd Btms





 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 323,Design For First Flush
               Design For First Flush

 The "first flush" is the runoff associated with the
 most frequent storms. "First flush" designs can be
 developed for :
   (1) The runoff produced by a one-inch (1") storm
       over the contributing site area.
   (2) 0.5 inch of runoff per impervious acre in the
       contributing site area (first flush).
   (3) The runoff per impervious acre produced by a
       one-inch (1") storm.

                   < Page Down >
*p
          Design For First Flush <Con't.>

   (4) 0.5 inch of runoff in the contributing site
       area (Virginia Stormwater Management
       Regulations).

 A large percentage of urban pollutants being
 discharged into receiving waters are associated
 with most frequent storms (normal rainfall).


 SEE ALSO:
   ^General Info^  ^Edit Keys^
*E

*B 999,How To Quit
                  Quit BMP Master

 This will take you back to the DOS environment. You
 will first be asked if you are sure that you want
 to quit.

   Possible Responses:

      'N' Key    :  Returns to main menu.
      ESCAPE Key :  Returns to main menu.
      ENTER Key  :  Returns to main menu.
      'Y' Key    :  Quits program.
 *E



                                       20








*B 1000l,Report Help
                 Report File Pathname

This is a file name or device name where you want
your reports to go.

   Common Uses are:

     PRN           :   The standard printer.
     BMP.RPT       :  A sample report file name.
     C:\TEST.FILE :  A sample full pathname.

 SEE ALSO:
   ^General Info'  ^Edit Keys'
*E
























































                                      21










U DESIGN GUIDANCE MANUAL

  I        ~~FOR HAMPTON ROADS


   I~~~~~~~~~~~~~~~~k






     PAEI I DTN IORENTN
~~~AIL I TIE


                   HAMPTON RODS PLANNIG DISTRICTCOMMISSIO
    I~~~~~~~~~~EEBR19













                               BEST MANAGEMENT PRACTICES

                                DESIGN GUIDANCE MANUAL

                                            FOR

     *                         ~~~~~~~HAMPTON ROADS VIRGINIA

                         PHASE II: DETENTION/RETENTION FACILITIES





            This report was produced, in part, through financial support from the Virginia
5          ~~~Council on the Environment pursuant to Coastal Resources Management
            Program Grant No. NA90AA-H-CZ796 from the National Oceanic and
            Atmospheric Administration and from the Chesapeake Bay Local Assistance
I         ~~~Department pursuant to Contract No. 91-42 of August 1990 and Unnumbered
            Contract of June 20, 1990.



            Preparation of this report was included in the HRPDC Work Program for FY
I         ~~~1990-9 1, approved by the Commission at its Executive Committee Meeting of
            March 21, 1990 and in the HRPDC Work Program for FY 1991-92, approved by
            the Commission at its Executive Committee Meeting of March 20, 1991.









     3                       ~~~~~~~~~Prepared by URS Consultants, Inc.
                              in cooperation with the Staff of the
                         Hampton Roads Planning District Commission

                                       December 1991








                                                                               AN INTERNATIONAL PROFESSIONAL SERVICES ORGANIZA1
                                             URS CONSULTANTS, INC.   TLANOST
                                                5606B VIRGINIA BEACH BOULEVARD   RUFFALO
                                                                               CL EVELAND
                                                        EXECUTIVE COVE CENTER  COLUMBUS
                                               VIRGINIA BEACH. VIRGINIA 23462-5631  DENVER
                                                                 (804) 499-4224  NEW YORK
                                                                               PARAMUS NJ
                                                              FAX: (804) 473-8214  NEW0ORLEANS
                                                                               SAN FRANCISCO
                                                                               SAN MATEO
                                                                               SEATTLE
                                                                               VIRGINIA BEACH
                                                                               WASHINGTON.  C

                                        February 4, 1992





Mr. John Carlock
Hampton Roads Planning District Commission
The Regional Building
723 Woodlake Drive
Chesapeake, Virginia 23320

RE:    Detention - Retention
        BMP Design Guidance Manual

Dear Mr. Carlock:

The attached document is submitted in accordance with our contract of November 4, 1991. The document
was produced in accordance with our discussion and follows the format established by the HRPDC.
Because one of the considerations was to have sections that could be removed and stand alone, there is
duplication of some parts of Sections 3 and 4.

As we discussed during the preparation of the document, the focus is on guidance rather than design
examples. It is expected that the user will have the knowledge and experience to prepare the design.
Further, the variables in design of retention and detention facilities and the need to have design flexibility
and innovation limit the value of details or step by step procedures. The state of the art is advancing rapidly
and computer software is becoming more sophisticated, which also strengthened the decision to produce
a guidance document.

This document is in a final draft form for the review of your staff and the committee. Consequently, the table
of contents and the lists of tables and figures have not been finalized because we are certain that changes
in page numbers will be made. Also, we are still in the process of receiving some data which we believe
would be valuable input, and we will include that data in the final report.

We have truly enjoyed the opportunity to work with you on this project and trust that the fruits of our labor
will be beneficial to all of the users.

                                        Very ruly yours,

                                        UR  CONSULTANTS, INC.





                                          mont W. Curtis, P.E.
                                        Vice President
Enclosures

LWC/kbr







                          TABLE OF CONTENTS

LETTER OF TRANSMITTAL

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

1.0 INTRODUCTION
      1.1 Purpose and Scope ................................... 1-1
      1.2 General Planning Considerations .......................... 1-2
      1.3 Regulations Regarding BMPs ............................ 1-4
           1.3.1 Stormwater Management Regulations ................. 1-4
           1.3.2 Chesapeake Bay Preservation Act .................... 1-5
           1.3.3 Virginia Erosion and Sediment Control Law and Regulations  . 1-6
           1.3.4 Virginia Dam Safety Act Regulations ................... 1-7
           1.3.5 USEPA Stormwater NPDES Permit .................... 1-7
     1.4 Current Practices by Jurisdictions ......................... 1-8
     1.5 Definitions ........................................ 1-9
     1.6 Hydrology ....................................... 1-10
     1.7 Outlet and Channel Hydraulics .......................... 1-11

2.0   GENERAL PLANNING AND ENGINEERING CONSIDERATIONS
     2.1 Stormwater Management Planning ........................ 2-1
           2.1.1 Elements of a Program ............................ 2-2
                2.1.1.1 .... Multi-Objective ........................ 2-2
                2.1.1.2 .... Structural ........................... 2-2
                2.1.1.3 .... Non-Structural ........................ 2-3
           2.1.2 Locations of Basins .............................. 2-3
           2.1.3 BMP Selection .................................. 2-4
     2.2 Hydrology ........................................ 2-7
           2.2.1 Rainfall Data ................................... 2-7
           2.2.2 Hydrographs and Peak Flows ....................... 2-9
           2.2.3 Low Flow/Base Flow ............................ 2-14
     2.3 Water Quality Enhancement ............................2-15
           2.3.1 Methods ..................................... 2-15
           2.3.2 Regulatory Requirements ......................... 2-17
     2.4 Retrofitting ....................................... 2-17

3.0   DETENTION BASINS
     3.1 Description ........................................ 3-1
     3.2 Applicability ........................................ 3-1
     3.3 Proposed Functions ................................... 3-1
           3.3.1 Stormwater Quantity Control ........................ 3-2
           3.3.2 Stormwater Quality Enhancement .................... 3-2
     3.4 Design Guidelines .................................... 3-3

 I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~







           3.4.1 Quantity Control Guidelines ......................... 3-3
                 3.4.1.1 .... Methodology of Computations ............. 3-4
                 3.4.1.2 .... Physical Features of a Basic Detention Basin  . . 3-6
           3.4.2 Quality Control Guidelines .......................... 3-6
                 3.4.2.1 .... Methodology of Computations ............. 3-7
                 3.4.2.2 .... Physical Features for Water Quality Enhancement 3-8
           3.4.3 Design Modifications for Water Quality Enhancement .   ..... 3-8
                 3.4.3.1 .... Extended Release ...................... 3-9
                 3.4.3.2 .... Infiltration Basins ...................... 3-10
                 3.4.3.3 .... Retrofitting Existing Facilities ............. 3-12
                 3.4.3.4 .... Wetland Area Establishment .............. 3-13
     3.5 Construction and Operation Issues ....................... 3-13
           3.5.1 Construction Guidelines .......................... 3-13
                 3.5.1.1 .... Infiltration Basin Construction Guideline ..... 3-15
           3.5.2 Cost Estimates ................................. 3-16
           3.5.3 Facility Life Expectancy ........................... 3-16
           3.5.4 Maintenance Requirements ........................ 3-16
     3.6 Plan Submittal Requirements ............................ 3-17
           3.6.1 Impacted Agencies .............................. 3-17
           3.6.2 Submittal Checklist .............................. 3-18

4.0   RETENTION BASINS
     4.1 Description ..... 4-1
     4.2 Applicability ........................................ 4-1
     4.3 Proposed Functions ................................... 4-1
           4.3.1 Stormwater Quantity Control ........................ 4-2
           4.3.2 Stormwater Quality Enhancement .................... 4-2
     4.4 Design Guidelines .................................... 4-3
           4.4.1 Quantity Control Guidelines ........................ 4-3
                 4.4.1.1 .... Methodology of Computations ............. 4-3
                 4.4.1.2 .... Physical Features of a Basic Retention Basin  .. 4-6
           4.4.2 Quality Control Guidelines .......................... 4-7
                 4.4.2.1 .... Methodology of Computations ............. 4-7
                 4.4.2.2 .... Physical Features for Water Quality Enhancement 4-9
           4.4.3 Design Modifications for Water Quality Enhancement ..... 4-10
                 4.4.3.1 .... Extended Release .................... 4-11
                 4.4.3.2 .... Water Quality Storage .................. 4-11
                 4.4.3.3 .... Retrofitting Existing Facilities ............. 4-11
           4.5 Construction and Operations Issues ................. 4-12
           4.5.1 Construction Guidelines .......................... 4-12
           4.5.2 Cost Estimates ................................. 4-13
           4.5.3 Facility Life Expectancy ........................... 4-15
           4.5.4 Maintenance Requirements ........................ 4-15
     4.6 Plan Submittal Requirements ............................ 4-15
           4.6.1 Impacted Agencies .............................. 4-16
           4.6.2 Submittal Checklist .............................. 4-16



 I                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ii







5.0   ESTABLISHMENT  OF  WETLANDS   IN  STORMWATER   DETENTION   OR
     RETENTION BASINS
     5.1 Goals and Criteria of Success ........................... 5-2
           5.1.1 Water Quality .................................. 5-3
           5.1.2 Wildlife Habitat .................................. 5-3
     5.2 Implementation Guidelines .............................. 5-4
           5.2.1 Required Data .................................. 5-4
           5.2.2 Physical Aspects of Basin .......................... 5-4
                5.2.2.1 Water Balance ............................ 5-4
                5.2.2.2 Wetland Size ............................. 5-5
                5.2.2.3 Wetland Permanent Pond .................... 5-5
                5.2.2.4 Modifications to the Incoming Flow of Water ....... 5-7
                5.2.2.5 Outlet Structure and Extended Detention ......... 5-7
           5.2.3 Biological Aspects of the Basin ...................... 5-8
                5.2.3.1 Wetlands Substrate ......................... 5-8
                5.2.3.2 Wetland Vegetation ......................... 5-8
                5.2.3.3 Planting Procedures ....................... 5-14
                5.2.3.4 Submerged Aquatic Vegetation ............... 5-15
     5.3  Maintenance and Monitoring of Wetland Basins .............. 5-15

6.0   CONSTRUCTION, OPERATIONS, AND MAINTENANCE
     6.1 Purpose ........................................ 6-1
     6.2   The Responsibilities of Ownership and Maintenance .      .     .......... 6-1
     6.3 Construction Inspection ................................ 6-3
           6.3.1. Preconstruction Phase ............................ 6-3
           6.3.2. Construction Phase .............................. 6-4
           6.3.3. Post-Construction Phase ........................... 6-4
     6.4 Maintenance ........................................ 6-8
           6.4.1 Preventive Maintenance Procedures ................... 6-8
           6.4.2 Corrective Maintenance Procedures .................. 6-10
           6.4.3 Aesthetic Maintenance Procedures .................. 6-12
     6.5 Equipment Requirements .............................. 6-15
     6.6 Maintenance Costs .................................. 6-17
     6.7 Training ....................................... 6-17

APPENDICES

     A.    CHECKLIST FOR SUBMITTALS

     B.    CHESAPEAKE BAY PRESERVATION ACT GUIDANCE CALCULATIONS

REFERENCES

ACKNOWLEDGEMENTS







                             LIST OF TABLES



Figure                   Title

1-1    Current Practices by Jurisdictions ......................... 1-7

2-1   Selection Evaluation Objectives ........................... 2-6

2-2          Typical "C" Coefficients For Rational Method ................ 2-11

2-3          Modified Rational Correction Factor (VDOT) ................. 2-13

2-4    Precipitation and Evaporation Data ....................... 2-14

2-5   Pollutant Removal Efficiencies ........................... 2-16

3-1   Location - Detention Requirement ......................... 3-2

3-2  Typical Infiltration Rates ............................... 3-11

4-1   Location - Retention Requirements ........................ 4-2

5-1          Benefits of Establishing Wetland Basins ..................... 5-2

6-1    Typical Preconstruction Meeting Topics ..................... 6-5

6-2          Summary of Construction Inspection Practices ................ 6-7

6-3   Maintenance Items by Categories ........................ 6-14

6-4          Equipment and Materials Common to Maintenance ............ 6-16
















                                   iv







                            LUST OF FIGURES



Figure                   Title                                       Following oaae

1-1          Urbanization Accelerates Stormwater Runoff ................. 1-1

1-2  Multi-objective Planning ................................ 1-3

1-3          Run-off Flows Resulting from Increased Paved Surface ......... 1-10

2-1          Stormwater Facility Area Versus Total Project Area ............ 2-4

2-2  Cost for Infiltration Trench ............................... 2-5

2-3  Cost for Grassed Swale ................................ 2-5

I 2-4  Cost for Grit-Oil Separator .............................. 2-5

2-5          Intensity Duration Frequency Curve (24 hours) ................ 2-7

2-6          Intensity Duration Frequency Curve (6 hours) ................. 2-7

2-7  Design Storm Mass Diagrams ........................... 2-7

2-8          Depth Versus 24-hour Storm, SCS Type II, 10-year ............ 2-8

2-9          Rates of Overall Rainfall to Maximum Point Rainfall ............ 2-8

2-10         Typical Assumed Hydrographs using Rational Method ......... 2-12

2-11         Post-Development Storage Volume Calculation .............. 2-12

2-12  Discharge - Duration Curve ............................. 2-12

3-1  Detention Basin Schematic .............................. 3-1

3-2          Detention Basin W/ Extended Release Schematic ............. 3-1

3-3   Detention Basin Outlet Structure .......................... 3-5

3-4   Schematic Infiltration Basin ............................. 3-10

3-5 Total Project Size Versus Facility Cost .................... 3-16

3-6          Construction Cost Versus Stormwater to be Stored ......... 3-16


                                    v







                          UST OF FIGURES (cont.)



Figure                   Title                                       Followina naae

4-1  Retention Basin Schematic .............................. 4-1

4-2   Retention Basin Outlet Structure .......................... 4-5

4-3          Total Project Size Versus Facility Cost .................... 4-14

4-4          Construction Cost Versus Stormwater to be Stored ........... 4-14

5-1    Design Parameters for Wetland Basins ..................... 5-3

5-2  Target Functions ..................................... 5-3

5-3    Wetland Communities and Water Balance ................... 5-5

6-1          Maintenance Cost Versus Facility Area For Detention Basins ..... 6-17

6-2          Maintenance Cost Versus Facility Area For Retention Basins ..... 6-17


























                                   i







1.0   INTRODUCTION

1.1   PURPOSE AND SCOPE

      The Hampton Roads Planning District Commission, on behalf of its member local
      governments has produced a Stormwater Management Strategy for managing and
      financing programs. The strategies were developed to assist the communities in
      preparing Stormwater Management Programs consistent with the USEPA NPDES
      Stormwater Program, the Chesapeake Bay Preservation Act, the State Erosion and
      Sediment Control Program, and the State Stormwater Management Regulations.
      To further assist, HRPDC has prepared two documents for guiding the design of
      on-site and regional best management practice (BMPs) stormwater facilities. This
      document addresses retention and detention basins for use as both local and
      regional BMPs.

      The purpose of this document is to provide general design guidelines. The focus
      is on selecting a design that will satisfy the Chesapeake Bay Preservation Act
      performance criteria, provide stormwater  management  consistent with the
      Stormwater Management Regulations, and be cost effective in construction and
      maintenance costs.

      Prior to the 1960's, the focus in stormwater management was on drainage and
      flood control. Getting the water off the streets and out of the yards was the
      primary concern. Little attention was paid to the consequences downstream within
      the developing area or to impacts on developed neighborhoods. Flood control
      was a problem relegated to the Government and to agencies such as the Corps
      of Engineers, Bureau of Reclamation and Soil Conservation Service. In the 1960's,
      increasing urbanization was recognized as a serious contributor to local flooding
      problems. These problems included taxing the capacity of the local streams,
      causing increases in flood stages downstream, erosion of channel banks, and
      causing peak flow problems at wastewater plants through infiltration and inflow into
      the sanitary sewers. The courts, in general, held little hope for the suffering
      downstream property owner in the eastern part of the United States. Figure 1-1
      pictures the problems associated with urbanization.

      To solve these problems, many turned to the concept of detention and retention
      basins. They had been used successfully on a large scale at the Miami
      Conservancy District in Ohio, the Tennessee Valley Authority, and were promoted
      for rural areas by the Soil Conservation Service. In those areas which started to
      use retention and detention, the initial concept was to store water to reduce peak
      storm flows to some manageable rate of flow.
























                     C  -  E-  -t                                       m - - -   - L                               m OAD

                                   LOW RUNOFF INFILTRATION          HIGH RUNOFF


   STREAM IN NATURAL FLOOD PLAIN LAKES    WOODED HILLSIDES          INTENSE URBAN DEVELOPMENT-PAVED SURFACES



            NATURAL AND CONTROLLED RUNOFF                                     ACCELERATED RUNOFF









                                                     FIGURE 1-1
                           URBANIZATION ACCELERATES STORMWATER RUNOFF
HnAPTON ROADS    (Source.- Ohio Stormwater Control Guidebook, Ohio Department of Natural Resources, 1980. p.l1)     CONSULTANTS






I            ~~~~Many political subdivisions and localitie's passed laws and wrote regulations
             requiring a reduction in flow rates to a pre-development level. A result of this was
             the proliferation of basins with no regard for the hydraulics of the watershed or the
I          ~ ~~potential compounding effect of the reduced but elongated outflow hydrographs
             as they are routed downstream. Further, another more basic question of who was
             going to maintain these basins when the developer moves on to his next project
I          ~ ~~was not always considered. These practices have shown that watershed planning
             is a critical element in locating regional facilities, and the ownership and
             maintenance of the basin needs to be addressed in the initial planning.
             The passage of the Clean Water Act, Public Law 92-500, in 1972 by Congress
             showed the nation's concern with pollution and water quality issues. Several
I          ~ ~~~studies resulting from that Act such as the Nationwide Urban Runoff Program, 208
             Water Quality programs, and the Urban Studies program pointed to non-point
             source runoff as a major contributor to the total water quality picture. Many
I          ~ ~~~investigations pointed to the benefits of detention storage and the settling that
             takes place as a positive attribute of detention and retention basins. More recently,
             the incorporation of wetland features have proved to be another added benefit of
             pollution reduction and habitat preservation. Further, landscape architects and
             urban planners have incorporated these basins into aesthetic and recreational
             features of developments.

             As a result, the design of a retention or detention basin ought to consider the
             advantages of a multi-objective facility. Research is ongoing and experiences, both
             successes and failures, are increasing our knowledge and advancing the state-of-
             the-art of detention and retention basin design. Consequently, it is expected that
3           ~~~this manual will be a dynamic document.  Changes will be made as experience
             dictates.

       1.2   GENERAL PLANNING CONSIDERATIONS

             Regional detention or retention basins control stormwater runoff from large areas.
             When best management practices for reducing non-point source pollution are
             added, the facility becomes a Regional Best Management Practice (BMP) or
             Stormwater Management Facility (SWMVF). In this document, detention or retention
3            ~~~~basins designed to service a large area or a regional area will be referred to as a
             Regional Stormwater Management Facility. The volume of water is typically greater
             than that which can be handled by on-site facilities. The watershed area is typically
I          ~ ~~~measured in acres and even square miles. There is no universal definition for a
             regional SWMVF, but in general, a regional SWMVF would control runoff peaks and
             enhance water quality from a watershed large enough such that a detailed
I          ~ ~~engineering study would be necessary to evaluate the hydraulic impact on
             downstream areas. The design usually incorporates multiple objectives of flood
             control, water quality enhancement, aesthetics, recreation, wildlife habitat
             protection, habitat management, and groundwater recharge and protection. Multi-
             objective planning is a change from the traditional approach. Not only does it

        U                                     ~~~~~~~~~~~~~1-2







             satisfy several objectives, but it enhances the neighborhood's and community's
             attractiveness and character. Figure 1.2 shows the concept of multi-objective
             stormwater management planning.

             The location and design of regional SWMFs need to be the subject of a watershed
             management plan. Location within a watershed and release rates need to
I           ~ ~~~consider the routing of flows so as not to accumulate rates of flow above those
             which cause erosion and flooding. A Regional Stormwater Management facility
             should not only protect against problems associated with further development but
I          ~ ~~should be designed whenever possible to reduce problems caused by prior
             development.    CBLAD  notes that regional facilities must  protect against
             downstream flooding and erosion regardless of "normal" conditions. Regional
I          ~ ~~~SWMFs also require more land, so it is incumbent upon the planner to identify
             future sites which can be protected or purchased. The selection of the design
             features to incorporate in the BMP depends upon the need for water quality
             enhancement, peak discharge reduction requirements, and other criteria of the
             multiple objectives selected.

I            ~~~The operation and maintenance issues of several smaller retention-detention
             facilities versus a larger regional facility needs attention in the selection process.
             This particular criteria is very important because neither the Virginia Stormwater
I          ~ ~~Management Regulations or the Chesapeake Bay Preservation Act speaks to the
              resolution of ownership and  maintenance  responsibility.   In general, both
              regulations require a commitment that maintenance be addressed, but do not
              require that the property owner or locality take on the responsibility.

              Section 3.8.8 of the Virginia Stormwater Management Regulations call for six
I          ~ ~~~planning steps as a minimum which are as follows:

                    1.    "Consideration  of the  locality's comprehensive  plan,  zoning,
                           government facility plans and similar planning tools."

  *                ~~~~~2.    "An analysis of the impacts of development on the watershed based
                           on hydrologic and hydraulic modeling. At a minimum, the 2-year,
                           10-year,  and  100-year  storms  shall  be  studied.    Ultimate
   *                      ~~~~~~~development of the watershed shall be assumed."

                    3.    "Recommendations  for locations, specified release rates,  and
    I                    ~ ~~~~~required  storage  capacities  of  needed  regional  stormwater
                           management facilities based on the modeling."

                    4.    "Consideration  of  future  expansion  of  regional  stormwater
                           management facilities based on the possibility that development
    *                     ~~~~~~~might exceed the anticipated level."

                    5.    "Requirements for necessary onsite stormwater managementfacilities

         I                                     ~~~~~~~~~~~~~1-3













        I                                                           ~~~~~~~~~~~~~~~~~~~~GROUNDWATER
                                                                             RECHARGE







                                                          HABITAT          HABITAT
                                                          PROTECTION       PROTECTION






                                       AESTHETICS      AESTHETICS       AESTHETICS

                                       RECREATION    RECEATION          RECREATION
        I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I










                    WATER           WATER            WATER            WATER
                    QUALITY         QUALITY          QUALITY          QUALITY
                    ENHANCEMENT     LZEMENT    [ZEMENT                ENHANCEMENT





 DRAINAGE &      DRAINAGE &       DRAINAGE &       DRAINAGE &       DRAINAGE &
 FLOOD           FLOOD            FLOOD            FLOOD            FLOOD
 CONTROL         CONTROL          CONTROL          CONTROL          CONTROL





TRADITIONAL                                                        NEW APPROACH
SINGLE PURPOSE                                                     MULTI-OBJECTIVE








                                       FIGURE 1-2
 ---~_                   MULTI-OBJECTIVE PLANNING
liAMPTrON TUO2ADS                                                            URS
HAM&NONI~ DCr COADS~  .(Source- Clinton River Watershed Planning Guidelines)  CONSULTANTS







                   and release rates."

             6.     "An implementation schedule and financing requirements."

      Within much of the Hampton Roads area, the flat topography limits the area that
      can be effectively drained to a regional SWMF. The high groundwater level and
      poor soils limit the use of infiltration type systems and also reduce the volume of
      storage available in basins. The high groundwater level will also impact the design
      of retrofitting basins in order to create a shallow pond for enhancing water quality
      improvement.

1.3    REGULATIONS REGARDING BMPs

      Within the Hampton Roads area, stormwater is regulated by the Commonwealth
      primarily under the Stormwater Management Regulations (VR 215-02-00), the
      Chesapeake Bay Preservation Area Regulation (VR 173-02-01) and the Virginia
      Erosion and Sediment Control Regulations (VR 625-02-00). The Stormwater
      Management Regulations result from Article 1.1 of Chapter 6 of Title 10.1. The
      Chesapeake Bay Act is Chapter 21 of Title 10.1, and the Virginia Erosion and
      Sediment Control Law is found in Title 10.1 Chapter 5 Article 4 of the Code of
      Virginia. In addition to these regulations, the Dam Safety Act may become
      important if the impoundment exceeds 50 acre feet and the dam is over 25 feet
      high. The USEPA Stormwater NPDES regulations will have an impact. Other
      regulations may include state and federal wetlands permits, 401 Water Quality
      Certification and State Water Control Board Regulations. Pending regulations of
      the Coastal Zone Management Act Reauthorization - Coastal Non-point Program
      may have a future impact.

      It has been recognized that these laws, regulations, and permits overlap, and the
      staffs of the state departments are working to consolidate the requirements to
      reduce any conflicts and to develop a checklist for submittals.

      A brief summary is included to provide a general description of the key regulations.

1.3.1  STORMWATER MANAGEMENT ACT AND REGULATIONS

      The purpose of the Virginia Stormwater Management Act, enacted in 1989, is to
      enable localities to inhibit the deterioration of existing water quality and maintain
      runoff at pre-development characteristics as nearly as practical.

      The Stormwater Management Regulations are applicable to those localities that
      establish a local stormwater management program and every state agency that
      disturbs land and soil. There are several exemptions including single family homes
      separately built, and land development projects that disturb less than one acre of
      land.


                                        1-4







I           ~~~The Regulations establish technical criteria for new development which local
             programs must meet as a minimum, encourages watershed planning, sets up
             administrative procedures, and establishes maintenance as an important feature
             that needs to be considered.
5           ~~~~Specific Technical Criteria of importance to retention - detention facilities are'

               Storage and Outlet Discharge - If retention or detention is used solely or in
               combination with other stormwater management practices, the end result is that
               the post development rates from the 2 year and 10 year storm do not exceed
               the pre-development rate, as nearly as practical.

1             ~~~~Water Quality Volume - Water Quality Volume is defined as the first 0.5 inches
               of runoff per acre of the land development project, which is the area subject to
I             ~~~~manmade changes. In a detention basin, this volume needs to be detained and
               released over thirty hours from the time of peak storage. In a retention basin,
               the permanent pool needs to be three times the water quality volume as a
5             ~~~minimum.

               Design Storm - Using Soil Conservation Service (SOS) methodology, the 24-hour
               rainfall distribution recommended by SCS is used. When using other methods,
               the rainfall intensity curves for the appropriate return interval are to be used with
               the duration of the design rainfall intensity occurring over a period equal to the
5             ~~~~time of concentration.  Other durations need to be checked for maximum
               volumes.

5           ~~~The Regulations also encourage watershed planning and require that permanent
             arrangements satisfactory to the approving agency be prepared for operation and
*           ~~~maintenance.

       1.3.2 CHESAPEAKE BAY PRESERVATION ACT AND REGULATIONS

I           ~~~The Chesapeake Bay Preservation Act was enacted in 1988 to protect water
             quality in the Chesapeake Bay. The Act and Regulations establish Land Use and
             Development Performance criteria to reduce the contribution of nonpoint source
             runoff being transported to the Chesapeake Bay by stormwater runoff. The
             objectives of the criteria (Section 4.1) are to:

 I                ~~~~~a.    "Prevent a net increase in nonpoint source pollution from new
                          development."

 I                ~~~~~b.    "Achieve a 10%  reduction in nonpoint source  pollution from
                          redevelopment."

 I                ~~~~~C.    "Achieve a 40%  reduction in nonpoint source  pollution from
                          agricultural and silvicultural uses."

        I                                     ~~~~~~~~~~~~~1-5







I           ~~~~The Performance Criteria include several elements. Among them are the following:

                  * "Where the Best Management Practices utilized require regular or periodic
                    maintenance in order to continue their functions, such maintenance shall be
                    ensured by the local government through a maintenance agreement with
 *                ~~~~~the owner or developer or some mechanism that achieves an equivalent
                    objective." (Section 4.2.3)

                  * "Stormwater  management  criteria which  accomplish  the  goals  and
                    objectives of these regulations shall apply. For development, the post-
                    development nonpoint source pollution runoff load shall not exceed the pre-
                    development load based upon average land cover conditions.
                    Redevelopment of any site not currently served by water quality best
                    management practices shall achieve at least a 10% reduction of nonpoint
                    source pollution in runoff compared to the existing runoff load from the site.
                    Post-development runoff from any site to be redeveloped that is currently
                    served by water quality best management practices shall not exceed the
                    existing load of nonpoint source pollution in surface runoff." (Section 4.2.8)

                  * Performance is measured by use of the "Keystone Pollutant" concept. The
                    concept simplifies the computations of loads and is generally accepted as
                    being a practical and realistic indicator of total nonpoint source pollution
                    loads. Total phosphorus has been selected as the keystone pollutant in
 3                ~~~~~~Virginia.

                  * A manual has been prepared which includes substantial detail on the
                    application of the regulations and on calculating the performance of BMPs
                    to determine compliance with The Chesapeake Bay Preservation Act. The
                    Guidance Calculation Procedure is included as Appendix B. The entire
  I                ~~~~manual  entitled Local Assistance  Manual  (November  1989) can  be
                    purchased from the Chesapeake Bay Local Assistance Department
                    (CBLAD). It has been provided to all affected localities by CBLAD.

       1.3.3 VIRGINIA EROSION AND SEDIMENT CONTROL LAW AND REGULATION

3           ~~~~The Erosion and Sediment Control Law, enacted in 1973, is established to control
             soil erosion, sediment deposition, and non-agricultural runoff. Minimum standards
             have been established by the regulations that require techniques and methods to
I          ~ ~~be employed to meet the criteria. The sections of the Regulations which pertain
             directly to retention-detention stormwater management facilities are:

 I              *~~~~ Sediment basins and other measures intended to trap sediment shall be
                    constructed as a first step. (Section 1.5.4)

 I              *~~~~ Surface runoff from three or more acres, which runs across disturbed areas
                    of 10,000 square feet or more, shall be controlled by a sediment basin.

         I                                     ~~~~~~~~~~~~~1-6







   1             .  ~~~~Downstream waterways shall be protected from damage due to increases
                      in volume, velocity and peak flows from two- and ten-year frequency storm
   *                 ~~~~~~events.

                    . A plan for maintenance needs to be approved.

   1                 ~~~~~A handbook providing guidance has been prepared and is available from
                      the Division of Soil and Water Conservation, Department of Conservation
                      and Recreation. The most recent edition is the second edition, dated 1980.
                      This handbook is currently being revised.
3       ~~1.3.4 VIRGINIA DAM SAFETY ACT AND REGULATIONS

               This Act and Regulations provide for the safe design, construction, operation and
               maintenance of impounding structures to protect public safety. The Regulations
               establish specific design criteria. The Act and Regulations include all dams which
               are equal to or greater than 25 feet high as measured as a vertical distance from
               the natural bed of the stream at the downstream end to the top of the impounding
               structure and which create a maximum impoundment equal to or greater than fifty
               acre feet. The top of the impounding structure is defined as the lowest point of the
               non-overflow section of the impounding structure.

         1.3.5 USEPA STORMWATER NPDES PERMIT

 B            ~~~Regulating stormwater has been a controversial subject since the 1972 Clean
               Water Act. When the Act was reauthorized in 1987 by the Water Quality Act of
               1987, provisions were included to govern stormwater discharge through a phased
               approach to establish permits for stormwater discharges. The final regulations
               were published on November 16, 1990, for the NPDES permit application
  3           ~~~~requirement. The Regulations require that pollutants in stormwater discharges, for
               both existing systems and new systems associated with development, be reduced
               to the "maximum extent practicable."

               The permit is applied for in two parts - PART I and PART II. In PART I, the section
               Source Identification is intended to identify possible sources of pollutants to the
  I          ~ ~~~separate storm sewer system and to identify possible locations for treatment based
               controls. In this section any retention or detention basin needs to be identified so
               that it can be studied for retrofit for use as a BMP. Also public lands need to be
               identified for new structures.
               Also in PART I is a section on Discharge Characterization for the purpose of
  I          ~ ~~~identifying existing short- and long-term water quality problems in stormwater
               discharges. The PART I section on Management Plans requires identification of
               existing structural and non-structural programs to control pollution from
               stormwater.   The  description  shall provide  information on  operation  and
               maintenance as well.


          1                                      ~~~~~~~~~~~~~1-7







      PART II of the application is submitted after PART I and goes into more detail. It
      requires submission of a proposed management plan describing how the applicant
      proposes to improve the water quality of its stormwater runoff. Examples of these
      programs include:

            Stormwater Regulations
            Erosion and Sediment Control Regulations
            Clean Water Connections to Sanitary Sewers
            Floodplain Management
            On-site Stormwater Control
            Stormwater Management Plans
            Non-Structural Controls
            Public Education Programs

1.4   CURRENT PRACTICES BY JURISDICTIONS

                                   TABLE 1-1

                    CURRENT PRACTICES BY JURISDICTIONS




 Chesapeake                                            in place
 Franklin                  Follows State               not applicable
 Hampton                                              in place
 Isle of Wight County                                  in place
 James City County                                     in place
 Newport News                                          in place
 Norfolk                                               in place
 Poquoson                                              in place
 Portsmouth                                            in place
 Smithfield                                           in place
 Southampton County                                    not applicable
 Suffolk                                               in place
 Virginia Beach             Follows State              in place
 Williamsburg                                          in place
 Windsor                                               in place (2/92)
 York County                                           in place

                                       1-8







 1.5   DEFINITIONS

      The Virginia Department of Conservation and Recreation, Division of Soil and Water
      Conservation has defined many elements of a stormwater management program.
      For the sake of consistency this manual will use many of the same definitions but
      will make modifications for clarity rather than substantial change.

      The definitions are applicable in a general sense. Each locality may have different
      regulations, and it must be realized that the design of these facilities needs to be
      in accordance with that specific locality's Stormwater Management Program.

      Reaional BMP: a facility or practice designed to function as a best management
      practice (BMP) for an area ultimately encompassing more than one property
      owner. These facilities are designed to control water from a large contributing
      area, although only portions of the watershed may experience land development.
      These facilities are best planned as a result of a regional or watershed-based
      stormwater management plan.

      Reaional stormwater management plan or reaional plan: means a document
      containing material describing how runoff from open space, existing development
      and future planned development areas within a watershed will be controlled by
      coordinated design and implementation of regional stormwater management
      facilities.

      Stormwater manaaement facilitv: means a device that controls stormwater runoff
      and changes the characteristics of that runoff including, but not limited to, the
      quantity and quality, the period of release or the velocity of flow.

      Stormwater management plan or plan: means a document containing material for
      describing how existing runoff characteristics will be maintained by a land
      development project and will comply with the requirements of the local program
      or the State regulations.

      Retention Basin: a stormwater management facility comprised of: a) a permanent
      pool of water which loses water primarily through infiltration and evaporation which
      may be increased in volume to enhance water quality; and b) additional capacity
      above the permanent pool for the storage of stormwater runoff. The facility
      discharges to the downstream conveyance system through an outlet structure
      designed to both release the runoff over a specified period of time and to maintain
      the permanent pool at a minimum level. These facilities are also called wet ponds
      or wet detention basins and can be used for both stormwater quantity and quality
      control.

      Detention Basin: a stormwater management facility which temporarily stores
      runoff, with discharge to the downstream conveyance system through an outlet
      structure designed to completely empty the facility over a short time period,

                                         1-9







      typically six hours or less. These facilities, also called dry ponds, are used
      primarily to control runoff quantity, however, some water quality enhancement
      occurs through sedimentation.

      Extended release: the use of a modified outlet structure in either a detention or
      retention basin to extend the stormwater runoff storage time beyond that typically
      used for quantity control and achieve water quality enhancement through nonpoint
      source pollutant removal.

      Wetlands Bottom:  the establishment of a wetland or shallow marsh area in
      detention or retention basins to enhance the removal of soluble pollutants,
      enhance sediment trapping, reduce sediment resuspension, and conceal trash and
      debris.

      Water Qualitv Storaae: the storage in the permanent pool of a retention basin to
      meet the regulatory requirements.

      Infiltration facilitv: means a stormwater management facility which temporarily
      impounds runoff and discharges it via infiltration through the surrounding soil.
      While an infiltration facility may also be equipped with an outlet structure to
      discharge impounded runoff, such discharge is normally reserved for overflow and
      other emergency conditions. Since an infiltration facility impounds runoff only
      temporarily, it is normally dry during non-rainfall periods.

1.6 HYDROLOGY

      The relationship between rainfall and runoff is modified by development, land use
      changes, and urbanization. The rainfall values for a design storm event and the
      intensity-duration-frequency curves for specific recurrence intervals are for all
      practical purposes constant.

      The runoff resulting from similar rainfall events will be modified by antecedent
      rainfall, time of the year, changes in land use and the amount of paved surfaces.
      If we look at the impact of paved surfaces in Figure 1.3, it is evident that as the
      paved area increases, runoff increases and the infiltration decreases. There are
      several variables in determining runoff from a rainfall event. Antecedent rainfall will
      fill up depression storage and reduce infiltration and evapo-transpiration. The time
      of year may cause conditions that will increase runoff such as frozen ground, ice
      cover, or snow. Snow melt is not a major problem in Hampton Roads, but it does
      have an impact when it occurs.   Construction activity which changes the
      topography, removes natural swales and depressions, removes tree and heavy
      forest cover, changes slopes, and introduces drainage systems has an impact by
      decreasing the amount of pervious area and by reducing the time of concentration
      in the watershed, both causing an increase in the peak flow.

      There are a number of methods that can be used to compute runoff from the

                                         1-10











I~~~~~~~~,-%

   I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~LL

   I~~~~~~~






                                *~~~~~~~~~~~~~~~~~~~~F 5 


                           0


                                                                         ........... ~-j
                                                                                     -j

                                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.   .....

I~~~~~~~~~~~0             .  ..           .........

              *~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. ..... ...~....




             NATURAL       20% PAVED      50% PAVED      90% PAVED
         GROUND COVER







      I                        ~~~~~~~~FIGURE 1-3
                                     RUN-OFF FLOWS
 H   HAMPTON    RESULTING FROM INCREASED PAVED SURFACEUR
             ~~~~ ~~(Source- Leopold-Hydrology for Urban Land Planning USGS 1988)  CONSULTM1Sn







I           ~~~~Ill. The Tidewater Virginia area falls within the generalized Type 1I storm, except
              for Virginia Beach which is influenced by coastal events and falls within the Type
              Ill category. The difference between Type II and Type Ill is minor, and Virginia
I          ~ ~~~Beach has elected to use the Type II storm for the purpose of standardization.
              Figure 2-8 shows the SOS Type II storm plotted as a mass diagram where the
              rainfall depth is plotted against twenty-four hours. As with Figure 2-7, the intensity
              over any duration of the SCS design storm can be found by the slope of an
              average line drawn between two points describing the duration around the
              steepest slope which occurs for this synthesized storm at about 11:30. For
              example, the intensity for a one hour duration storm event would be found by
              measuring the depth of rainfall occurring between 1 1:00 and 12:00 or one hour.
              That depth would be the inches per hour of intensity for a one hour duration and
              correlates closely to the intensity for a one hour duration storm of the same
              recurrence interval from Figure 2-1 and 2-2.

1           ~~~~This concept is important to understand because the intensity rainfall value used
              in the Rational method is the duration value which is selected by computing the
              time of concentration of runoff in the watershed at the place where the runoff value
              is being computed. When using the Rational Method in the Hampton Roads area,
              the intensity value is taken from Figure 2-5 or 2-6.

              Several studies have been done to locate the period of most intense rainfall during
              storm events to make the design storm more closely resemble an actual event.
              Studies in Chicago, Cleveland, Boston and New York show a consistence when
              the most intense part of rainfall event usually occurs in the 30-40% time frame of
              the rainfall distribution.   However, the SOS Type II storm follows a more
              standardized bell-shaped distribution around the twelfth hour. Since the intensity-
              duration values do not change regardless of when the most intense rainfall occurs,
              no attempt was made to skew the design storm, shown on Figure 2-7, for the
              Tidewater Virginia area.
              Figures 2-5 and 2-6 show rainfall data for a site specific rainfall gage or station, that
              being the National Weather Service station at Norfolk International Airport. It has
U           ~~~~long been recognized that rainfall is variable over a wide area. As the storm front
              moves, the intensity of rainfall moves. Consequently, the distribution of an equal
I          ~ ~~~intensity of rainfall occurring may or may not be widespread over an area. The
              variation of the depth of rainfall and intensity over the area is referred to as areal
              distribution. To account for the fact that rainfall may not be occurring over the
I          ~ ~~~entire watershed at an equal intensity, Figure 2-9 provides a factor which modifies
              the intensity based upon watershed size. Figure 2-9 is derived from a variety of
              sources dating back to the early work of F.A. Marston. This factor is simply
I          ~ ~~~multiplied times the site specific intensity value for the selected duration, and then
              used as the intensity value in the rational method. It should be recognized that the
              Rational Method should not be used for watersheds over 200 acres.



        I                                     ~~~~~~~~~~~~~2-8







U           ~~~precipitation data. Many variables are based upon assumptions by the designer
             or provided by historic data for the specific site under consideration or which can
             be transferred from similar sites. The methods in general practice include the Soil
I          ~ ~~Conservation Service's TR20 and TR55, the Rational method and variations, and
             a wide variety of proprietary software programs. Also, the runoff block of the EPA
             SWMM model is gaining increased usage.

             The output of these computations will produce hydrographs of the runoff resulting
             from a rainfall event under a specific set of circumstances. The first set of
I          ~ ~~circumstances defines the pre-development conditions. The second set defines
             the post-development condition based  on full development.   Intermediate
             circumstances may be important if the development time is going to be lengthy.

       1.7   OUTLET AND CHANNEL HYDRAULICS

I           ~~~With the hydrographs developed for pre- and post-development runoff, the type
             of outlet structure needs to be selected to reduce the discharge flows to the
             selected value whether it be to maintain a discharge value equal to or less than
             pre-development flow rates or some lesser value necessary to prevent downstream
             problems with accumulations due to routing, because of channel limits, or to meet
             a water quality limitation. There may be cases where the post development
             discharge may not need to be reduced as low as the pre-development discharge
             or where it may not be desirable to have a long extended release period. To
             evaluate the overall impact of the facility under design, a basin or watershed model
             needs to be used. The purpose of the watershed model is to determine the
             impact of reduced flows over a longer period of time on discharges from other
*           ~~~~facilities existing or planned and the other uncontrolled areas of the watershed.

             A retention or detention basin reduces the peak flow by storing the water and
             releasing it at a lower rate for a longer period of time. The actual volume of water
             is nearly the same with some additional losses due to infiltration and evaporation
             at the basin. The exception, of course, are those basins designed with no outlet
3           ~~~~and the only release is through infiltration and evaporation. The analytic technique
             to move the hydrograph downstream is flood routing. In addition to the outlet
             structure, the basin will need an emergency spillway to prevent the dam or
I          ~ ~~embankment from being overtopped and susceptible to being washed out causing
             serious flooding problems. In the streams or rivers, water surface profiles may
              need to be computed to determine bank full capacity, to establish flooding limits
I          ~ ~~~for specific frequencies of flood events, and to obtain velocity data for erosion
             control practices.

*           ~~~~The basin and outlet should be configured to control a range of rainfall events so
             the overall effectiveness can be increased. It has been found from other studies
             that an outlet structure designed to provide control from the 2- and 1 0-year rainfall
             events is sufficient to provide control from other recurrence frequencies as well.
              However, local standards, particularly those related to the Federal Emergency

         1                                     ~~~~~~~~~~~~~1-11







Management Agency, National Flood Insurance program, generally require the
specific analysis and outlet sizing for other storm frequencies, such as the 100-year
storm.













































                                  1-12







      2.0   GENERAL PLANNING AND ENGINEERING CONSIDERATIONS

             This section contains general planning and engineering data and methods which
             are common to any of the stormwater management practices or facilities. The
             planning consideration are considered under Stormwater Management Planning.
             The engineering considerations are considered under the Hydrology and Water
             Quality Enhancement Sections. The fourth section entitled "Retrofitting" covers the
             issues which are similar to either retention or detention basins.

             The planning discussed in this section is focused on stormwater management
             facility design. It must be recognized that the comprehensive planning of land use
             and zoning is also an integral part of total stormwater management.   The
             appropriate use of land development methods, thoroughfare plans, landscaping,
             open space requirements, cluster development and other comprehensive planning
             methods and tools need to be part of the total program to reduce non-point
             source pollution loads and mitigate drainage and erosion problems.
      2.1    STORMWATER MANAGEMENT PLANNING

             Stormwater Management Planning is done at several levels within the watershed.
             Starting at the single property site, the planning typically is done by the developers
             and includes on-site facilities which may incorporate small retention or detention
             basins, grass swales, infiltration basins or trenches, underground storage and
             such. These facilities are constructed and generally later operated and maintained
             by the owner with periodic inspections being performed by the locality. Generally,
             open on-site storage facilities are incorporated into the landscaping and have only
             a specific site benefit. The next level involves multiple sites which may be done by
             the developer and considers a small watershed system. Typically, stormwater
             management practices or facilities at this level are going to be retention or
             detention facilities. At this level of planning more than one property owner is
             usually involved and an agreement must be reached regarding construction and
             maintenance.

             As the size of the system and the number of properties increases, the next level
             is approached. From the standpoint of a community, this level of planning may be
I          ~ ~~limited by political boundaries;  consequently the level of planning  is only
             community-wide. Beyond the community boundary limitations, the stormwater
             planning is regional or watershed-wide. In the watershed plan, there may be
I          ~ ~~~several retention or detention basins which have been planned or constructed as
             well as other on-site structural BMVPs and non-structural methods being used as
             established by local regulations.

             It has been recognized by many planners and engineers that the use of retention
             and detention basins to serve multiple sites is more efficient and cost effective than
             several on-site basins serving individual properties. It has also been recognized
             that the proliferation of basins~ without watershed or regional planning causes

                                               2-1







 I           ~~~problems just as much as development without any retention or detention basins
              at all.

I      ~2.1.1  ELEMENTS OF THE PROGRAM

I      ~2.1.1.1  MULTI-OBJECTIVE PLANNING

              As discussed in the introduction, there are multiple benefits which can be derived
              from retention-detention basins by advanced planning considerations. For the
              Chesapeake Bay Preservation Areas, the primary objective is water quality
              protection, whereas in areas regulated by the Stormwater Management Act, the
              primary objectives are peak flow control and water quality enhancement. A
              retention or detention facility can satisfy the requirements for both of these Acts
              plus requirements for erosion and sediment control.   With added features
              incorporating plantings, water quality can be further enhanced, and a wetlands
              area can be provided for habitat management. The addition of a permanent pool
              further increases pollutant removal and adds an aesthetic value and possible
              recreation, especially fishing.

              By staring water over a longer period of time than it would be traveling in an open
              channel, there may be additional infiltration into the groundwater. Typically, this
              is an advantage; however, care must be taken not to cause an increase in
              groundwater contamination.

              With proper planning, the land surrounding and within the dry areas of the basin
              can be utilized for other activities such as active recreation, soccer, baseball,
              softball, football, badminton, volleyball, or passive recreation activities such as
              picnicking and bird watching.

 *            ~~~~Multi-objective planning can add many benefits to the community and be a useful
              tool in quelling the negative comments raised by the local public. Proper design
              and maintenance will eliminate the complaints often voiced by the public about
 I          ~ ~~~retention-detention basins, such as child safety, mosquito breeding, aesthetic and
              health concerns. They can in fact reverse the priority of the objectives in the
              public's viewpoint of these basins from stormwater management to recreation.

        2.1.1.2 OTHER STRUCTURAL ELEMENTS

 I           ~~~~Retention and detention basins are not the only structural elements that can be
              part of the plan. The use of grass swales, open ditches, infiltration basins, parking
              lot storage, roof storage and in-line storage are among the myriad of methods that
 I          ~ ~~can be used.  Many are discussed in the companion document.  Retention and
              detention basins can be local in nature, that is, on a single site to satisfy the
              stormwater management for that site or parcel. They also may be part of a system
              or regional facility serving many sites and larger areas. The location of the basins
              needs to be considered in a system or watershed planning approach because the

          U                                      ~~~~~~~~~~~~~2-2







 I           ~~~~hydraulics of the watershed can compound flows or increase the length of time of
              a higher stage flow, thereby increasing flooding and erosion problems. Secondly,
              in many areas, there are not many suitable sites that are available for a retention
 I          ~ ~~or detention basin.  For those watersheds, the sites need to be identified and
              protected from other uses.

I      ~2.1.1.3  NON-STRUCTURAL ELEMENTS

              As retention-detention basins are not the only structural solution, structural
              solutions in turn are not the only element in stormwater management planning.
              The use of non-structural elements can provide a significant impact on reducing
              flows and pollution problems and offer the major advantage of not being as
 I          ~ ~~~expensive to build or maintain. As discussed in the introduction to this section,
              comprehensive land use planning and the appropriate zoning has the potential to
              be a very cost effective method in reducing non-point source pollution and
              mitigation drainage and flooding problems.   Non-structural elements include
              restrictive or creative zoning, use of floodplain zoning and management, and
              consideration of building code changes. One of the primary building code
              changes to consider is parking requirements for commercial and industrial
              development. The number of spaces per unit of development can often be
              reduced. The use of isolated portions of the parking lot for temporary storage
              should be considered if it is currently restricted.
        2.1.2 LOCATION OF BASINS

              Several factors are important in locating detention and retention basins. Several
              studies and papers have discussed the problems of random multiple basins
              throughout a watershed. In general, these studies have shown that randomly
              placed basins can control peak flow from large and infrequent storms, but for the
              more frequent storms such as 2-year and less, the impact is only seen immediately
              downstream of the facility, with occasions where the impact causes greater peak
              flows as other sub-areas join and the hydrographs accumulate, thereby
              compounding downstream flooding. This points to the need for watershed master
              planning to locate basins within the watershed. Generally, basins used to control
              peak flow are best placed in the upper reaches of the watershed rather than in the
              downstream areas. However, the shape of the basin, the need to reduce peak
              flows in downstream reaches, and the aggregate effects of lengthened discharge
              hydrograph must be considered. Retention and detention basins are usually used
              on sites with a watershed of ten acres or more. This is not a hard and fast rule,
 H           ~~~~but cost comparisons with other methods usually show other on-site structural or
              BMVP methods to be more cost-effective.

              A specific basin location needs to consider the community. The basin must
              become part of the landscape and be aesthetically pleasant. Topography, soils,
 I          ~ ~~environmentally sensitive areas or areas of cultural importance need to be
              considered. The objective of the basin becomes important. If multi-objective uses

         U                                      ~~~~~~~~~~~~~2-3







 I           ~~~are  being  planned,  then the  opportunities  for recreation,  wildlife habitat
               management, and water quality enhancement need to be considered. The basin
               needs to be structurally safe, the embankment needs proper engineering, and
 I          ~ ~~methods to by-pass rare storms need to be designed into the structure.  The
               design needs to consider protecting people from harm by proper design of side
               slopes and permanent pools. The outlet structures need to be designed to protect
 I          ~ ~~people from getting hurt.  Fences and signs have limited value.  Better design
               features include minimizing the visibility, putting the outlet structure away from
               shore, using trash racks and extending them totally over the outlet are some
               methods. The Task Committee report "Stormwater Detention Outlet Control
               Structures," ASCE, New York, 1985, is a good reference.

I      ~~2.1.3  BMP SELECTION

               Selecting the appropriate regional facility depends upon the objectives and the
 I          ~ ~~~location. Ownership and maintenance should be a consideration, because if there
               is any chance that maintenance will not be performed, it should not be built. This
               fact in itself points heavily to the need for municipal ownership (or easement) and
               maintenance, or at least a method of routine inspection and enforceable
               regulations and ordinances.

 I           ~~~~Table 2-1 compares the multiple objectives discussed earlier with the detention and
               retention basins with the various modifications and provides a relative value of
               meeting the objective opportunities. The opportunities for meeting the objective
               increase as the modifications are added. Certain modifications such as extended
               release and water quality storage may required if the basin is in a locality regulated
               by the Virginia Stormwater Management Act or the Chesapeake Bay Preservation
               Act. A review of Table 2-1 would indicate that the best solution for a multi-purpose
               or multiple objective basin is a retention basin with extended release, the added
               water quality storage, and a wetlands fringe, and in general, it is most likely the
               best solution if the space is available, funds are available, and if in fact all of the
               objectives need to be met. In some areas, there will not be space for a permanent
 I          ~ ~~~pool. In some areas, there may not be the need to consider aesthetics, recreation,
               or habitat protection or management. For example, small retention or detention
               basins may not suitable for these added features. Basins in heavy industrial areas
 I          ~ ~~~or other areas of limited public use need a different evaluation. In order to provide
               data an typical cost and land area requirements, several scenarios were developed
               as part of this work by URS Consultants, Inc. Figure 2-1 shows typical land
 I          ~ ~~~requirements in an area with Hampton Roads' topography for the various types of
               facilities. The size for a retention or detention facility varies from roughly 2% to 5%
               of the watershed area. This is based on basins averaging 6' deep and allows area
 I          ~ ~~~for access roads and a buffer strip. In a multipurpose basin, additional land has
               been allowed for buffer, recreation, and a permanent pool to meet the water quality
               volume requirement. Three different development scenarios were used for Figure
               2-1 of 200, 500, and 1000 acres.

          U                                      ~~~~~~~~~~~~~2-4



-I - -                               --                         -  -                 ----  -














                    40                                                         X




                os
                Lu



                0- 0
                w

                 <20 
                ,.J
                )-J


                    I-






                                 200        400         600         800         1000

                                             TOTAL PROJECT AREA (ACRES)


                                                   FIGURE 2-1
     _o,., .o^Ds                                                                                              URS
     =,.---t..ï¿½.,A.o,       STORMWATER FACILITY AREA VS. TOTAL PROJECT AREA                                   CONSULTANTS







              Cost curves shown for some typical BM Ps in Figures 2-2 to 2-4. compare various
              methods of managing stormwater for a range of watershed or project areas.
              Figure 2-2 is for an infiltration trench, Figure 2-3 is for a typical swale design, and
I           ~ ~~~Figure 2-4 is for an oil-water separator. These were based on a design of a facility
              to control a 10-year runoff from areas with C-factors as shown. The designs were
              based upon typical standards and an assumed lot development of the different
              acreages of 1,3, and 5 acres.
              If the water quality performance criteria of the Chesapeake Bay Preservation
I           ~ ~~~Regulations can be met in other ways, the basin modifications can be simplified.
              The end result should be a cost-effective design to meet the specific objectives at
              that site and the regulatory criteria established.
















        U~~~~~~~~~~~~~~-







                       TABLE 2-1


              SELECTION EVALUATION OBJECTIVES











Detention Basin
     Basic Basin 1 4 4 1  4 4 4
     add extended release 1  3 4 1 4 4 3
     add wetlands bottom 1  3 2 1  3   3 3
R~etention Basin
     Basic Basin                         1  4   3   1 3 3 3
     add extended release 1 3 3 1 3 3 4
     add water quality storage 1 2 2 1 2 2 4
     add wetlands fringe 1 2 1   1   1   1  4



1 - Excellent
2 - Good
3 - Fair
4 - Poor












                          2-6
















                  % AREA. C = 0.60                         -30



                                                  -30

    2 0                                                       -25




     20i
    5r





  o                  In                                        -15 I





                                                  -10
    50G o                   0  _









              I         2           3          4          5
                    TOTAL PROJECT AREA (ACRES)













                        FIGURE 2-2
HAiM .OcO=~As INFILTRATION TRENCH (WITH 20' FILTER STRIP)          CONSULTANTS
      I~~~~~~~~~~~R



-  -_ ---  -                                                       ----    ----










               4,000




                                                                                                0









                                                                                            -4  0
                                 3                                                                       u~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~,





               2,000
                                       ,0.30                                            3'
                                                                                                Lu
                                                                                                I-




                                                                                   QC,



               2,000
                                                                                            I-








                                              2              3                    4
                                            TOTAL PROJECT AREA (ACRES)


                                                 FIGURE 2-3

     ...N     S                            SWALE FOR 2 & 10 YEAR STORiM UCO0.U0A-2





















        80,000








   .J
   0



   o   40,000






        20,000







                             1            2             3             4             8            6
                                                    TOTAL PROJECT AREA (ACRES)
                                                                                                             C = .7 AND HIGHER


                                                           FIGURE 2-4
HAiMPTON RQAOUD        GRIT-OIL SEPARATOR FOR HIGHLY DEVELOPED INDUSTRIAL SITES
                                                 G  SEPARATOR                                                                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~CONSULTANTS







I       ~2.2   HYDROLOGY

                In stormwater management, the aspect of hydrology we are most concerned with
               is the rainfall and subsequent runoff. Rainfall data is available from NOAA, National
               Weather Service and has been statistically synthesized into events of specific return
               intervals. The runoff resulting from precipitation has been measured by using
 I          ~ ~~~gaging stations along major water courses. A variety of methods are available to
               estimate or account for the difference between the total rainfall and that which runs
               off. These methods allow computation of the peak flow and many will generate
 I          ~ ~~data for hydrograph development. Stormwater drainage systems in the past have
               basically dealt with the peak flow; however, with stormwater management
               practices, it is necessary to develop a hydrograph so that the volume of runoff is
               known. With the design of permanent pools in retention basins, we also need data


I       ~~2.2.1  RAINFALL DATA

               The rainfall data is commonly described in an Intensity - Duration - Frequency
 I          ~ ~~~(IDE) curve which is available for site specific rain gages throughout the area from
               the National Weather Service. The Norfolk IDF curve is shown on Figure 2-5. This
               data has been plotted for a 6-hour duration on Figure 2-6. One of the important
               characteristics to recognize in these two curves is that the time value is the
               duration of a rainfall event at an intensity equal to or greater than the number
               derived by selecting an intensity from the frequency curve. The time value has no
               relationship to the time from the beginning of the rainfall event. If a value of
               intensity is read for a duration for a specific frequency, it means that from the
               rainfall data collected over the period of record that specific intensity over that
               duration of time has occurred equal to or greater than that value that frequently.
               Although common practice is to discuss frequency as a yearly event, that is, once
               in five years or once in fifty years, it is statistically more appropriate to use a
               percentage of recurrence interval. A five year storm event has a 20% chance of
               occurring in any one year. A fifty year frequency has a 2% chance of occurring
 *           ~~~~in any one year.

               The National Weather Service has taken the site specific rainfall data and
               generated a variety of data presentations useful for large scale planning, but for
               the purposes of stormwater management planning at this level, Figures 2-5 and 2-6
               provide the needed basic data. Variations of Figure 2-5 include rearranging the
 I          ~ ~~~data to create a design storm where the time scale is time from the beginning of
               the rainfall event. The purpose of this exercise is to obtain values of antecedent
               rainfall and to be able to compute hydrographs that show the impact of long
 I          ~ ~~duration storms and allow volumes of these storms to be computed.  Figure 2-7
               shows a six-hour design storm for Norfolk. In this figure, the rainfall depth has
               been plotted for a six-hour storm for the frequencies of return intervals shown.
 I          ~ ~~~The Soil Conservation Service has developed typical design storms for the United
               States. These storms have been categorized as Type I, Type IA, Type II, and Type

          U                                     ~~~~~~~~~~~~~2-7














   20.0

    15.0 _




    8.0    _ .     

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                                      (MINUTES) DURATIO'NNN  (HORS



                        ~~~~~~~~~~~~~~~~-  35               ue17)CNSUT,
                                   (Sorc- N A A T e c nial emranumNW$ HYDRO                                                 UR








































=Z 3
z




   2







                                                                          10 YR
                                                                          2 YR



                   ! 2                        3             4             5 6
                                     DURATION (HOURS)


                                       FIGURE 2-6

                   INTENSITY DURATION FREQUENCY CURVE-
            s______           NORFOLK,VA (0-6 HOURS)
 HAMPrTON ROADS (Source- NOAA Technical Mememorandum NWSS Hydro-35, June 1977)           CRS
                      Pt*~~~~~~~~NI~~~~~~ OUT1C1 WM.40.4 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ C,~L,,~











  4.00





  3.50






  3.00






,2.50

























  0.00-
  0.50














  0 O0




       ï¿½             1             2              3             4             5             6
                                          DURATION (HOURS)
                                          FIGURE 2-7                                       URS
 *  ___RoFGR2-                                                                          URS
 ~riHAMPTON ROADS2o        DESIGN STORM  MASS  DIAGRAMS                                 CONSULTANTS
                                           NORFOLK, VIRGINIA














        5.00




        4.00



      UJ                                                         /
      o 3.00




        2.00




        1.00





                I   I   I        I  I     I        I   I                 I     I
                    2       4        6        8        10      12       14       16       18       20       22       24

                                                       TIME (HOURS)





 _~~~                                                    FIGURE 2-8
HAMPON R.OADS                   DEPTH VS. 24 HOUR STORM, SCS TYPE II, 10-YEAR                                            URS


















                                                       6 HOUR DURATION









9L  -.



LL

0
w

u.














                                  AREA ACRES (1000)



                                     FIGURE 2-9                                    Ct~~AT
  J~QAPSRATES OF OVERALL RAINFALL TO MAXIMUM POINT RAINFALLUS







1  ~ 2.2.2 HYDROGRAPHS AND PEAK FLOW COMPUTATIONS

              The design of retention and detention facilities requires the development of
              hydrographs to arrive at volumes of water running off the watershed and the peak
              discharge of that runoff. The pre-development hydrograph becomes the measure
              of performance because it defines the peak flows for the two- and ten-year
              frequency storms which may need to be maintained.  The post-development
              hydrograph before the stormwater management practices are put in place defines
              the additional volume of runoff and the increase in peak flow.  The post-
              development hydrograph becomes the inflow hydrograph for the retention -
              detention facility after any non-structural or on-site BMPs have been accounted for.

 ~~I  ~Routing the inflow hydrograph through the retention - detention basin by
              considering the storage elevation curve and the outlet hydraulics will produce the
              outflow hydrograph. The retention - detention basin storage elevation curve is
              governed by the topography of the site and height of the impoundment. This
              curve is developed by plotting the available flood storage against the
              corresponding pool elevation. The storage is determined by measuring the surface
              area flooded at contour elevation intervals and computing the volume between the
              intervals.

              The hydraulics of the outlet or principal spillway control the discharge. Since the
              hydraulic head over the outlet structure is the primary energy source, the pool
              elevation can be related to the hydraulic head over the outlet structure and a
              corresponding outflow-elevation relationship can be developed. The type of outlet
              structure and size will provide the other design data to compute the discharge with
 *I~ ~the given elevation head.

              By selecting the appropriate outlet structure type and size and with sufficient
              storage, the outflow can be controlled to limit the two- and ten-year runoff events
              resulting from development to those that existed prior to development, or to those
              established by watershed/regional SWM plans, thereby satisfying the requirements
              of the Virginia Stormwater Management Regulations and the Erosion and Sediment
              Control Regulations with regard to peak flow control. It should be noted that the
              CBPA regulations require that post-development runoff pollutant loads not exceed
              pre-development runoff pollutant loads.

              There are a number of methods which can be used to compute peak flows and
              to develop hydrographs:

              Rational Method: The Rational Method is widely used for computing peak rates
              of runoff from areas generally less than 150 to 200 acres. Virginia Department of
              Transportation (DOT) allows the Rational Method for watersheds up to 200 acres.
              Others have limited it to watersheds as small as fifty acres, but general agreement
              is found in the 150-200 acre range. The method produces the maximum discharge
              from a given uniform rainfall event when the entire watershed is contributing runoff

                                                2-9






I           ~~~~to the outflow at the point of design.  In order for the entire watershed to
             contribute, the uniform rainfall event must occur for a length of time that it takes
             water to flow from the most remote part of the watershed to the point of design.
I          ~ ~~~This time is called the "time of concentration." The intensity value is selected from
             the Intensity - Duration - Frequency curve for the selected frequency of storm for
*           ~~~~the duration of time which is equal to the time of concentration.
             Table 2-2 shows typical values of 'IC" to be used in the Rational Method for a
             variety of land uses. These factors do not correlate exactly with the percentages
I          ~ ~~of impervious area because they have been increased to account in a general
             fashion for slope, infiltration, and intercepted flow. Table 2-2 was generated from
             a wide variety of sources, and the present impervious data are averages computed
             for a comparison.















        I~~~~~~~~~~~~21







                            TABLE 2-2

      TYPICAL UC" COEFFICIENTS FOR RATIONAL METHOD

                                             Runoff             Percent
Land Use in Area                            Coefficients        Impervious

Business
    Downtown areas                         0.70-0.95              90%
    Neighborhood areas                     0.50-0.70              60%
Shopping Malls
    Regional - enclosed                    0.65-0.85
    Strip Malls                            0.70-0.90
Business Parks                              0.60-0.75
Industrial Parks                            0.65-0.80
Residential
    Single Family Suburbs                  0.30-0.50             20-25%
    Single Family Urban                    0.40-0.60             30-35%
    Multiunit detached                     0.40-0.60
    Multiunit attached                     0.60-0.75              75%
    Planned Unit Development               0.30-0.50             20-30%
    Apartments - Urban                     0.50-0.80              75%
industrial
    Light areas                            0.50-0.80
    Heavy areas                            0.60-0.90
Parks, cemeteries                           0.10-0.25              15%
Playgrounds                                 0.20-0.35              20%
Railroad yard areas                         0.20-0.40
Unimproved areas                            0.10-0.30
Streets
    Asphaltic                              0.70-0.95
    Concrete                               0.80-0.95
    Brick                                  0.70-0.85
Drives and walks                            0.75-0.85
Roof                                        0.75-0.95
Lawns; Sandy Soil:
    Flat, 2%                               0.05-0.10
    Average, 2-7%                          0.10-0.15
    Steep, 7%                              0.15-0.20
Lawns; Heavy Soil:
    Flat, 2%                               0.13-0.17
    Average, 2-7%                          0.18-0.22
    Steep, 7%                              0.25-0.35




                               2-11







I           ~~~~In order to select the appropriate "C" factor, typical percentages of impervious area
             of the entire area should be measured. The placement and contours of pervious
             areas needs to be considered. If the pervious areas drain to low pervious swales
I          ~ ~~~or ditches, the runoff will be contained.  If curbs separate the impervious areas
             from higher pervious islands or median strips, the pervious areas will have less
             impact on reducing runoff. Areas with higher slopes will have a higher runoff "C"
             value. Pervious areas that are grass covered will hold more water from running
             off than bare soil, and the type of soil has an impact. All of these factors need to
             be considered when selecting a "C" factor. For final design, a field visit to the site
             and watershed along with a study of recent aerial photographs is a significant help
             in making the estimate.

U           ~~~~For small watersheds less than 160 acres, the Rational Method has been used to
             develop hydrographs. The two methods most commonly used both make
             assumptions that the hydrograph is triangular. The method developed by A.S.
             Paintel assumes the hydrograph shape has a rising limb equal in time to the time
             of concentration and the recession limb of the hydrograph equal to 1.5 times the
             time of concentration. The second method often used merely assumes the failing
             limb is an image of the rising limb in a triangular fashion. Both of these methods
             need to check volumes of runoff by calculating the impact of longer duration
3           ~~~storms of the same return interval to verify the maximum storage requirements.
             Figure 2-10 illustrates the concepts. When the discharge is computed for
             durations other than the time of concentration for storms of a like return interval
3           ~~~~the simplified hydrograph takes the shapes as shown in Figure 2-1 1.  By
             computing the volume of total runoff for a variety of storms with varying durations
             of the same return interval a storage curve can be plotted which will show the
3           ~~~~maximum storage volume required for that specific site for the selected "C" values
             for the selected rainfall intensity frequency curve or return interval.

3           ~~~~This storage curve can be used to find the required volume for a variety of outfall
             discharges by plotting the discharge - duration curve as shown on Figure 2-12.

1           ~~~~Modified Rational Method: The Virginia Department of Transportation has modified
             The Rational Method by adding a correction factor to account for the influence of
             antecedent rainfall. As discussed earlier, the duration factor in the selection of the
I          ~ ~~~intensity has no relationship to the time from the beginning of rainfall. The design
             storm concept further illustrates this fact and in order to account for reduction in
             infiltration, transpiration, evaporation, and depression storage, VDOT has derived
I          ~ ~~~a correction factor for storms with a frequency of greater than 10 years as shown
             in Table 2-3. This factor is multiplied times the discharge computed using the
3           ~~~~regular Rational Method.






        1                                     ~~~~~~~~~~~~2-12



























              oil
              IL
              I-
              o















                  etL           /  as %--TRIANGULAR METHOD
                             /j       ~%9~~ ~     A.S. PAINTEL METHOD








                              Tc              2.0 TC    2.5 Tc

                                    DURATION (MINUTES)















                              FIGURE 2-10

                     TYPICAL ASSUMED HYDROGRAPHS
          LTONRADS           U    G    T      L      H-URS
                         USING RATIONAL METHOD URS
*~~~~~~~~~~X~ E-rc- rlnCNULAT












    35-



                                                                           * TRIANGULAR METHOD OF HYDROGRAPH ASSUMPTION
    30-

                                              To /   \\



    z25-





  I-
  20-                                                     Tc+20



                    10         20          30         40          50          60          70         80










                                                      DURATION (MINUTES)
  LU
















                                                      DURATION (MINUTES)
                                                       _   FIGURE 2-11
HAMPTON RASPOST-DEVELOPMENT STORAGE VOLUME CALCULATION                                                    URS
                                          USING*W~~~~~~~~~I~~~~~~~)~~  THED~~~~~~$rlc~  RATIONAL    M T O CO S E I G T E I E O CO C T A ON ( T O S D RCONSULTANTS
                USING THE RATIONAL METHOD CONSIDERING THE TIME OF CONCENTRATION (NOT CONSIDERING DISCHARGE)















   40-                                                                               -t          _  




              54~~~~~~~~o0-/
   40-                                      0.- .





 o 30-                 /                 MAXIMUM.
                                         STORAGE
 8                    //                 REQUIRED
 _                   /o




 -,                    /
 o 20-





   so_-    /






                  I          I            I           I           I           I           I           I
                  10         20           30         40           50          60          70          80
                                                DURATION (MINUTES)
                                                  FIGURE 2-12
HAMPoTo.N                               DISCHARGE - DURATION CURVE                                            URS
                                       DISCHARGE - DURATION CURVE                                            CONSULTANTS







                                           TABLE 2-3

                                 MODIFIED RATIONAL METHOD
     I                            ~~~~~~~CORRECTION FACTOR


  I                     ~~~~~~~Frequency or              Correction
                          Recurrence Interval             Factor

  1                      ~~~~~~~~10 years or less        1.00
                          25 years                       1.10
                          50 years                       1.20
                          1 00 years                      1.25


             The Anderson Formula: The VDOT Manual includes a method described as The
             Anderson Formula. This method was developed for use in Northern Virginia and
             because of the localized data analyzed to arrive at the formulation, it should not
             be used in the Hampton Roads area. This method is generally used on
             watersheds greater than 200 acres.

             Snyder Method: The Snyder Method is described in the Virginia Department of
             Transportation as another method of computing peak flows. It is valid throughout
             Virginia, and is generally used on watersheds greater than 200 acres.
             SOS Technical Release 55 and 20: TR55 can be used to compute peak flows,
             generate hydrographs, and perform routing. However, when routing through a
             pond or reservoir the TR55 methodology is an approximation. It is suitable for
3           ~~~planning but not for final design. TR20 should be used for final design.  TR20
             allows the direct input of the outflow hydraulic characteristics from the outflow-
             elevation curve.

             EPA SWMM - Runoff Block: This method may be used to compute stormwater
             runoff. The most recent version allows continuous simulation and also will estimate
             pollution loads. The program is complex and requires experience.

             Other Methods. There are a number of other methods which can be used;
3           ~~~~however, since the staff from localities and the state do not have access to all of
             the proprietary software, it is incumbent upon the designer to supply sufficient
             detail for the local or State staff to check and verify the design calculations. A
I          ~ ~~checklist has been provided in Appendix A as a guide to the material and data
             needed.




        U                                    ~~~~~~~~~~~~2-13







        2.2.3  LOW FLOW/BASE FLOW

              When a permanent pool is established, the base flow and runoff from frequent
              rainfall events need to be sufficient to maintain the pool. Evaporation, transpiration,
              and infiltration losses must be overcome. Evaporation is the greatest in the
              months of April through November as is transpiration. Infiltration losses occur
              constantly if the basin bottom is above the groundwater table; however, infiltration
              losses should decrease over time as the sediments seal off the bottom. Infiltration
              can also be reduced by use of a clay or geotextile liner.

               Rainfall occurs on an average of once every three days in the Hampton Roads
              area. These frequent rainfalls are usually of a low intensity lasting for several hours
              which result in a low total depth of precipitation. Generally, for low-intensity, low-
              duration storms, only the rainfall that falls on impervious area directly connected
              to the stormwater drainage system will become runoff. The designer needs to
              compute the expected volume of runoff from  the impervious area directly
              connected per month and compare that to the volume in the permanent pool and
              the expected evaporation from the pool surface area. Evaporation for this area is
              shown on Table 2-4 along with monthly rainfalls data.

                                             TABLE 2-4

                             PRECIPITATION AND EVAPORATION DATA
                                     Norfolk, Virginia, 1948-1990
                       (Source: NOAA - Climatic Summary of the United States)

                          Days of rainfall        Normal
            Month        areater than a trace    Precipitation  Minimum   Evaporation

              J                 10.4                3.72         1.05           --
              F                 10.4                3.28         0.86           --
              M                 11.0                3.86         0.75           --
              A                 10.1                2.87         0.43         6.42
              M                 10.0                3.75         1.41          6.98
              J                  9.3                3.45         0.37         7.73
              J                 11.2                5.15         0.77         7.69
              A                 10.5                5.33         0.74          6.60
              S                  7.8                4.35         0.26         4.92
             0                  7.6                 3.41         0.57         3.57
              N                  8.0                2.88         0.49          2.53
              D                  9.0                3.17         0.67           --

          ANNUAL               115.3               45.22

I   ~~~~~~~~~~*'
         Evaporation is measured at Holland, Virginia, and the period of record is 11 years.

                                                 2-14







 I           ~~~~In normal months, the precipitation will be'sufficient to maintain a permanent pool
               if infiltration is insignificant. However, the average basin will lose permanent pool
               storage in the months of minimum  precipitation.   Based on a very rough
 I          ~ ~~~calculation, a basin with a contributing watershed of about 35% impervious surface
               will not be able to maintain a permanent pool if the monthly rainfall is less than 1.5
               inches per month. This situation can be expected to occur about once every five
 I          ~ ~~years in the summer months when evaporation is the highest and the situation
               would be the most critical. A study of drought flows would better these values.
 3           ~~~~Statistical data on minimum rainfall is not readily available.

        2.3   WATER QUALITY ENHANCEMENT

 I           ~~~~Water quality can be enhanced in a retention or detention basin by increasing the
               time the water is stored under quiescent conditions. The sedimentation of total
               suspended solids also removes other pollutants such as lead, zinc, copper, and
               some organic priority pollutants. Further enhancement is found when a permanent
               pool is established that has a storage time in general greater than 24 hours. More
               recent studies have shown that additional removal of pollutants, specifically
               nutrients, soluble phosphorus nitrates and nitrates can be accomplished by aquatic
               plants to grow in the permanent pools or in shallow marshes. There are many
               variables in determining the efficiency of a basin to remove pollutants such as
               particle size distribution, initial concentration, pH of the water, and configuration of
               the basin.

 U           ~~~~The concentration of pollutants will vary with the time between rainfall events,
               character of the area, and degree of air pollution.

I      ~2.3.1  METHODS

 3           ~~~Several studies have generated typical ranges of efficiencies for retention and
               detention basins for various pollutants, and this data has been summarized in
               Table 2-5. These are expected removal rates for a basin where the pollutant
 1            ~~~stormwater concentrations are typical of the NURP data, and the basins are
               configured for effective removal consistent with standard design parameters.

 3            ~~~~In basic detention and retention basins, the ability of the basin to capture sediment
               is referred to as the trap efficiency.










          U                                      ~~~~~~~~~~~~2-15







                                   TABLE 2-5

                      POLLUTANT REMOVAL EFFICIENCIES




                                                  Percent Removal
                                                       Heavy
                                       TSS    TP       Metals    Organics    N

 Detention Basin
       Basic (6 hr storage)            30-60  20-50    25-85        30-60
       extended release (30 hr         60-90  20-60    60-85        60-90
       storage)
       wetlands bottom                          60
 Retention Basin
       Basic                           30-60  35-65    25-85        30-60
       extended release                60-90  30-70    60-85        60-90
       water quality storage                    65
       wetlands fringe


TSS                Total suspended solids
TP                 Total phosphorus
Organics           Proportional to TSS
Heavy Metals-      Median EMC concentration value from NURP
N                  Nitrogen


      The percent removals shown have been obtained from several sources and show
      removal efficiencies within the range one could expect. The data was compiled
      from many types of basins, and removal efficiencies recommendations are not
      consistent in the literature. When used for a preliminary design, the lower numbers
      should be used. Actual design values should be computed using 1986 EPA
      recommendations or other similar methods.






                                      2-16







E      ~2.3.2  REGULATORY REQUIREMENTS

               In Virginia, the State Stormwater Management Regulations require a thirty hour
              release from detention basins of the water quality volume of 0.5 inches of runoff
              and permanent pool storage in retention basins of 1.5 inches of runoff from the
               project area, or 3 times the water quality volume.

              The Chesapeake Bay Preservation Act establishes a performance criteria for the
              selected keystone pollutant, Total Phosphorus. These are discussed in Sections
               1.3.1 and 1.3.2. The methods to provide these requirements are discussed in
              Sections 3 and 4 separately.

I      ~2.4   RETROFITTING

               Retrofitting existing stormwater management facilities that were designed for the
              single purpose of drainage or flood control into a facility that will improve water
               quality to some degree can generally be accomplished without major difficulty.

               Retrofitting existing detention and retention basins by extending the release period
              for frequent rainfall events can be done by modifying the outlet. Adding a wetlands
               bottom or wetlands fringe can be done with only minor construction. Detailed
               discussion of retrofitting can be found in Sections 3.4.3.3 and 4.4.3.3 of this
               manual. A detailed analysis needs to be done to make certain the modifications
               do not impact storage requirements; however, usually any extension of the storage
               time will result in a water quality improvement.











          I~~~~~~~~~~~~21







I      ~3.0   DETENTION BASINS

I      ~~3.1    DESCRIPTION

              Detention basins temporarily store stormwater runoff and discharge it to the
              downstream conveyance system through an outlet structure designed to
              completely empty the facility over a relatively short time period, usually six hours
              or less. In their basic form, they have historically only controlled stormwater
              quantity, but recent studies have shown that extending the stormwater release time
              to 24 to 30 hours can significantly enhance stormwater quality, making them
              suitable for Best Management Practices of stormwater management.

I      ~~3.2   APPLICABILITY

              Detention basins are applicable for controlling the quantity and quality of runoff
 I          ~ ~~from residential, industrial, and commercial developments, highways, or other
              areas of urbanization where excess runoff must be detained and released at
              controlled rates so that discharges and pollution levels are maintained within the
              capacities of existing downstream systems and do not exceed pre-development
              levels.

 I           ~~~~Detention basins can be designed to control runoff from an individual development
              site, multiple development sites, or entire drainage areas. Regional planning, as
              described in Section 2, is the best method of selecting locations for basins that will
              serve more than one development site. It has been shown in other studies that
              individually designed and randomly located basins may actually create or
              exacerbate downstream flooding by the combination of discharges. Figure 3-1
              shows a detention basin schematic with a small forebay at the inlet which also
              serves as a wetlands area. Figure 3-2 shows a detention basin schematic with
 *           ~~~extended release.

I      ~3.3   PROPOSED FUNCTIONS

               Detention basins can be designed to either control stormwater runoff quantity,
              enhance stormwater runoff quality, or both. The location of the proposed basin
 I          ~ ~~will determine the minimum required performance standards. Table 3-1 describes
              the four primary basin location scenarios, and the associated detention basin
              functions existing within the Hampton Roads area.









                                  TOP OF BERM-




                                                          PLAY ARE 

                             --                      ~~~~~~LOW FLOW CHANNEL 



INFLOW ENERGY DISSIPATOR                  'TRASH RACK


                                                   W .STORAGE               SI








                                                                                CONCRETE BOX OUTLET
                                                                                STRUCTURE ALTERNATIVE


                                     FLOOD CONTROL STORAGE

                         W/BAFFELS    -~  WATER  rA QUALITY STORAGE        EBA OTE NKER
                   RUINDOW                                                                        DISSIPATOR

                          6o to 186 DEEP
                          SHALLOW WETLANDS.   LOW FLOW CHANNEL INVERT 
                          (OPTIONAL)          S-1-2%LA






                                                 FIGURE 3-1
            _____                       ~~~~DETENTION BASIN SCHEMATICUR
 MAMPTNlml 1~                       (Source- Stormwater Detention. Stahre, Urbonas, 1990)                     CONSULTANTS







                                    TABLE 3-1

                    LOCATION - DETENTION REQUIREMENTS
.................................................  ........ ..                  . .. ... .
               LOCATION..............            DEENT. ON.REQUlREMENT.:::-':::":':':':::'::
 Locality with no CBPA or local             None
 stormwater management plan
 Locality with local stormwater             Quantity and quality controls under local
 management plan                            guidelines (must be Commonwealth of
                                           Virginia requirements at a minimum)
 Locality with CBPA requirements only       Quality controls under CBPA Guidelines
 Locality with both CBPA and local          Quantity controls under local Guidelines.
 stormwater management requirements    Quality controls under CBPA Guidelines

      If both programs are in place, the more stringent water quality requirement - CBPA
      or local stormwater - will govern.

      The design guidelines for each proposed basin function must be considered
      individually and then integrated if necessary for the overall final basin design. The
      following paragraphs describe the two primary basin functions, quantity and quality
      control, and how those functions must be addressed.

3.3.1  STORMWATER QUANTITY CONTROL

      Temporarily storing or detaining excess stormwater runoff and then releasing it at
      a regulated rate has been a fundamental principle in stormwater management.
      The primary functions of a detention basin for stormwater quantity control is to
      reduce the post-development runoff from a development site or drainage area to
      a specified level, such as the pre-development rate.  The state Stormwater
      Management Regulations require such a control strategy for at least the 2-year and
      10-year design storms for basins constructed by State agencies and within
      localities that have implemented stormwater management  programs.   The
      regulations also include a water quality control component.

3.3.2 STORMWATER QUALITY ENHANCEMENT

      The design of detention basins for stormwater quality enhancement is still a
      relatively new process.   Collection of qualitative data supporting the design
      guidelines used to date has only recently begun. Water quality enhancement
      functions of detention basins are a primary consideration in both the state
      Stormwater Management Regulations and Chesapeake Bay Preservation Act
      Regulations; however, as shown in Table 3-1, any locality adopting requirements
      for detention basins must require some level of water quality enhancement. The

                                        3-2







 I           ~~~Stormwater Management Regulations require the first one-half inch of runoff from
              the total development area to be stored and released over a minimum of thirty
              hours from the time of peak storage of the one-half inch of runoff.   The
              Chesapeake Bay guidance establishes a performance criterion. 
              Detention basins can provide pollutant removal by both physical and biochemical
 I          ~ ~~processes.  Larger, suspended pollutants such as sediment and other solids are
              mostly removed by settling under relatively quiescent conditions. Extending the
              stormwater detention time to a period much longer than required for basic quantity
              control allows the necessary quiescent conditions to develop within the basin along
              with providing additional time for settling to occur. Smaller sizes pollutants and
 I           ~~~~those which are more likely to be dissolved in the stormwater, such as nutrients,
              require biochemical activity for removal. The establishment of wetland areas, or
              shallow marsh areas in the basin bottom provides a region for that activity to
              occur. The determination of the primary pollutant of concern, as well as the
              proposed basin location, are the key considerations in the selection of basin
              features necessary to provide the required water quality enhancement function.

I      ~3.4    DESIGN GUIDELINES

 *           ~~~This section contains the recommended design guidelines for detention basin
              BMPs. These guidelines are the result of compiling design data that is in use
              within the HRPDC area, design data from other areas of the state and country, the
              requirements of the Virginia Stormwater Management Regulations and the
              requirements of the Chesapeake Say Preservation Act. The guidelines are
              intended to provide the general procedures necessary for designing detention
              basin BMPs to achieve the required quantity and quality control functions. The
              guidelines are not intended to stifle the innovative engineering processes
              necessary in basin design or to supersede local requirements.

              As such, this manual does not provide step-by-step, "cookbook" design
              procedures or individual design examples. The guidelines are intended for use by
              a professional engineer experienced in drainage design and stormwater
              management who can apply the appropriate knowledge and insight to produce an
              effective and efficient basin design.

              The following sections describe the methodology of the computations and the
              minimum standard physical features required for successful detention basin design
 I          ~ ~~~for both quantity and quality control. Also discussed are design modifications and
              alternatives that can be implemented to provide different basin operational
              functions, if desired.

       3.4.1  QUANTITY CONTROL GUIDELINES

 I           ~~~~This section contains design guidelines for detention basins to be used for quantity
              control. They should be utilized in conjunction with the quality control guidelines

          1                                      ~~~~~~~~~~~~3-3







      described in Section 3.4.2 where necessary to ensure the basin performs all of its
      required functions.

3.4.1.1  METHODOLOGY OF COMPUTATIONS

      Stormwater quantity control in detention basins is primarily a function of watershed
      hydrology and basin hydraulics. The primary design computations necessary
      involve the inflow and discharge hydrographs and the outlet structure hydraulics.
      Basic to these calculations are the selection of an appropriate design storm and
      the basin outlet rate. The allowable outlet rate may be based on historic, or pre-
      development, runoff levels, or the discharge capacity of the downstream system.
      Placing limits on the volume of runoff allowable may also be considered.

      Desian Storm: The design storm is a primary component in basin design. The
      storm return period and duration should be chosen to both reflect the
      characteristics of the watershed and to meet local regulatory requirements. It is
      recommended that, unless local hydraulic conditions require other specific control
      standards, all detention basins be designed to control the 2-year and 10-year
      storms through the outlet structure, with an emergency outlet or spillway capable
      of passing the flows from the 100-year storm as discussed in Section 1.7.

      The design storm duration, if not otherwise specified by the locality,should be
      greater than or equal to the drainage area time of concentration. It has been
      shown in other studies that the 6-hour design storm provides good representation
      of the watershed drainage characteristics and allows for proper downstream
      routing of stormwater runoff. If the watershed time of concentration is between 1
      and 6 hours, a 6-hour design storm duration may be used for hydrologic
      calculations. If the drainage area time of concentration is greater than 6 hours, a
      24-hour design storm should be used. The SCS method uses a 24-hour design
      storm.  The purpose of using the design storm is to provide rainfall data to
      compute the volume of runoff and in turn to evaluate storage capacity.

      Hvdroaraph Calculation:  Each selected design storm must then be utilized to
      calculate the inflow and discharge hydrographs of the proposed detention basin.
      It is important to determine the entire hydrograph and not just the peak runoff rate
      since the detention basin must be capable of controlling both the rate and volume
      of runoff from a drainage area. In the following discussion, the calculation of
      hydrographs pertains to each design storm which requires control by the basin.

      The pre-development and post-development hydrographs for the drainage area
      should be calculated using an appropriate method such as those described in
      Section 2.2.2 of this manual. The pre-development hydrograph calculations should
      be based on the assumption that the land area prior to development exhibits
      hydrologic conditions typical for that type area. The post-development hydrograph
      calculations should be based on available predictions of the ultimate development
      for the entire drainage area tributary to the basin. This is especially important

                                        3-4







when the basin is intended to serve as a regional facility, and may be developed
over a period of time of several years.

The post-development hydrographs from the drainage area become the inflow
hydrographs to the detention basin.   Inlet facilities must be designed to
accommodate the range of flows expected from all of the design storms. The
peak runoff rates indicated by the pre-develooment hvdroaraDhs typically become
the limiting basin discharge for each selected design storm. For example, if the
peak pre-development flows for the 2-, 10-, and 100-year storms for a drainage
area are 10, 50, and 200 cubic feet per second (cfs), respectively, the basin and
outlet structure would be designed to release flow at or below those levels under
post-development conditions for each design storm, if required by the locality and
if practical.

However, the state Stormwater Management Regulations also state that a
developer may have to reduce post-development outflow rates to levels less than
the pre-development rate in order to prevent flooding or erosion downstream.
Localities may only impose this type of additional requirement if a watershed study
has been done.

Basin Outlet Desian:  In order to accommodate the above design storm and
hydrograph requirements, the inflow hydrographs must be hydraulically routed
through the basin and a multi-stage outlet structure must be evaluated and
designed. Routing provides a defined estimation of the timing of the flows into and
out of the basin, along with predicting the actual volume of water requiring storage
at any time during the storm. The outlet structure can include weirs, orifices,
pipes, or a combination of these and other flow controlling configurations to
provide the level of quantity control required for the appropriate design storms.
An example of such a multi-stage outlet structure is shown in Figure 3-3.

The inflow hydrographs can be hydraulically routed through the basin by a number
of manual and computerized procedures. One of the manual processes widely
utilized is the Storage/Indication method, also called the Modified Puls method.
There are also a number of commercially available programs for personal
computers that increase the speed of the calculations and allow for relatively quick
alternatives analysis.

The primary data required for any of the above methods or programs is the basin
depth-versus-storage information and an initial estimate of the outlet structure
configuration.  The depth/storage data can be developed based upon the
proposed size and shape of the basin, computing the volume of water stored for
each increment of basin depth. The depth/storage data is then combined with the
proposed outlet structure to determine the timing (routing) of the stormwater inflow,
storage, and outflow.   This will be an iterative process involving multiple
calculations, alternative outlet structure designs, and other variations until the
correct quantity control functions are met. By plotting the inflow hydrograph and

                                  3-5






                               TIE DOWN BOLTS
                              ,-- SUPPORT BAR

                                       c;   ;   |  ANTI-VORTEX DEVICE OR TRASH RACK,
                         T            .ASPHALT COATED CORRAGATED METAL
           10 YEAR                   PIPE OR BARS WELDED WITH CORROSION
         DESIGN LEVEL                  .CONTROL COATING








        ROCK
                               .**CONCRETE PIPE, PRECAST MANHOLE
                         ,.   .j











                                       ':-~0    u  SECTION PREFERRED OVER METAL
                  -  A<  A~,:        .'..'.  ,/

       GEOTEXTILE  
        FILTER FABRIC      :          i.


             OUTLET  4

    U~~~~~~~~~~~~~~~~~~~_ b %*:..:.
                         _'  .:

                       I , *. ':'.  *' D   '  I       



            ~~~/EMERGENCY \PILLWAY-                           10 YEAR DESIGN LEVEL
I     ~EMERGENCY SPILLWAY                                        TRAS RACK


                        RIP ADWALL  Ace t     \         /      ~TRAASH      RACKE

            ~~~~_RIP RAP OR    /  /r wa                        v LEVEL FOR WATER
       PAVED SECTI REUALITY STORAGE
                                                               FIXED ORIFICE, ROUND OR
                                                               RECTANGULAR
        I ,       - 1                               |           W IAS REQUIRED BY
                     XG APES                     :>J           LJ ~~~~~~~~RELEASE RATE

                           '3'

                                      ALTERNATE TYPE ENERGY DISSIPATOR OUTFALL
                           LI          US BUREAU OF RECLAMATION
I~    A  m     *Y~      . .  '.    F  -





                                      FIGURE 3-3
                           TYPICAL DETENTION BASIN OUTLET
         HAMPTION ROATUrDS  STRUCTURES FOR EXTENDED RELEASE CONSULTANTS







 I            ~~~~outflow hydrograph as developed from the outfall structure discharge at various
               storage elevations, the volume of required storage can be determined.

I      ~3.4.1.2  PHYSICAL FEATURES OF A BASIC DETENTION BASIN

               There are certain basic physical features of detention basins that have been found
 I          ~ ~~~to increase the efficiency and effectiveness of the basin operation. The following
               design guidelines describe those features that can optimize the stormwater
               quantity control function and facilitate maintenance of the facility. These guidelines
               should be used as minimum requirements to produce satisfactory basin designs.
               Side Slopes: The side slopes of the basin should be at a maximum slope of 3:1
               horizontal to vertical for maintenance and ground cover control. If steeper slopes
               are required, they should be paved.

 U           ~~~Low Flow Channel: A low flow channel should be provided through the basin to
              transport any dry weather flows and storm flows less than the minimum controlled
               design storm. The minimum slope through the basin for this channel should be
               0.5 percent if paved and 2.0 percent if grass lined. In flat topography, a 2 percent
               slope may be difficult to obtain and the designer may have to modify the channel
               cross sections to optimize low flow velocities. The entire bottom should drain to
               stay dry.

 3           ~~~Embankment: The basin height should allow for aminimum ofl1foot of freeboard
               above the elevation of maximum water storage. If the height of the embankment
               exceeds 25 feet from the downstream toe to the top, and the basin capacity is
 I          ~ ~~greater than 50 acre-feet, the Commonwealth of Virginia Dam Safety Regulations
               must be addressed.

 3           ~~~Configuration: Oblong shapes are best with a minimum length to width ratio of
               2:1.

 3           ~~~~Inflow structure: The design must consider protection against entrance erosion by
               paving or lowering the entrance channel so the flow enters the pool. The inflow
               needs to be distributed evenly into the pond to avoid stagnant zones and also to
               avoid short circuiting of the inflow directly to the outfall structure.
              Outlet structure: Several types are possible. Safety needs to be considered.
              Trash racks should be installed, or a gravel or stone encasement be used.
I      ~3.4.2  QUALITY CONTROL GUIDELINES

              This section contains design guidelines for use when detention basins are to
              achieve stormwater quality control as discussed in Section 2.3. They should be
              utilized in conjunction with the quantity control guidelines described in Section 3.4.1
              where necessary to ensure the basin performs all of its required functions.

                                                 3-6







3.4.2.1  METHODOLOGY OF COMPUTATIONS

      Stormwater quality control or enhancement in detention basins is primarily a
      function of the detention time available for solids and other pollutants to settle out
      of the flow. Sedimentation is the key process for removal of pollutants in detention
      basins. Alternative measures to provide for additional detention time and other
      potential treatment processes are described in Sections 3.4.3 and 5.0.

      The requirements for a detention basin to provide water quality control will depend
      upon its location.   As shown in Table 3-1, stormwater quality control or
      enhancement is required in areas under the jurisdiction of either a local stormwater
      management program developed under the Virginia Stormwater Management
      Regulations or the Chesapeake Bay Preservation Act. The minimum levels of
      stormwater quality control required, and the procedures for calculating and
      designing those levels, also depend upon the regulations to which the basin must
      conform.

      Commonwealth  of  VirGinia  Stormwater  Manaaement  Reaulations:    The
      Commonwealth of Virginia Stormwater Management Regulations (VR 215-02-00),
      while recommending planning on a regional or watershed basis, also impose some
      minimum restrictions on the enhancement of water quality through the use of
      detention basins. These requirements must be met in any locality adopting a
      stormwater management program in accordance with the Regulations.

      The Virginia Regulations require that detention basins store a minimum "water
      quality volume" equal to the first 0.5-inch of runoff over the entire development
      area. The water quality volume must be released from the basin over a minimum
      30-hour period to provide the desired retention and pollutant removal time. The
      remainder of any storage requirements in the basin will depend on the quantity
      control function. The primary quantity control issue addressed by the Regulations
      is the requirement that the post-development release from the basin not exceed
      the pre-development runoff from the development site for the 2-year and 10-year
      design storms, as nearly as practical.

      Chesapeake  Bav  Preservation Act Reauirements:   The  Chesapeake  Bay
      Preservation Act establishes criteria relating to performance standards, best
      management practices, and planning and zoning concepts to protect the quality
      of state waters while allowing appropriate use and development of the land. While
      the standards do not directly address detention basins, the performance of any
      best management practice implemented within preservation areas designated
      under a local program must meet these requirements.

      In general, the water quality enhancement goals of the CBPA include:

        I   For new development, the post-development nonpoint source pollution
             runoff load shall not exceed the pre-development load based upon average

                                        3-7







   ~~~~I  ~land cover conditions.

                  The redevelopment of any site not currently served by water quality best
                    management practices shall achieve at least a 10 percent reduction of
                    nonpoint source pollution in runoff compared to the existing runoff load from
                    the site. Post-development runoff from any site to be redeveloped that is
                    currently served by water quality best management practices shall not
                    exceed the existing load of nonpoint source pollution in surface runoff.

 ~~I  ~The CBLAD Local Assistance Manual includes a Guidance Calculation Procedure
              that outlines the steps needed to determine if a BMP meets the criteria. The
              Guidance Calculation Procedure is included in this manual as Appendix B.
              Because nonpoint source pollution can include many different contaminants and
              compounds, the calculation procedure is based upon the "keystone pollutant"
              concept. The keystone pollutant is an indicator pollutant, the existence of which
              provides an estimate of the total level of pollution in the runoff. The keystone
              pollutant for the Tidewater Virginia area is total phosphorus.

              When a stormwater management facility is proposed outside of the CBPA (RPA or
              RMA), it is recommended that the projected water quality enhancement be
              calculated.  Although other methods can be used, the Guidance Calculation
              Procedure provides an estimation. It has been recognized that the CBLAD
              procedure should not be used without understanding its limitations and lack of
              historical data. Long-term monitoring of all types of structural and non-structural
              BMPs will allow more detailed calculations of removal efficiencies for a variety of
              pollutants. It is important to evaluate each situation and not to apply blanket
 ~~I   ~      requirements arbitrarily. This is especially critical if the procedures or methods are
              used for regulatory or enforcement purposes.

1    ~  3.4.2.2  PHYSICAL FEATURES FOR WATER QUALITY ENHANCEMENT

              The physical design of detention basins for water quality enhancement is a
              relatively new procedure. There are, however, some features and configurations
              that have been shown to provide successful results to date. Since the minimum
              requirement for water quality enhancement in detention basins designed for
              programs adopted under the Virginia Stormwater Management Regulations is to
              release the "water quality volume" over a period of at least 30 hours, all such
              facilities will come under the heading of detention basins with extended release.
              The physical features requirements of these basins are described in Section
              3.4.3.1.

1    ~  3.4.3  DESIGN MODIFICATIONS FOR WATER QUALITY ENHANCEMENT

              There are modifications that can be made to the basic design of detention basins
              to improve the removal of nonpoint source pollution. The selection of the
              appropriate modification for any particular site should be based on the type and

                                               3-8







 I           ~~~degree of pollution removal desired. The CBPA Guidance Calculation Procedure
              includes steps for the proper selection of a BMP.

I      ~3.4.3.1  EXTENDED RELEASE

              Extending the retention, or flow release, time of a detention basin is an effective
              means of implementing stormwater quality enhancement. Longer detention times
              allow for quiescent conditions to occur in the basin, facilitating settling and other
              pollutant removal  processes.    Extended  release,  used  as  water  quality
 I          ~ ~~enhancement modification, is most effective for the removal of the larger,
              particulate pollutants. A release time of at least 24 hours has been shown to
 *           ~~~achieve as much as 90 percent removal of these materials.

              As with other detention basins, there are certain design guidelines that can
              facilitate the performance of a basin with extended release. The following
              paragraphs describe some of - the guidelines that have been found to be
              successful.

 I           ~~~~Release Times: The release time is the primary factor in the removal efficiency of
              an extended release basin. The Virginia Stormwater Management Regulations
              provide good minimum standards to evaluate during the basin design. The
              Regulations require a minimum release time for the "water quality volume" portion
              of the basin contents of 30 hours starting at the time of peak basin storage. This
 *           ~~~should result in an average detention time for all the flow of about 12 hours.
               Release times must be analyzed in conjunction with removal efficiencies that may
              be required if the basin is located in a CBPA regulated area.

               Basin Confiouration: The basin size and shape has a direct effect on the flow-
              through and settling characteristics of the stormwater. Oblong shaped basins are
              most effective. A minimum length to width ratio of 2:1 is recommended to help
              prevent short-circuiting of the flow through the basin. Side slopes should be no
              steeper than 3:1 horizontal to vertical for maintenance and slope protection.

              Forebay: A forebay is a section of the inlet area of the basin designed to intercept
              the larger particles for settling and thus facilitate their eventual removal by keeping
 I          ~ ~~them. out of the deeper portions of the basin. The forebay should include a baffle
              of wood or concrete or some other appropriate material on its downstream side
              to slow the flow, improve the particulate capture efficiency, and aid in the
  I          ~ ~~prevention of short-circuiting. The forebay is indicated in Figure 3-1. Since the
              forebay area acts like a sediment basin or trap, the area will need frequent
              maintenance. The area needs to be easily accessed by an all-weather roadway
  I          ~ ~~for heavy vehicles.  Plantings or wetlands mitigation need to be avoided in this
              area. In small basins, this area could be paved for easy maintenance. The use
  *           ~~~of rip-rap in these areas should be avoided.



          1                                      ~~~~~~~~~~~~3-9







I      ~3.4.3.2  INFILTRATION BASINS

               The design of the basic detention basin can also be modified so that it functions
               as an infiltration basin. An infiltration basin is a detention facility without a primary
               outlet structure so that the stormwater runoff infiltrates into the ground. It functions
               in a similar manner to a detention basin when the basin inflow exceeds the
 I          ~ ~~~infiltration capacity, and water is stored until it can infiltrate. Infiltration basins can
               be effective in removing both soluble and fine particulate pollutants. Larger
               pollutants must typically be removed from the flow before it enters an infiltration
 U          ~ ~~~basin. The overall infiltration basin design is similar to other detention facilities, with
               the goal being that the facility will contain the design inflow without overflowing.
 3           ~~~~A schematic of an infiltration basin is shown in Figure 3-4.

               The use of infiltration basins in the Tidewater Virginia region is restricted by state
               Stormwater Management Regulations to areas where the basin invert can be at
               least 4 feet above the local high groundwater level. While this effectively eliminates
               many areas in the Hampton Roads vicinity, infiltration basins remain, in concept,
               a viable, if limited, stormwater BMP for the Hampton Roads area. Guidelines for
               their basic design and configuration, therefore, have been included in this manual.
               Site Selection: The soils and groundwater levels must be investigated specifically
               for a potential basin site. Each soil core must extend at least 5 feet below the
               proposed basin floor elevation. Soils within this zone should have a minimum field
               infiltration rate of 0.5 inches/hour as a desirable rate; however, rates as low as
               0.25 inches per hour have been considered acceptable, especially for smaller
               facilities such as infiltration trenches. A table of infiltration rates for smaller facilties
 3           ~~~can be found in the Phase I companion manual to this document.  The lowest
               measured infiltration rate, as measured by a percolation test, indicated at the site
               of the proposed basin should be used for the design calculations.

               As described above, and as required by the state Stormwater Management
               Regulations, the basin floor must also be at least 4 feet above the seasonally high
 3           ~~~~local groundwater level. Additionally, a site must not be used for an infiltration
               basin if any of the following conditions exist:

   *             *  ~~~~bedrock is within 4feet of the basin floor

   *             .  ~~~~~the site is over fill material

                    . the surface and underlying soils are classified in the SCS Hydrologic Soil
   3                ~~~~~Group "D".

               As an example, Table 3-2 below lists some SCS soil groups, typical soils within
 3           ~~~~each group, and a general average infiltration rate for each soil.



          I                                      ~~~~~~~~~~~3-10









                                                              SIDE SLOPE MINIMUM 3:1
            EMBANKMENT-/ Y                                 1 


                                  FLAT BASIN FLOOR                                  INLET
                                  WITH DENSE GRASS           "       'URF     . 
                             J1  vI [ .  ste  - ,0  . 0 ',,,  INFLOW ENERGY
                                    / .  .  .    6h:~pb  DISSIPATOR REQIE

                                             BACK-UP UNDERDRAIN A AN ALTI 
        RIPRAP OUTFALL                                e        4       
        PROTECTION        U        EMERGENCY SPILLWAY C-    IL d  T





                                                     AS SHOWN
                                                     ON INFILTRATION - STORAGE CURVE BELOW

                             STORAGE REQUIRED

       SCREEN                    If \ II/AI4Ai/iIIJTAI  I



                                                     UNDERDRAIN AS AN ALTERNATIVE INLET

                          CALCULATING INFILTRATION AND PERCOLATION FACILITIES





                                        EXCESS INFILTRATION CAPAC TY

                        -   MASS DIAGRAM OF INFILTRATION 

                         MASS DIAGRAM OF RUNOFF                     i
                         VOLUME FROM INFLOW
                   Gu,   HYDROGRAPH


                          _    -MAX. NFILTRATION                   I-
                      - /        VOLUME TO BE STORED    .


                               /  INFILTRATED



                                         DURATION (MINUTES)







         _y__ oFIGURE 3-4
         ^HAMPTON RQos          SCHEMATIC INFILTRATION BASIN                         CONSULTANTS
~~~~~~~~NIN    I T C I Iq ('ICOSLAS







                               TABLE 3-2
                    TYPICAL INFILTRATION RATES


                                                     Infiltration Rate
       SCS Soil Grouo and Soil                           (in/hr)

      A. Sand                                             8.0
      A. Loamy Sand                                       2.0
       B. Sandy Loam                                       1.0
       B. Loam                                             0.5
      C. Silt Loam                                        0.25
      C. Sandy Clay Loam                                  0.15
       D. Clay Loam and Silty Clay Loam                    <0.09
       D. Clays                                            <0.05

(Source: Stormwater Detention, Stahre and Urbonas, 1990.)


The basin should be capable of completely infiltrating the first 0.5 inch of runoff per
impervious acre of contributing watershed. This minimum requirement can achieve
significant pollutant removal and downstream channel protection.

Infiltration basins should only be used on drainage areas less than or equal to 25
acres in size.

Basin Confiauration: The preliminary basin size can be estimated by the general
rule that the infiltration surface area should not be smaller than one-half of the
tributary impervious surface area. Final size needs to be calculated considering
the infiltration rate. The basin size can also be estimated by plotting the runoff
hydrograph and the estimated stormwater infiltration volume versus time. The
greatest difference between the runoff and infiltration plots indicates the maximum
amount of water that will have to be stored which is also shown on Figure 3-4.

In general, the storage depth should be adjusted so that the basin completely
drains within 48 hours.

The side slopes, like detention basins, should have maximum slopes of 3:1
horizontal-to-vertical to allow for maintenance and bank stabilization.

The basin floor should be graded as flat as possible to permit uniform ponding and
infiltration. Low spots and depressions should be leveled out.

Basin Inlets: The inlet pipe or channel leading to the basin should discharge at the
same elevation as the basin floor.


                                  3-11







I           ~~~~All basins should also have sediment forebays or riprap aprons that dissipate the
             velocity of the incoming flow and trap larger sediments before they reach the basin
             floor.

      3.4.3.3 RETROFITTING EXISTING FACILITIES

I           ~~~Many detention basins exist that do not have an outlet structure to allow for
             detention for periods of time approaching the thirty hours required to meet the
             extended release requirements for water quality enhancement. These basins often
             have side slopes that are steeper than 3:1 and bottoms that are flat, do not have
             a low flow or trickle flow channel, and are not graded to prevent pools of standing
             water. Most of these basins have an outlet structures that controls one flood,
             usually the five- or ten-year storm. The steps to retrofit should include:
                    1 )    Regrade slopes to 3:1 or less;

                    2)    Regrade bottom to prevent standing water;

                    3)    Install low flow channel;

 *                ~~~~~4)    Investigate inflow structures for erosion and repair;

                    5)    Evaluate the outfall structure to incorporate the possibility of
   I                   ~ ~~~~~~controlling a two-year storm or a second event to improve control
                          over a wider variety of recurrence intervals;

 1                ~~~~~6)    Evaluate storage capacity for extended releases. Often the outfall
                          structure is designed for pre-development flows from a I10-year storm
                          and storage may be available for extended release of lower
   I                   ~ ~~~~~~frequency storms even though it may not be possible for the design
                          storm. If this is the case, the outlet structure can be modified to
                          control a 2-year storm with extended storage and release;

                    7)    Consider the construction of a forebay near the inflow for capturing
                          heavier sediments. The forebay should be sized to store a volume
                           equal to the annual runoff event for five minutes of detention. The
                          bottom of the forebay should be stabilized for easy maintenance.

 I                ~~~~~8)    Consider adding a wetland bottom or a wetland finger along the low
                          flow channel. This bottom area can be placed anywhere in the basin
                          from the inlet to the outlet. It may be part of the forebay area if the
                          forebay is not stabilized, and it is recognized that maintenance will
                          disrupt the wetlands planting.

  I                ~~~~~9)    Consider increasing storage by adding height to the embankment.


                                                3-12







I      ~3.4.3.4  WETLAND AREA ESTABLISHMENT

               Establishing a wetlands area in a detention basin involves creating a shallow marsh
               located within the basin bottom or along the low flow channel. The low flow
               channel can meander through the basin with a marsh fringe on either side.
              Virginia does not have any guidelines for a shallow permanent pool in a detention
 U          ~ ~~basin designed for a wetlands area. The wetlands pool should be considered as
               a permanent pool in a retention basin where the volume is governed by the factor
               of three (3) times the water quality runoff. The wetlands pool has a different
              function and needs to be considered for its own benefits. Further details on
               design methods can be found in Section 5 of this manual.

I      ~3.5   CONSTRUCTION AND OPERATION ISSUES

              The construction, operation, and Maintenance of all types of detention facilities are
 I         ~ ~~primary factors in their success rate and longevity.  A basin can be designed
               utilizing proven criteria and state-of-the-art techniques, but unless it is constructed
               according to that design and maintained so that it continues to emulate the original
               design, it will not be able to operate efficiently and achieve its desired water
               quantity and quality control functions.

              The following sections contain guidelines for successful construction, operation,
               and maintenance of detention basin BMPs. It is incumbent on the administering
               locality that these guidelines, along with other proven and accepted techniques,
               be adhered to throughout the operating life of the facility.

I      ~3.5.1  CONSTRUCTION GUIDELINES

               Construction Seauencina: The detention basin is typically part of the site plan as
 I         ~ ~~well as the erosion control program.  Since it is designed to trap sediment from
               upstream development, the detention basin should be constructed in the early
               stages of the project. Grading operations should be scheduled in a manner which
 I          ~ ~~will limit the soil's exposure to erosion.   Grade only those areas ready for
               immediate development. Promptly reseed bare soil upon completion of sitework.
               Additionally, protect downstream areas by installing graveled construction
 U          ~ ~~~entrances, silt fence, check dams and other temporary measures until the facility
               is completed. Inspect the basin after it has been stabilized for premature silting,
 *           ~~~channel scour and other defects.

               Site Layout and Prercaration: The outside perimeter of the detention facility should
               be staked out before any clearing and grading begins. The embankment and any
               appurtenant work like stream bank stabilization should also be staked at this time.
               At a minimum, the following layout stakes, marked for grade, should be used:

  I             .~~~~ top of slope of the basin excavation
                 0   bottom of slope of the basin excavation

          U                                    ~~~~~~~~~~~~3-13







I~ ~~ .   centerline of embankment
                  front and back toe of slope or embankment
*I           ~ï¿½ . several grade stakes through the basin floor

            The first stakes to be set should be the centerline of the embankment and the top
            of the slope of the basin excavation.

            The outlet control structure should be staked, constructed, and backfilled before
5I~ ~general earthmoving is started.

            The site must be dry for successful excavation to take place. If a site is wet, or if
            the site is expected to be wet during construction, measures should be taken to
            ensure proper conditions. These measures could include direct drainage trenches
            to points of lower elevation or the collection of runoff and surface water in sumps
*I~  ~      that require pumping.

             Embankment  Construction:   Good  fill material, suitable soils, and proper
Ia  ~       compaction  techniques  are  imperative  for the  construction  of a  stable
             embankment. Increasing the embankment breadth and decreasing the slope can
             also be important measures.

             Placing embankment fill should be performed in sequential lifts of 6- to 8-inches
             each. An entire lift across the embankment should be completed before the next
It~ ~        lift is begun. This allows any moist soils to dry and additional compaction to occur
             from the application equipment.

*1~ ~       The proper construction of a cutoff trench is imperative to prevent any undermining
             of the embankment. A cutoff trench is a trench excavated along the centerline of
             the embankment before the fill materials are placed. It must be constructed from
             a relatively impermeable soil. The cutoff trench can be constructed wide enough
             for the bulldozer or other equipment to work within it. The impermeable soils
             should be placed in 6- to 8-inch lifts. The cutoff trench must extend from several
             feet below the existing grade up into the embankment fill.

             The placement of antiseep collars at the point where the outlet pipe passes
             through the embankment to prevent soil piping failures is of key importance. An
             antiseep collar is a metal, concrete, or masonry shield placed around the pipe
             within the fill embankment. The backfill material around the outflow pipe should
             also be properly placed and compacted to help prevent embankment failure.

  I  ~~Inflow and Outflow Structures: The inflow structure is generally less critical than
             the outflow structure, but it still requires accurate vertical placement.  Slope
             protection should be used ahead of a stream inflow structure, downstream of an
             inflow "spillway" of any length, and at the basin discharge point. The inflow control
             structure must be constructed so that it directs the flow into the basin forebay, or
             across the basin floor as intended by the basin design. Riprap should be grouted

                                               3-14







 I           ~~~to make future maintenance operations easier.

              The outflow structure may contain several key components that must work in
              concert with each other. These may include weirs, orifices, grates, or other flow
              control sections. These must all be properly constructed and placed at accurate
              elevations. If the structure is constructed offsite, it must be inspected carefully
              upon delivery to the site for any defects or misalignments of any of the
              components. The final placement and/or construction must be exactly as shown
 *           ~~~on the construction documents.

              Construction Ooerations: A retention basin is most subject to externally caused
              damage during its construction. Since most basins will be located at the low
              points of a site, they must be protected from extreme rainfall events that may occur
              during construction. Vegetative cover and the emergency spillway must also be
 *           ~~~completed as quickly as possible during the construction phase.

              The use of an inspector is one of the best methods of ensuring that the detention
              basin is constructed as designed.   This inspector may  be an  in-house
              representative, someone from the designing firm, or from an outside consultant or
              inspection company. The inspector may be full time or part time. The primary
 *           ~~~focus of the inspections should include:

                   * embankment fill placement
   *            *  ~~~~embankment fill material
                   * implementation of adequate erosion and sediment control

 *           ~~~Additional details can be found in Section 6of this document.

E      ~3.5.1.1  INFILTRATION BASIN CONSTRUCTION GUIDELINES

              Proper construction techniques are extremely important to the successful
              installation and operation of infiltration basins. The most common cause of
 U         ~ ~~~infiltration basin failure is premature loss of infiltration capacity, which is often linked
              to the procedures followed during construction. The following guidelines, along
              with those described above, must be addressed during the construction of an
              infiltration basin.
                  * Heavy equipment traffic must be restricted from the basin area to prevent
   I              ~ ~~~~excessive soil compaction.
                  *If the basin is not intended to function as a sediment basin during
   I               ~~~~~construction, proper erosion and sediment control measures must be
                     implemented prior to initiating the basin construction to keep excessive
                     sediment from entering the basin.
                  * If the basin is to be used as a sediment basin during construction, initial
                     grading should be completed to within only two feet of the final basin floor
                     elevation. The final two feet can then be removed with the collected

          I                                    ~~~~~~~~~~~~3-15







   I                ~~~~~sediment when the site is completely stabilized.
                *    The basin should be excavated using light earth-moving equipment with
                     tracks or over-sized tires. Since some compaction will still occur, the basin
   I              ~ ~~~~~floor should be tilled with a rotary tiller or disc harrow. The floor can then
                     be smoothed and leveled after the excavation is complete.
                *    Slope stabilization with vegetation should be completed as soon as possible
                     after construction.
I      ~3.5.2  COST ESTIMATES

              The graph in Figure 3-5 shows the average cost to construct a detention basin of
               a size which would be required for a given total project area. Likewise, the graph
               in Figure 3-6 compares the construction cost associated with the volume of
               stormwater which must be detained. These figures are generic and should only
 *           ~~~be used as guidelines and planning purposes.

              The costs shown in Figure 3-5 and 3-6 are based on generalized development
 *           ~~~scenarios which were also used in developing Figure 2-1. The basins were
              designed for a 10-year runoff. The retention basin includes water quality storage.
              The cost includes site preparation, earthwork, inlet, outlet structures, discharge
              energy dissipators, seeding, and a contingency for other costs.

              The pre-development C-factors used for the associated hydrographs ranged from
 3           ~~~~0.35 for a small 1 -acre site to 0.20 for a larger 50-acre site. Post-development C-
              factors were determined by the types of development typically associated with
              various sizes of land. These post-development C-factors ranged from 0.80 for
 I         ~ ~~~small sites to 0.50 for larger sites. The costs of constructing detention facilities
              were generated from recent contractor estimates of various basin sizes and were
              compared with Means Sitework and Landscape Cost Data, 1991.
        3.5.3 FACILITY LIFE EXPECTANCY

 *           ~~~~The life expectancy of a detention facility is directly proportional to the quality of
              construction and maintenance. If properly placed in stable soil, concrete can be
              expected to last up to fifty-years. The embankment properly constructed would
 I         ~ ~~have an indefinite life probably exceeding all other parts of the facility.  Metal
               portions can be expected to last twenty-years or more if properly maintained.
              Aluminum alloy products will have a longer life if properly specified. Preventative
 I         ~ ~~~and corrective maintenance are crucial to the success of the forebay and the basin
              bottom.

P~~3 .5.4  MAINTENANCE REQUIREMENTS

              The agency responsible for long term maintenance must be identified during the
              planning stages. Even though a detention facility only performs its design role on
              an occasional basis, it must be constantly prepared to do so. A comprehensive,


          N                                    ~~~~~~~~~~~3-16








      45
           NOTE: COST ARE CONSTRU'TION COST, IM 1991,
           LAND, ADMINISTRATIVE, ANtD FINANCING CCISTS
           ARE NOT IN(CLUDED.


      40





      35





 93.


 I /




  /30
 O    2











 0     15/





      1C





      5






                   la 20                     30            40            50
                         TOTAL PROJECT SIZE (ACRES)
                               FIGURE 3-5URS
........... ,-.,--~    TOTAL PROJECT SIZE VS. FACILITY COST                         CONSULTANTS
















                NOTE: COST ARE CONSTRU,,TION COST, lb~ 1991.
                LAND, ADMINISTRATIVE. ANIP FINANCING CC'STS
                 ARE NOT INCLUDED.

     40




     3j  0


 0 
 a


 IA-
0
z


     I-




        ~~~~~204 0s                                                       0     1      2           90ISO
             0 ~ ~ ~ ~ ~ ~~SOMAE OB TRD(HUAD FGLOS
                   0 ~ ~ ~ ~ ~ ~ ~ ~ ~ F G R                         -

    20VO  Q-D                                                                                                          R
PI-1. a~~-NC N T U T O                      OTV.SOMAE                             O      B          T     R D CNLAT







 I           ~~~~regularly-scheduled maintenance program is the key to any successful stormwater
              management facility. Such a program is comprised of funding, maintenance,
 3           ~~~~inspection, training and program reviews.

              A detention basin will be useless if funding for its maintenance is not adequate.
              Funding considerations include: staffing, equipment, and material needs; facilities
              for storage of materials; storage, maintenance and replacement of equipment;
              training and administrative costs; seasonal effects; long-term capital improvements;
 3           ~~~and emergency appropriations for unforeseen problems.

              The physical portion of the maintenance program should include aesthetic,
 3           ~~~~preventive and corrective measures.

              Regular, major and informal inspections should be performed. Major inspections
 3           ~~~~should be performed semi-annually and after each major storm.   Regular
              inspections should be conducted to determine the need for and the effectiveness
              of maintenance work. Informal inspections should be conducted during every visit
 3           ~~~~to the facility by maintenance personnel, and, if possible, prior to the occurrence
              of a major storm. Section 6 of this manual provides additional details on
              inspections.

              A training program should include: maintenance and inspection techniques,
              proper record keeping,and stormwater program goals and objectives. Particular
 3           ~~~attention should be paid to the purpose and operation of stormwater management
              facilities, the importance of thorough maintenance, and the health, safety and other
 *           ~~~consequences of maintenance neglect.

              Additional information can be found in Section 6 of this documnent.

E      ~3.6   PLAN SUBMITTAL REQUIREMENTS

              Submittals for detention facilities need to be made to the locality in accordance
              with local ordinances and regulations for Stormwater Management, Erosion and
              Sediment Control, and Chesapeake Bay Preservation Ordinances. Only State
              agencies need to submit plans to the Division of Soil and Water Conservation and
              the Chesapeake Bay Local Assistance Department.
I      ~3.6.1  AGENCIES

              The Department of Conservation and Recreation Division of Soil and Water
              Conservation can provide assistance on the Erosion and Sediment Control Law,
              Stormwater Management Act, and Dam Safety Act and Regulations.
              The State Water Control Board, Permits Section can provide information on the
              stormwater NPDES permit.


          I                                    ~~~~~~~~~~~3-17







 I           ~~~The Chesapeake Bay Local Assistance Department can provide information on the
              Chesapeake Bay Preservation Act and Regulations.

 I           ~~~~The Norfolk District Corps of Engineers Construction Operations, Regulatory Permit
              Section, issues permits for wetlands disturbance and navigable water crossings.

 I           ~~~VMRC and local Wetlands Boards need to be contacted for wetlands or projects
              impacting the shoreline.

 I           ~~~If a permit is needed for construction because of wetlands disturbance or
              interference with a navigable stream, then a permit would probably be needed for
              maintenance dredging. If no permit is issued or needed for construction, then no
              permit would probably be needed for maintenance dredging. Dredge materials
              under either circumstance would probably not be regulated. At this point in time,
              there are no requirements for disposing of materials dredged from retention or
              detention basins; however, it would be prudent to discuss the issue with the Corps
              of Engineers prior to dredging.

E      ~3.6.2 SUBMITTAL CHECKLIST

 *           ~~~A checklist has been prepared for overall guidance and can be found in Appendix
              A. Localities may have their own checklist which the applicant would need to
              follow. In addition, the Division of Soil and Water Conservation has developed a
 *           ~~~checklist to be used by State Agencies.







I      ~4.0   RETENTION BASINS

I      ~4.1   DESCRIPTION

              A retention basin is a stormwater management facility comprised of: a) a
              permanent pool of water to enhance water quality which loses water primarily
              through infiltration and evaporation; and b) additional capacity above the
              permanent pool for the storage of stormwater runoff. The facility discharges to the
              downstream conveyance system through an outlet structure designed to both
              release the runoff over a specified period of time and maintain a minimum level of
              the permanent pool. These facilities are also called wet ponds or wet detention
              basins and can be used for both stormwater quantity and quality control.

        4.2   APPLICABILITY

              The use of retention basins as BMPs is most applicable in residential or
              commercial developments where there is a reliable source of water to maintain the
              permanent pool. Like detention basins, they can serve the dual function of
              stormwater quantity and quality control. They also can be an aesthetic, attractive
              feature in a development if designed. constructed, operated, and maintained
              correctly.
I      ~4.3   PROPOSED FUNCTIONS

              Retention basins can be designed to either control stormwater runoff quantity,
              enhance stormwater runoff quality, or both. The location of the proposed basin
 I          ~ ~~will determine the minimum required performance standards. Table 4-1 describes
              the four primary basin location scenarios and the associated retention basin
              functions existing within the Hampton Roads area.










          I~~~~~~~~~~~~~~~-



- - -                                                - m - m m - ----- m m  











                  SIDE SLOPES MAX 3:1  

                                                                SAFETY BENCHKï¿½E










                                          DISSAR                                                WITh _-o 4 D










               ZA s                              .~~~~~~~~~N~~~L FIGURE 4-1MN                                                     URS
           HAMrONROADS                              RETENTION BASIN SCHEMATIC                                                    CONSULTAS
                                        RIGHT--0F-~~~~~~~~~~~~~?ï¿½::::ji::i:l ii~:~~~j :ii  iii~i:ii~iP::'(::::~I
               SE13IMENT    TO  SEDIME                                                 T~lli~ii~i~~iii~li~'''"'''''''"'
            FOREBAY          -F                                        I                                        /EMER4ENCY SPILLWAYiiij  iii~i~i    ~~liii
                STABILIZED W~~~~~~~~:::::t:::::::::I   ~    i~~iiii               ::::..:::::....;~ ~  :~
                      BOTTOM UFFE.,R AROUND POO~~~~~~~:iiS~iiii~ib~iiiX3 ~i~i:i~~iiiih~~i~~~i~  ~ ~ il
               PREFERRED~~~~ï¿½ai~i~~xj~~i~ii~j~S~S




                      AM"ON RQAF)S   F I    G      U     R     E        4 -1                                                      URS::::i::i~::::::::~: 
                               ~~~~~~~~~EETO AIN SCHEMATIC CONSULTANTS







                                    TABLE 4 -1

                    LOCATION - RETENTION REQUIREMENTS



 Locality with no CBPA or local            None
 stormwater management plan
 Locality with local stormwater            Quantity and quality controls under local
 management plan                           guidelines (must be Commonwealth of
                                          Virginia requirements at a minimum)
 Locality with CBPA requirements only      Quality controls under CBPA Guidelines
 Locality with both CBPA and local         Quantity controls under local Guidelines.
 stormwater management requirements    Quality controls under CBPA Guidelines

      If both programs are in place, the more stringent water quality requirement - CBPA
      or local stormwater - will govern.

      The design guidelines for each proposed basin function must be considered
      individually and then integrated if necessary for the overall final basin design. The
      following paragraphs describe the two primary basin functions, quantity and quality
      control, and how those functions must be addressed.

4.3.1  STORMWATER QUANTITY CONTROL

      Temporarily storing or detaining excess stormwater runoff and then releasing it at
      a regulated rate has been a fundamental principle in stormwater management.
      The primary functions of a Retention basin for stormwater quantity control is to
      reduce the post-development runoff from a development site or drainage area to
      or below pre-development levels. The state Stormwater Management Regulations
      require such a control strategy for at least the 2-year and 10-year design storms
      for basins constructed by State agencies and within localities that have
      implemented stormwater management programs. The regulations also include a
      water quality control component.

4.3.2 STORMWATER QUALITY ENHANCEMENT

      Retention basins effect removal of pollutants from stormwater by two primary
      mechanisms, sedimentation (settling) and biological uptake. The relatively long
      detention times allow for the settlement of many of the larger suspended pollutants
      and the permanent pool and perimeter promote the conditions necessary for the
      biological activity to take place.

      Theoretically, the inflowing stormwater displaces water out of the pond which is

                                        4-2







 I            ~~~~then stored until the next storm. The suspended pollutants settle out of the flow
               with the permanent pool acting to prevent their resuspension. The larger and
               coarser particles settle first, with the smaller and finer materials taking longer.
 I          ~ ~~~Factors which can reduce the effectiveness of settling in retention basins are short-
               circuiting of the flow through the basin and an inflow volume that is greater than
 *            ~~~the permanent pool volume.

               Aquatic plants and algae that can live in retention basins can remove significant
               quantities of soluble nutrients. They convert the nutrients into a mass that will
               settle into the sediments at the bottom of the pond.
*       ~~4.4    DESIGN GUIDELINES

               The following sections contain guidelines for the design of retention basin BMPs.
               Since retention basins can perform both quantity and quality control functions,
               guidelines are presented that specifically relate to each issue. The quantity and
               quality control functions must be evaluated separately and integrated into the final
 *            ~~~~basin design.

               This manual does not provide step-by-step, "cookbook" design procedures or
               individual design examples. The guidelines are intended for use by a professional
               engineer experienced in drainage design and stormwater management who can
               apply the appropriate knowledge and insight to produce an effective and efficient
               basin design. A schematic of a retention facility is shown on Figure 4-1.

               The following sections describe the methodology of the computations and the
 *            ~~~~minimum standard physical features required for successful retention basin design
               for both quantity and quality control. Also discussed are design modifications and
               alternatives that can be implemented to provide different basin operational
               functions, if desired.
        4.4.1  QUANTITY CONTROL GUIDELINES

               This section contains design guidelines for retention basins to be used for quantity
               control. They should be utilized in conjunction with the quality control guidelines
 U          ~ ~~~described in Section 4.4.2 where necessary to ensure the basin performs all of its
               required functions.

I      ~4.4.1.1  METHODOLOGY OF COMPUTATIONS

               Stormwater quantity control in retention basins, while somewhat more complex
               than in detention basins, is still primarily a function of watershed hydrology and
               basin hydraulics. The primary design computations necessarily involve the inflow
               and discharge hydrographs and the outlet structure hydraulics. Inherent in these
               calculations is the selection of an appropriate design storm or storms and the
               decision of how to limit basin outlet rates. The allowable outlet rates may be

          I                                      ~~~~~~~~~~~~4-3







based on historic, or pre-development, runoff levels, the discharge capacity of the
downstream system, or some specific value. Placing limits on the volume of runoff
allowable may also be considered.

Desian Storm: The design storm is a primary component in basin design. The
storm return period and duration should be chosen to both reflect the
characteristics of the watershed and to meet local regulatory requirements. It is
recommended that, unless local hydraulic conditions require other specific control
standards, all detention basins be designed to control the 2-year and 10-year
storms through the outlet structure, with an emergency outlet or spillway capable
of passing the flows from the 100-year storm as discussed in Section 1.7.

The design storm duration, if not otherwise specified by the locality, should be
greater than or equal to the drainage area time of concentration. It has been
shown in other studies that the 6-hour design storm provides good representation
of the watershed drainage characteristics and allows for proper downstream
routing of stormwater runoff. If the watershed time of concentration is between 1
and 6 hours, a 6-hour design storm duration may be used for hydrologic
calculations. If the drainage area time of concentration is greater than 6 hours, a
24-hour design storm should be used. The SCS method uses the 24-hour design
storm.  The purpose of using the design storm is to provide rainfall data to
compute the volume of runoff and in turn to evaluate storage capacity.

Hvdroaraph Calculation:  Each selected design storm must then be utilized to
calculate the inflow and discharge hydrographs of the stormwater storage portions
of the proposed retention basin. It is important to determine the entire hydrograph
and not just the peak runoff rate, since the retention basin must be capable of
controlling both the rate and volume of runoff from a drainage area while
maintaining the permanent pool level during non-storm conditions. In the following
discussion, the calculation of hydrographs pertains to each design storm which
requires control by the basin.

The pre-development and post-development hydrographs for the drainage area
should be calculated using one of the appropriate methods described in Section
2.2.2 of this manual. The pre-development hydrograph calculations should be
based on the assumption that the land area prior to development exhibits
hydrologic conditions typical for that type of area.  The post-development
hydrograph calculations should be based on available predictions of the ultimate
development for the entire drainage area tributary to the basin. This is especially
important when the basin is intended to serve as a regional facility and may be
developed over a period of several years.

The post-development hydrographs from the drainage area become the inflow
hydrographs to the retention basin.   Inlet facilities must be designed to
accommodate the range of flows expected from all of the design storms. The
peak runoff rates indicated by the pre-development hvdroaraohs typically become

                                  4-4







the limiting basin discharge for each selected design storm. For example, if the
peak pre-development flows for the 2-, 10-, and 100-year storms for a drainage
area are 10, 50, and 200 cubic feet per second (cfs), respectively, the basin and
stormwater control outlet structure would be designed to release flow at these or
other approved levels under post-development conditions for each design storm
if required by the locality and if practical.

However, the state Stormwater Management Regulations also state that a
developer may have to reduce post-development outflow rates to levels less than
the pre-development rate in order to prevent flooding or erosion downstream.
Localities may only impose this type of additional requirement if a watershed study
has been done.

Basin Outlet Desion: In order to accommodate the above design storm and
hydrograph requirements, the inflow hydrographs must be hydraulically routed
through the basin and a multi-stage outlet structure must be evaluated and
designed. This will be somewhat different than the design of a detention basin
outlet structure since the "low flow" or smallest storm release level will still be
above the permanent pool level. Routing provides a defined estimation of the
timing of the flows into and out of the basin, along with predicting the actual
volume of stormwater requiring storage at any time during the storm. The outlet
structure can include weirs, orifices, pipes, or a combination of these and other
flow controlling configurations to provide the level of quantity control required for
the appropriate design storms. An example of such a multi-stage outlet structure
is shown in Figure 4-2.

The inflow hydrographs can be hydraulically routed through the basin by a number
of manual and computerized procedures. One of the manual processes widely
utilized is the Storage/Indication method, also called the Modified Puls method.
There are also a number of commercially available programs for personal
computers that increase the speed of the calculations and allow for relatively quick
alternatives analysis. It must be remembered that the method used must be able
to accurately simulate a reservoir, since there will always be water already in the
retention basin when the stormwater flows enter it.

The primary data required for any of the above methods or programs is the basin
depth-versus-storage information and an initial estimate of the outlet structure
configuration.  The depth/storage data can be developed based upon the
proposed size and shape of the basin, computing the volume of stormwater stored
for each increment of basin depth above the permanent pool. The depth/storage
data is then combined with the proposed outlet structure to determine the timing
(routing) of the stormwater inflow, storage, and outflow. This will be an iterative
process involving multiple calculations, alternative outlet structure designs, and
other variations until the correct quantity control functions are met.



                                  4-5















                    FENCE

            - ~  EMERGENCY SPILLWAY

   r//? [re ;/j      -DESIGN LEVEL

       /' -,,* f '',TRASH RACK

             3  .. tvPERMANENT POND - WATER QUALITY VOLUME


      (---' Z"~                   DRAIN GATE
       ~,OUTLET...






                   3-  TRASH RACK OR GRATE

                     ...i r-,        v 10 YEAR STORM LEVEL
MESH OR 
RACKS                    ORIFICES

             (. )
                 3_____      i       - PERMANENT POND WATER
                    " ;~'.  rQUALITY VOLUME

    ROCK---w!' '".;4                       CONCRETE PIPE, PRECAST MANHOLE
                                        SECTION, PREFERRED OVER METAL



            | 1 A ' *   An' '            41 DRAIN GATE














                               FIGURE 4-2
                       TYPICAL RETENTION BASIN
Ia^M[oN RoDS HP O UTLET STRUCTURES WITH PERMANENT POOL                   MURS
Pt.NNIN~ iD~rllCl COMZUO I  -N                                           CONSULTANTS







1       ~4.4.1.2  PHYSICAL FEATURES OF A BASIC RETENTION BASIN

               There are certain basic physical features of retention basins that have been found
               to increase the efficiency and effectiveness of the basin operation. The following
               design guidelines describe those features that can optimize the stormwater
               quantity control function and facilitate maintenance of the facility. These guidelines
               should be used as minimum requirements to produce satisfactory basin designs.
               Basin Side Slooes: The side slopes of the basin should be at a maximum slope
               of 3:1 horizontal to vertical for maintenance and ground cover control. It steeper
               slopes are required they should be paved.

 I            ~~~~Permanent Pool Side Slooes: The side slopes entering the water should taper off
               at a slope of 5:1 or less for the first ten feet where the slope can be increased to
 3            ~~~~3:1. This is to reduce the possibility of people or animals not being able to get out
               of the water should they fall in.

  3            ~~~~Embankment: The basin height should allow for aminimum oflIfoot of freeboard
               above the elevation of maximum water storage. If the height of the embankment
               exceeds 25 feet from the downstream toe to the top, and the basin capacity is
  3            ~~~~greater than 50 acre-feet, the Commonwealth of Virginia Dam Safety Regulations
               must be addressed.

  I            ~~~~Pond Confiouration: The pond should be configured to minimize short-circuiting
               of the stormwater flow. The most direct way to achieve this is to maximize the
               distance between the inlet and the outlet. A minimum length to width ratio of 3:1
  I          ~ ~~~is recommended.  If the local site conditions inhibit the construction of a relatively
               long narrow facility, baffles or gabions or other materials should be placed within
               the pond to "lengthen" the stormwater flow path as much as possible.

               The invert elevation of all inlet pipes should be at or within one foot below the
               surface of the permanent pool.

               The low flow stormwater outlet orifice should be negatively sloped so that it draws
  3           ~~~~water from at least one foot below the permanent pool surface.

               Reinforced concrete should be used in the construction of all risers, barrels, and
               pipes in the stormwater outlet structure to provide for greater longevity.
               Corrugated metal should not be used.
               The riser should be located within or at the face of the embankment where
               possible. This facilitates future maintenance and prevents flotation problems.
               All retention basins should have a maintenance drain to allow the basin to be
               completely emptied for maintenance and repairs and sediment removal. This
               should be a gate valve or slide gate with a positive seating head.

                                                  4-6







      Maintenance access must be provided. The access way should be a minimum of
      10 feet wide, with widths of 15' being common, have a maximum slope of 5:1
      (H:V), and never cross the emergency spillway. Slopes less than 5:1 are preferred
      and 10:1 are not uncommon. The steeper slopes will result in more rutting and
      access road maintenance.

      Additional volume should be provided for sediment accumulation. A rule of thumb
      is to add 25% of the volume, but detail calculations based on watershed sediment
      yields and basin trap efficiency will provide a more accurate volume.

      A sediment forebay should be constructed near the inlet to trap sediments entering
      the basin with the stormwater.  Methods to calculate a required forebay volume
      can be found in the references, but in general the forebay should be able to detain
      the seasonal average inflow for about five minutes.

4.4.2 QUALITY CONTROL GUIDELINES

      This section contains design guidelines for use when retention basins are to
      achieve stormwater quality control. They should be utilized in conjunction with the
      quantity control guidelines described in Section 4.4.1 where necessary to ensure
      the basin performs all of its required functions.

4.4.2.1  METHODOLOGY OF COMPUTATIONS

      Stormwater quality control or enhancement in retention basins is a function of the
      detention time available for solids and other pollutants to settle out of the flow and
      the biological action that can take place in the permanent pool.  Alternative
      measures to provide for additional detention time and other potential treatment
      processes are described in Sections 4.4.3 and 5.0.

      The requirements for a retention basin to provide water quality control will depend
      upon its location.   As shown in Table 4-1, stormwater quality control or
      enhancement is required in areas under the jurisdiction of either a local stormwater
      management program developed under the Virginia Stormwater Management
      Regulations or the Chesapeake Bay Preservation Act. The minimum levels of
      stormwater quality control required, and the procedures for calculating and
      designing those levels, also depend upon the regulations to which the basin must
      conform.

      State Stormwater Manaaement Reaulations: The state Stormwater Management
      Regulations (VR 215-02-00), while recommending planning on a regional or
      watershed basis, also impose some minimum restrictions on the enhancement of
      water quality through the use of retention basins. These requirements must be
      met in any locality adopting a stormwater management program in accordance
      with the Regulations.


                                        4-7







U1~ ~The Virginia Regulations require that the permanent pool volume in retention basins
             is at least 3 times the "water quality volume" (the first 0.5-inch of runoff over the
             entire development area).   The remainder of any storage requirements in the
             basin will depend on the quantity control function.

             Chesapeake  Bav Preservation  Act Requirements:   The  Chesapeake  Bay
             Preservation Act establishes criteria relating to performance standards, best
             management practices, and planning and zoning concepts to protect the quality
             of state waters while allowing appropriate use and development of the land. While
             the standards do not directly address retention basins, the performance of any
             best management practice implemented within preservation areas designated
I)~ ~under a local program must meet their requirements.
             In general, the water quality enhancement goals of the CBPA include:

               ï¿½   For new development, the post-development nonpoint source pollution
                   runoff load shall not exceed the pre-development load based upon average
 *I~ ~~ ~land cover conditions.

                   The redevelopment of any site not currently served by water quality best
 *1~ ~ ~management practices shall achieve at least a 10 percent reduction of
                   nonpoint source pollution in runoff compared to the existing runoff load from
                   the site. Post-development runoff from any site to be redeveloped that is
                   currently served by water quality best management practices shall not
                   exceed the existing load of nonpoint source pollution in surface runoff.

31~ ~        The CBLAD Local Assistance Manual includes a Guidance Calculation Procedure
             that outlines the steps needed to determine if a BMP meets the criteria. The
             Guidance Calculation Procedure is included in this manual as Appendix B.
             Because nonpoint source pollution can include many different contaminants and
             compounds, the calculation procedure is based upon the "keystone pollutant"
             concept. The keystone pollutant is an indicator pollutant, the existence of which
             provides an estimate of the total level of pollution in the runoff. The keystone
             pollutant for the Tidewater Virginia area is total phosphorus.

~~I  ~When a stormwater management facility is proposed outside of a CBPA (RPA or
             RMA), it is recommended that the projected water quality enhancement be
             calculated. Although other methods can be used, the Guidance Calculation
             Procedure provides an estimation.  It has been recognized that the CBLAD
             procedure should not be used without understanding its limitations and lack of
             historical data. Long-term monitoring of all types of structural and non-structural
             BMPs will allow more detailed calculations of removal efficiencies for a variety of
             pollutants. It is important to evaluate each situation and not to apply blanket
             requirements arbitrarily. This is especially critical if the procedures or methods are
             used for regulatory or enforcement purposes.


                                              4-8







I      ~4.4.2.2  PHYSICAL FEATURES FOR WATER QUALITY ENHANCEMENT

              The physical design of retention basins specifically for water quality enhancement
              is still a relatively new procedure. There are, however, some features and
              configurations that have been shown to provide successful results to date.

 I           ~~~Permanent Pool Volume:  The size of the permanent pool in a retention basin
              compared to the size of the contributing watershed is a primary factor in basin
              performance. While larger ponds are typically more successful, there seems to be
              a certain threshold size after which further water quality improvement by
              sedimentation is negligible. From the standpoint of time, most pollutants have
              settled out at their maximum level after 24 hours. After 48 hours, little benefit is
              gained by sedimentation.
              The state Stormwater Management Regulations require that the permanent pool
              be at least 3 times the water quality volume for a development area in size. Other
              agencies and localities base their criteria on retention time or some other factor.
 3           ~~~~It is recommended that the initial basin sizing conform to the State Regulations at
              a minimum, with further calculations and evaluations based on the desired pollutant
              removal efficiency.

              The permanent pool needs to be sustained by low flows either generated by
              groundwater or rainfall. This is discussed in Section 2.2.3 of this document. If the
 3           ~~~permanent pool is at groundwater level, the seasonal fluctuation needs to be an
              item of concern especially if a wetlands fringe is established.

 3           ~~~~Pond Shape:  Short circuiting can affect the water quality function of a retention
              basin. The basin, therefore, must be designed to prevent short circuiting. This is
              done by maximizing the length between the inlet and outlet. The recommended
              minimum length to width ratio for a retention basin is 3:1. Long, narrow, and
              irregular shapes for retention basins also reduce the surface area exposed to wind,
              which, especially for shallower basins, prevents the resuspension of previously
 I          ~ ~~~settled material. For purely aesthetic effects, irregularly shaped basins also appear
              more natural, or less "engineered."

 3           ~~~~Pond Depth: Pond depth is an important design criteria since most of the pollutant
              removal is accomplished by settling. Since very shallow basins may be prone to
              resuspension of materials by wind or flow effects, and deep basins can be subject
 I          ~ ~~~to thermal stratification and can release pollutants back into the water, an average
              pond depth of 3 to 6 feet is recommended. Deeper ponds are being used and
              with proper design to account for public safety and groundwater problems, they
              could be considered. Thermal stratification is not usually a problem until depths
              become greater than 30 feet.
 I           ~~~A 10-foot wide shelf with a 5:1 slope is recommended around the perimeter of
              basin to provide a margin of safety for the public. If aquatic plants or a wetland

          U                                      ~~~~~~~~~~~~4-9







 I           ~~~fringe is provided on the bench, the depth should be about 1' or a 10:1 slope.
              The stability of soils should be considered.

 I           ~~~~In general, the outlet structure should be located in the deeper portion of the basin
              so that the basin naturally drains to it and the maintenance drain can completely
 3           ~~~~empty the basin if necessary.

              Vegetation: The growth and establishment of vegetation around the perimeter of
              a retention basin can:

                   * enhance pollutant removal
                   * provide a habitat for wildlife and waterfowl
                   * protect the shoreline from erosion
                   * trap incoming sediment
                   * provide an environment for microorganisms that can remove pollutants from
                     the stormwater biologically
 3           ~~~~Additional details are provided in Section 5of this manual.

              Site Reauirements: Retention basins are not recommended in watersheds of less
              than 10 acres unless a natural spring exists on site. Maintaining a permanent pool
              is difficult in small drainage areas. Proposed basins in drainage areas of less than
              30 acres should be evaluated carefully. A general rule of thumb is that 4 acres of
              contributing watershed is needed for each acre-foot of storage.

               If soils beneath the proposed basin are permeable, such as SOS soil groups "A"
 3           ~~~and 'IS", a liner may be necessary to prevent the basin from emptying through
                 infltatin.Liners can consist of clay or other impermeable soils or geotextile
              materials. Dense compaction of the soils may also be successful.

              A buffer area of about a 25-foot width should be included around the retention
               basin. This buffer area should receive vegetation including trees and be managed
 I          ~ ~~as a meadow, not as a lawn.  No trees, however, should be planted in the
              embankment structures.

I      ~4.4.3  DESIGN MODIFICATIONS FOR WATER QUALITY ENHANCEMENT

              This section contains descriptions of modifications to typical retention basin design
              that can be made to enhance the stormwater quantity and quality control functions.
              The modifications primarily relate to the basin storage and release facilities. An
              additional modification which can be considered, especially for a retention basin,
 I          ~ ~~is the establishment of a wetland or shallow marsh area in and/or around the
               basin. These aspects are described in section 5.0. The design guidelines
               included for these modifications are in addition to and should be used in
              conjunction with the other retention basin guidelines described above.


          I                                     ~~~~~~~~~~~4-10







1      ~4.4.3.1  EXTENDED RELEASE

               The use of extended release in a retention basin relates directly to the stormwater
               quantity control function of the basin, and also impacts the water quality
               enhancement function. In this case, the stormwater outlet structure is designed
               to extend the release of the runoff flow. Instead of being located at the bottom of
               the basin, however, the extended release orifice is the low flow stormwater quantity
               control orifice, located above the permanent pool elevation.

 I           ~~~~One method of providing extended release in a retention basin is by having the low
               flow outlet pipe at a negative slope from the riser to the outlet structure. The pipe
               opening is located about 1 foot below the permanent pool surface. This keeps the
               floating debris from blocking the inlet pipe. The orifice can be protected by wire
               mesh for further protection.

I      ~4.4.3.2  WATER QUALITY STORAGE

 3           ~~~The Commonwealth of Virginia Stormwater Management Regulations require the
               release of the "water quality volume" of stormwater over a minimum 30-hour time
               from the point of peak stormwater storage in a detention basin. However, in a
 3           ~~~~retention basin the water quality storage is a permanent pool with a volume equal
               to three times water quality volume computed the same way as for a detention
               facility. That is, the volume is equal to the 0.5" of runoff multiplied by the total drain
 3           ~~~area. The flood storage above the water quality storage has no specific release
               time and is governed by the hydrology of the event and the hydraulics of the
               system.

        4.4.3.3 RETROFITTING EXISTING FACILITIES

 3           ~~~~For retention basins, most retrofitting tasks involve modifying the outlet structure
               or improving the storage capacity of the basin. These can include:

  3              .~~~~ excavating the existing basin to create additional storage capacity;
                 *   adding to the elevation of the embankment, also creating additional storage
                     capacity; or
   I             *  ~~~~constricting or modifying the outlet orifices, thus changing the release rates
                     and storage configurations.

 I           ~~~The new storage capacity can be used to improve the quantity control function by
               releasing flow at a lower rate, extending the release time of the stormwater runoff,
               increasing the permanent pool volume, creating a shallow marsh or wetland area,
              or a combination of all of the above.
              The basic design guidelines for any of these tasks will be the same as if the basin
              were being constructed originally. The hydrologic and hydraulic analyses must,
              however, be performed with exact information concerning any limits that the

                                                 4-11







 I           ~~~~existing facility configuration will have on the desired functions of the basin.

I      ~4.5   CONSTRUCTION AND OPERATIONS ISSUES

               The construction, operation, and maintenance of all types of retention facilities are
               primary factors in their success rate and longevity. A basin can be designed
               utilizing proven criteria and state-of-the-art techniques, but unless it is constructed
               according to that design and maintained so that it continues to emulate the original
               design, it will not be able to operate efficiently and achieve its desired water
               quantity and quality control functions.
 3           ~~~~The following sections contain guidelines for successful construction, operation,
               and maintenance of retention basin BMPs. It is incumbent on the administering
               locality that these guidelines, along with other proven and accepted techniques,
 *           ~~~~be adhered to throughout the operating life of the facility.

I      ~4.5.1  CONSTRUCTION GUIDELINES

               The construction of retention basin BMPs must be completed both according to
               the original design and utilizing practical knowledge about the intended function of
 I          ~ ~~the facility.   The  following  guidelines  include techniques,  methods,  and
               recommendations formulated from construction and operation experience.

 3           ~~~~Construction Schedule: Because it is typically a portion of the site utilities, and it
               is often used as a sediment basin during the construction of the upstream
               development, a retention basin should be one of the first facilities planned and
 I          ~ ~~~constructed on the site. Additionally, any temporary drainage or erosion control
               facilities should be constructed during the initial phases. Temporary facilities can
               most eff iciently be used to keep stormwater or erosion and sediment damage from
               occurring to the permanent facilities.
               Site Layout and Preoaration: The outside perimeter of the retention facility should
  I          ~ ~~be staked out before any clearing and grading begins. The embankment and any
               appurtenant work like stream bank stabilization should also be staked at this time.
               In general, at a minimum the following layout stakes, marked for grade, should be
               used:
                    * top of slope of the basin excavation
   I             *  ~~~~bottom of slope of the basin excavation
                    * centerline of embankment
                    * front and back toe of slope or embankment
                    * several grade stakes through the basin floor
               The first stakes to be set should be the centerline of the embankment and the top
               of the slope of the basin excavation.


                                                  4-12







~~I  ~The outlet control structure should be staked, constructed, and backfilled before
             general earthmoving is started.

UI~ ~The site must be dry for successful excavation to take place. If a site is wet, or if
            the site is expected to be wet during construction, measures should be taken to
             ensure proper conditions. These measures could include direct drainage trenches
            to points of lower elevation or the collection of runoff and surface water in sumps
            that require pumping. If a typically wet site has been selected for ease in
             maintaining the permanent pool, appropriate dewatering techniques should be
             used.

             Embankment  Construction:   Good  fill material, suitable soils, and  proper
            compaction  techniques  are  imperative for the  construction  of a  stable
            embankment. Increasing the embankment breadth and decreasing the slope can
             also be important measures.

             Placing embankment fill should be performed in sequential lifts of 6- to 8-inches
31~ ~each. An entire lift across the embankment should be completed before the next
             lift is begun. This allows any moist soils to dry and additional compaction to occur
            from the application equipment.

            The proper construction of a cutoff trench is imperative to prevent any undermining
             of the embankment. A cutoff trench is a trench excavated along the centerline of
            the embankment before the fill materials are placed. It must be constructed from
             a relatively impermeable soil. The cutoff trench can be constructed wide enough
            for the bulldozer or other equipment to work within it. The impermeable soils
             should be placed in 6- to 8-inch lifts. The cutoff trench must extend from several
            feet below the existing grade up into the embankment fill.

3I~ ~The primary location of concern in terms of potential embankment failure is the
             point where the outlet pipe passes through it. The placement of antiseep collars
            to prevent soil piping failures is of key importance. An antiseep collar is a metal,
             concrete, or masonry shield which is placed around the pipe within the fill
             embankment. The backfill material around the outflow pipe should also be
3I~ ~properly placed and compacted to help prevent embankment failure.

             Inflow and Outflow Structures: The inflow and outflow control structures must be
             constructed correctly in order for the basin to operate as intended and designed.
             Both structures require accurate surveys to ensure they are placed at the correct
             elevations.

Is~ ~       The inflow structure is generally less critical than the outflow structure, but it still
             requires accurate vertical placement. Slope protection should be used ahead of
            a stream inflow structure, downstream of an inflow "spillway" of any length, and at
            the basin discharge point. The inflow control structure must be constructed so
            that it directs the flow into the basin forebay, or across the basin floor as intended

                                              4-13







 I           ~~~~by the basin design.

              The outflow structure may contain several key components that must work in
               concert with each other. These may include weirs, orifices, grates, or other flow
               control sections. These must all be properly constructed and placed at accurate
               elevations. If the structure is constructed offsite, it must be inspected carefully
 I          ~ ~~upon delivery to the site for any defects or misalignments of any of the
               components. The final placement and/or construction must be exactly as shown
 *           ~~~on the construction documents.

               Construction Ooerations: A retention basin is most subject to externally caused
               damage during its construction. Since most basins will be located at the low
               points of a site, they must be protected from extreme rainfall events that may occur
               during construction. Vegetative cover and the emergency spillway must also be
 *           ~~~completed as quickly as possible during the construction phase.

              The use of an inspector is one of the best method of ensuring that the detention
               basin is constructed as designed. This inspector may an in-house representative,
               someone from the designing firm, or from an outside consultant or inspection
               company. The inspection may be full time or part time. The primary focus of the
 *           ~~~~inspections should include:

                   . embankment fill placement
   *            .  ~~~~embankment fill material
                   * implementation of adequate erosion and sediment control

 *           ~~~~Additional details can be found in Section 6of this document.

I      ~4.5.2 COST ESTIMATES

              The graph in Figure 4-3 shows the average cost to construct a retention basin of
               a size which would be required for a given total project area. Likewise, the graph
 I         ~ ~~in Figure 4-4 compares the construction cost associated with the volume of
              stormwater which must be detained. These figures are generic and should only
 *           ~~~be used as guidelines.

              The pre-development C-factors used for the associated hydrographs ranged from
              0.35 for a small I1-acre site to 0.20 for a larger 50-acre site. Post-development C-
 I         ~ ~~factors were determined by the types of development typically associated with
              various sizes of land. These post-development C-factors ranged from 0.80 for
              small sites to 0.50 for larger sites. Retention for the water quality volume was also
 I         ~ ~~~included in these estimates, as was the permanent pool in a retention facility with
              a volume equal to three times the water quality volume. The costs of constructing
              retention facilities were generated from recent contractor estimates of various basin
              sizes and were compared with Means Sitework and Landscape Cost Data, 1991.


          U                                    ~~~~~~~~~~~4-14







        45
              NOTE: COST ARE CONSTRIU.TION COST, Il 1991,
              LAND, ADMINISTRATIVE, ANDI FINANCING CCOSTS
              ARE NOT INCLUDED.


        40






        35






  :  30




 0








 I
 0
 II









 0 I s


        5
 I      5 /






















                        10           20            30           40             50
                                TOTAL PROJECT SIZE (ACRES)
 __A___D                              FIGURE 4-3                                      URS
                  o RAS TOTAL PROJECT SIZE VS. FACILITY COST                          CONSULTANTS
,r
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ONSLAT
















         NOTE: COST ARE CONSTRU1' TION COST. Ilb 1991.
         LAND. ADMINISTRATIVE, ANDI FINANCING CCSTS
         ARE NOT INCLUDED.


.j   40
0
13
LA.
0


  30 
  30

0
z 2


  20





o0
       I,






                 20          40      so           so                1OO          120           0t0
                             STORMWATER TO BE STORED (THOUSANDS OF GALLONS)
                                             FIGURE 4-4

                            .t R12~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~cW4LA DS R
                      CONSTRUCTION COST VS. STORMWATER TO BE STORED                                              URWAANS







I      ~~4.5.3  FACILITY LIFE EXPECTANCY

               The life expectancy of a retention facility is directly proportional to the quality of
               construction and the maintenance. If properly placed in stable soil, concrete can
               be expected to last up to fifty years. Metal facilities can be expected to last twenty
               years or more if properly maintained. Aluminum alloy products will have a longer
               life if properly specified. Preventative and corrective maintenance are crucial to the
               success of the forebay and the basin bottom.

U      ~4.5.4 MAINTENANCE REQUIREMENTS

               The agency responsible for long term maintenance must be identified during the
 I          ~ ~~~planning stages. Even though a retention facility only performs its design role on
               an occasional basis, it must be constantly prepared to do so. A comprehensive,
               regularly-scheduled maintenance program is the key to any successful stormwater
               management facility. Such a program is comprised of funding, maintenance,
               inspection, training and program reviews.

               A retention basin will be useless if funding for its maintenance is not adequate.
               Funding considerations include: staffing, equipment, and material needs; facilities
               for storage of materials; storage, maintenance and replacement of equipment;
               training and administrative costs; seasonal effects; long-term capital improvements;
               and emergency appropriations for unforeseen problems.

               The physical portion of the maintenance program should include aesthetic,
               preventive and corrective measures.

               Regular, major, and informal inspections should be performed. Major inspections
               should be performed semi-annually and after each major storm.   Regular
 *           ~~~inspections should be conducted to determine the need for and the effectiveness
               of maintenance work. Informal inspections should be conducted during every visit
               to the facility by maintenance personnel, and, if possible, prior to the occurrence
               of a major storm. Further details can be found in Section 6 of this manual.
               A training program should include: maintenance and inspection techniques;
 I         ~ ~~proper record keeping,and stormwater program goals and objectives. Particular
               attention should be paid to the purpose and operation of stormwater management
               facilities, the importance of thorough maintenance, and the health, safety and other
               consequences of maintenance neglect.
 *           ~~~Additional information can be found in Section 6of this document.

        4.6   PLAN SUBMITTAL REQUIREMENTS

  I           ~~~Submittals for detention facilities need to be made to the locality in accordance
              with local ordinances and regulations for Stormwater Management, Erosion and

                                                4-15







             Sediment Control, and Chesapeake Bay Preservation Ordinances. Only State
             agencies need to submit plans to the Division of Soil and Water Conservation and
             the Chesapeake Bay Local Assistance Department.

       4.6.1 AGENCIES

             The Department of Conservation and Recreation Division of Soil and Water
             Conservation can provide assistance on the Erosion and Sediment Control Law,
3           ~~~Stormwater Management Act, and Dam Safety Act and Regulations.

             The State Water Control Board, Permits Section can provide information on the
             stormwater NPDES permit.

             The Chesapeake Bay Local Assistance Department can provide information on the
3           ~~~Chesapeake Bay Preservation Act and Regulations.

             The Norfolk District Corps of Engineers Construction Operations, Regulatory Permit
3           ~~~~Section, issues permits for wetlands disturbance and navigable water crossings.

             VMRC and local Wetlands Boards need to be contacted for wetlands or projects
3           ~~~~impacting the shoreline.

             If -a permit is needed for construction because of wetlands disturbance or
             interference with a navigable stream, then a permit would probably be needed for
             maintenance dredging. If no permit is issued or needed for construction, then no
             permit would probably be needed for maintenance dredging. Dredge materials
             under either circumstance would probably not be regulated. At this point in time,
             there are no requirements for disposing of materials dredged from retention or
             detention basins; however, it would be prudent to discuss the issue with the Corps
3           ~~~~of Engineers prior to dredging.

       4.6.2 SUBMITTAL CHECKLIST

I           ~~~~A checklist has been prepared for overall guidance and can be found in Appendix
             A. Localities may have their own checklist which the applicant would need to
3           ~~~~follow. In addition, the Division of Soil and Water Conservation has developed a
             checklist to be used by State Agencies.










         I                                    ~~~~~~~~~~~~4-16







5.0   ESTABLISHMENT OF WETLANDS IN STORMWATER DETENTION OR
      RETENTION BASINS

      Well-planned stormwater management practices will reflect the natural processes
      of the environment. The artificially-created or planned fringe wetland, in a basin
      designed to also provide stormwater control, makes practical sense.

      The design recommendations described within this section are geared toward a
      functional use of available resources. The wetland basin can be part of a broad
      planning approach which combines many values and uses within an area
      traditionally considered wasted space. Therefore, the most desirable approach is
      to provide a balance of functions and not strictly "wetland creation" or "mitigation
      effort." A clearly stated purpose and operating plan during planning and design
      will address the future maintenance needs and purpose of the basin, which is
      ultimately stormwater control.

      The establishment of wetlands in stormwater detention or retention basins is
      another step in providing multi-objective functions.  The benefits of creating
      wetlands in new or existing detention basins include improved water quality of
      stormwater discharges, increased urban wildlife habitats, and improved
      environmental awareness in the local communities. The success of the wetland
      basin depends on long-term commitment to monitoring and maintaining the facility.
      The most effective way for local jurisdictions to manage these facilities is through
      community cooperation in a joint 'project team' relationship. Local subdivision
      organizations, clubs, scout troops, or environmental action groups serve as project
      sponsors to provide routine maintenance and watch dog services, while the local
      jurisdiction provides periodic maintenance.

      Stormwater basins can be attractive, functioning wetland habitats. Standard basin
      design is generally not appropriate for wetland establishment, but creative designs
      that consider both the criteria for stormwater management and the wetland
      establishment are possible. The driving force behind any wetland is the hydrology.
      If the appropriate conditions in the basin can be established and maintained
      through the growing season, then wetland creation in the basin is possible. Care
      must be taken to choose plant species that are compatible with stormwater basin
      functions. Above all, an aggressive maintenance program should be instituted and
      adhered to.

      A wetland basin should always be considered where the proper hydrologic
      conditions are present, sufficient land is available for creation of the basin, and 404
      Permit requirements for subsequent maintenance or expansion of the facility are
      not too restrictive.







                                        5-1







                                    TABLE 5-1
                 BENEFITS OF ESTABUSHING WETLAND BASINS

      Socioeconomic Values                          Environmental Qualitv Values
      * Flood control                               * Water quality maintenance:
      * Erosion control                                   Pollution filter
      * Groundwater recharge and water                    Sediment removal
         supply                                           Oxygen production
      * Fishing                                           Nutrient recycling
      * Recreation                                        Chemical and nutrient
      * Aesthetics                                           absorption
      * Education and scientific research           * Aquatic productivity
                                                    * Microclimate regulator
      Fish and Wildlife Values                      ï¿½ World climate (ozone layer)
      * Fish habitat
      * Waterfowl and other bird habitat
      * Furbearer and other wildlife habitat


5.1   GOALS AND CRITERIA OF SUCCESS

      In the initial stages of planning, the decision can be made to create a new wetland
      or establish a wetland in an existing basin being retrofitted. This decision will be
      made based upon defined goals that the basin system should be expected to
      accomplish. These goals must be realistic, reflect the nature of the surrounding
      community, and achieve success measurable against set criteria of performance.
      The nature of the facility and the criteria of performance will be defined by five
      major issues.

             1. Stormwater Control: Stormwater control will be the primary goal of the
             basin. The basin may be required for floodwater storage. The need for
             stormwater control probably will have been previously established, and will
             drive the nature and size of the facility.

             2. Water Quality: The water quality of stormwater runoff is increasingly a
             major concern to local municipalities. A wetland basin can significantly
             improve the water quality of the stormwater runoff.

             3.  Wildlife Habitats:   Wetland basins will naturally become  wildlife
             sanctuaries. The type and quality of the habitat can be controlled by design
             considerations.

             4. Community Impacts: The type of wetland basin implemented must
             reflect the neighborhood and community wherein it is to be placed.

             5.  Subsequent 404 Permit requirements must not prevent adequate
             maintenance or improvement of the facility for its primary purpose --
             stormwater management.

                                        5-2







 I           ~~~~Some of the parameters that will influence the goal setting procedure are-

                     * What are the drainage requirements?
                     * Will the facility be a retention or detention basin?
                      * Is the topography of available land compatible with the proposed basin?
                      * Will the facility serve as a wildlife area?
   I               ~ ~~~~* Is the neighborhood compatible with the establishment of a wetland
                        basin?
                      * Will runoff conditions negatively impact the wetland basin?
                       *Are there special maintenance concerns?
                      * Is community involvement expected?

 I           ~~~These parameters can be rated to develop the basis of design for the wetland
               basin. Additionally, long-term monitoring needs to be addressed and included in
              the design methodology in order to judge the success of the basin, and provide
               corrections as necessary. Figure 5-1 shows the inter-relationship of the design
               parameters. Figure 5-2 illustrates the target functions of the basin, stormwater
               control, water quality, and wildlife habitats.

        5.1.1 WATER QUALITY

 I           ~~~Wetland detention basins can significantly improve overall water quality of
               stormwater runoff. The basin will take advantage of physical, chemical, and
 *           ~~~biological processes to remove dissolved as well as suspended pollutants.
               Sedimentation of suspended solids, uptake by algae and rooted wetland plants,
               oxidation of organics, and absorption of nutrients and heavy metals will all occur
 *           ~~~~within the wetland basin.

I      ~5.1.2 WILDLIFE HABITAT

              The creation of the wetland basin can have a secondary benefit in creating a
               habitat for wildlife. A major concern of the continuing loss of wetland areas is the
               loss of wildlife habitat. Drainage basins and drainage channels can become part
               of a successful urban wildlife management plan. The incorporation of islands or
               pockets of wildlife habitat with corridors connecting to a 'core' wildlife area
 I          ~ ~~~provides a vital step in restoring wildlife habitats. The corridors can be drainage
               channels, power lines, or utility easements. The core areas are parks or other
               open area such as swamps or larger wetland areas. The shallow water of the
 I          ~ ~~~wetland basin will become the home to a variety of animals and insects, increasing
               the available food chain. The wetland basin can become a classroom for school
               children in the community, increasing their awareness of the delicate structure of
               the environment.








                                              FIGURE 5-1
                        DESIGN PARAMETERS FOR WETLAND BASINS



       DATA COLLECTION AND                             HYDROLOGY                    N             STANDARD
       ANALYSIS OF DRAINAGE                      Can a stabilized water balance                       BASIN
           REQUIREMENTS                                be maintained?                               DESIGN



       *                                                       I1
                           ~~~Shallow Detention Basin y~ ~Extended Detention Basin
I          ~Shallow Detention Basin          NWTRQALIYwt   grsiewtad.
          with aggressive wetland WATER QUALITY with aggressive wtlands
               w g veg etati n                 Is water quality the main goal?          deepwater pool and submerged
                                                                                 aquatic vegetation.


       I  -
          Basin layout and design                  WILDLIFE HABITAT                  y         Diverse wildlife habitats
          with wetland vegetation                s the basin connected to other            designed to encourage wildlife
         oriented toward minimum                 wildlife areas or open parks?                   Deepwater pools to
              *  maintenance.                        basin to       as birDeattract waterfowl.
              ma intenance.Is basin to serve as bird
                                                  sanctuary?




                                         I
         Heavily d eveloped residential     N       COMMUNITY TYPE                    y         Aggressive wetland basin
                 areas may not be compatible Is neighborhood compatible                   of maximum size, with deepwter
       with aggressive wetland designs                with wetland basin?                   pools and wildlife habitat diversity.
        Consider shallow, open basins,
                                            Is watershed industrial or            _______
         less aggressive vegetation,                                        I .
                                  with r eulraitlow density residential?
          with regular maintenance                  l   d







                                STORM WATER CONTROL


                              .~~ RAINFALL


      I  ~ ~      ~      ~      ..~~~~~.HYDROGRAPH WIO STORAGE


                                    MAXIMUM RETENTION
              0u


                      00                         ~~~~~~~~~~HYDROGRAPH   z
                                                 WITH STORAGE



                                      TIME -
                                    WATER Q3UALITY


                                            SE MENT TRAPPED BY VEGETATION
















     I                             ~~~~~~~~~WILDLIFE- HABITATS
            WRkENS-      -  HO RTBILL -
          PLO VERS-,-UPLAND        - LONGBILL-
                         -KILLDEER
H           ~~~~TERNS --BLACK~


                                        - LEASTE-ERO
                DUCKS-  ~      MALLAR RUDD

             RAILS-             - SORO -      OT_      ORIOLE

          ICTERIDS- - BOBOLINK-  -VRIA GALLINULE
                       ADOWLARK  -RED WI   YE WHEAD    GRACKLE
                                                       REDWING   MN

                UPLAND  LOWLAND SEDGE
                GRASSES GRASSES        CATTAIL. HARDSTEM            LMUSKRAT
                                          MUSKRAT'


                                                        Source:
                                    FIGURE 5-2          Burke, et. al.,
      H!Al .~.gns                                         Protectina Nontldal  URS
             _____RQ41~~~~       TARGET FUNCTIONS    Wetads, 1988.







5.2   IMPLEMENTATION GUIDELINES

5.2.1  REQUIRED DATA

      The following will be required in the development and design of a stormwater
      wetland basin.

             * Soil borings (classification of subgrade)
             * Exfiltration (percolation) tests
             * Seasonal groundwater elevations
             * Drainage area (size in acres)
             * Land use characteristics
             * Discharge conditions
             * Inlet invert
             * Topographic map of proposed site

5.2.2 PHYSICAL ASPECTS OF BASIN

      The physical aspects of the wetland basin are; the size of the wetland area, the
      size (shape and depth) of the deeper water pools in a retention basin, control of
      inlet velocity, control of short-circuiting, the outlet structure, and the water balance
      of the wetland.

5.2.2.1  WATER BALANCE

      The most important aspect of establishing the wetland basin is adequate control
      of the water balance. Most wetland plants have narrow water depth tolerances.
      Properly functioning basins will often have sufficient water levels in the spring, but
      during drought periods, water levels drop drastically. It is crucial to verify that pool
      elevations can be maintained during drought periods.

      Water inputs can include stormwater runoff, base flow, and groundwater. In the
      Tidewater region, seasonal groundwater elevations can vary widely. Groundwater
      infiltration can produce a basin which has an unacceptable depth variation to the
      narrow water depth requirements of most wetland plants. Most natural upland
      wetlands are standing depressions with an impervious substrata.  This can be
      artificially replicated in a designed wetland basin by using a clay layer, or a plastic
      pond liner.

      Recognizing that wetland vegetation has a narrow range of water depth tolerances,
      it is critical to calculate properly the water balance. Yearly, or average, total water
      budget calculations for the basin are not sufficient to determine whether water will
      be present in the basin during drought periods. Outlet structures which are
      designed to hold water longer than 24-30 hours which will flood and cover wetland
      vegetation can kill the wetland plant material.  The important calculation is to
      determine the wetland area which can be maintained during low, dry summer

                                         5-4







      month flows.  It is also important to control how long water will be detained at
      elevations which will totally flood the plants.

      Most evaporation calculations rely on           de termining factor. Shallow
      water is generally warmer and evaporates faster. In addition, the wetland basin will
      be vegetated, so transpiration must be considered. Data on evaporation and
      monthly rainfall can be found in Section 2.2.3. Figure 5-3 illustrates water balance
      calculation.

5.2.2.2  WETLAND SIZE

      It is recommended that as much of the basin bottom as possible be utilized for the
      wetland creation and still meet the other multi-purpose objectives. The contribution
      of the wetland to water quality enhancement will vary with the surface area
      available for the wetland because increased wetland area improves vegetative
      uptake and solids settling characteristics.   For those basins that use large
      increases in depth, instead of surface area, to control peak flows, the wetland will
      not contribute as significantly to water quality. The greatest benefit to wildlife and
      pollution control is achieved when the maximum area is used for the shallow
      wetland basin.

      Although it is strongly recommended that wetland basins be used in conjunction
      with extended release detention, that may not always be possible. If not, it is
      recommended that the surface area of the wetland account for a minimum of 3
      percent of the area of the subwatershed draining into it.

5.2.2.3  WETLAND PERMANENT POND:

      1. Freauentlv Flooded Areas: As mentioned above, the surface area of the
      wetland should be maximized in relation to the surface area of the entire
      stormwater basin. However, all wetland basin facilities should include a transitional
      area that is not entirely wetland and not entirely upland. This is part of the flood
      control and extended release volume or bordering areas that will be flooded
      whenever stormwater runoff enters the basin, but that will not contain standing
      water after the extended release period. This area will support a diverse group of
      plants that will thrive on the damp soil and which can tolerate the brief periods of
      flooding associated with extended release detention. However, most of these
      species would not be likely to tolerate constant inundation. The plant community
      in this area will provide cover and a food source (seeds) for certain nesting bird
      species. The transition zone should include the area within 10 to 20 feet from the
      edge of the permanent pond.

      2. Shape and Deoth of the Permanent Pond: Shallow water (i.e., < 12 inches)
      promotes the growth of most species of emergent wetland vegetation. Since
      emergent vegetation is a contributing factor in the pollution removal capability of
      the wetland, and provides value to wildlife, the water of the wetland should be

                                         5-5



  mm - - -- ---- ---  -  m -








                       WATER BALANCE
            CALCULATED DURING DROUGHT CONDITIONS

BASE FLOW - GROUNDWATER GAIN + LOW INTENSITY RAINFALL( =
           EVAPORATION 4 TRANSPORTATION + INFILTRATION

(1) AT OTHER TIMES, EXCESS FLOW ABOVE OUTLET CONTROL ELEVATION WILL BE DISCHARGE.
(2) FOR LOW INTENSITY SUMMER RAINFALL EVENTS, ASSUME 0.2 INCHES OF RUNOFF OVER IMPERVIOUS AREA OF WATERSHED EVERY 4 DAYS.



        : ^   b a   FRINGE WETLANDS         SHALLOW WETLANDS                EDGE WETLANDS

                       INNUDATED          ONLY OCCASIONALLY                 PERMANENTLY
                DURING RAINFALL EVENTS            DRY                            WET


                                                                 OUTLET CONTROL ELEVATION       .

                                         AMAY~Ua.  a REQUIRE  TO M   JWATER BALANCE R





   8EA80NAL GROUNDWATER FLUCUATIONO C. .                                                ..- .  ..... ...  

   LOW GROUNDWATER LEVEL                                         /                                                      TN
                                                                     PERMANENT POOL- IN DETENTION BASIN,
             IMPERVIOUS CLAY LAYER OR PLASTIC POND LINER  SMALL VOLUME AND 8HALLOW DEPTH, IN
             MAY BE REQUIRED TO MAINTAIN WATER BALANCE.                   RETENTION BASIN EQUAL TO 3 X WATER QUALITY
                                                                     VOLUME INCLUDES WATER BALANCE VOLUME..



                                                   FIGURE 5-3
                               WETLAND COMMUNITIES AND WATER BALANCE                                              CONSULTANTS







I           ~~~limited to the depths conducive to the growth of emergent vegetation.

             Approximately 75% of the wetland should have water depths less than 12 inches
             and 25% of the wetland should have depths ranging from 2 to 3 feet. The deeper
             depths will result in open water, which will make the entire wetland more attractive
             to waterfowl.

             Waterfowl seem to prefer a habitat with both cover and open water (Weller 1978).
             In addition, the deeper water will favor the growth of submerged aquatic
             vegetation, another food source for waterfowl. It is important to note that it may
             not be appropriate to attract waterfowl to the wetland basin if water quality is an
3           ~~~overriding concern. This is because waterfowl can add excessive nutrients and
             bacteria to the water column with their excrement. The additional nutrients
             promote algal growth, which in turn causes eutrification of the system. Eutrification
3           ~~~leads to oxygen depletion and poor water quality.

             The deeper area of the wetland should include an outlet structure design such that
             sediment does not block the outlet pipe. A basin forebay should be established
             at the pond inflow points to capture larger sediments.
             After passing through the forebay, the incoming runoff should pass through
             shallow areas of emergent vegetation in order to maximize sedimentation and the
             mixing of runoff with the shallow pond water. As much vegetation, and as much
3           ~~~~distance, should separate the basin inlet from the outlet as possible as discussed
             in Sections 3 and 4 of this document. This will avoid a "short-circuiting" effect
             whereby stormwater runoff flows out of the wetland with only minimal treatment by
3           ~~~the wetland.

             Seventy-five percent of the wetland should be 12 inches deep or less. Of this area
3           ~~~one third should range from 6 inches deep to 12 inches deep, and the remaining
             two thirds should be 6 inches or less in depth. The water depth should slope
             gradually but regularly from the 12 inches depth at the edge of the deep area to
I          ~ ~~the 6 inches depth, and then fix the basin depth at roughly 6 inches throughout
             most of the remaining two thirds of the shallows. The water should gradually get
             shallower about 10 feet from the edge of the pond.

             It is necessary to have precise depths and a uniformly graded substrate. Grading
             also will "soften up" the basin soil enough that no supplemental disking or plowing
I          ~ ~~~will be required. In retrofitting existing facilities, if the basin does not need grading
             or excavating the soil should be broken up with a disk or chisel.

I           ~~~~Several wetland layouts are possible. Basins can have the inlet and the outlet at
             opposite ends of the basin. If this is not possible the incoming stormwater runoff
             should be channeled away from the outlet structure into the stand of emergent
             vegetation.


         1                                     ~~~~~~~~~~~~~5-6







1       ~5.2.2.4  MODIFICATIONS TO THE INCOMING FLOW OF WATER

               As discussed above, if the outlet and the inlet structures must be located close
               together it is recommended that the incoming stormwater runoff be channeled
               away from the outlet structure. If runoff enters the wetland with high velocity,
               bottom scour and damage to the emergent vegetation could result. If high water
 I          ~ ~~~velocity is a potential problem, some type of energy dissipating device should be
               installed. For channel flow rip-rep placed along the channel should be installed as
               needed. For basins in which runoff enters directly through a pipe the runoff should
 I          ~ ~~be directed at an energy dissipating structure.  This would be a particular
               requirement for inflow pipes which discharge from above the surface of the
 *            ~~~~wetland.

        5.2.2.5 OUTLET STRUCTURE AND EXTENDED DETENTION

 I            ~~~~The requirements of an outlet structure for a wetland basin include damming up
               the volume of water needed for wetland creation, detaining a certain volume of
               water for extended periods of time, and permitting water to flow from the wetland
               without blockage. Outlet structures include the common barrel and riser type and
               a simple damming device for retrofitting existing peak flow attenuation devices.
               The structures operate by damming up the base flow that previously left the basin
               at ground level. The structures have an orifice (or orifices) that permits the runoff
               to leave the basin. Above the orifice is a section of the outlet structure that detains
               a certain volume of runoff during storm events as runoff enters the basin faster
               than it can leave the basin through the orifice. Once the storm event has ended,
               the water level in the basin drops as the detained runoff flows out of the basin
 *            ~~~~through the orifice.

               The Washington, D.C. area portion of the U.S. Environmental Protection Agency's
 3            ~~~Nationwide Urban Runoff Program (MWCOG 1983) demonstrated the value of
               "extended detention for dry stormwater basins" or extended release from detention
               basins. The low volume of permanent water storage in shallow wetlands in
 *            ~~~comparison to deep ponds and lakes makes it important to incorporate extended
               detention into the wetland basin design. Based upon previous studies, it is
               recommended that the runoff from a one year storm be detained for at least 30
 *            ~~~~hours for wetlands design considerations.

               The orifices used for extended detention will be vulnerable to blockage from plant
               material or other debris that will enter the basin with stormwater runoff. Some form
               of protection against blockage will be necessary. The device used to protect the
               orifice must allow water to pass freely through the orifice, therefore some type of
 *1         ~ ~~non-corrodible wire mesh is recommended.  Wire mesh laid directly across the
               orifice will not suffice since the mesh can become blocked almost as easily as the
               orifice itself. Instead, the mesh should have some depth to it, so that the area that
               must be blocked in order to interfere with water flow is relatively large. Given
               enough time, most of the floating organic materials (e.g., wood, vegetation, paper)


          1                                      ~~~~~~~~~~~~~5-7







      will lose buoyancy and sink. Hemispheric, pyramid and box-like structures are
       possible, among others. The devices should be made of mesh and reinforced so
      that they do not collapse and lose their functional surface area. They also should
       be attached very securely to the outlet structure.  Routine maintenance of the
       protective device will be required. The smaller the mesh on a protective cover, the
       more efficient it will be at protecting the outlet structure, and the more likely it will
      clog. Design also should take into account possible damage and ice and freezing
      of the strainer device.

5.2.3 BIOLOGICAL ASPECTS OF THE BASIN

5.2.3.1  WETLANDS SUBSTRATE

      Wetland soils that have supported wetland vegetation will generally contain a pool
      of plant propagules. This type of soil can prove valuable when constructing an
      artificial wetland, since many of these propagules can be expected to generate
      vegetation in the new wetland. However, it is unlikely that a site for an artificial
      wetland will contain a preponderance of wetland soil, or that such soil will be
      valuable for application to the basin. On the other hand, it is likely that dry basins
      scheduled for conversion to wetland basins will contain at least isolated pockets
      of such soil. These basins will have base flow, and the presence of base flow
      generally results in wetland vegetation. The vegetation may be located along the
      channel that contains the base flow, or there may be pockets of wetland vegetation
      in low areas of the basin that do not drain completely.

      Opinions differ as to whether this type of soil should be preserved when the
      wetland basin is created. If excavating is necessary the wetland soil should be
      saved and then spread over the graded basin in the areas planned for shallow
      water only if it can be done without stockpiling. Stockpiling and mechanical
      handling of the existing wetland soil will have detrimental biological effects on the
      soil. However, the best choice would be to keep the wetland soil in its present
      position with no disturbance, although this will only be effective if the elevation of
      the wetland soil is compatible with the final elevation of the permanent pond.

      Most of the soils that will be available for constructing artificial wetlands in
      stormwater basins will be acceptable for the establishment of wetland vegetation.
      Soil depth may be more important than soil type in establishing vegetation, since
      the plants must be anchored securely to the substrate to avoid being uprooted.
      A soil depth of at least 8-18 inches is recommended for the shallow wetland basin.
      If there is insufficient substrate depth on the basin, the remainder can be made up
      using sand or other available soil material.

5.2.3.2 WETLAND VEGETATION

      1. Plantina Wetland Veaetation: The long term effects of artificially established
      wetland vegetation on the vegetative development of the wetland is not yet clear.

                                         5-8








~~I  ~       However, data show that artificial establishment does influence the short-term
             development of the wetland. In addition, volunteer species are not likely to occur
             in large numbers for at least several years after establishment of the wetland. For
             this reason, it is recommended that wetland vegetation be artificially introduced on
             newly constructed wetland basins.

I~ ~       ~2. Desirable Species Characteristics:  Shallow wetlands will not be completely
             homogeneous even if the construction plans call for homogeneity. Differences in
             soil types, depth, water circulation, and other aspects will result in habitat variations
             on the wetland. To take advantage of this heterogeneity, more than one species
             of wetland vegetation should be established on the wetland. A greater number of
             sites on the wetland thus can be utilized, which will increase the probability that
             more of the wetland will be vegetated. There are other benefits also, including an
              increase in the diversity of food for wildlife, which will likely result in utilization by
             more wildlife species. In addition, the growth cycles of wetland species differ, with
             some species reaching peak biomass in the late spring while others will not reach
             peak biomass until later in the summer. A mix of such species will result in
              maximizing the presence of vegetation on the wetland throughout the growing
             season.

*I~ ~The most important species to be established on the wetland are the ones that
              spread aggressively. Aggressive species will spread to other sections of the
             wetland and by their increase in numbers make it more likely that the
              establishment will be successful. In addition, rapidly spreading species will make
              it unnecessary to plant vegetation in all parts of the wetland, thereby resulting in
              a savings of money and manpower.

              It is recommended that all the aggressive wetland species (which will be termed
              "primary" species here) be established in quantity on the wetland. This should
~~I   ~      ensure that the artificially established vegetation will spread and influence the
              species composition of the wetland for several years.

              In addition to the primary species, it is recommended that 3 additional species
              (termed "secondary" species) be planted on the wetland, although in far lesser
              numbers than the primary species. These species should have wildlife, aesthetic
              and other values but need not be as aggressive as the primary species. This small
              pool of secondary species may provide the wetland with larger populations of
*I~ ~these important species in later years.

              3. Selection of the Wetland Soecies: The primary qualities that are needed in a
              species for establishment on an artificial wetland are aggressiveness in spreading
              and value to wildlife. Although the productivity, and thus the nutrient uptake of
              species may be important to water quality improvements, there is not yet enough
*I~ ~information available to select species by nutrient uptake rates.

              Not all species have both aggressiveness and value to wildlife. Two species,

                                                5-9







Typha spp (cattail) and Phragmites australus (common reed) are aggressive
wetland species but do not appear to have good wildlife value. This is particularly
true of Phragmites. Other species such as Peltandra virginica (arrow arum), have
good wildlife value but are reported to spread slowly on the wetland. In Peltandra
this is probably a result of very little vegetative propagation with the preponderance
of spreading the result of seed germination. This does not mean that the species
of wetland plants that are not rapid spreaders are unsuitable for artificial wetlands.
Such species may be quite valuable to the long-term vegetation of the wetland and
may even come to dominate it. However, they may not be suitable for establishing
vegetation on the wetland quickly.

Two species, Typha spp (cattail) and Phragmites australus (common reed) are
special cases. These species may be the best choice for water quality concerns,
but, they are very aggressive and may completely dominate a wetland. Their low
wildlife food value and the density of the plant communities they form can result
in a wetland with a low value to wildlife in terms of both food and habitat. The high
biomass production of these plants will quickly 'fill-in' a wetland, thereby reducing
storage volume and increasing maintenance. It is recommended that cattail and
common reed not be planted on artificial wetlands.

Non-persistent, perennial, herbaceous vegetation is probably the best plant
material for basin use. The reasons for this are as follows:

1. Many of these plants are good colonizers, and are efficient at removing
nutrients from the water.

2. The above ground parts of the plants decompose rapidly in the Fall, and are
exported from the basin. The decomposed material has less of a chance of
clogging the outlet structure. The leafy, aboveground material which is exported
is rapidly decomposed and may provide food sources for downstream aquatics.
Harvesting of aboveground plant materials may be feasible in some cases, but
extreme disturbance of the root mass should be avoided.

3. Since the above ground material is exported from the basin, less material
accumulates, and problems associated with plant material changing the volume
capacity of the basin is reduced.

A recommended plant list is as follows:

Primarv Species                  Deoth (in feet)    Available Commercially

Saoittaria latifolia                0 - 2.0                yes
(duck potato)
Peltandra virainica                 0 - 1.0                yes
(arrow-arum)
Pontederia cordata                  0 - 1.0                yes
(pickerel weed)
                                  5-10








Saururus cernus                     0 - 0.5                yes
(lizards tail)
Scirous americanus                  0 - 0.5                yes
(S. pungens, common three-square)

Secondarv SDecies                Deoth (in feet)     Available Commercialvly

Acorus calamus                      0 - 0.25               yes
(sweet flag)
Cephalanthus occidentalis           0 - 2.0                yes
(button bush)
Hibiscus Moscheutos                 0 - 0.25               yes
(rose mallow)
Leersia orvzoides                   0 - 0.25               yes
(rice cutgrass)
Scirous validus                     0 - 1.0                yes
(softstem bulrush)


Newly established plants generally will not have good survival rates at the lowest
depth ranges and generally survive better in the upper two thirds of the range.
Established colonies will generally expand into the deeper ranges. Plants should
be spaced 1.5' - 2.0' on center.

In the areas above pool elevation that flood during storm events, several other
species should be considered for their wildlife values. The plants listed below are
plants that can tolerate dry conditions with periodic episodes of inundation. All are
available commercially. This list contains suggested species but is not exhaustive.
It has to be kept in mind that these plants will contribute to the organic load of the
basin, their fallen leaves can clog outlet structures, and they may increase the
maintenance time needed for mowing the basin sides. All woody plants to be
used as a buffer or for landscaping should be purchased in containers. Bare root
plant material has a much lower survival rate than containerized stock.


Frinae Soecies                   Common Name

Panicum viroatum                 Switch Grass
Androooaon viroinicus            Broomsedge
Cornus sp.                       Dogwoods
Lonicera tatarica                Tartarian Honeysuckle
Rosa rugosa                      Rugose Rose
JuniDerus virainiana             Red Cedar
Euonvmous americanus             Strawberry Bush
Rubus sp.                        Blackberries

                                  5-11








             Maintenance mowing of the basin sides can be reduced by planting wildflowers or
             other meadows instead of lawn grasses. Generally meadows only need to be
             mowed once a year, but the mowed material should be raked from the site so that
             it does not enter the basin and clog outlet structures.

I            ~~~~4. Number of Individuals of Each Species to Plant: There is limited information on
             which to base recommendations for the number of individual plants to establish on
             a wetland. This includes both influencing the long-term species composition and
             the immediate success of the artificial establishment. However, it probably can be
             safely assumed that if the habitat conditions on the wetland are not conducive to
             the growth of a certain species, that species will not do well even if planted at high
             population densities. Therefore, the first consideration is to have the proper water
             depths for the species that are going to be planted.

K            ~~~~If habitat conditions are suitable, the entire area planned for vegetation need not
             be planted.   Aggressive species will colonize the areas not planted.  It is
             recommended that 30 percent of the shallow (1 2 inches or less) area of the basin
             be planted with wetland vegetation. This vegetated area should be divided into
             sites whose surface areas are roughly equal. Mixing species should be avoided
             in order to reduce competition within the planted areas. Each area should be
             located in that part of the wetland conducive to the growth of the species it will
             contain, but in addition, the areas should be placed as far apart as possible while
             still fulfilling the habitat requirements of the individual species. Within each area the
             individual plants should be spaced 1.5 - 2.0 feet on center.

             In addition to these sites, small clumps of the primary species should be planted
             throughout the rest of the wetland to increase the probability and rate of
             colonization of unplanted areas. It is recommended that 40 clumps per acre per
I          ~ ~~species be set out in this fashion.  Each clump should contain one or more
             individuals of a single species. The clumps should be equally divided among the
             primary species. Of course, species should not be planted where water depths
I          ~ ~~are not conducive to the growth of that particular species.  Place the clumps
             throughout suitable areas of the wetland.

             Based on the above discussion, the number of individuals of each species to be
             planted is a function of the total area and shape of the site to be planted. The
             planted areas should be made as square as possible within the design of the
I          ~ ~~~wetland, rather than long and narrow. The greater area to perimeter ratio of the
             square design may help to preserve the homogeneous populations of the planted
             species by reducing colonization.

              Besides the primary species to be planted in abundance, secondary species also
             should be planted on the wetland. It is recommended that 50 individuals of each
             of these additional species be planted per acre on the new wetland. These plants
             should be set out in 10 clumps of 5 individuals each. The clumps should be

        I                                      ~~~~~~~~~~~~5-12







planted within 6 feet of the edge of the pond in the shallow area leading up to the
edge of the pond. In addition, the clumps should be spaced as far apart as
possible, but there is no need to segregate species to different areas of the
wetland.

5. Deoth Reouirements of the Selected Species: Emergent vegetation appears to
grow best in water less than 12 inches deep, with depths of roughly 6 inches or
less showing high growth rates. The three species mentioned above as primary
species will all do well in water 6 inches or less deep. This is not to say that these
species will have low survival rates in somewhat deeper water, however. They are
often found in deeper water in natural wetlands, although the requirements for
successful growth in deeper water may be more stringent than in shallower water.
Thus, the probability of successful establishment is greater in shallower water. The
plants recommended for wetland establishment are suited for the recommended
wetland depths.

6. Plant Prooaoule and Dates for Plantina: Wetland plant establishment using
seeds has been shown to have a low success rate because of the exacting
germination requirements of seeds. By contrast, growing plants and dormant
underground plant parts are much more amenable to transplanting. The latter two
categories of plant propagules are the primary means of plant establishment
recommended here.

The growth cycle of perennial plants determines what form of propagule will be
suitable for planting at particular periods during the year. At the end of the
growing season, generally sometime in October or November, the above ground
portion of the plant dies, while the below ground portion of the plant becomes
dormant. The following spring the underground portion of the plant breaks
dormancy and, using food reserves built up during the previous growing season,
sends up new shoots. This growth generally begins in April.

From the above brief discussion it can be seen that the natural period of dormancy
of wetland plants is also the correct time for planting dormant underground parts.
Likewise, growing plants should be set out on the wetland during the growing
season. In general, plants can be established on the wetland at any time of the
year except late summer and fall. There are two reasons for this. First, plants
must have sufficient growing time to store up food reserves in the below ground
parts. Plants grown in bulk in the nursery have a difficult time doing this; thus they
must be set out in the wetland sufficiently early to store up below ground reserves.
It is recommended that July be the latest month for setting out actively growing
nursery plants.

Dormant, below-ground plant parts are usually available in late November or
December, although some dormant material may be available even earlier in the
year. The primary time of the year for planting dormant material is from November
through April, although actively growing material may be available in April.

                                  5-13







I            ~~~Not all species are desirable to plant 'in the dormant state.  This may be
              attributable to the high probability of wintering waterfowl to detect and eat the
              planted material, or the low percent germination of dormant material. Since these
I          ~ ~~~factors may vary it is recommended that the probability of successful establishment
              be discussed with the seller of the plant material. It is recommended that plant
I            ~~~~material be set out in the spring and summer, using actively growing material.

       5.2.3.3 PLANTING PROCEDURES

1            ~~~~1. Preoarino the Site for Plantina: The only site preparation that is necessary for
              the actual planting (besides flooding the basin) is to ensure that the substrate is
              soft enough to permit easy insertion of the plants. If the basin has been graded
              or excavated there will be no problem. However, there could be difficulty if the
              wetland is to be created by simply flooding a previously dry basin. Such basins
              may have a compacted substrate or the overlying vegetation may have formed a
              dense root mat or a sod that could make planting difficult. It is recommended that
              this type of basin be "softened up" by disking or some other type of physical
              disturbance before the basin is flooded. Disturbance to the upper 6 inches of
              substrate should be sufficient.
              2. Plantina Procedures: The planting procedure begins when the final site
              preparations have been completed. These site preparations will include the
              flooding of the wetland to the proper depths, or if planting is to be done on the
              unflooded wetland, final plans to flood the wetland as soon as planting is complete.

              Dormant below-ground plant parts are easier to handle. They are stored dry,
              usually in mulch, at temperatures slightly above freezing to maintain the dormant
              state. These propagules should be kept dry until the time of planting, and freezing
              should be avoided. Planting consists of burying the dormant material to the proper
              depth in the substrate.
              When growing plants are to be established on the wetland one of the primary
I          ~ ~~~concerns is to keep the roots moist. If the roots dry out they will be damaged and
              decrease the probability of a successful planting. Plants will be received from the
              nursery either growing in small peat pots or bare rooted. Bare rooted plants will
I          ~ ~~~very likely have some form of root protection when received, including moisture
              retaining plastic bags or water filled tubs. These plants can be kept for many days
              in this condition, although they should be kept out of direct sunlight. However, this
I          ~ ~~~type of environment is not a good one for maintaining healthy plants. They should
              be planted as soon as possible.

I            ~~~~The potted plants will be received in very small peat pots that can be planted to
             facilitate root spreading. When the peat pot is in the hole the surrounding
I            ~~~~substrate in pressed down firmly around it.

              Bare rooted plants are treated similarly to peat potted plants, although the holes

        U                                      ~~~~~~~~~~~~5-14







 I           ~~~should be made large enough to accomm'odate the roots. The roots should be
               spread out as much as possible when the plants are set into the hole, as opposed
               to being bunched together. The plants should be placed deep enough that the
 U          ~ ~~~wetland substrate is level with the point on the plant where stem becomes root.
               This point is usually easily identified. Thus the final arrangement of the roots in
               these plants will be shallow but spread out, with an overall depth of approximately
               4 to 6 inches.
               If the planting is to be done before the wetland is flooded no more than 24 hours
 I          ~ ~~~should elapse before flooding occurs if bare root plants are involved. If flooding
               cannot occur within that time other arrangements, such as diverting the base flow
               through the planted area or using water from a fire hydrant, should be employed.
               If only peat potted plants are involved not more than 72 hours should elapse
               between planting and flooding without using other means of wetting the plants.

1      ~5.2.3.4  SUBMERGED AQUATIC VEGETATION (SAy)

               Submerged aquatic vegetation is an important food for waterfowl and may aid in
 I          ~ ~~~improving the quality of stormwater runoff passing through the wetland. However,
               there appear to be few if any commercial SAV sources for fresh water sites at the
               present. Therefore, the artificial introduction of submerged aquatic vegetation is
               not being recommended now. However, the recommendation for a deep pond in
               the artificial wetland (see above) was made, in part, to provide suitable habitat for
               SAV. It is quite likely that SAV will be brought to the wetland by migrating
               waterfowl or other means.
               Although the introduction of SAV is not being recommended at this it is also not
               being discouraged. If a source of SAV is known a small amount can be
               introduced into the deeper part of the wetland. Although these and most other
 3           ~~~~SAV species can tolerate water deeper than the 2 to 3 feet discussed earlier, that
               depth range should be maintained because of the turbidity that seems to occur in
               stormwater basins. Deeper, turbid water might eliminate SAV habitat in stormwater
 *           ~~~~basins.

        5.3   MAINTENANCE AND MONITORING OF WETLAND BASINS

               The following is a list of maintenance and monitoring issues which should be
               addressed at the design stage of implementation.

                      * Is there a project sponsor?
                      a Who will provide monitoring? How often?
   I                 *~~~~~ Who will provide mid-course corrections?
                      * How will biomass clogging and siltation be removed?
                      * How will nuisance plant species be controlled?
                       *Who will provide periodic replanting?
                       *Who will provide regular outlet maintenance?

          1                                     ~~~~~~~~~~~~5-15








             There are two types of organisms that could affect the acceptance of the wetland
             basin by the people in the surrounding community. These include thick surface
             algal growths and mosquitoes. Neither of these factors are likely to be a problem
             in the first years of the wetland's existence.   However, they could arise if
             sedimentation changes the flow characteristics in the basin. The primary cause
             of either algal mats or mosquitoes probably would be standing water in portions
             of the wetland that were prevented from draining properly. These areas could
             accumulate nutrients which would favor algal growth, while the fluctuating water
             level or other extreme habitat conditions could decrease and mosquitoes probably
             will not be a problem around artificial wetlands. The correct maintenance
             procedures will restore the planned drainage characteristics and very likely
             eliminate any algal or mosquito problems.

             Solids will accumulate on the wetland by two means. These include sedimentation
             of suspended solids carried to the basin in stormwater runoff and base flow, and
             the accumulation of plant material. Accumulation of this material could result in a
             loss of the area of ponded water available for emergent vegetation. Two remedies
             are available. The first is to raise the elevation of the water level in the permanent
             pond by raising the height of the orifice in the outlet structure. The loss of peak
             storage volume by increasing the water level will be minimal considering the
             shallowness of the wetland. If the original design accounted for additional volume
             for sedimentation, then the loss is not a problem for several years.

             The second remedy is to remove the accumulated solids by excavation. This will
             require draining some of the water from the wetland and could involve
             considerable disruption to the vegetative community, which could require
             replanting. However, much of the accumulated solids probably will be in the
             deeper forebay of the basin inlet where the runoff first loses velocity upon entering
             the wetland. This area should be easy to excavate. In addition, not all the water
             will have to be removed from the wetland since the overall ponded area
             (approximately 25% of the total wetland area) should not accumulate solids as
~~1  ~rapidly as the forebay. Thus the deeper pond may not require excavating and can
             remain flooded when the shallower areas are drained.

             At this time it is not clear how long shallow wetland basins will function without
             maintenance. To avoid maintenance as much as possible it is recommended that
             wetland basins be installed on stabilized watersheds and not be used for sediment
             control. In addition, the maintenance procedure recommended here is that the
             outlet structure be modified to raise the elevation of the permanent pond when
             solids accumulate. This procedure can be repeated until the peak storage volume
             requirements of the basin are in danger of being compromised, at which time
             excavation will be required.




                                               5-16







                                        MONITORING REPORT

Project:                                                                                   Stormwater Wetland Basin

City/County:                                                   Reviewer:

Date of Report:

Description of Wetland Basin:



Stormwater Control Type:

Acres Created:

Monitoring Agency:

Upland Type:

Plant Species Used:

Outflow Control Depth:

Date Wetland Created:

Comments:



                                          Monitoring and Management

Duration:                                             Inspector:

Phone:                                                Sampling Methods:



Stormwater Control Functions:

        Sediment:

        Outflow Structure:

        Inlet Control:

Wetland Functions:

        Plant Communities:

        Nuisance Species:

        Fill-in:

        Maintenance:

        Wildlife Observed:

Mid-course Corrections Required:

        General Cleaning:

        Re-planting:

        Water Balance Control:

        Sedimentation Removal:

Action Required:

Comments:









                                                    5-17







1      ~6.0   CONSTRUCTION, OPERATIONS AND MAINTENANCE

               This section has been taken substantially from the Ocean County Demonstration
               Study, Stormwater Management Facilities Maintenance Manual, State of New
               Jersey Department of Environmental Protection.

1      ~~6.1    PURPOSE

               This section addresses construction and maintenance procedures for stormwater
               management facilities (SWMF). Although the focus of this document is on
               detention and retention basins, the principles apply to all types of stormwater
               management facilities; consequently, the discussions will address these facilities
               (SWMF) in general. When insufficient attention is paid to these elements, retention
               - detention basins have resulted in poor performance as measured by water quality
               and flood control, as well as an increased threat to public health and safety. The
               guidelines are intended to be used to define minimum requirements and to assist
               in implementing effective and comprehensive maintenance programs. The goal of
               SWIVFs is to mitigate the adverse hydrologic impacts of land development, protect
               downstream areas from flooding and erosion, and prevent stormwater-caused
               water quality degradation. In order to achieve the design goals, retention and
 3           ~~~~detention basins require thorough maintenance performed on a regular basis.

        6.2   THE RESPONSIBILITIES OF OWNERSHIP AND MAINTENANCE

 U           ~~~The owner of a SWMF usually comes by that ownership as the result of some
               course of action other than a direct desire for ownership of the SWMF itself. A
 I           ~~~public agency may acquire or construct a SWMF  in order to alleviate a
               downstream flooding condition. A private individual or corporation may construct
               a SWMVF as a matter of necessity in order to obtain municipal and/or county
 3           ~~~approval for a development and to mitigate the project's downstream runoff
               impacts. In some cases, the SWMVF is worked into the landscaping package, while
               in other cases the facility is constructed in a portion of the site with low visibility to
 3           ~~~the owner and the general public. The adage "out of sight, out of mind" often
               applies. The success of SWMF cannot be fully achieved without proper facility
               maintenance. Therefore, the owner must be aware of the facility's purpose and
 3           ~~~~needs, as well as the absolute importance of proper maintenance. Failing that, the
               owner must be closely regulated by an agency which does.

 I           ~~~~To insure proper maintenance of the facility, the owner must have the necessary
               institutional, managerial, and financial resources. Even where maintenance of a
               private facility is enforceable by a governmental entity by means of regulations and
  I          ~ ~~ordinances,  consistent  performance  of facility maintenance  will  not  be
               accomplished unless the owner has adequate resources for this task.




          3                                     ~~~~~~~~~~~~~~6-1







E           ~~~Actual ownership of a SWMF may change throughout the life of the facility. The
             private individual or corporation which ultimately becomes responsible for
             maintenance of the facility may not be the one which originally planned, designed
I         ~ ~~and constructed the SWMF. This is particularly true in subdivisions where the
             people responsible for maintenance were never involved in the design or
             construction and may have little appreciation for its purpose, function, or
             maintenance. When ownership of a SWMF changes over the facility's life, the
             success or failure of that facility is often determined before it even exists.

I           ~~~Therefore, basic arrangements for SWMF ownership and maintenance need to be
             evaluated during the planning, design, and review phases of the project.
*           ~~~~Ownership and maintenance responsibility fall into three categories:

                          a)    public ownership with public maintenance;
                          b)    private ownership with public maintenance;
                          c)    private ownership with private maintenance.
3           ~~~Public ownership with public maintenance is the most desirable solution.  This
             should be the goal of all localities. Charges or user fees should be established for
             maintaining the facilities. The stormwater utility concept is very viable and can be
             used in Virginia. It has been successfully used in many localities and the Hampton
             Roads Planning District Commission has published a report discussing the subject.
*           ~~~Private ownership with public maintenance should be considered in situations
             where the owner is unlikely to have the institutional, managerial, or financial
             resources to properly maintain the facility. Individual homeowners and single-
3           ~~~~family homeowners associations generally fit into this category. These groups tend
             to lack the incentive, knowledge, equipment and resources necessary to
             adequately maintain the facility. In such situations, public ownership and/or
3           ~~~~maintenance of the SWMF, as well as access to the facility and financial liability,
             should be considered as part of the criteria for the design of the facility.

3           ~~~~Private ownership with private maintenance is the least desirable situation. If the
             private owner is concerned with the proper functioning and appearance of the
             SWM F and must maintain other facilities, then he is more likely to properly maintain
I          ~ ~~~the SWMF. Private corporations are generally capable of and willing to maintain
             SWMFs. Corporations are usually conscious of their public image and community
             status. They can be expected to have the manpower, financial resources and
3          ~ ~~equipment required for proper maintenance. Likewise, condominiums and co-op
             apartments can also generally be considered self-sufficient, since they collect funds
             and maintain grounds, roads and other facilities.






        1                                     ~~~~~~~~~~~~6-2







 I            ~~~~Within the organization, associate, or corporation, an individual should be held
               responsible for the performance of SWMF maintenance. This person should be
               vested with sufficient authority to establish procedures and priorities for the
 I          ~ ~~maintenance personnel. This person should also have a thorough understanding
               of the purpose and function of the SWMFs along with an appreciation for the
 *            ~~~~consequences of facility failure and the role maintenance plays in preventing such
               occurrences.

*      ~6.3   CONSTRUCTION INSPECTION

               A well-designed and well-built retention - detention facility should require the least
               amount of maintenance effort and expense.   Proper inspection is crucial to
               achieving that goal. Poor construction can lead to many serious maintenance and
               safety problems, including isolated pockets of water, slope erosion, channel scour,
               mosquito breeding and structural failure of dams, embankments, slopes and outlet
               structures.

               Other SWMF maintenance problems that arise due to poor construction include:

                         *  ground settlement
                         *  cracked, spalled or deteriorated concrete
                         *  incorrectly installed fittings and appurtenances
                         *  incorrect elevations, grades and dimensions
    5                *    ~~~~~missing, damaged, or hidden components

        6.3.1  PRECONSTRUCTION PHASE

               Effective facility construction inspection begins in the preconstruction phase. The
               inspector should become familiar with the facility plans, specifications and other
 5            ~~~related documents.  Special attention should be paid to complex components,
               difficult site conditions and other potential problem areas. In addition, the inspector
               should attend all preconstruction meetings. It is at these meetings that the
               inspector has the opportunity to become more familiar with the nature of the
 U            ~~~project and the key personnel involved.  The preconstruction meeting should
 *            ~~~~address:

                      I1)    The project's overall purpose and objective;

   *                 ~~~~~~2)    Specific areas or details of the project that are particularly complex
                            or that otherwise require special attention;

   *                 ~~~~~3)    Construction schedules and deadlines;

   3                 ~~~~~4)    The establishment of a chain of command for problem solving.



          5                                       ~~~~~~~~~~~~6-3







 I           ~~~~A detailed list of recommended preconstruction meeting topics is summarized in
               Table 6-1.

1       ~6.3.2 CONSTRUCTION PHASE

               During this phase, it is necessary for the inspector to understand and inspect the
 I          ~ ~~~current construction activity . During this phase, some of the inspector's key
               responsibilities should include:

   1                ~~~~~1)    Daily Reports:  A brief summary which includes all construction
                            activity, the weather and working conditions, vehicle arrival and
    3                      ~~~~~~~departure times, equipment, materials and key project personnel.

                      2)    Shop Drawings:  Shop drawings should be required for all facility
                            components. Experience has shown that problems solved on paper
                            prior to construction can prevent major problems later in the field.
                      3)    Progress Meetings: These meetings afford all parties the opportunity
                            to discuss current or anticipated problems.
                      4)    Extra Work and Change Orders: A design which may work on paper
                            may not always be successful in the field. The contractor may have
                            suggestions which may aid in the progress of the work or enhance
                            the quality of the design. The inspector should review change orders
                            and extra work orders to determine their legitimacy. Quick resolution
                            of these requests can improve communications, relations, and
    3                     ~~~~~~workmanship.

                      5)    Final Inspection and Punch List: The Punch List is an effective tool
    3                     ~~~~~~~~that the inspector can use to help insure that all facility construction
                            is complete and correct before final payment is made to the
                            contractor.

        6.3.3 POST-CONSTRUCTION PHASE

 3           ~~~The new facility should be warranted by the contractor.  During this time, the
               inspector should perform periodic inspections of the facility and immediately bring
               any problems to the contractor's attention. This is the last opportunity to correct
               construction flaws before they become the owner's maintenance problems.
               Table 6-2 summarizes recommended construction inspection practices.






                                                  6-4







                                   TABLE 6-1

                 TYPICAL PRECONSTRUCTION MEETING TOPICS



A.    GENERAL INFORMATION

       1.   Attendance
      2.   Purpose of Project and Background Information
       3.   Emergency Phone Numbers
       4.   Construction Photograph Requirements
       5.   Project Sign Requirements
       6.   Starting Date
       7.   Review of Contract Documents, including Insurance Certificates, Bonds and
            Subcontractors Documents
       8.   Field Office Requirements
       9.   Responsibility for Notifications of Affected Property Owners and Residents
      10.   Chain of Command for Communications and Correspondence
      11.   Construction Schedules
      12.   Key Personnel and their degree of involvement in the Project (Inspector,
            Owner, Engineer, Agencies, etc.)

B.    POLICE AND FIRE DEPARTMENT CONCERNS

       1.   Traffic Control
       2.   Barricades and Signs Conforming to the Uniform Manual
       3.   Noise Ordinance Considerations
       4.   Working Hours, including Weekend and Holidays
       5.   Vandalism and Preventative Measures
       6.   Flagmen and Traffic Control Officers
       7.   Equipment Storage and Vehicle Parking
       8.   Emergency Vehicle Access
       9.   Underground Tank Locations and Precautionary Construction Procedures
      10.   Storage and Use of Hazardous Materials

C.    UTILITIES

      1.   Utility Locations and Mark-Outs
      2.    Coordination of Utility Relocations
      3.    Emergency Phone Numbers of Utility Companies






                                      6-5







       I                            ~~~~~~~~TABLE 6-1 (CONTINUED)

*      ~D.    FUNDING AND PAYMENTS

              1.    Funding Sources and Availability
              2.    Procedures and Dates for Payment Estimates
 I         ~ ~~3.    Dates for Payments to Contractor
              4.    Breakdown of Lump Sum Items for Partial Payment
              5.    Policy for Payment for Materials on Site at the Close of a Payment Period
              6.    Retained Monies during and after Construction
              7.    Requirements of Funding Agencies

I      ~E.    CHANGE ORDERS AND EXTRA CLAIMS

              1.    Requirements for Additional Work and Submittal of Change Orders
 I          ~ ~~2.    Procedures and Schedule for Review and Recommendations of Change
                    Orders
 3           ~~~3.    Procedures for Negotiating Extra Claims and Change Orders

        F.    CONSTRUCTION ACCESS AND EASEMENTS

 I           ~~~1.    Easement Locations and Maps
              2..   Responsibility for Locating and Staking Easements
              3.    Available Survey Data for the Site
              4.    Access Requirements and Staging Areas
              5.    Easement Restrictions and Restoration Requirements

        G.    CONSTRUCTION DETAILS

 *           ~~~1.    Unique or complex Aspects of the Project
              2.    Testing Laboratories and Sampling Procedures
              3.    Cold and Hot Weather Protection Measures
 *           ~~~4.    Blasting Requirements
              5.    Dump Site Location for Construction Related Materials
              6.    Shop Drawing Requirements and Review Procedures
 *           ~~~7.    Specific Construction Techniques and Procedures
              8.    Review of Technical Section of the Specifications










         3                                    ~~~~~~~~~~~~~6-6







                                   TABLE 6-2

            SUMMARY OF CONSTRUCTION INSPECTION PRACTICES



A.    PRECONSTRUCTION

       1.   Review Purpose of the Project
       2.   Review Plans and Specifications
       3.   Obtain Pertinent Permit Documents
       4.   Review Permit Conditions
       5.   Obtain Pertinent Easement Documents
       6.   Review Easement Conditions and Restrictions
       7.   Schedule and Conduct Preconstruction Meeting
       8.   Obtain List of Emergency Phone Numbers
       9.   Obtain List of Key Personnel

B.    CONSTRUCTION

       1.   Observe All Pertinent Construction Activity
       I2.   Be Familiar with Construction Procedures
       3.   Anticipate Problems
       4.   Keep a Diary of all Pertinent Activities
       5.   Write Daily Construction Reports
       6.   Review Shop Drawings
       7.   Consult with the Contractor Frequently
       8.   Conduct Progress Meetings
       9.   Review Change Orders and Extra Claims
      10.   Prepare Punch List
      11.   Conduct Final Inspection

C.    POST CONSTRUCTION

       1.   Perform Periodic Inspections
       2.   Notify Contractor of Necessary Work
       3.   Inspect Corrected Work
       4.   Prepare Record Plans
       5.   File all Pertinent Contract and Inspection Records








                                      6-7







I      ~6.4   MAINTENANCE

              The majority of the maintenance tasks at a well-designed and constructed
               retention-detention facility should be simple and routine. The physical aspect of
               maintenance can be broken down into three areas -- preventative, corrective and
               aesthetic. Preventative and aesthetic measures minimize the need for costly
 I          ~ ~~corrective measures.  Routine tasks such as lawn mowing, maintenance and
              'trimming keep multi-purpose facilities from becoming eye-sores and enhance their
               attractiveness to the general public while ensuring that they still serve their
 I          ~ ~~~stormwater control goals. Single purpose facilities, particularly those in areas of
               low visibility, may not require the level of service of these preventative or aesthetic
               measures. The frequency of preventative and aesthetic maintenance is governed
 I          ~ ~~~by the multiple objective of the facility and its use by the public.  Corrective
               measures are used to rehabilitate portions of a retention-detention site which will
               degrade in spite of preventative measures. These three categories are described
               in further detail below and summarized in Table 6-3. Equipment which can
               improve the quality of maintenance is listed in Table 6-4. Training should be given
              to maintenance personnel.  It increases manpower productivity and gives
               employees a sense of purpose in purpose in performing their tasks. This can
               result in more thorough, less expensive maintenance.

I      ~6.4.1  PREVENTIVE MAINTENANCE PROCEDURES

               Preventive maintenance, like aesthetic maintenance, is proactive. Its purpose is
 I         ~ ~~to ensure that the facility remains operational and safe at all times.  When
              conducted properly, preventive maintenance minimizes the need for costly
              emergency and other corrective maintenance. The following items, which are
              summarized in Table 6-4, should be incorporated into preventive maintenance
              procedures:

              Grass Cuttina: This activity should be minimized by planning to limit areas where
              lawn type areas are needed.   Generally these are contained to areas of
              recreational activity. Lawn mowing and trimming can amount to 15 to 25% of
              maintenance costs. A regularly scheduled program of mowing and trimming of
              grass at SW`MFs during the growing season will help to maintain a tightly knit turf,
              and will also help to prevent diseases, pests and the intrusion of weeds. The
              actual mowing requirements of an area should be tailored to the specific site
              conditions, grass type, and seasonal variations in the climate. In general, grass
 I          ~ ~~~should not be allowed to grow more than I to 2 inches between cuttings (probably
              once a week during the growing season in this area). Allowing the grass to grow
              more than this amount prior to cutting it may result in damage to the grass'
 I          ~ ~~~growing points and limit its continued healthy growth. Agencies such as the local
              Soil and Water Conservation District, Local Extension Service office and the
              Chesapeake Bay Local Assistance Department (CBLAD) can provide valuable
              assistance in determining optimum grass selections and mowing frequencies.

          U                                    ~~~~~~~~~~~~6-8







I           ~~~~Grass Maintenance: Grassed areas require periodic fertilizing and soil conditioning
             in order to maintain healthy growth. Additionally, provisions should be made to re-
             seed and re-establish grass cover in areas damaged by sediment accumulation,
             stormwater flow, or other causes. This maintenance should be incorporated into
             the schedule as a spring and fall procedure.

I           ~~~~Veaetative Cover: Trees, shrubs, and ground cover require maintenance, including
             fertilizing, pruning, and pest control in order to maintain healthy growth. This
*           ~~~~should be done in the spring and fall.

             Removal and Disposal of Trash and Debris: A regularly scheduled program of
             debris and trash removal from retention and detention basins will reduce the
             chance of outlet structures, trash racks and other components becoming clogged
             and inoperable during storm events. Additionally, removal of trash and debris will
             prevent possible damage to vegetated areas and eliminate potential mosquito
             breeding habitats. Disposal of debris and trash must comply with all local waste
             flow control regulations. For simplicity and effectiveness, trash collection should
             occur on at least a weekly basis. In high visibility/usage areas, trash pick-up may
             need to be conducted on a daily basis. In areas of low visibility or limited access,
             this activity may not be required on a frequent basis. In such cases, periodic
             inspections should be performed to ensure that the collection is frequent enough
             to be effective.

             Sediment Removal and Disoosal: Accumulated sediment should be removed
             before it threatens the operation or storage volume of a SWMF. Disposal of
             sediment must comply with all local, county, state, and federal regulations. Only
             suitable disposal sites should be utilized. The sediment removal program in
             infiltration facilities must also include provisions for monitoring the porosity of the
             sub-base, and replacement or cleansing of the pervious materials as necessary.
I          ~ ~~Sediment should be disposed of in accordance with State Water Control Board
             and Army Corps of Engineers (COE) guidelines. A dredging permit will probably
             be required from the COE, The Virginia Marine Resources Commission, and local
I          ~ ~~~Wetlands Boards if this facility lies in tidal wetlands or other Resource Protection
             Areas. Basins should be checked for accumulation on a semi-annual basis.
             Sediment removal will probably need to be performed on a bi-annual (2-year)
             basis.
             Mechanical Components: Valves, sluice gates, pumps, fence gates, locks, and
U          ~ ~~access hatches,- should remain functional at all times.  Regularly scheduled
             maintenance should be performed in accordance with the manufacturers'
             recommendations. Additionally, all mechanical components should be operated
             or exercised at least once every month to assure their continued performance.







             Elimination of Potential Mosauito Breedina Habitats: The most effective mosquito
             control program is one that eliminates potential breeding habitats. Almost any
             stagnant pool of water can be attractive to mosquitoes and the source of a large
             mosquito population. Ponded water in areas such as open cans and bottles,
             debris and sediment accumulations, and areas of ground settlement provide ideal
*           ~~~~locations for mosquito breeding.

             Pond Maintenance: A program of monitoring the aquatic environment of a
             permanent pond should be established. Although the complex environment of a
             healthy aquatic ecosystem will require little maintenance, water quality, aeration,
             vegetative growth, and animal populations should be monitored on a regular basis.
             The timely correction of an imbalance in the ecosystem can prevent more serious
             problems from occurring. Additional information on pond maintenance can be
             obtained through agencies such as the U.S. Fish and Wildlife Services, State Water
             Control Board, VIMS, and others.

             Inspection: Regularly scheduled inspections of the facility should be performed by
             qualified inspectors. For multi-objective facilities, this should be done an a weekly
             basis. Single-purpose facilities should be inspected quarterly and after each major
             storm. The primary purpose of the inspections is to ascertain the operational
             condition and safety of the facility, particularly the condition of embankments, outlet
             structures, and other safety-related aspects.   Inspections will also provide
             information on the effectiveness of regularly scheduled Preventative and Aesthetic
             Maintenance procedures, and they will help to identify where changes in the extent
             and scheduling of the procedures are warranted. Finally, the facility inspections
             should also be used to determine the need, for and timing of Corrective
             Maintenance procedures.  It should be noted that, in addition to regularly
             scheduled inspections, an informal inspection should be performed during every
             visit to a SWMF by maintenance or supervisory personnel.

             Reporting: The recording of all maintenance work and inspections provide
             valuable data on the facility condition. A quarterly review of this information will
I          ~ ~~~also help to establish more efficient and beneficial maintenance procedures and
             practices. Along with the written reports, a chain of command for reporting and
             solving maintenance problems and addressing maintenance needs should be
I          ~ ~~~established. From field personnel to the maintenance director, everyone should
             be encourage to report any problems or suggest any changes to the maintenance
3           ~~~program.

       6.4.2 CORRECTIVE MAINTENANCE PROCEDURES

             Corrective Maintenance is required on an emergency or non-routine basis to
             correct problems or malfunctions and to restore the intended operation and safe
             condition of a SWMF.



                                               6-10







I(~ ~Removal of Debris and Sediment: Sediment, debris and trash which threatens the
             discharge capacity of a SWMF should be removed immediately and properly
             disposed of in a timely manner. Equipment and personnel must be available to
             perform the removal work on short notice. The lack of an available disposal site
             should not delay the removal of trash, debris, and sediment. Temporary disposal
             sites should be utilized if necessary.

             Structural Repairs: Structural damage to outlet and inlet structures, trash racks,
             and headwalls from vandalism, flood events, or other causes must be repaired
             promptly. Equipment, materials and personnel must be available to perform these
             repairs on short notice. The immediacy of the repairs will depend upon the nature
             of the damage and its effects on the safety and operation of the facility. The
             analysis of structural damage and the design and performance of structural repairs
             should only be undertaken by qualified personnel.

             Dam. Embankment. and Slope Repairs: Damage to dams, embankments, and
             side slopes must be repaired promptly. This damage can be the result of rain or
             flood events, vandalism, animals, vehicles, or neglect. Typical problems include
             settlement, scouring, cracking, sloughing, seepage, and rutting. Equipment,
             materials and personnel must be available to perform these repairs on short notice.
             The immediacy of the repairs will depend upon the nature of the damage and its
             effects on the safety and operation of the facility. The analysis of damage and the
             design and performance of geotechnical repairs should only be undertaken by
*I~ ~        qualified personnel.

             Dewatering:  It may be necessary to remove ponded water from within a
             malfunctioning SWMF. This ponding may be the result of a blocked principal outlet
             (detention facility), inoperable low level outlet (retention facility), loss of infiltration
             capacity, or poor bottom drainage. Portable pumps may be necessary to remove
             the ponded water temporarily until a permanent solution can be implemented.

             Pond Maintenance: Problems such as algae growth, excessive siltation, and
             mosquito breeding, should be addressed and corrected in a timely manner. The
             sooner the problem is corrected, the easier it will be to restore a balanced
             environment in the pond. Due to the complex environment in a pond, it is
             recommended that agencies such as the U.S. Fish and Wildlife Service be
             consulted for corrective maintenance procedures.

~~I  ~Extermination of Mosauitoes: If neglected, a SWMF can readily become an ideal
             mosquito breeding area. Extermination of mosquitoes will usually require the
             services of an expert, such as the appropriate local city or county department.
             Proper procedures carried out by trained personnel can control the mosquitoes
             with a minimum of damage or disturbance to the environment. If mosquito control
             in a facility becomes necessary, the preventative maintenance program should also
             be re-evaluated, and more emphasis placed on control of mosquito breeding
             habitats.

                                              6-11







U            ~~~~Erosion Repair: Vegetative cover or other protective measures are necessary to
             prevent the loss of soil from the erosive forces of wind and water. Where a re-
             seeding program has not been effective in maintaining a non-erosive vegetative
             cover, or other factors have exposed soils to erosion, corrective steps should be
             initiated to prevent further loss of soil and any subsequent danger to the stability
             of the facility. Soil loss can be controlled by a variety of materials and methods,
             including riprap, gabion lining, sod, seeding, concrete lining and re-grading. The
             local Soil and Water Conservation District can provide valuable assistance in
*            ~~~recommending materials and methodologies to control erosion.

             Fence Repair: Fences are damaged by many factors, including vandalism and
             storm events. Timely repair will maintain the security of the site, however, use of
             fences should be minimized.

3            ~~~~Elimination of Trees. Brush. Roots and Animal Burrows:  The stability of dams,
             embankments, and side slopes can be impaired by large roots and animal
             burrows. Additionally, burrows can prevent a safety hazard for maintenance
             personnel. Trees and brush with extensive, woody root systems should be
             completely removed from dams and embankments to prevent their destabilization
             and the creation of seepage routs. Roots should also be completely removed to
             prevent their decomposition within the dam or embankment. Root voids and
             burrows should be plugged by filling with material similar to the existing materials,
             and capped just below grade with stone, concrete or other material. If plugging
3            ~~~~of the burrows does not discourage the animals from returning, further measures
             should be taken to either remove the animal population or to make critical areas
             of the facility unattractive to them.

             Snow and Ice Removal: Accumulations of snow and ice can threaten the
             functioning of a SWMF, particularly at inlets, outlets, and emergency spillways.
I          ~ ~~Providing the equipment, materials and personnel to monitor and remove snow
             and ice from these critical areas is necessary to assure the continued functioning
*            ~~~~of the facility during the winter months.

       6.4.3 AESTHETIC MAINTENANCE PROCEDURES

I            ~~~Aesthetic Maintenance, although not required to keep a SWMF operational, will
             maintain the visual appeal of a facility and will benefit everyone within the local
             community. This is particularly true for those SWMFs that are also used by
I          ~ ~~members of the community for athletic and recreational purposes.  Aesthetic
             Maintenance can also reduce the amount of required Preventative and Corrective
             Maintenance.   A  comparison  of Aesthetic and  Preventative  Maintenance
I          ~ ~~procedures reveals how both can readily be combined into an overall SWMF
             maintenance program.




                                               6-12







I            ~~~Graffiti Removal:  The timely removal of this obvious eyesore will restore the
             aesthetic quality of a SWMF. Removal can be accomplished by painting or
             otherwise covering it, or removing it with scrapers, solvents or cleaners. Timely
             removal is important to discourage further graffiti and other acts of vandalism.
             Grass Trimmina: Although time consuming, trimming of grass edges around
             structures and-fences will provide for a neat and attractive appearance of the
             facility. Grass trimming should be scheduled to coincide with grass cutting.

*            ~~~Control of Weeds: Although a regular grass maintenance program will keep weed
             intrusion to a minimum, some weeds will invariably appear. Periodic weeding will
             not only help to maintain a healthy turf, but will also keep grassed areas looking
             attractive. The application of chemical weed control needs to be carefully
             considered and monitored.

I            ~~~Details:  Careful, meticulous, and frequent attention to the performance of
             maintenance items such as painting, tree pruning, leaf collection, debris removal,
             and grass cutting will result in a SWMF that remains both functional and attractive.















        I~~~~~~~~~~~~61







                           TABLE 6-3

              MAINTENANCE ITEMS BY CATEGORIES

A.    PREVENTATIVE MAINTENANCE

            1.   Grass Cutting
            2.   Grass Maintenance
            3.   Vegetative Cover
            4.   Removal and Disposal of Trash and Debris
            5.   Sediment Removal and Disposal
            6.   Mechanical Components
            7.   Elimination of Mosquito Breeding Habitats
            S.   Pond Maintenance
            9.   Inspection
            10. Reporting

B.    CORRECTIVE MAINTENANCE

            1.   Removal of Debris and Sediment
            2.   Structural Repairs
            3.   Dam, Embankment, and Slop Repairs
            4.   Dewatering
            5.   Pond Maintenance
            6.   Extermination of Mosquitoes
            7.   Erosion Repair
            8.   Fence Repair
            9.   Elimination of Trees, Brush, Roots, and Animal Burrows
            10. Snow and Ice Removal

C.    AESTHETIC MAINTENANCE

            1.   Graffiti Removal
            2.   Grass Trimming
            3.   Control of Weeds
            4.   Details











                              6-14







1      ~6.5   EQUIPMENT REQUIREMENTS

               Table 6-4 lists the Equipment and materials which are typically required to maintain
 I          ~ ~~a SWMF.  It is presented as a general guide to assist owners, maintenance
               directors, designers, and financial planners in establishing specific facility
               maintenance programs. Actual equipment and materials requirements must be
               determined on an individual basis for each facility. Equipment that is used
               infrequently could be rented, come from a city wide pool,or be shared with other
 3           ~~~~~localities.

               Factors to consider in the selection of equipment:

   *~~~~~~ 1.    Frequency of Usage - Renting or contracting should be considered
                            if the equipment is rarely used.
                     2.    Ease of Operation - The average laborer should able to use the
                            equipment safely without significant training.
                     3.    Economy of Operation - Consider the fuel consumption per hour and
    *                     ~~~~~~~the rate of production that the machine offers (i.e. - a riding mower
                            uses x - # of gallons per hour and travels at x - # of feet per minute)
                     4.    Attachments - A machine which can perform a variety of functions
     3                     ~~~~~~~can be very cost-effective.
                     5.    Warranty, Service, Parts Availability - Compare warranty periods.
                            Also, equipment which can be serviced by only one local dealer may
                            not be attractive. Finally, ensure that either the dealer stocks a
                            variety of parts for the product line in question or that parts are
                            commonly available on the local market. If the dealer stocks parts
     I                    ~ ~~~~~~for the entire product line, he is more likely to stock a wide range of
                            parts for each product in that line. Commonly available parts reduce
                            the need to rely on one dealer. Down time can slow down or stop
     I                    ~ ~~~~~maintenance.
                     6.    Transportation - Equipment which can be moved by one employee
                            is ideal.   Equipment which  requires special trailers or other
     I                    ~ ~~~~~transporting devices should be avoided if possible.  The proper
                            selection of equipment can increase the production and efficiency of
                            maintenance personnel. This can mean a significant reduction in
                            overall maintenance costs.







                            TABLE 6-4
     EQUIPMENT AND MATERIALS COMMON TO MAINTENANCE

A.    GRASS MAINTENANCE EQUIPMENT
      1.   Tractor-Mounted           6.   Seed Spreaders
            Mowers                   7.   Fertilizer Spreaders
      2.   Riding Mowers             8.   De-Thatching Equipment
      3.   Hand Mowers               9.   Pesticide and Herbicide
      4.   Gas Powered                    Application Equipment
            Trimmers                 10.   Grass Clipping and Leaf Collection
      5.   Gas Powered                     Equipment
            Edgers

B.    VEGETATIVE COVER MAINTENANCE EQUIPMENT
      1.   Saws                      3.   Hedge Trimmers
      2.   Pruning Shears            4.   Wood Chippers

C.    TRANSPORTATION EQUIPMENT
      1.    Trucks for Transportation of Materials, equipment, and personnel
      2.    Trailers for Transportation of equipment

D.    DEBRIS, TRASH, AND SEDIMENT REMOVAL EQUIPMENT
      1.    Loader      3.    Grader
      2.    Backhoe

E.    MISCELLANEOUS EQUIPMENT
      1.   Shovels                   9.   Tools   for   Maintenance   of
      2.   Rakes                           Equipment
      3.   Picks                    10.   Office Space
      4.   Wheel Barrows            11.   Office Equipment
      5.   Fence Repair Tools       12.   Telephone
      6.   Painting Equipment       13.   Safety Equipment
      7.   Gloves                   14.   Tools for Concrete Work (Mixers,
      8.   Standard Mechanics              Form Materials, etc.)
            Tools                    15.   Welding Equipment (for Repair of
                                          Trash Racks, etc.)
F.    MATERIALS

      1.   Topsoil                   6.   Mulch
      2.   Fill                      7.   Paint
      3.   Seed                      8.   Paint Removers for Graffiti
      4.   Soil   Amenities          9.   Spare Parts for Equipment
            (Fertilizer, Lime)       10.   Lubrication
      5.   Chemicals                11.   Concrete
            (Pesticides,
            Herbicides,)

                               6-16







I       ~6.6   MAINTENANCE COSTS

               The Figures 6-1 and 6-2 are provided for budgetary purposes. The projected
               costs were determined for facilities which receive a comprehensive level of
               maintenance which may be more ideal or thorough than typical. Estimating a
               relatively high initial budget gives the maintenance director the necessary time and
               funding to properly establish his maintenance program. As actual needs are
               experienced, the budget can be adjusted. The date for Figures 6-1 and 6-2 were
               estimated from data generated from performance standard studies done by URS.
               hours by various maintenance tasks were multiplied by salary rates including fringe
               benefits. No allowance for overhead was made. Equipment costs were based on
               hourly rates typical of rental costs. The maintenance items covered are those
               listed in Table 6-3.

        6.7   TRAINING

               Training of the maintenance personnel is very important since they are normally
               the most frequent visitors to the site.  A training program should include:
               maintenance and inspection techniques; proper record keeping, and stormwater
               requirements. Particular attention should be paid to the purpose and operation of
               stormwater management facilities, the importance of thorough maintenance, and
               the health, safety and other consequences of maintenance neglect.













         I~~~~~~~~~~~~61




















             $90,000

              $8,000

             $7,000           WV

            0
                                                            P-51- 
              $6,000                            I



                         S~~~~~~~~~~~~~~~~~~~~~~~~~~11~~ $5,000              1                      - 
                                                             r- ~      ~~~~~ No
              $4,000                 v.--,

                  '~~~~~~C





              $2,000                        -&'








                                               MAINTE NANCE AREA (ACRES)
                  $1,000       1~           2           3            Al5                     6UR
            _____                                 ~~~~~~~FIGURE 6-1UR
HAPT"ON ROADS         MAINTENANCE COST VS. FACILITY AREA FOR DETENTION BASINS                                      CONSULTANTS



















                                                                                                                                           0-
                    $10,000                                                    __                              -


                     $8,000

                      $7,000                                                             -

                      $6,000                         '-           ~       c~                                 

                  0

                      $5,000
                  co

                      $4,000



                      $3,000            ..
                                                   -h




                      $2,000                              4-



                                            - a




                                                                          MAINT1iNANCE AREA (A~CRES)


                                                                ______  FIGURE 6-2                                                               URS
HAMPT~ON ROADS            MAINTENANCE COST VS. FACILITY AREA FOR RETENTION BASINS                                                        CONSULTANTS








                                     ACKNOWLEDGEMENTS

This document was prepared by URS Consultants, Incorporated, Virginia Beach, Virginia under a contract
with the Hampton Road Planning District Commission. The funds were provided by the Chesapeake Bay
Local Assistance Department and the Virginia Council on the Environment. The Project Director for the
HRPDC was Mr. John Carlock.

In the office of URS, the project was managed by Lamont W. Curtis, P.E. Contributing authors include
Robert Arnold, P.E., Timothy Clarke, and Philip Rinehart. Dr. Mark Kraus of Environmental Concern
contributed to the section on Establishment of Wetlands in Stormwater Detention on Retention Basins.








                                           REFERENCES

Peter Stahre and Ben Urbonas, Stormwater Detention, Prentice - Hall, 1990

Viessman, Knapp, Lewis, Introduction to Hydrology, Harper & Row, Second Edition, 1977

Haun and Barfield, Hydrology and Sedimentology on Surface Mined Lands, University of Kentucky, 1978

EPA, Results of the Nationwide Urban Runoff Program, Final Report, U.S. Environmental Protection Agency,
NTIS Accession No. PB84-185552, December, 1983

EPA, Methodology for Analysis of Detention Basins for Control of Urban Runoff Quality, U.S. Environmental
Protection Agency, EPA440/5-87-001, September, 1986

Urban Runoff Quality-Impacts and Quality Enhancement Technology, Proceedings of an Engineering
Foundation Conference, ASCE, 1986

Stormwater Detention Facilities, Proceedings of an Engineering Foundation Conference, ASCE, 1982

Urban Runoff Quality-Impact and Quality Enhancement Technology, American Society of Civil Engineers,
New York, 1986

Proceedings of Twelfth International Symposium on Urban Hydrology, Hydraulics, and Sediment Control,
University of Kentucky, Lexington, Ky., 1985

ASCE, Stormwater Detention Outlet Control Structures, Final Report of the Task Committee on the Design
of Outlet Structures, American Society of Civil Engineers, New York, 1985

Guidelines for Constructing Wetland Stormwater Basins, Sediment and Stormwater Division Water Resource
Administration, Maryland Department of Natural Resources, 1987

Wetland Creation and Restoration, The Status of Science, Kusler and Kentula, Island Press, 1990

Protecting Nontidal Wetlands, Burke, Meyers, Tiner, Groman, American Planning Association, 1988

Urban Stormwater Treatment at Coyote Hills Marsh, Association of Bay Area Governments, December 1986

Stormwater Management Facilities Maintenance Manual, Ocean County Demonstration Grant, New Jersey
Division of Water Resources, June 1989

A Framework for Evaluating Compliance with the 10% Rule in the Chesapeake Bay Critical Area, Thomas
Schueler and Matthew Bley, MWCOG, October 1987

Controlling Urban Runoff, Thomas Schueler, MWCOG, June 1987

Design of Urban Runoff Quality Control, Proceedings of an Engineering Foundation Conference, ASCE, 1988

Highway Runoff Water Quality Training Course, USDOT, FHA, February 1986







     I                                    ~~~~~~~~APPENDIX A

                                        CHECKLIST FOR

                           STORMWATER MANAGEMENT FACILITIES


     PROJECT STATEMENT
I                 ~~~~~~Brief description of the overall project

               *  Sequence of Construction:  Date project is to start, expected dates of soil
I                ~    ~~~~~stabilization, expected date of completion

*                 ~~~~~Brief description of erosion and sediment control program

               *  Brief description of stormwater control program
     SITE CONDITIONS - PRE-DEVELOPMENT

               *  Nature and extent of existing vegetation

               *  Description of soils on site:

                      *  Include name, texture, slope, depth, drainage and surface area of
                         each type of soil.

               *  Brief description of sensitive environmental areas located within or in
                  proximity to the site. Such areas include, but are not limited to: resource
I                ~ ~~~~protection areas and resource management areas, including floodplains,
                  streams, lakes, ponds, wetlands, weak soils, steep slopes, etc.

*           *    ~~~~~Impact analysis to briefly to discuss the ramifications of development.

               *  See the HRPDC report "Environmental Assessment Procedures", dated 1991
I               ~ ~~~~for guidance.

     STORMWATER MANAGEMENT PLAN

     Description of Plan:

I           *    ~~~~Brief analysis of problems posed by stormwater runoff on downstream
                  areas
I           *    ~~~~Pre- and post-development nonpoint source (N PS) loading conditions for
                  CBPA areas and other areas as required by locality

                                              A-i








I                 ~~~~~Selected Best Management Practices (BMPs) or other control procedures
                   and how they were determined; efficiency of such practices; also:

 I                      ~~~~~~~Note if use to affect RPA or RMA

                      *  Procedures for implementing non-structural stormwater BMPs

      Hydrologic Calculations:

*           *    ~~~~Map with existing and proposed drainage areas: Note overland flows over
                   200' used for computing time of concentration.

I                 ~~~~~~Rainfall data: Include a copy of the Intensity Duration Frequency chart used
                   and provide a list of the intensities used for the selected duration and
                   frequencies. If the Type 1I SOS rainfall distribution was used, so state.

                *  Surface runoff coefficients or runoff curve numbers

                *  Runoff Computation:  Note if SOS, rational or other method is used.  If
                   some other method is used, supply supporting documentation for that
U               ~ ~~~~method.  Note the times of concentration and how they were determined.
                   Supply calculations sufficient for independent review.

*           *    ~~~~~Base flow for facilities with a permanent pool show calculation for base flow
                   to maintain the required volume for the objective.

                *  Infiltration: for all storage facilities, show computations for infiltration losses
                   with respect to time.

1           *    ~~~~Hydrographs and peak flow data:  plotted hydrographs from the runoff
                   calculations showing pre-development hydrographs and post-development
                   hydrographs for 2-, 5- and 100-year storms.

      Hydraulic Calculations:

I           *    ~~~~~Retention -Detention:

                       *  Storage volume curves

                       *  Hydraulic calculations for spillways and outlets

  I                *    ~~~~~For regional or system  networks show  routing procedure for
                          evaluating impact of discharges on downstream facilities.




                                                A-2







             Pipe or culvert structures:

             ,,    Inlet and outlet elevations, slopes

             ï¿½ Length

             I  *  Diameter or height

             ï¿½     Mannings roughness coefficient

             *     Verification of inlet/outlet control conditions

             ï¿½ *   Design flows

             Streams or channels:

             *     Map of area and location of cross sections not to exceed 1000' apart
                   with 500' distances preferable. The accuracy of the water surface
                   profiles is greatly dependent upon the selection of cross-sections.
                   The cross sections need to reflect hydraulically controlling cross
                   sections, show overbanks and floodplain limits and project obstacles
                   in the floodplain that may influence flow within the reach between
                   cross-sections.

             ï¿½ Profile showing stream bottom, top of bank, 2 and 10 year water
                   surface profile; and other profiles as required by locality, e.g., 1000-
                   year floodplain

             ï¿½ Mannings Roughness Coefficient

             ï¿½ Velocity

             *  *  Method of computing water surface profile

Structural data for retention - detention facilities:

      *     Location and design of planned stormwater facilities including verification of
             structural soundness by a Professional Engineer

      ï¿½     Cross-sections of structures involving embankments; design elevations
             including freeboard allowances; the composition of core material; include
             cross-sections of outlet structures; vegetative cover or enclosure

      ï¿½ Cross-sections of infiltration facilities; composition of materials and any type
             of vegetative cover


                                         Ao3







   I                 ~~~~~~Soil boring data which supports the viability of such facilities

                   *  Statement of applicability of Virginia Dam Safety Act

        EROSION AND SEDIMENT CONTROL PLAN

I    .~       ~~ Vegetative cover to be disturbed

         0     Estimate of soil loss by use of Universal Soil Loss Equation or other acceptable
 I          ~ ~~~method.  If a different method is used, supply supporting documentation.

            aThe volume of first flush from the project area and the upstream watershed

         *     Peak runoff from 10G- and 1 00-year frequency storms based on present and future
               conditions and according to the existing hazards and degrees of protection
               required. For watersheds under one square mile, the peak runoff for the 1 00-year
               storm is not necessary.

               Methods of calculation

 3   .       ~~~~Phasing of land-disturbing activities:

                      Sequence of land clearing operations

                   *  Removal and stockpiling of topsoil

   *           *    ~~~~Major earth moving and grading

   *           *    ~~~~~Control facility installation

            *  Temporary erosion and sediment control

  *            *~~~~ Types of measures and facilities and the rationale for using them

                4     Location of each measure or facility with a description of upstream and
   I               ~ ~~~~downstream areas affected
  3            *~~~~ Cross-sections or other self-explanatory drawings

                0     Calculations supporting these measures

  I  .        ~~~Permanent erosion and sediment control

   3           *    ~~~~Types of measures and facilities and the rationale for using them

                   *  Location of each measure or facility with a description of upstream and

          1~~~~~~~~~~~~-







   I                 ~~~~~downstream areas affected

                  *  Cross-sections or other self-explanatory drawings

                  *  Calculations supporting these measures

*       ~CHESAPEAKE BAY PRESERVATION ACT

           *  Type of Development (IDA, New, Redev.)

           *  All components of the RPA (including wetlands and buffers) and the extent of the
              RMA.
           *  If compliance is not necessary, supply statement and documentation supporting
 *            ~~~exemption.
           *  CBPA Guidance Calculations or locally specified calculations.

           *  Water Quality Impact Assessment as required.

           *  Other locality specific documentation.













          I~~~~~~~~~~~~-



I
I
I
                       APPENDIX B
I
   I ~GUIDANCE CALCULATION
                     PROCEDURE
I
I
I
I
I
I
I
I
I
I
I
   I ~~~~SOURCE: CHESAPEAKE BAY LOCAL ASSISTANCE DEPARTMENT.
I
I






I                                                   IA A~.1ï¿½~\~~1111~**~:~~I1~'UIa  I  W


          INTRODUCIION

                 This procedure is designed to help applicants determine compliance with a locality's
          Chesapeake Bay Preservation Act program. This procedure does not supplant any informa-
          tion or requirement of other stormwater management programs, namely any local initiative
          adopted pursuant to either the Erosion and Sediment Control (ESC) Law [ï¿½ 10.1-560, et. seq.]
          or the Stormwater Management (SWM) Law [ï¿½ 10.1-603.1, et. seq.]. While all three programs
          are intended to protect water resources from further degradation, each requires separate
          engineering analysis. In general, these programs require calculations as follows:

          ï¿½     a CBPA program: stormwater quality

          ï¿½     a SWM program: stormwater quantity and quality

          *     an ESC program : two-year design storm runoff volumes and velocities

          Many localities may combine all aspects into one, comprehensive program. This calculation
          procedure would then be just one aspect of that program and a development proposal's
          submittal.


           | STEP ONE: |    Determine ifthe site is in a Chesapeake Bay Preservation Area.


                 The Regulations' require localities to designate Chesapeake Bay Preservation Areas
          (CBPAs). Guidelines for local designation are contained in Chapters II and m of the Local
          Assistance Manual and Part III of the Regulations. CBPAs consist of two different classifica-
          tions: Resource Protection Areas (RPAs) and Resource Management Areas (RMAs). The
          stormwater management criteria apply equally to both RPAs and RMAs.

                 While localities have flexibility to determine their own CBPAs, those areas will
          generally include the following land features:

          In RPAs:    tidal wetlands, nontidal wetlands contiguous to tidal wetlands, tidal shores,
                       tributary streams, a buffer area (of not less than 100 feet), and other lands as
                       designated by the locality;


          In RMAs:    floodplains, highly erodible soils, highly permeable soils, nontidal wetlands not
                       in the RPA, and other land as designated by the locality.








      Determine from the locality's designation maps and criteria if the site is subject to this
procedure. Localities mUï¿½ require the entire site to comply with the Regulations even if only
a portion of the site is in a CBPA. Determine the locality's requirement on total site compliance.



                    Determine if the site is classified as new development or
                    redevelopment.


The Regulations provide the following definitions:

Development means the construction, or substantial alteration of residential, commercial, industrial,
institutional, recreational, transportation, or utility facilities or structures.

Redevelopment means the process of developing land that is or has been previously developed.


      Check with the locality to see if further clarification is provided concerning redevelop-
ment.


 NOTE:      Any site in an Intensely Developed Area is automatically classified as redevel-
             opment, regardless of the site's present or previous condition.
             [ï¿½ 3.4 of the Regulations]

      For development, the post-development nonpoint source pollution runoff load cannot
exceed the pre-development load based on "average land cover conditions." This standard can
be referred to as a "no net increase" standard. STEP THREE will further discuss "average land
cover conditions."

      For redevelopment sites not served by BMPs, the post-development non-point source
pollution runoff load must be 90 percent or less of the pre-development load for that site. This
standard can be referred to as a "10 percent reduction" standard. Redevelopment criteria are
not based on average land cover conditions.

      For redevelopment sites with BMPs, the following provision(s) must be satisfied to
constitute "being served by water quality best management practices":

      (1)   In general, runoff pollution loads must have been calculated and the BMP
             selected for the expressed purpose of controlling NPS pollution. However, if
             existing facilities can be shown to achieve the current standard of NPS pollution
             control, local authorities may consider the site as being served by water quality
             BMPs.

                                            *  B-2






     ~~~~~~~~~~~~~~~~~I                                94KI W" big                 R A


      (2)   If BMPs are structural, facilities must currently be in good working order, per-
             forming at the design levels of service. The local authority may require a review
             of both the original structural design and maintenance plans to verify this pro-
             vision. A new maintenance agreement may be required to ensure consistency
             with the locality's SWM requirements.



 STEP           THREE. Determine the relative pre-developmentpollutantload of the Keystone
                    Pollutant (L.).

      The Keystone Pollutant for Tidewater Virginia is total phosphorous. The selection of
total phosphorous as the keystone pollutant is discussed in Attachment A. For the remainder
of this procedure, "pollutant" or "pollutant loading(s)" will mean total phosphorous.

      Following development or redevelopment, impervious cover is the key determinant in
the levels of pollutant export. Up to 90 percent of the atmospheric pollutants deposited on
impervious surfaces are delivered to receiving waters.2 So, for STEPS THREE and FOUR, the
site designer need only determine the amount of total area subject to these criteria and the
proposed amount of impervious cover (or equivalent). Guidance on determining equivalents
is given in Attachment B. Worksheets A and B will help with these next two steps.

      The zoning classification or proposed density of a site will allow applicants to estimate
impervious cover. Compliance and final engineering calculations, however, should be based
on impervious cover shown on the final site plan. Even so, localities and applicants are
encouraged to "err" conservatively, as properties tend to become more impervious with time,
e.g. the expansion of a structure, paving a driveway, adding more parking spaces. A
conservative estimate indicates more rather than less, impervious cover. Localities may wish
to set a minimum for particular land uses but require the determination of proposed impervi-
ous cover and use the higher number. Representative land use categories and associated
pollutant exports are shown in Table 1.


FOR DEVELOPMENT:

Average Land Cover Conditions (Iwt..~)

      Just as a locality must designate CBPAs, a locality must also establish baseloads for
watersheds within its jurisdiction. Once set, the baseload will not change unless technology
provides a more precise answer. Watershed delineations serve as the baseline for a calculation
procedure and do not constitute an additional regulatory step. The two options available to
localities are:


                                       B-3





       I ~ ~ ~ ~    ~     ~    ~     ~    ~ ~ ~ tMII ROOM__oil)_____


       1.    A locality will designate watersheds within its jurisdiction and calculate the
             average total phosphorus loading and equivalent impervious cover for each
             individual watershed, or

      2.    A locality will declare its entire jurisdiction as part of Virginia's Chesapeake Bay
             watershed with an average total phosphorus loading (FvA) of 0.45 pounds/acre/
             year and an equivalent impervious cover ('VA) of 16 percent.


       Some localities may begin with OMIoN Two while they gather the necessary data for
OPION ONE. Guidance on how a locality should calculate individual watershed loads is
provided in Attachment B. Discussion of the default loadings is in Attachment C.


      With Iwatehe, Lr   can be calculated using the Simple Method.  The derivation of the
Simple Method can be found in Appendix A of Controlling Urban Runoff:A Practical Manual
forPlanning and Designing Urban BMPs, published by the Metropolitan Washington Council
of Governments.

      JLr = P x P. x [0.05 * 0.009(lw.,.,a]xCxAx27  2
      PMI hpgP~X00+* (w~t)] xC xA x2.72 / 12j

where:

L  = relative pre-development total phosphorus load (in lbs/yr)
P     average annual rainfall depth (in inches)
      = 40 inches for Northern Virginia area
      = 43 inches for Richmond Metropolitan area
      = 45 inches for Hampton Roads area
Pi =   unitless correction factor for storm with no runoff = 0.9
Iwatershed= equivalent impervious cover for watershed,
      or "average land cover conditions" (percent expressed in whole numbers)
C =   flow-weighted mean pollutant concentration (in mg/l)
      =0.26 mg/l when It    < 20
      = 1.06 mg/l when I   >20
A =   applicable area of site (in ac)

 NOTE: 12 and 2.72 are conversion factors


FOR REDEVELOPMENT:

Pre-development loads for redevelopment sites are not based on average land cover condi-
tions. Instead, pre-development loads are based on the site conditions at the time of plan
submittal. Therefore, determine existing impervious cover or equivalent.

                                         B-4





I



        With I,, L   can be calculated using the Simple Method.

              | Lp= = P x Px[0.05 + 0.009(i,,,p))] xC xAx2.72 / 12

        where:

        Lp = relative pre-development total phosphorus load (in lbs)
        P =   average annual rainfall depth (in inches)
               = 40 inches for Northern Virginia area
               = 43 inches for Richmond Metropolitan area
               = 45 inches for Hampton Roads area
        P. =   unitless correction factor for storm with no runoff = 0.9
        Ip, = equivalent pre-development impervious cover of the site
              (percent expressed in whole numbers)
        C =   flow-weighted mean pollutant concentration (in mg/1)
               = 0.26 mg/1 when Isite (p,) < 20
               = 1.06 mg/i when Iste- , > 20
        A =   applicable area of site (in ac)

          NOTE:      12 and 2.72 are conversion numbers


         |STEP FOUR: I   Determine the relative post-development pollutant load (L).

               Just as with STEP THREE, the designer needs to know the post-development impervi-
        ous cover (or equivalent). For both new development and redevelopment, post-development
        loadings are site-specific.


        FOR NEW DEVELOPMENT

        Again, the Simple Method is used.

              ILpat = P x Pi x [0.05 + 0.009(I1,t,,0)] x C x A x 2.72 / 12

        where:

         Lt = relative post-development total phosphorus load (in lbs)
         P =   average annual rainfall depth (in inches)
               = 40 inches for Northern Virginia area
               = 43 inches for Richmond Metropolitan area
               = 45 inches for Hampton Roads area
         P. =   unitless correction factor for storms with no runoff = 0.9
                                             B-5









      I,,o = equivalent post-development impervious cover
             (percent in whole numbers)
      C = flow-weighted mean pollutant concentration (in mg/l)
            * For OPTION ONE: LOCALLY DESIGNATED WATERSHEDS
            = 0.26 mg/l when Ie~po < 20
            = 1.06 mg/l when I      >,( , > 20
            I  FOR OPTION Two: VA CHESAPEAKE BAY DEFAULT
            =0.26 mg/l for all Ite(p, -,,
      A = applicable area of site (in ac)

       NOTE:    12 and 2.72 are conversion factors

      FOR REDEVELOPMENT:

      Again, the Simple Method is used.

             L -t = P x PX  x [0.05 + 0.009(I,tt,)]) x C x A x 2.72 / 12

      where:

      Lot = relative post-development total phosphorus load (in lbs)
      P =   average annual rainfall depth (in inches)
            = 40 inches for Northern Virginia area
            = 43 inches for Richmond Metropolitan area
            = 45 inches for Hampton Roads area
      P. =   unitless correction factor for storms with no runoff = 0.9
I~~~~~~
      Ise(po = equivalent post-development impervious cover
             (percent in whole numbers)
      C =   flow-weighted mean pollutant concentration (in mg/l)
            = 0.26 mg/l when %t,(e w < 20
            = 1.06 mg/l when I   _> 20
      A =   applicable area of site (in ac)

       NOTE:      12 and 2.72 are conversion factors


      | STEP FIVE:  I   Determine the relative removal requirements (RR).

      Remember from STEP TWO, the performance standards are different.

      FOR DEVELOPMENT:

                                      RR = L   -, L,

                                               B-6





I



       FORREDEVELOPMENT:

                                         RR = L   - 0.9(LP.)


       If the calculated number isless than or equal to zero, STOP. Note thatin watershedsusingthe
        Tidewater weighted average, FVA = 0.45 Ibslaclyr, new single-family home parcels one acre
       or greater do not require BMPs.

              If no BMPS are required, the applicant need only submit documentation to support his
       or her findings. If such findings are found correct by local officials, the applicant has then
       satisfied the stormwater management criteria. The state Stormwater Management Law and
       the Erosion and Sediment Control Law also deal with other water resource related provisions,
       such as quantity-related requirements.

              If removal efficiencies are required, continue on with STEP SIX.



              STEP SIX      Identify BMP options for the site.


              Best Management Practices (BMPs) can be used to remove pollutants. BMPs are not
       always structural. For instance, trash removal can drastically reduce the amount of solid
       wastes that reach our streams. However, for the purpose of this discussion BMPs will mean
       any structural or mechanical device capable of preventing or reducing the amount of pollution
       from nonpoint sources.

              The use of certain BMPs may be limited on some sites by soils, topography, area and
       other physical characteristics. Most BMPs can only be applied under restricted site conditions.
       Improperly sited, a BMP cannot perform as designed and may become a chronic maintenance
       problem. A poorly maintained BMP may even contribute pollutants, e.g. an eroding pond
       embankment sends sediment into the receiving stream.

              BMPs and their associated pollutant removal efficiencies are shown in Table 2. This list
       is by no means a complete listing of available BMPs, nor does appearance on this list indicate
       appropriateness for a given situation.







                                              B - 7









3LTEP SEVEN:    Determine if feasible BMP options can meet the pollutant
                    removal requirement.


      If noff from the entire site passes through the BMP, the applicant need only select a
BMP with arn efficency rating equal to or greater than the efficiency required [as determined
in STEP FIVE]. If, as is usually the case, only portions of the site are covered by BMPs, a
weighted summation must be made.

      Localities mniy allow pollutant reduction credits for serving off-site areas which drain
through BMPs on the subject site. However, while applicants might claim pollutant reduction
credits for serving off-site areas, applicants MAY-NOT claim credit for one or more off-site
BMPs serving their property (even if, in fact, they do). Neither the Act nor the Regulations
allow for such an off-set program.

      Worksheet C will help with this step of the procedure.

      If no combination of BMPs can meet the required standard, the applicant must consider
a different site design. Increasing the proportion of site area covered with vegetation is one of
the best ways of lowering the required removal efficiencies. A different site layout may make
a more appropriate BMP possible; for example, placing structures on "tight" soils may leave
more permeable soil for infiltration areas.









ENDNOTES

I  Chesapeake Bay Local Assistance Board, Final Regulations: VR 173-02-01 Chesapeake Bay
Preservation Area Designation and Management Regulations. September 1989.

2 Thomas R. Schueler, Controlling Urban Runoff: A Practical Manual for Planning and
Designing Urban BMPs (Washington, D.C.: Metropolitan Washington Council of Govern-
ment, Department of Environmental Programs, 1987), 1.4.

3 Ibid, 1.9-1.13.





































                                     B-9










ANNUAL STORM PHOSPHOROUS EXPORT                                                                    TABLE 1

                                  For Existing Urban Land Uses
                                       (in pounds/acre/year)

                                                                ANNUAL RAINFALL
                                IMPERVIOUS                               (in)
                                    COVER
       LAND USES                       (%)             40       41      42     43      44      45


                                       0             0.11   -.11    0.11  ,0.11    0.12   ::.12
       5.0 acre residential lots        5              0.20   i021ii   0.21  0:.22   0.22   i23
       2.0 acre residential lots       10              030   :.30  0.31  :f0.32    0.33    0.33
       1.0 acre residential lots       15              0.39    A0.40   0.41  !:0.42:   0.43  i.-0.44..
                                      16             0.41    0.42    0.43    0.44    0.45    O.46
                                      17             0.43   .0.44   0.45    0.46-   0.47  :0.48
                                      18             0.45    0.46    0.47    048     0.49  it0.51
                                      19             0.47 A0.48S  0.49    0.50    0.52  i':i0.53 
       0.50 acre residential lots      20              2.03  i.2.08-1111 2.13   :2.18    2.23    2.28
       033 acre residential lots       25              2.42  :2.48    2.54   i2.61    2.67  i:2.72
       0.25 acre residential lots      30              2.82  Ii2.89!i  2.96  ::13.03:i: 3.10    3.17
                                   f  35             3.22   :3.30!i  3.38    3.46   3.54    3.62
       Townhouses                      40              3.61  ~i3.70:/:::! 3.79  3i  :i3.88    3.97    4.06
                                      45             4.01  ::4.11I: 4.21  i .431    4.41 I451
                                      r50             4A41    452    4.63    4.74    4.85  :4.96
       Garden Apartments               55              4.80    4.92    5.04    5.16:  5.28  :iA5.40..:
                                      60             5.20   '533    5.46  5:59    5.72    ..:85
                                      65             5.60   !5.74    5.88   6.02    6.16    630
       Light                           70              5.99    6.14: 6.29   :6.44' 659  *: 6.74
       Commercial/Industrial           75              6.39    655    6.71  ::6.87:0i  7.03  i':7.19i
                                      80             6.79    6.96:  7.13  :29    7.46    7.63
       Heavy                           85              7.98  ?i8.17::: 837  ::'8.57;  8.77    8.97
       Commercial/Industrial           90              7.58    7.96  i .i15    8.34   ;83
                                      95             7.98    8.17i 837  :.57   8.77    8.97
       Asphalt/Pavement                100             8.37  -!::858    8.79  :!9.00   9.21  ii9.42


                                     For Non-Urban Land Uses
                                        (in pounds/acre/year)

                                                  SILT LOAM   LOAM   SANDY LOAM
            LAND USE                                  SOILS         SOILS          SOILS

            Conventional Tillage
            Cropland                                   3.71          2.42           0.83

            -Conservation Tillage   ::::-  :l'
            Cropland                 . -:; -- - 00i:   ..    2 i2.32  1.5'           0.52

            Pasture Land                               0.91          059            0.20

            ForestLand  i  ::  :::                 :    .19       :0.12          ; 0.04

                                                 B-10









STRUCTURAL BMPs FOR CHESAPEAKE BAY PRESERVATION AREAS                                   TABLE 2


                                                                      Average
                                                                       Total P
                                                                      Removal
                     Acceptable BMP                                   Efficiency

              A.     Extended Detention

                      (1) Design 2 (6-12):                              20%

                      (2) Design 3 (24 hours):                          30%

                      (3) Design 4 (shallow marsh):                     50%


              B.     Wet Pond

                     (1) Design 5 (0.5 in/imp.ac):                     35%

                     (2) Design 6 (2.5 V):                           40-45%

                     (3) Design 7 (4.0 V):                             50%


              C.     Infiltration

                     (1) Design 8 (0.5 in/imp. ac):                    50%

                     (2) Design 9 (1.0 in/imp. ac):                    65%

                     (3) Design 10 (2-year storm):                     70%


              D.     Grassed Swale

                     (1) Design 15 (check dams):                      10-20%



These designs are taken from Metropolitan Washington Council of Governments, Controlling Urban Runoff:
A Practical Manual for Planning and Designing Urban BMPs, ,1987

Effeciency ratings are taken from John P. Hartigan, P.E., Three Step Process for Evaluating Compliance with
BMP Requirements for Chesapeake Bay Preservation Areas, 1990

                                         B-11










WORKSHEET A: NEW DEVELOPMENT OPnom ONE: LociLYDEsINawEm WATRSEEDSmms



[1 Compile site-specific data and determine site imperviousness (I,.).

                               POST-DEVELOPMENT
       A* =                                    acres
           I:**   structures      =            acres
               parking lot        =            acres
               roadway            =            acres
               other              =            acres
                                  =---    __ acres
                                 =   _____ acres

               total I,           =           acres

       !.b = (total I,/A) X 100   =            (percent expressed in whole numbers)

       *Although the area subject to regulations may be only the area actually in a CBPA, some localities
         may require all of the site to comply with criteria.
       * I. represents the actual amount of impervious area.

       Determine the average land cover conditions (Iw,,,th,d).

       Use I.lw.W as determined by the locality. If I,,md < 20, use Cp = 0.26mg/i. If I, ,thd> 20, use Cp, =
       1.08 mg/l.



   [  Determine need to continue.

       I,,    = __ __%  (from Step 1)
         Iw   -,_,_ =       %  (from Step 2)

       I f I.1 < Iwa,,d, STOP and submit analysis to this point.
       If It, > 'waterhed CONTINUE.


     Set constants.   '

       Pi     = unitless rainfall correction factor  P      = annual rainfall depth in inches
               = 0.9 for all of Tidewater Virginia          = 40 inches for Northern Virginia area
                                                           = 43 inches for Richmond Metropolian area
       CpON    = flow weighted mean concentration           = 45 inches for Hampton Roads area
                 of total phosphorus
               = 0.26 mg/l for IAs < 20
               = 1.08 mg/1 for I,~ > 20.


       12 and 2.72 are used in the equation as unit conversion factors.




                                               B e-12










WORKSHEET A: NEW DEVELOPMENT OpoNE: LOCAuYDESICNATIED WATERSHEDS

      Calculate the pre-development load (Lpd.

      LPr   = P X P.X [0.05 + (0.009 X I )XC XCXAX2.72/12

             -      X 0.9 X [0.05 + (0.009 X   X      X      X 2.72 / 12

             I  ________pounds per year



      Calculate the post-development load (LMP.).

      Lp3-     P X Pi X [0.05 + (0.009 X I X Cp,, X A X 2.72 /12

             -     x 0.9 X [0.05 + (0.009 X --)I X    X      X 2.72 /12

             1           pounds per year


      Calculate the pollutant removal requirement (RR).

      RR       LP.. - pP.



                         pounds per year


      To determine the overall BMP efficiency required (%RR) when selecting BMP options:

      %RR   =RR/ L X 100

             I  - (    3X 100




















                                       B- 13







~~~~~~~~~~~~~~WilI                               bil ,                              i s Kw 

WORKSHEET A: NEW DEVELOPMENT   OmION Two: VA. CIfESAPEAE BAYDEFauT


     Compile site-specific data and determine site imperviousness (I,,).

                               POST-DEVELOPMENT
       A*                         =             acres
       I.:**   structures         =             acres
               parking lot        =            acres
               roadway            =             acres
               other              -             acres
                                  = __        acres
                                  ____=  acres

               total I            =          acres

       I,,t = (total Ia/A) X 100  =             (percent expressed in whole numbers)

       *Although the area subject to regulations may be only the area actually in a CBPA, some localities
         may require all of the site to comply with criteria.
        **I. represents the actual amount of impervious area.

       Determine the average land cover conditions (IWterhed).

       Use Iw.tered = IVA=16 because F.. = 0.45 lbs/ac/yr for Virginia's Chesapeake Bay Watershed. Use
       Cp = 0.26 mg/l.



[~     Determine need to continue.

       I,.     =             %  (from Step 1)
       Itenhed =     16      %  (from Step 2)

       If I,, < I.wh,,   STOP and submit analysis to this point.
       If I'. > I.rhed, CONTINUE.

     Set constants.

       Pi    = unitless rainfall correction factor     P     = annual rainfall depth in inches
               = 0.9 for all of Tidewater Virginia           = 40 inches for Northern Virginia area
                                                           = 43 inches for Richmond Metropolian area
       C       = flow weighted mean concentration            = 45 inches for Hampton Roads area
                 of total phosphorus
               = 0.26 mg/l for all I,..

       12 and 2.72 are used in the equation as unit conversion factors.





                                            B-14









WORKSHEET A : NEW DEVELOPMENT                             Olmov Two: VA. ChESAPEAKE BAY DEFA Ur

U      Calculate the pre-development load (Lur).

       L Pr   =M  X 1.X [0.05 + (0.009 X I,)X C  XAX2.72 / 12

              -      X 0.9 X [0.05 + (0.009 X  )] X 0.26X    X2.72 / 12

              I           pounds per year



1      Calculate the post-development load (L.,d,

       L~    = PM F?    X [0.05 + (0.009 X I X C X A X 2.72 /12

              - ___ X 0.9 X [0.05 + (0.009 X __)IX 0.26 X     X 2.72 /12

              1           pounds per year


       Calculate the pollutant removal requirement (RR).

       RR       LP. - LP-



                          pounds per year

       To determine the overall BMP efficiency required (%RR) when selecting BMP options:

       %RR   =RR/ LpX 100

             I  - (   /)XIoo




















                                            B-15









 WORKSHEET B: REDEVELOPMENT

3i ~Compile site-specific data.
                                      PRE-DEVELOPMENT                  POST-DEVELOPMENT
              A*                         =            acres               =            acres
              I,:    structures          =            acres               =            acres
                      parking lot        =            acres               =             acres
                      roadway            =                                            acres
                      other              =            acres               =             acres
                                        =__           acres              =            acres
                                        -= _acres                        =            acres

                      total I.           =            acres               =             acres

              I = (total I./A) X 100     =            percent expressed  =              percent expressed
              Rt = 0.05 + (0.009 X I)                 in whole numbers                 in whole numbers
                                        -= ___       unitless            =            unitless
              C:     I > 20 = 1.08 mg/l
                      I < 20 = 0.26 mg/l  =           mg/l                =            mg/l

       * Although the area subject to regulations may be only the area actually in a CBPA, some localities
         may require all of the site to comply with criteria.

       Set constants.
           Pi = unitless rainfall correction factor     P  = annual rainfall depth in inches
              = 0.9 for all of Tidewater Virginia          = 40 inches for Northern Virginia area
                                                           = 43 inches for Richmond Metropolitan area
                                                           = 45 inches for Hampton Roads area
            12 and 2.72 are used in the equation as unit conversion factors.
       Calculate the pre-development load (Lp,.).

       Lw,    = P X PIX R,) X Cp. X A X 2.72 / 12

               =       xo.9 X       X       X       X2.72 / 12

                            pounds per year

[ Calculate the post-development load (Lp.,).

       Lpo.t   = P X PI X Rpo.) X C0p, X A X 2.72 /12

               =      X0.9X         X      X        X 2.72 /12

               =            pounds per year

       Calculate the pollutant removal requirement (RR).

       RR     = L,  - (0.9 X LX,)                   %RR   = (RR / L,) X 100

               =            -(0.9     X       )             = (-/  )X100

                            pounds per year                =             %

                                          B-16









WORKSHEET C: COMPLIANCE


        Select BMP options using screening tools and list them below. Then calculate the load
        removed for each option. DO NOT LIST BMPs IN SERIES HERE.
                                               Fraction of
                                             CBPA Drainage
                           Removal            Area Served                             Load
           Selected        Efficiency   X      (expressed in   X      L        =    Removed
           Option           (%/100)           decimal form)         (Ibs/yr)         (lbs/yr)











        Estimate parameters for non-CBPA drainage areas on the project site (if the locality
        does not require complete compliance for the whole site). If the locality requires
        compliance for the whole site, omit this step.

        A (on site, non-CBPA)         =            acres
        I.:    structures             =            acres
                parking lot           =             acres
                roadway               =             acres
                other                 =             acres
                                      =- _____acres
                                      I= -   _________acres

                total I               =             acres

        I = (total I./A) X  100       =             %
        R = 0.05 + (0.009 X I)        =

        C:     I > 20 = 1.08 mg/l     =             mg/l
                I < 20 = 0.26 mg/l

        When using VIRGINIA CHESAPEAKE BAY DEFAULT (FV. = 0.45 lbs/ac/yr), C=0.26 mg/l for all I,,.

        Calculate post-development load for on-site non-CBPAs.


        Lp.o~.(.)      = PXPXRV XCXAX2.72/12

                       =       X0.9X        X       X       X 2.72/12

                       =           pounds per year
                                                                                        Revised 7/90

                                           B-17











BI11   Determine loadings for off-site areas if the locality allows this option.
       I,..1d, = from locality OR I, ,, =  IvA = 16

       If I .,,,hd < 20, use C,, = 0.26 mg/l.
       If I,..,h > 20, use C,,. = 1.08 mg/l.
       If Is ,, = IvA use C. = 0.26mg/l.


       Loff,,   = P X Pj X [0.05 + (0.009 X Iwm,,a.)] X Cof, X Aofi, X 2.72 / 12

               =   _   X 0.9 X [0.05 + (0.009 X     )] X      X        X 2.72 / 12-

               =             pounds per year

       Total non-CBPA pollutant loading.

       Step 3     +   Step 4   =   total non-CBPA loading

                   __+____      =             pounds per year


       Calculate credits if the locality allows this option.


                            Removal                                    Load
          Selected         Efficiency    X         L=                Removed
           Option           (%/100)               (Ibs/yr)            (lbs/yr)










       Calculate overall compliance.

       Step 1     +    Step 5  =   total load removed

                   +            =             pounds per year

        If total load removed > removal requirement, criteria are satisfied.







                                                                                            Revised 7/90

                                                 B-18








ATTACHMENT A

      Many different pollutants can be identified in our streams and water bodies. The
Regulations merely require the control of "nonpoint source (nps) pollution." The Model
Ordinance defines NPS as pollution consisting of constituents such as sediment, nutrients, and
organic and toxic substances from diffuse sources. Trying to deal with all the possible
pollutants would make any calculation procedure complicated and expensive. To simplify the
calculations needed, a "keystone" pollutant can be selected. A keystone pollutant shares the
general characteristics of most other pollutants. By removing the keystone pollutant, other im-
portant pollutants will be simultaneously removed. Chapter 2 of A Framework for Evaluating
Compliance with the 10% Rule' reviews each of the major pollutants found in urban runoff for
their suitability as the keystone pollutant, based on the following three criteria:

1.    The pollutant must have a well-defined adverse impact on the Chesapeake Bay.

2.    The pollutant should exist in a "composite" form, i.e. in a roughly equal split between
      particulate and soluble phases.

3.    Enough research data must be available to provide a reasonable basis for estimating
      how keystone pollutant loads change in response to development and to current
      stormwater control measures.

      The only urban pollutants that appear to meet all three criteria for suitability as a
keystone pollutant are: total phosphorus, total nitrogen and zinc (Table 3). Of these three, total
phosphorus exists in the most equivalent proportions of soluble and particulate forms (40/60).
Total nitrogen and zinc are less proportionate, at 20/80 and 25/75, respectively.



             TABLE 3

                                   Well-Defined        Composite  Adequate
              Pollutant          Impacts on the Bay?     Form?        Data?

              Sediment                yes                 no           no
              Total Phosphorous        yes                yes          yes
              "TotalNitrogen.          yes                yes-         yes
             *-Coliform   Bacteria .  :.-ii fy - yes- i :n-:o-:: n-:io:i:i:::oX   :i :i-:  :X-
              BOD/COD                 yes                 yes          no
              Oil/Grease              yes                 no           no
              Zinc                    yes                 yes          yes yes  yes
              Lead                    yes :nof                         y
              Toxics                   no                 no           no


                                        B -19









      By removing total phosphorus, an equal or greater level of removal for most other urban
pollutants is simultaneously obtained. An equal or higher level or removal is possible for
nearly every other pollutant, except total nitrogen. Total nitrogen is primarily found in soluble
form, which is much more difficult to remove with current techniques. Nevertheless, by
removing phosphorus, a reasonable degree of nitrogen is still removed as well.

      Based on this review, total phosphorus was selected as the best candidate for the
keystone pollutant in Tidewater Virginia. In doing so, Virginia will target the same pollutant
as Maryland, preserving some consistency in our multi-state Bay preservation effort.


ENDNOTE:

 Schueler, Thomas R. and Matthew R. Bley, A Framework for Evaluating Compliance with
the Chesapeake Bay Critical Area (Washington, D.C.: Maryland Critical Area Commission
and Maryland Department of the Environment, 1987).






























                                          B-20








        ATTACHMENT B

              The Regulations require new development stormwater management criteria be based
        on "average land cover conditions.' Watershed designations serve as the baseline for a
3      ~~calculation procedure and do not constitute an additional regulatory step. Localities will have
        two options:

        1.    A locality will designate watersheds within its jurisdiction and calculate the average
        phosphorus loading and impervious cover for each individual watershed, or

I      ~~2.    A locality will declare its entire watershed as part of Virginia's Chesapeake Bay
        watershed with an average phosphorus loading of 0.45 pounds/acre/year and impervious
*      ~~cover of 16 percent.

        A locality may begin with Option Two while they gather the necessary data for Option One.
        Figure I shows how Fairfax County could break up its watersheds. This discussion revolves
        around Option One. Option Two is discussed in Attachment C.
        To determine average land cover conditions within a watershed, the locality must follow a
        three-step procedure:

        1.    Evaluate individual watersheds. We recommend a minimum watershed area of 100
              acres. Localities may wish however, to use watershed delineations used for other
              aspects of its work, e.g. a sanitary sewer master plan.

        2.    Know existing land use data. The Regulations are based on present land uses, not
              proposed land uses. A comprehensive plan is more future oriented than a zoning map.
              Still, a zoning map does not always indicate present use. A locality may also be able to
              use current aerial photographs. Data may be cross-referenced with Commissioner of
              Revenue information.

        3.    Compute a weighted average of impervious cover (or its equivalent). The Simple
 I         ~ ~~~Method (and the nonpoint source pollution load) is highly dependent on the percent of
              impervious cover. Some land uses contribute nonpoint source pollution but do not
              have "'impervious covers," e.g. forest and agriculture lands. Therefore, conversions, or
 I         ~ ~~equivalents, must be determined. Use Table I to find equivalent loading/impervious
              factors for non-urban uses. Localities may use other documented loading factors,
              especially if found to be more appropriate to that locality, as long as the factors are used
              consistently.
              Weighted averages are frequently computed for quantity related analyses and this
              process is identical. Figure 2 shows how average land cover conditions might be
              calculated for a 100-acre watershed.


         3                                   ~~~~~~~~~~B-21









I   Possmiu FAnRFAx CourNTY WATERSHEDs                                                         Fxc uRE I





                           SUGARLANO/  NICHOLS
                              RNRUN   POND
                                                BRANCH
                                                           BUL NECK RUN
                                                               SCOTTS SUN
                       rL~~T of>                                    DEAD RUN
                                                                       TURKEY RUN-


                         HORSNoEN         DIFFICULT RUN
    I                I      ~~~~~~~CREEK 

             I~~~~~~~~~~~~~~~~~~~~~~IM



I           ~~~~RUN                                .




           LITTLE ROCKY HAVN
I        ~ ~~~JOHNNY MOORE CREEK     WOL    C HICK CREE
              OLD MILL BRANCH   SAN                                     CU   ~ITN







                -   Watershed BoundaryKA E CEHPON
           BULL RUN  Watershed Name
              I~~~~~~~~~~~~~~~~~~~~~~~~~~SV


 ISource: County of Fairfax, 1987 Annual Report on the Environment (Fairfax, Va.: Environmental Quality
     Advisory Council and Office of Comprehensive Planning, 1987), p. 16







                                                   B-22









CALCULATING AVERAGE LAND COVER CONDITIONS                                       FIGURE 2


100 acre Watershed
Wooded         = 20acres
                                                  Woode  no i         Woode
                                             Low-density

Residential    = 20 acres            ......
(1-acre lots)

Agriculture
 Pasture       = 30 acres
 Conservation
       tillage  = 15acres                         Agricultural
  Conventional
      tillage   =  15 acres

Total acreage    100 acres





Land Use           Loading: *          # of Acres      Weighted Load:
                   lbs/acre/year                         lbs/year

Wooded                0.12               20                  2.4
1-acre lots           0.42               20                  8.4
Pasture               0.59               30                 17.7
Conventional          2.42               15                 36.3
Conservation          1.52               15                 22.8

                                         100               87.6

* Phosphorous; based on rainfall of P=43 inches/year and loam soils.

I   = Sum of weightedloadings
           total acreage

  = 0.12(20) +0.42(20) + 0.59(30) + 2.42(15) + 1.52(15) = 88 lbs per year = 0.88 lbs per acre per year
             20 + 20 + 30 + 15 + 15                100 acres

                 Equivalent Impervious Cover  =  Iwatershe    =    19

                                      B-23








ATTACHMENT C

       Not all localities will have the ability to designate individual watersheds and compute
an average watershed baseload. For that reason, the department has determined a default load
for Tidewater Virginia.

Following the procedure outlined in Attachment B:

1.    Designate watershed.

       The department chose the entire Virginia portion of the Chesapeake Bay watershed -
       not just Tidewater Virginia (as defined by the Chesapeake Bay Preservation Act). The
       department encourages multi-jurisdictional cooperation among localities to designate
       large-scale watersheds as well.

2.    Evaluate existing land use data.

       Existing land use data is given in Virginia's Chesapeake Bay Initiatives: First Annual
      Progress Report (September 1985) produced by the Virginia Council on the Environ-
      ment. This breakdown is shown in Figure 3.

3.    Compute a weighted average of impervious cover (or its equivalent).

       Because urban areas are most likely to adopt Option One, urban areas are excluded from
      the weighted average. In addition, loading rates for "urban" areas are highly variable.

  FVA = relative total phosphorus load for Virginia's Chesapeake Bay watershed

  Fo = relative total phosphorus load for any land use (X)

  FVA = %FOR(FFOR) + %PAST(FpAsr) + %CST(Fcr) + %CVT(FvT)

       = 0.66(0.12) + 0.21(0.59) + 0.07(1.52) + 0.06(2.42)

       = 0.45 lbs/ac/yr


      Use Table I to determine the equivalent impervious cover. The average loading, FVA =
      0.45 lbs/ac/yr, falls between impervious covers of 16 to 18 percents. Because of the
      differing annual rainfall across the state, the department has choosen the most conser-
      vative value of 16.

         Fv = 0.45 lb/ac/yr <=> IVA = 16%


                                           B-24










Therefore, the default load for Virginia's Chesapeake Bay watershed is 0.45 lb/ac/yr with an
equivalent impervious cover of 16 percent. Localities are encouraged, but not required, to
customize this aspect of the procedure, even if computing individual watersheds is not
feasible. The Town of Herndon might use IvA = 18, Caroline County might use IvA = 17 and Isle
of Wight County would retain IVA = 16.




VIRGINIA LAND USE DATA                                                              FIGURE 3




              .area   %    aea    %                    area    %   area    %    area
River Basin   :(sq.mi:  URB  u(sqmi) FOR(sqn.mi.) PAST   (sqmi.) CST (sq.mi.) CVT :m(squni.)

Potomac       :14670   7    1027:::-..: 56  :8215:  26   :3814   7  :1027:.    4  ::.S::::587:
Rappahannock .2630   1    I26-      4  16 84    20        26     8  .210       7    1
York          ..980  0.2  -6    70  20           13      388   10.1  3-02      6.7  200.
James         10495...i 3   -.::315:i:  73  766 -    14   1:469-i::::  6  :630: .    4    :. 420
EasternShore   1000   1.5   15:   50   -00    805       :S         90    31   310
Total (w/urban) 31781'Si:   5    1389: : 63 20150:::::  20  ::6286    7   2259  5   1701
Total (w/ourban)30398:: n/a l:n:a;:  66 20150::  21   :::6286    7  i12259:::::::   6   1701

URB = urban land uses
FOR = forest cover
PAST = pasture land
CST = conservation till acreage
CVT = conventional till acreage

Source: Commonwealth of Virginia, Council on the Environment, Virginia's Chesapeake Bay
        Initiatives: First Annual Report (Richmond, Va.: Council on the Environment, 1985).


















                                        B-25