Guide to stormwater management practices in New Jersey

[From the U.S. Government Printing Office, www.gpo.gov]

A Guide to Stormwater Management
Practices In New Jersey,

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1986
April 1986
lew Jersey Department Of Environmental Protection/Division of Water Resources

A GUIDE TO

STORMWATER MANAGEMENT

PRACTICES IN NEW JERSEY

Property of C-.C Library

Division of Water Resources
Department of Environmental Protection
March 1986

U S DEPARTMENT OF COMMERCE NOAA
COASTAL SERVICES CENTER
2234 SOUTH HOBSON AVENUE
CHARLESTON , SC 29405-24 1

TABLE OF CONTENTS

Introduction

Chapter 1 General Planning Guidance ........................... J.

Chapter 2 Dual Purpose Detention Basins ....................... 3

Chapter 3 Planninq Considerations, Infiltration and Volume
Controls ........................................... 1.7

Chapter 4 Hydrologic Design Methods, Infiltration and
Volume Controls .................................... 80

Appendix A Stormwater Management Regulations .................. 130

Appendix B Model Ord4nance for Phase I Compliance with
State Stormwater Management Regulations ........... 144

INTRODUCTION

Under provisions of the New Jersey Stormwat,er Management Act (PL
1981, c. 82) and its implementing regulations, (N*.J.A.C. 7:8
1.1 et. seq.), procedures and standards for stormwater
management have been established, for implementation by counties
and municipalities. These regulations are attached as Appendix
A. The mandatory requirement for these local agencies to
control stormwater management is contingent upon the availability
of a grant to cover 90 percent of the necessary planning costs.
The policy of the Department is to request such funding only to
cover the higher priority situations, where economic growth is
rapid enough and the consequences of flooding downstream serious
enough to warrant early imposition of storm water management
controls. Such priorities have been made known to agencies
concerned. In general, all rapidly developing areas, except
those on barrier islands, need such controls.

The Division of Water Resources is aware that there is a great
diversion in stormwater management controls already in effect.
The greater part of the structures built so far consists of
single purpose detention basins; and many consulting engineers
and municipal engineers are entirely familiar with the technology
involved. Moreover, many texts and reference books cover the
subjedt. However, conditions in urbanizing areas of New Jersey,
require general use of dual purpose detention basins, in the
interest of water quality; and the use of infiltration basins and
other volume controls is also of increasing interest. Therefore,
this report provides general guidelines.and some technical
guidance for use of such structures, with reference to
publications where further details may be found.

These standards and specifications are provided as-design guides
to engineers familiar with the fechnical aspects of stormwatdr
hydrology, hydraulics and soils engineering. Engineers are not'
limited to utilizing only these techniques in the preparation of
storm water management control plans. The use of other
techniques such as roof top or parking lot storage, underground
storage etc. are suitable alternati 'ves when properly used.
Additionally, the design methods presented are not the only
methods that can be used to design the techniques discussed.

Stormwater management control techniques are sometimes defined as
either rate or volume controls. Rate controls are-primarily'used
to reduce the peak rates of runoff, and thereby to provide local
control of runoff problems. Volume controls are used to actually
reduce the volume of runoff and are effective for both local and
regional runoff control. The most effective management
techniques (such as those of New Jersey), combine rate and volume
controls. The New Jersey State Stormwater Management Regulations
require rate control of a range of design storms and volume
control of a water quality design storm. The rate controls are

based on reproducing predevelopment peak rates, the water quality
controls require the volume of runoff be retarded so as to reduce
the transport of pollutants as well as reducing flooding
downstream. The water quality requirements can be satisfied by
providing prolonged retention in a rate control facility (i.e.
detention basin) or by the production of zero runoff through a
volume control (i.e. infiltration techniques). The combination
of these requirements provide local and regional flood control as
well as enhanced water quality.

The designer and the planner should review the overall objectives
of the stormwater management plan or policy to insure that the
control techniques chosen provide the desired level of control.

With only a few exceptions, existing local stormwater management
controls in New Jersey, exemplify a site-by-site approach. It
has been known for some time, as stated in the regulations, that
much better results can be obtained by planning systems which are
adopted to an entire drainage basin. With the cooperation of the
U.S. Soil Conservation Service and Hunterdon County, a
demonstration planning project has been carried out, the results
of which are summarized in areport, "Regional Stormwater
Management Planning, South Branch Rockaway Creek", March 1986,
which is being distributed separately.

In preparation of the present report, contributions of the work
group and participating agencies for the regional study provided
major input, particularly those of the U.S. Soil Conservatioh
Service. The major portion of information presented herein on
infiltration techniques was taken from "Standards and
Specifications for Infiltration Practices" prepared by the
Maryland Department of Natural Resources, Water Resources
Administration Stormwater Management Division. The New Jersey
Division of Water Resources appreciates the assistance supplied
by the Maryland Division of Natural Resources in providing this
information on the design of infiltration techni-ques for storm
water control.*

Although an effort has been made to be consistent with the
policies of all participating agencies, the finall working of this
document is that of the Division of Water Resources, Department
of Environmental Protection.

CHAPTER 1

General Planning Guidance

The planning of stormwater management structures under the
regulations is governed by performance standards. Provided
certain criteria are-met, the State policy allows considerable
latitude in accomplishing the desired objectives.

However, such flexibility is not without its limits; and there
are some practices which, while nominally effective, have
inherent limitations greatly limiting their ability. Also, there
are some secondary effects, such as environmental aspects, which
may control. Any guidance in-this document is subject to various
other legitimate objectives, each important in its own field,
including such items as groundwater p6llution, insect vectors,
endangered species and wetlands.

Since the basic purpose of stormwater management is to compensate
for the added storm runoff and the runoff pollution caused by
development, some form of storage or detention of stormwater
runoff is the characteristic feature of the program. The term
detention (or rate control) usually refers to the holding of
runoff for periods not over a few hours, while retention (or
volume control) refers to methods of infiltrating runoff into the
ground or otherwise holding it back for a longer period.

Most stormwater management ordinances-in the past established
criteria only for peak flows, on a site-by-site basis. For
example, the peak flow from a 100 year storm after development
should'not exceed that from the same storm prior to development.
When faced with such criteria, competent engineers, considerate'
of costs for their clients, naturally designed single purpose
detention basins with a single outlet. The demonstration project
analysis (previously referred to) clearly indicates the inherent
deficiency in such designs - the effects of site-by-site rate
controls even a few miles downstream are completely ineffective.
Recognizing this relationship, the state stormwater management
regulations require the prolonged storage of a small design storm
which, besides favoring flood control downstream, reduces the
pollution from urban runoff. The state standard is a minimum
standard for flood control and water quality of "dual purpose"
basins. Single purpose detention basins are only acceptable when
combined with some other measures which reduce the runoff
pollution, such as grassed swales.

infiltration-basins have been widely used in areas with porous
soils, and under appropriate circumstances they may be fully
effective for water quality retention as well as rate control of

major stormi. Volume*contro .ls, such as infiltration techniques,
are usually limited to use with small runoff volumes, therefore
they will be most effective when applied to small design storms.
Attempts to design such controls for design storms above the Y
2-year event are usually impractical. As indicated in detailed
guidance below, infiltration basins must be carefully analyzed
for permability of soil, seasonal high water table, and necessity
for miintenance of depbsited sediments, in order to prevent
clogging by colloidal particles. Even more important, it is
absolutely essential that possible groundwater contamination be
taken into account. Urban runoff usually contains considerable
concentrations of hydrocarbons, and often of nitrates, which may
unacceptably pollute groundwater. Not only infiltration basins,
but also systems of infiltration trenches and dry wells, should
be subjected to an analysis of the anticipated pollutants and
their effect upon groundwater before such systems are approved.
Vegetated swales are less likely to cause problems of this sort.

Although infiltration basins can be cleaned out to remove
accumulated sediments, this is not the case for infiltration
trenches and dry wells. Urban runoff generally contains
considerable sediment. Such structures are only permanently
usable for stormwater management when designed for runoff from
.roofs or grassed areas not likely to contain suspended sediment.
If, due to space limitations, there is no other alternative,
provision should be madelfor additional capacity capable of
holding an estimated 20 years of suspended sediment load.

Planners should carefully consider the lessons to be learned from
the report on basin wide demonstration project. Although the
results cannot be applied automatically elsewhere, they indicate
some practical points which may reduce the scope and cost of
engineering studies required to find an optimum-solution. In
particular, the combination of at-site measures to control small
storms combined with regional basins to control the larger
storms, is apt to be widely useful. It is almost certain to be
more satisfactory and more economical than a simple site-b@-site
approach imposed uniformly by an ordinance.

of course, the first stage of control for a municipality must be
an ordinance, which can then be replaced by specific basins plans
as fast as they can be developed. A model ordinance suitable for
Phase.I planning is attached as App.endix B.

CHAPTER 2

2.1 DUAL'PURPOSE DETENTION BASIN

(DB)

Description

A dual purpose detention basin is a temporary water impoundment
made by constructing a dam or an embankment or by excavating a
pit or a dugout in undisturbed soil. The purpose of the basin is
twofold; to promote the settlement of runoff pollutants by
retaining the first flush for a prolonged period and to
temporarily store the surface runoff from a selected desiqn
storm or a series of design storms by restricting outflows to a
predetermined rate.

Applicability

A dual purpose detention basin will be generally used where a
significant volume of runoff is to be controlled. Detention
basins are the most popular control techniaue for managing
storm water runoff. The detention b8sin will typically be'
utilized for drainage areas from one acre up to several hundred
acres. Detention basins may be combined with an infiltration
basin (IB) or a wet pond by raising the outlet pipe @o allow
predetermined volume of stored runoff to infiltrate, or to be
retained. where possible the infiltrated volume should be equal
to the runoff from the water quality design storm.

Planning Considerations

'On small sites with large percentages of impervious cover,
detention basins can occupy significant portic'ns of the site-.
In cases where the conventional basin design results in
unacceptable revisions to the plan layout, other control
practices should be investigated.

A typical dual purpose detention basin will range from three to
twelve feet in depth. Depth is often limited by groundwater
conditions or by the need for po 'sitive drainage from excavated
basins. The depths are often limited by the need to minimize
the*in inundation area or "take line" associated with the top of
the embankment'. A typical basin will be constructed with a.
combination of excavated and diked storage.

The nature of the dual purpose detention basin concept, the
collection of site runoff, will u 'tually recruire the facility to
be located at the lowest elevation of the contributorv drainage

area. On many sites, this means locatinq the detention basin
along a stream or within.-a flood hazard area. When so
located, the outlet st Iructure can be influenced*by backwater
from the stream or flood hazard area and the intended effect of
the'detention basin partly or entirely nullified. The New
Jersey State Storm Water Management Regulations contains
criteria for such situations.

Design Criteria

The following design criteria should be considered as
minimum requirements. Where applicable or desirable,
additional criteria mav be added to or required in place of
these minimum criteria, as long as such additions or
replacements equals or exceeds the degree of water quality and
flood control achieved bv these minimum requirements.

Desian Storms

All hydrologic and hydraulic calculations shall be based on the
design storm criteria provided in the New Jersey Storm Water
.Management (SWM) Regulations (NJAC 7:8). These design storm 's
shall be defined as either.a 24-hour SCS Type III storm or as
the estimated maximum rainfall for the estimated time of
concentration when using the Rational Method. The regulations
specify the 2-year, 10-year and 100-year storms as flood control
design storms and the 1-year storm or a one and one cruarter inch
two hour rainfall as the water quality design storm.

Release Rates

The -release rates for *dual purpose detention basins shall
be determined in accordance with the water quality, flood-and
erosion control requirements of the New Jersey SWIM
regulations (NJAC 7:8). The water Quality design storm must-be
retained and released such that no more than ninety percent of
the runoff is evacuated in less than 18 hours for residential
developments and no less than 36 hours for all non-residential
projects. For flood and erosion control, the regulations
require the runoff from the 2-vear, 10-year, and 100-year storms
be controlled such that peak flows from a site shall not
increase as a result 'of development. Also, when computing
runoff peaks, all lands in the site shall be assumed, prior to
development, to be in good hydrologic condition Specifically,
whet using Table 1.2 of Technical Release-55 predeveloped lands
will be assumed to be in qood condition if the lands are
pastures, lawns, or parks; with good cover, if wooded; or with
conservation treatment if cultivated regardless of the
conditions existing at.the time of computation.

The 8ischarqe rates and volume of the discharge from
detention basins can be fairly large, especially in relation
to the cavacitv of the receiving,streams or swales. Concerned
efforts should be made to set release rates from the basins
to.levels that will reduce downstream flooding and
erosion potential. Release rates are often tied to@
"predeveloped" rates of runoff. Recognizing the fact that these
predeveloped rates may be damaging, many communities have used
release rates based. on other than the existing condition rate
of runoff. The use of a percentage reduction, such as 50% of
the predeveloped rate or predeveloped condition of the site
be considered in a "natural" state, such as meadow, are two of
the more popular methods used to reduce basin outflows. The use
of these rate reductions schemes can often be justified by the
need to reduce downstream impacts or by the fact that detention
basins do not reduce the increased volume of runoff caused by
the development and the rate control must be adjusted to
safely control the increased volume.

Alternatively, the release rates may be tailored to a regional
or a state approved Phase II SWM plan. In such cases,
downstream impacts must be evaluated in detail to insure
that minimum requirements equivalent to those of the New Jersev
SWM regulations are equalled or exceeded.

Water Table and Ground Water Conditions

At the location of the proposed dual purpose detention
basin, the depth to the seasonal. high water table (SFVTT)
should he identified. II the basin is to intercept the
ground water table, the effects o"f seepage on the
facility should be investigated. Basins that do intercept
the SH`WT may have unstable side slopes and may create
maintenance problems due to the seepage especially where mowing
is required.

.Runoff Filtering

In order to reduce maintenance costs, inflows to dual purpose
detention basins may be filtered prior to entering the
facility. Filtering can be accomplished through the use
vegetated swales or filter strips. Removal of grease, oil,
organics and sediments from the runoff will serve to increase
the efficiency of the detention basin as a water quality
enhancement technique, and will also reduce maintenance
problems. Heavy sediment loads should not be allowed to
enter the detention basin unless adea uate provisions are made to
remove accumulated sediment-Known pollutants loads such as
floating-petroleum should not be routed into a (@etention basin,
but be controlled at source.

Principal Spillway Systems

The principal spillway-systems for a detention basin should be
designed to accommodate the required design storms. The spillway
structure should be located such that it is readily
accessible for maintenance. Easement or rilght-o-1-7-way access
should be provided.

From a stfuctural standpoint the outlet structure should be
designed to withstand all anticipated pressures or
loadinqs. If heavy equipment will be required for
maintenance, the outlet structures should be designed
accordingly. Provide vehicle access so that debris can be
removed from a spillway when in operation.

Dual purpose detention basins designed according to the New
Jersey SWM regulations will usually- involve multi-stage
outlet systems. The lowest outlet will be desiqned to
achieved prolonged retention requirements for water quality
enhancement. The invert of the first flood ccntrol outlet (2-
year control) will then correspond to the maximum water surface
elevation produced by @the water quality design storm. The
-principal spillway system should be sufficient to pass all the
required design storms.

Outlets from detention basins Shall be designed to function as
designed without manual, electrical or mechanical controls.

Anti-seep or cutoff collars should be used to avoid potential
piping along the outlet conduits. The use of good quality fill
and sufficient compaction around the conduit and collars is
essential. All conduit joints should be water tight and the
number of conduits through the embankment should be minimized.
Where thin walled conduits are used throuqh the embankment, a
protective exterior encasement should be used. The desian of
the conduit should attempt to minimize internal water pressu .re.

Emergency Outlets

All drainage facilities should be equipped with emergency
overflow systems to ensure safe passage of flows 'larger than
those from the design storms. Detention basins should be
designed to include an nonerosive emergency spillway. The
design of the spillway will depend on the size and location of
the basin. At a minimum the emergency spillway should be
designed to pass the maximum design storm with sufficient
freeboard assuming no flow through principal spillway. This
would apply to all detention basins not classified as a dam.
All dams, as defined by the New.Jersey Safe Dam Act and the
New Jersey Dam Safety Standards NJAC 7:20-1.1, must have

emergency spillways designed in accordance with the Dam
Safety Regulations.

Vegetated emergency spillways should have side slopes not
exceeding 1 vertical to 4 horizontal. Velocities in vegetatpd
spillways should not exceed the permissible non erosive
velocities listed in Table 3-3 and the criteria contained in
Hydraulic Enaineering Circular No. 15 - Desian of Stable
Channels with Flexible Linings or Standards for Soil Erosion
and Sediment ControT___I_nNew Jersey.

Dams and Embankments

All detention basins involving the construction of an
embankment that raises the water level five feet or more above
the usual, mean, low water heicrht when measured along the
downstream toe-of-dam to the emergency spillway crest is
classified as a dam and must be designed, constructed and
maintained in accordance with the N.J. Dam Safety Standards
(NJAC 7:20). All other detention basins with embankments shall
he designed in accordance with the following criteria.

Top Width: The minimum top width of the embankment
shall be 10 feet.

Side Slopes: The side slopes of.the settled embankment
should not be less than 4 hori7ont@al to 1 vertical.

Freeboard: The minimum elevation to the top of the
settled embankment shall be one foot above the water
surface in the detention basin with the emerqency spillway
at the maximum design flow or a minimum of-two feet.above
the crest of the.emerqencv spillwav, which ever is higher.

Settlement: The design height of the basin embankment
should be increased by ten percent where hauling equipment.
is used for compaction and five percent where compaction
equipment is used. All earth fill shall be free from
brush, roots and other organic material subject to
decomposition. The fill material in all earth dams and
embankments should be compacted to at least 95% of the
maximum density obtained from compaction tests
performed by the appropriate method in ASTM D698.

Vegetation

Water tolerant species of vegetative cover for pond
surfaces should be used to maintain high infiltration
rates and aid decomposition of settled particulates. Forage
and fodder crops, such as canary grass, fescue, perennial
rve, orchard grass, and bermuda grass can be used to
successfully treat large amounts of runoff, and are
tolerant of variations in water quality.

Water Ouality

A unique aspect of the New Jersey storm water management
regulations is the requirement for water quality control. While
water quality enhancement may be achieved through a variety of
SWM technicues, detention basins are the most widely used in New
Jersey. Dual purpose detention basins designed according to the
standards of the N,7SWM regulations provide significant
reductions in runoff pollutant loadings. Prolonged retention of
the runoff from the water quality design storm 'Promotes the
settlement of particulates. Sorbed to the surfaces of these
particulates are a varietv of runoff bourne pollutants; of
particular concern are petroleum based organics, heavy metals,
-and undissolved phosphates.

Construction Specifications

The construction of all detention basins should complv with the
Standards for Soil Erosion and Sediment Control in New Jersey
and the additional criteria provided below.

Schedule

The'seauence of various phases of basin construction should
be coordinated with the overall project construction schedule
and the approved soil erosion and sediment control plan.
Scheduling rough excavation of the basin with the rough grading
phase of a project permits the use of the excavated material a's
fill in earth work areas. The partially excavated basin could
serve as a sedimentation basin during construction. However,
-basins near final stages of completion should not be used for
the disposal of runoff from exposed surfaces, as this will
result in.larqe sediment-loads being deposited in the basin.
Final grading, seeding and cleaning of accumulated sediment in
the basin ana the outlet structure should not occur until all
disturbed areas have been permanently stabilized.

Specifications for basin construction should state: (1) whether
the basin will be used for sediment-control, (2) at what point
the basin will be converted to a detention basin and (3) how
the transition will be accomplished. The last stateirent.
should include a list of activities to be performed.such as

removal of temporary riser pipes, clearing of sediment,
final seeding, etc.

Maintenance

Schedule

All drainage systems must be routinely inspected to ensure
proper operation s. Inspections should be scheduled for all
facilities after malor storms for routine maintenance and at
least bi-annually for structural inspections.

Sediment and Debris Accumulation

All detention basins will accumulate sediment and debris.
Basin designs should include storage for at least on years
accumulation of sediment to ensure the s 'taqe-storage-disc'harge
characteristics of the facility are not-adversely affected by
routine sedimentation.

Debris and most sediment will accumulate around the outlet
for the basin. To protect the outlet from clogging, adeauate
trash racks and proper outlet designs are necessary. Trash racks
.should be arranged in series to provide, two stage protection.
openings in these screens should be small enough to restrict
debris vet large enough to avoid excessive clogging and
interference with the hydraulic function of the outle-11---.
Inclined trash racks are preferred as debris will tend to
ride up with the water level. All protective screen/racks must
be hinged to facilitate the removal of accumulate debris and
sediment.

To prevent the re-suspension of deposits of sediment in the
basin, energy dissipators should be employed at all inflow
points. Reduced inflow velocities will.also help promote
sedimentation.'

The New Jersev Department of Environmental Protection and Ocean
County are currently conducting an investigative study on the
maintenance of storm water management facilities. A major goal
of the study is the development of a maintenance manual that
will include information of the financial, legal and engineering
aspects of long term maintenance of storm water management
faiclities. A special_emphas-is will be placed on dual purpose
facilities. The manual is scheduled for completion in the
Spring of 1988.

2.2 DESIGN CONS IDERATIONIS FOR DUAL PURPOSE DETENTION BASIN

A dual purpose detention basin is an impoundment area made by
constructing an embankment and/or excavating a pit. The purpose of
the basin is to temporarily store storm water runoff in order to
promote the settlement of runoff pollutants and to restrict outflows
to predetermined levels to reduce local and downstream flooding. The
basin stores runoff from a developed site or upland area and rainfall
that falls on the impoundment surface. Dual purpose detention basins
are usually designed as either dry or wet basins. Dry basins are
expected to fully evacuate all stored runoff. Wet basins will retain
a permanent pond between storm events. Dual purpose basins may also
be built with an infiltration-feature rather than a wet basin.

Site Layout

The location of a dual purpose detention basin is extremely important
to it's effectiveness. To be able to effectively reduce peak rates of
runoff, the basin must be situated to intercept a majority of the site
runoff. For effective water quality control, the basin must collect
all runoff from the impervious areas of the site. The most
important of these are roadways and parking-lots. The majority of the
key pollutants that are removed by dual purpose detention basins
originate on these surfaces.

Runoff from areas uphill or upstream from the development site may be
passed through or around the site without detention or storage. This
may also be applied to onsite areas that have not been developed such
that the runoff potential is increased or the water quality of runoff
is degraded..

Site conditions may require a storm water management plan that results
in certain developed portions of the site bypassing the storm water
control measures. With traditional flood control detention basins, an
equivalent volume of off site or upstream runoff can be controlled to
offset the release of uncontrolled onsite runoff. Similar exchanges
of runoff are possible with dual purpose detention basins. However,
the excharige@ waters must have equivalent water quality
characteristics. An acceptable method of evaluating potential water
quality.for this purpose is to utilize*land use categories. For
example if the runoff from a parking lot bypasses the dual purpose
basin, an equivalent volume of runoff from an upstream parking lot may
be substituted. For similar land use categories, the extent of
impervious coverage should be used to establish equivalency. Density
may be substituted in the use of residential land uses, if
appropriate. The procedure is intended to allow exchanges of runoff
with similar water quality. However land use may not always be the
sole determinant of pollution potential. For example the runoff from
the parking-lot of an industrial chemical shipping yard can not be

considered to have the sarne water quality potential of runoff from an
office building parking lot.

Additional Design-Considerations

In order to provide the level of water quality and flood control
required by the New Jersey Storm Water Management Regulations, multi
stage outlet control structures are usually necessary. A typical .
discharge structure for a dual purpose detention basin is illustrated
in Figure 2-1.

Principal Outlets

The design of principal outlets of most storm water controls is based
on reproducing predeveloped flow rates. The New Jersey SWM
regulations require the 2-year, 10-year and 100-year predeveloped
rates of runoff be used for outlet release rates. Added to the rate
control requirement is a standard for water quality enhancement of the
runoff of the 1-year storm.

The discharge structure will typically have three principal outlets.
The water quality outlet (dl) will usually be small in comparison to
the other outlets. The use of a restrictor plate containing an
orifice desiqned to provide the required retention should be used in
place of a small pipe in the structure. The restrictor plate can be
removed to facilitate maintenance or to allow future alterations of
the water quality outlet.

The second and third stage outlets are sized to provide the required
reductions in the larger storms. In most cases, the second stage
outlet will be designed to act as the primary control for the 10-year
storm. The two(2)-year, being so close to the one(l)-year in runoff
volume, will be primarily controlled by the water quality outlet and
despite some overflow through the second outlet, peak rates are
adequately controlled. The third stage outlet is designed to control
the 100-year storm. The overflow is normally provided in case of
failure of the lower stage outlets, for example due to clogging by
debris.

Outlet Protection

In order to reduce the possibility of clogging of the various outlets,
trash racks should be installed for each. Two examples of trash racks
are shown in Figure 2-1. The inclined vertical bar rack is most .
effective for the lower stage outlets. Debris will ride up the trash
rack as water levels rise. This design also allows for removal of
accumulated debris with a rake while standing on top of the structure.
Cage type racks or racks with horizontal members inhibit this type of
debris removal.

The surface areas of all trash racks should be maximized and the trash
racks should be designed to be as far away from the protected outlet

SECTION

(@OVERFLOW GRATE
MAXIMUM DESIGN STORM ELEVATION

t
THIRD STAGE
OUTLET
d 3

TRASH TRASH
GRILLES GRILLES
HINGE

SECOND STAGE
WATER QUALITY DESM STORM ELEV jd2 OUTLET
ATION

OU rFLO
RESTRICTOR PLATE
W/ORIFICE
F@ I OUTLET
WATER QUALITY
ild

INFLOW ELEVATION
SECTION.
L.1 ... L L-L
Alm

.FIGURE 2-1 -DIAGRAM MULTIPLE STAGE OUTLET STRUCTURE
FOR A DUAL PURPOSE DETENTION BASIN

as possible. This is done to avoid interference with the hvdraulic
capacity of the outlet. Spacing of the rack bars should be wide
enough to also avoid interference. However, the spacing should be
close enough to.provide the level of clogging protection-required.
In o@der to facilitate removal of accumulated debris and sediment from
around the outlet structure, the racks should have hinged connections.
If the rack is bolted or set in concrete; i@,will preclude removal of
accur@ulated material and will eventually adve@rsely effect the'
hydraulics of the outlet.

since sediment will tend to accumulate around the lowest stage outlet,
the inside of the outlet structure should be depressed below the water
quality outlet to minimize clogging of this opening due to
sedimentation. Depressing the outlet bottom to a depth below the water
quality outlet equal to the diameter of the outlet is recommended
(Figure 2-1)..

Basin Configuration

The dual purpose detention basin relies on sedimentation for removal
of runoff pollutants. In order to maximize the degree of
sedimentation achieved, the basin should be designed to lengthen flow
paths and increase detention time. The use.of long, narrow basin
configurations with length to width ratios of 2:1 to 3:1 is
recommended. The use of basin designs that are'shallow and have large
surface areas will also provide better removal efficiencies than small
d6ep basin designs. The ratio of total inflow volume to detained
volume is ajso a significant factor in the removal efficiency of the
basin.

The inflow points to the basin should be as far removed from the
outlet structure as possible. This will avoid sho-rt circuiting of
runoff by maximizing flow paths in the basin. Reducing inflow
velocities will help lengthen detention times. Rip rap or other
energy dissapators should be used at all inflow points. Reduced.
inflow velocities will minimize resuspension of settled pollutants and
increase sedimentation for incoming runoff.

Low Flow Channels

Low flow channels are often required when erosion of the basin floor
is a concern. A review of dual purpose detention basins in New Jersey
has shown that erosion is no worse in basins without low flow channels
than in basins with low flow channels. Where low flow channels are
used, gabion lined channels with underdrains are encouraged.
Impervious channel linings such as concrete or asphalt are discouraged
as they reduce detentiontimes by increasing flow velocities.
Pervious channel linings also promote interaction of storm runoff
with the soil and grass which increases sorbtion of pollutants to
particulates.

Impervious channel linings have been often prescribed for low
maintenance , however experience with dfial purpose basins in New
Jersey has indicated just the opposite. These channels are often
undermined by runoff flow; differential settlement is common and th;O-s(
channel linings transport sediment loads to the outlet much more,
readily than vegetative or gabion lined channelg. In the long term,
impervious channel linings are probably more of a maintenance problem
than not having any low flow chanoel.

References

1. Shanane, Ashok, N.,Estimation of.Pre- and Post-
Development Nonpoint Water Qualitv LoadincTs,-Water
Resources-Bulletin, Volume IP, Number 2, April 19R-2.

2. Wanielista, M.P., Y.A. Yousef, and W.,. McLellon, Nor
Point Source Effects on Water Quality, Journal o'F Water
Pollut.ion Control Federation 49:441-@Bl, 1972.

3. Fardee, J., R.A. Miller, and H.C. Mattraw, Storm Water
Runoff Data for a Multifamily Residential Area, Dade
County, Florida, U.S.G.S ReDort 79-1725, 1971).

4. Whipple, W. Jr., S.D. Faust, W. Renwick, and N.K. Wiegand,
Flood Control Effectiveness of Svstems of Dual Purpose
Detentfon -Basins, Center for Coastal and Environmental
Studies, Rutgers LTniversity, Januarv 1983.

5. Whipple, W. Jr., J.17. Hunger and S.L. Yu, Runoff Pollution
from Multiple Family Housing, Water Resources Fulletin,
Volume 1.4, Number 2, April 1978.

6. Whipple, W. Jr., and J.V. Hunter, Petroleum Hydrocarbons
in Urban Runoff, Water Resources Bulletin, Volume 15,
Number 4, August, 1979.

7. Ferrara, R.A., P: Witkowski, Stormwater Quality
Characteristics in Detention Basins, ASCE Journal of
Environmental Engineering, Volume 109, Number 2, April 1983.

8. Davis, W.J., R.J. McCuen, and G.E. Kamedulski, The Effects
of Stormwater Detention on Water Oualitv, Proceedings
Tnternal Symposium on Urban Stormwater Management
University of Kentuckv, July 1978.

9. Oakland,*P.J., An Evaluation of Urban Storm Water
Pollutant RemOj7al through Grassed Swale Treatment,
Proceedings 1983 International Symposium on Urban
J4ydroloqy, Rydraulics, and Sediment Control, University of
Kentucky, 1983.

10. Kropp, R.H. Water Ouality Enhancement Design Techniques,
Proceedings - Conference on Storm Water Detention
Facilities, ASCE, 1982.

11. Randall, C.W. Stormwater Detention Ponds for Water Oualitv
Control, Proceedings - Conference on Stormwater Detention
Facilities, ASCE, 1982.

12. Department of Environmental Management, Farfax Countv,
Virginia, Design_Manual for BMP Facilities, August 1980.

13. Wiqington, P.J. Jr., C.W. Randall, and T.J. Grizzard
Accumulation of Selected Trace in Soils of Urban Runoff
.Detention Basins , Water.Resources Bulletin, Volume 1�-,
Number 5, October 1983'.

14. Harper, H.H., Y.A. Yousef and M.P. Wanielista, Fate of
Heavv Metals in.Stormwater Management Systems, Universitv
of Central--Florida, 1983.

15. Ferrara, R.A., K.,. Salvage, Stormwater Pollutant
Settleabilitv, Department of Civil Enqineering,
Princeton, New Jersey, 1982.

16. Whipple W. Jr., J.V. Hunter, 'Settleability of Urban Runoff
Pollution, Water Resources Research Institute, Rutgers
Univ@Tr-sity, April 1980.

17. EPA - Results of the Nationwide Urban Runoff Proaram,
USEPA - NTIS, December 1983.

18. Whipple, W. Jr., W.H. Clement and S.D. Faust, Modeling of
Alternative Criteria for Dual Purpose Detention Basins,
Water Resources Research Institute, Rutgers University, May
1981.

CHAPTER 3

PLANNING CONSIDERATIONS, INFILTRATION AND VOLUME CONTROLS

3.1 INFILTRATION FEASIBILITY TESTS

In order to comply the requirements of the New Jersey Storm
Water Management Regulation's or other local requirements, manv
designers are using infiltration techniques to control runoff.
When properly used, the techniques can provide satisfactory
results. However, these techniques are not applicable for all
control situations.

A number of feasibility requirements can be tested to determine
whether a SWM infiltration practice may be constructed on a
specific site and to what extent it may be applied. These
feasibility tests include:

1. Soil textural classes with minimum infiltration rates that
permit adequate percolation of stored runoff.

2. Maximum allowable ponding or storage time within the
structure.

3. Available depth between the bottom of the infiltration
practice and the seasonal high groundwater table or depth
to bedrock.

4. The topographic character of the site including the slope,
nature of the soil (natural or fill), and proximity of
building foundations, water supply wells and septic fields.

The usage of-, infiltration practices will depend upon a careful
site investigation to determine the conditions in which the
feasibility tests will indicate positive results. Each of the
above feasibility conditions are to be investigated and each are
equally important ensuring the proper functioning of the
proposed infiltration practice. Should a site investigation
reveal that any one of the feasibility tests is not adequate,
the implementation of infiltration practices should not be
pursued.In these cases, dual purpose detention basins usually
offer the most feasible alternative.

Soil Textures

The hydrologic design methods presented in Chapter 4 are based
on the utilization of two hydrologic soil properties, the
effective water capacity (C ) and the minimum infiltration
W
rate (f) of the specific soil textural groups, as shown in Table
3-1. The effective water capacity of a soil is the fraction of

TARLE 3-1 HYDROLOGIC SOI!s.PPOPERTIES CLASSTFIFT) PY SOIL
TPXTTJRE*

Effective Minimum
Water Infiltration Hydrologic
Texttre Class Capacity (C w Rate (F) Soi) Grouping

(in. per in..) (in.'per hr.)

SanO. 0.35 8.27 A

ToamY Sand 0.31 2.41 A

Sandy Loam 0.25 1.02 B

Loam 0.19 .52 P

Silt Loam 0.17 .27 C

Sandy Clay Toam 0.14 .09 C

Clay Loam 0.14 17 C

Siltv Clav Loam 0.11 .06 D

Sandy-Clav. 0.09 .05 D

Silty Clay 0.09 .04 D

C 1 av 0.08 .02 D

Source: Rawls, Brakensiek and Saxton, 19P2

Textural Triangle U.S.D.A.

100% clay

go to

so 20

70 clay 30

60 40

so Silty
so

40 Samcly 60
Y y

Aos
So y /clay
30
loom 70

20 'Loam 80

silt
10 ndy 90
loom loam Silt
a Y &JI
100% $and 90 so 70 go so 40 30 20 10 100% $Ut
percont *and

Figure 3-1. U.S.D.A. Textural Triangle
go 10
8 AO2 0
Clay

7
A 40
0 Z
0 Silty
@s a @M@
7/-7
s At t

the void spaces available for water storage, measured in terns
of inches per inch. The minimum infiltration rate is the final
rate that water passes through the soil profile during saturated
conditions, measured in terms of inches per hour. The hydrologic
soil properties are obtained by identifying the soil t6xtures by
a gradation test for each of the changes in soil profile. The
soil textures presented in'Table 3-1 correspond to the soil
textures of the U.S. Dept. of Agriculture (USDA) Textural
Triangle presented in Figure 3-1.

The data presented in Table 3-1 are based on the analysis of
over 5,000 soil samples under carefully controlled procedures hv
the USDA. The use of the soil properties established in Table 3-
1 for design and review procedures will offer two advantages.
First, it will provide for consistency of results in the design
procedures. Second, it will eliminate the need for the
laborious and costly process of conducting field and laboratory
infiltration and permeability tests.

Based on the soil textural classes and the corresponding minimum
infiltration rates, a restriction is established to eliminate
unsuitable soil conditions. Soil textures-with minimum
-infiltration rates of 0.17 inches per hour or less are not
suitable for usage with infiltration practices. These include
soils that have a 30 percent clay content, making these soils
susceptible to frost heaving and structurally unstable, in
addition to having a poor capacity to percolate runoff. Soil
textures that are recommended for infiltration svstems include
those soils with minimum infiltration rates of 0.27 inches per
hour or greater, which include silt loam, loam sandv loam, loamy
sand, and sand.

Maximum Allowable Ponding or Storage Time

The feasibility criteria for using storm water management
infiltration systems can also be based upon the concept of'a
maximum allowable ponding time (T ) for surface storage or a
maximum allowable storage time (TP) within a subsurface stone
aggregate reservoir. The concept swill vary depending upon the
storage mechanism of each practice which will govern the
available storage within the'structure. The established maximum
ponding and storage time i"s a 3 day or 72 hour period in which
stored runoff within the structure should be completely drained.
The maximum ponding time for vegetated swales is 24 hours. The
use of the maximum allowable time frame in conjunction with a
specific soil minimum infiltration rate (f) will dictate the
maximum allowable design depth (d ax ) of the structure. The
maximum depth of an infiltration Basin and a vegetated swale may
be defined as:

dmax fT p (3-1)

The maximum depth of an infiltration trench and dry well, will
depend upon the void ratio (V ) of the stone aggregate
reservoir and rnav be defined Ks:

fT
dmax ___s--- (3-2)
V
r
The maximum allowable design depths for various soil textures
and pondinq or storage times are given in Table 3-2 for the
criteria in Equations 3-1 and 3-2. The portion of the table
that is shaded represents soil infiltration rates that are
unacceptable. The maximum design depths will obviously become
greater as the minimum infiltration rates of a soil texture
class becomes greater. The depth of the structure required for
design W411 depend upon the site characteristic and the level of
control. The values of d in Table 3-2 represent only the
upper allowable range formfiesign depth-which may be less where
other limitations exist (i.e.", distance to groundwater).

Depth to High Groundwater Table and Bedrock

An additional feasibility criterion of a storm water management
infiltration structure consists of determining the safe distance
between the bottom of the structure and the location of the
seasonal high groundwater table. This distance is also important
in protecting against the flooding of the structure due to the
rise of the water table, rendering the structure inef@fective.
The U.S. Environmental Protection Agency's (EPA) criteria for
onsite wastewater treatment and disposal systems specifies that
a 2 to 4 foot distance be provided between the bottom of the
waste disposal system and-the water table or bedrock (EPA,
1980). Thus, it is recommended that infiltration structures be
located onlv in areas where the bottom of the structure will be
2 to 4 feet above the seasonally high groundwater table and/or
bedrock.

Data on the application of secondary effluent to land disposal
systems suggests that a 2 to 3 foot depth is generally.
sufficient to remove most pollutants (COG, 1980). Experience on
Maryland's Eastern Shore, where high water table conditions are
common, indicates that septic systems can be successfully
designed and built with less distance than generally recommended
bv rules of thumb. In addition, the use of vegetated swales on
t@e-Eastern Shore is a common drainage-technique which has been
used successfully to convey runoff, although during high water
table conditions they become saturated. Thus, the use of swales
as a storm water management infiltration practice may'still be
encouraged in cases where the distance to the water table is on
the order of 1 to 2 feet. The surface geology and soil textures
of the coastal plain of New Jersey are sufficiently similar to

TABLE 3-2. MAXIMUM -ALLOWABLE DEPTHS (INCHES) OF STORAGE FOR
TWO CRITERIA, SELEcTED MAXIMUM PONDING OR 'STORAGE TIMES (TP OR TS IN HOURS) ,
MINIMUM INFILTRATION RATES (INCHES/HOURS)

SOIL TEXTU E/F (INC ES/HOUR)
Sandy
Tp or Loamy Sandy Silt clay Clay clay Sandy Silty
Sand Sand Loam Loam Loam Loam Loam Clay Clay clay
Ts I Loam
Criterion (hrs) b.27 2.41 1.02 .52 .27 .17 .09 .06 .05 .04 .02

fTp 24 198 58 24 13 .6
48 , 397 116 49 25 13 4
72 595 174 73 37 19
fTs/Vr 24 496 145 6 1 31 16 3
for 48 992 290 122 62 32 4
(Vr 0.4) 72 1489 434 183 93 49 3
z

Note: Tp = Maximum allowable ponding time

TS = Maximum allowable storage time

Vr = Voids ratio

These values represent unfeasible solutions

Ma ryland's Eastern Shore that these gui(felines are applicable
anO should be used for design criteria in New Jersey.

Topographic Conditions

The topographic conditions of the site represent feasibility
factors that need to be examined p@7ior to incorporating
infiltration svstems on specific sites. These include the
slope, nature of the soil (natural or fill), and the proximity
of building foundations, water supply wells and septic systems.
If local health authorities document existing problems recrardina
water supply well pollution resulting from infiltration
practices, infiltration shall not be utilized.

The use of a particular practice will place a restriction on the
allowable slope. The use of vegetated swales, for example,
requires a relatively level or gentlv sloping area not to excess
of 5 percent (20h:lv). All other infiltration practices shall
be located in areas in which the slope does not exceed 20
percent (5h:lv). Application of infiltration.practices on a
steep grade increases the chance of water seepage from the
subgrade to the lower areas of the site and reduces the aynount
which infiltrates.

DeveloDments occurring on sloping and rolling sites often require
the use of extensive cut and fill operations. The use of storm
water management infiltration systems on fill material is not
recommended due to the possibility of creating an unstable
subgrade. Fill areas can be very susceptible to slope failure
due to slippage along the interface of the in-situ and fill
material. This condition could be aggravated if the fill
material is allowed to become saturated by using infiltration
practices.

The proximity of building.foundations should be at least.10 feet
up gradient from infiltration systems to prevent the possibility.
of flooding basements. The proximity of septic systems is also
a concern and local health officials should be consulted for
guidance 'on minimum setbacks. Additionally, the location of
infiltration practices shall be a minimum of 100 feet from any
water supply well where the runoff is from commercial or
industrial impervious parking areas.

Other Considerations

In addition to the feasibility criteria presented above, there
are a.number of other factors to be considered in evaluating the
use of infiltration practices. A number of localities have
regulations which require the use of curb and gutter for all
subdivision development. Unless this requirement is changed, it
will cause a conflict,with the-use of vegetated swales. Some

@23

localities have regulations which prevent the connection of roof
drains to dry wells. Also, some localities have requirements
that all'structures have a positive drain outlet. These
requirements seriously limit the use of infiltration practices.

Construction

Regardless of the type of' storm water management practice to be
constructed, careful con-siderations must be given in advance of
construction to the effects of the work sequence, techniques,
and equipment employed on the future maintenance of the practice.
Serious maintenance problems can be averted, or in large part,
mitigated, by the adoption of relatively simple measures during
construction.

Previous experience with infiltration practices in New Jersey
has shown that storm water management infiltration practices
must not be constructed until the drainaae areas contributing to
the structure have been adequately stabilized. When this
precaution has not been taken, the infiltration structure has
often become clogged with sediment from the construction
operations upland and thus failed to operate from the outset.

Specific construction methods and specifications are provided in
the remainder of this chapter for each individual storm water
management practice.

Maintenance

The maintenance requirements of storm water management practices
are an important aspect which are often not addressed in the
planning and design of these structures. Detention and
infiltration basins, vegetated swales, and vegetative buffers
can be visually inspected and easily maintained. Infiltration
trenches and dry.wells, once installed, are very difficult to
inspect and maintain. Consequently, these latter practices
should be designed using inlet structures, sediment and grease
traps and vegetated filters which will protect the integrity of
.the practice, ensure a long functional life, and provide readily
accessible structures for maintenance.

Specific maintenance specifications and'recommendations are
provided for each individual storm water management practice in
the remainder of this chapter.

The New Jersey Division of Water Resources and Ocean County are
conducting a study on the maintenance considerations of storm
water management controls. A maintenance manual will be
developed during this study. The study (and manual) will cover
a number of issues, including the relationship of designs to
maintenance, the legal and institutional aspects of maintenance

and long term financing of maintenance programs. The study is
scheduled-for completion in the Spring of 1988.

References

1. SCS National Engineering Handbook, Section 8, Engineering
Geology, Soil Conservation Service, U.S. Department of
Agriculture, 1978.

2. SCS National Engineering Handbook, Section 18 Ground Water,
Soil Conservation Service, U.S. Department of Agriculture,
1968.

3. SCS Technical Release No. 36, Ground Water Recharge,
Engineering Division, Soil Conservation Service, U.S.
Department of Agricutlure, 1967, 22. p.

4. Pettylohn, W.A., Introduction to Artificial Ground Water
Recharge, Robert S. Kerr Environmental Research Laboratory,
U.S. Environmental Protection Agency, Ada, Oklahoma,
published by the National Watersell Association,
Worthington, Ohio, 1981, 44p.

5.. Bianchi, W.C. and D.C. Muckel, Ground Water Recharge
Hydrology, ARS 41-161, U.S. Department of Agriculture,
1'970, 62p.

6. Schmidt, C.J. and E.V. Clements, Reuse of Municipal
Wastewater for Groundwater Recharge,-U..S. Environmental
Protection.Acrency, 1977, 183p.

7. Cedergree, H.R., Seepage, Drainage, and Flow Nets. John
Wiley and Sons, Inc., N.Y., 1967.

8. Davis, S.N. and R.J. DeWiest, Fydrogeology, John Siley and
Sons, N.Y., 1966.

9. Ferris, J.G., D.B. Knowles, R.H. Brown, and R.W. Stallman,
Theory of Aquifer Tests, U.S. Geological Survey, Water
Supply Paoer No. 1536-E.

10. Bentall, R., Methods of Collecting and Interpreting Ground
Water Data, U.S. Geological Survey Water Supply Paper 1544-
H., 1963. 97p.

11. Todd, D.K., Ground Water Hydrology, John Wiley and Sons,
Inc., N.Y., 1959.

12. Wenzel, L.K. Methods of-Determining Permeability of Water
Rearinq Materials, U.S. reoloqical Survey Water Supply
Paper 887, 1942.

.13. Hannon, J.G., Underground Disr)osal of Storm Water Runoff,
Design Guidelines'Manual, prepared by the California
Department of Transportation in Cooperation with the
Federal Highway Administration, U.S. Department of
Transvortation, Washington, D.C. (FRWA-TS-80-218), Februarv
1980.

14. Dingman, R.S., G. Mever, R.O. Martin, The Water Resources
of Howard and Motc.romerv Counties, Bulletin 14, Department
of Geoloqy, Mines and Water Resources, Board of Natural
Resources, State of Maryland, 1954.

15. Vokes, F.E., J. Edwards, Jr., Geography and Geology of
Marvland, Bulletin No. 19, Maryland Geological Survey '
Department of Natural Resources, State of Maryland, lq74.

16. New Jersey Department of Environmental Protection,
Standards for the Construction of Individual Subsurface
Sewaqe Disposal Systems (Chapter 1991, July 1, 1978.

3.2 INFILTRATTON RASI@T

(IB)

Description

An infiltration basin is a water impoundment made b.v
constructing a dam or an embankment or by excavating a pit or a
duaout in or down to relatively permeable soils. The purpose of
the basin is to temporarily store the surface runoff for a
select'ed desicrn storm and to maintain or increase ground water
recharge by infiltration through the bed and sides of the basin.

Applicabilitv

An infiltration basin will qenerallv be used in the same manner
as a detention basin. The infiltration basin will t 'ypically be
constructed in drainage areas of up to 50 acres. An infiltration
basin may be constructed icintly with a detention basin by
raising the outlet pipe.

Plannina Considerations

An infiltration basin has relatively large surface area require-
ments in comparison to either an infiltration trench or a dry
well. Whereas a tr(@nch or a drk7 well is qenerally associated
with small drainage areas of 1 acre or less, infiltration basins
are better suited to control larger drainage areas generally
ranging from 5 to 50 acres in size.

A typical infiltration basin will range from 3 to 12 feet in
depth. The seasonally hiqh groundwater table should be located
at least 2 to 4 feet below the bottom of the basin. Similarly,
bedrock should also be located at least 2 to 4 feet below the
bottom of the basin.

The permeability or final infiltration rate of the various soil
types will determine how rapidly the stormwater ponded in the
basin at the end of the storm will be infiltrated into the
ground. Table 3-2 provides the maximum allowable depth of
pondinq for the various soil textures for a range of allowable
ponding times. The table indicates that for soil textural
classes with final infiltration rates M of 0.52 in/hr and
larger allow for the design of basins with a ponding depth of
approximately 3 feet and deeDer, provided that the criteria for
depth to higher water table and bedrock are satisfied.

The soil textural class with an f value of 0.27 inches per hour
(silt loam) may have some limited suitability for a verv shallow
infiltration basin. This shallow-basin will of course control

a smaller drainage area than the deeper basins anO thus will
require more surf 'ace are to provide an eduivalent level of
control. Due to the high cost of land being converted to urban
uses, the designer or developer will generally seek to minimize
land costs by using fewer, smaller and deeper basins. This
constraint will make soils with infiltration rates less 0.27
inches per hour' unsuitable for use of infiltration basins. The
infiltration capacities of soils which allows them to infiltrate
36 inches of stored-runoff over a 3 day period makes them well
suited for an infiltration basin.

Design Criteria

The design of infiltration basins will follow the criteria
outlined in Chapter 4 or subsequent revisions for embankment
design, in conjunction with an adequate, non-erosive outlet
channel,.and the additional criteria set forth below.

Design Storm

All hydrologic and hydraulic calculations shall be based on the
design storm criteria provided in the New Jersey State Storri
*Water Management.Regulations N.J.A.C. 7:8.

Pondina Time

All *infil:tration basins shall be designed to completely drain
stored runoff within 3 days following the occurrence of a storm
event. Thus an allowable maximum ponding time T of 72 hours
shall be used. P

Water Table, Bedrock, and Groundwater Conditions

Infiltration basins should be located only in areas where the
bottom of the basin will be at least 2 to 4 feet above the.
seasonally high groundwater table or bedrock at all times.. Also.
infiltration basins shall be located at least 100 feet
horizontallv awav from any water supply well.

Concerns related to the development of a groundwater mound below
the infiltration facility as well as the iootential 'for polluting
downaradient groundwater supplies often arise when infiltration
facilities are considered. The data base evaluated in the
preparation of these specifications indicates that due to the
intermittent natur@! of precipitation patterns and the associated
operation of infiltration practices, groundwater mounding has
not been observed to be a problem where such facilities have
been properly designed and constructed. Also, the data base
indicates that groundwater pollution has not been observed to be
a problems with most residential and commercial land uses.-

Runoff Filtering

Grease, oil, floatable organic materials, and settleable solids
should be removed from runoff water before it enters the
infiltration basin. T hese materials can take up storage
capacity and reduce infiltration rates.

Runoff filtering devices such as vegetative filters (see Section
3.6), sediment traps, and grease traps can be used to remove
objectionable materials. In addition, modified basin designs
such as illustrated in Figures 3-2a and 3-2b can be used to
enhance and prolong the infiltration capacity of the basin
bottom. Even when the basin bed becomes clogged by layers of
accumulated sediment, infiltration can still be achieved through
the sides of the basins, as shown in Figure 3-3, provided that
the side materials are relatively permeable.

When a runoff filtering system or structure is included in the
design, the maintenance requirements and schedule of the filter
structure must be included.

Principal Spillway for Combination Structure

When*basins are designed to infiltrate the water quality design
storm runoff the bottom elevation of the low-staap. orifice
should be designed to coincide with the volume of runoff
produced by the wate-r quality design,storm. If the watpr volume
requirements exceed the 3-dav infiltration capacity of the
system, additional or alternate water quality controls must be
investigated. All other aspects of the principal Spillway
design will follow the guidelines prol7ided in-the standards and
for Dual Purpose Detention Basins.
speci

Emergency Spillway
An emergency spillway s'hall be providecT for all basins created
by an embankment. All excavated basins shall have a nonerosive
outlet channel. The emergency spillway design shall comply with
the requirements of the standards and specifications for Dual
Purpose Detention Basins.

Vegetation

The embankment, emergency spillway, spoil and borrow areas, and
other disturbed areas shall be stabilized and planted in
accordance with the appropriate vegetative measure standards in
the New Jersey Standards and Specifications for Soil Erosion
and Sediment Control. The emergency spillway is often the most
critical area. Additional, a grass strip or other vegetative
buffer at least 20 feet wide shall be provided around the basin
to protect against erosion.

Extent of Basin
Auxi iary
inf il trat ingf- -
Storm- 1... .0
0.
ru n o f f area
_!Ila 7,,,0
Settling area C,
inflow 0'.
pipe
0

Fiaure 3-2a. Two-level Infiltration Basin.
.Source: Aronson and Seaburn, 1969

Extent of Extent of
-recharge basin Fri@ff_ention b9s
Ove
rflow Water@eve@l
condu@it
ts
@ @-n
@Sd
deposited on runoff
basin floor inflow pipe

Figure 3-2b. Retention Basin and the Adjoining Infi ltration
Basin.
Source: Modified After Aronson and Seaburn, 1969

Extent of basin

water level
caused s orm runoff
fte tp rn_ s@aS @.r_ I eve I
Layer of accumulated sediments
of low hydraulic conductivity Deposits of high
hydraulic conductivity
Water table

Figure 3-3. Water-containing Infiltration Basin illustrat"incT
Storm Water Disposal through the Sides of the Basin
above the Level of accumulated Material of Low
Hvdraulic ConductivitY.

Source: Aronson and Seaburn, 1969

Fencing

The embankment and basin shall be fenced where and when deemed
necessary by the land developer or local jurisdiction to
provide public safety or protection of vegetation.

Hydrologic Design Method

A recommended hydrologic design method based on SCS TR-55
procedures is provided in Chapter 4, Hydrologic Design Methods,
Infiltration and Volume Controls.

Water Quality

The effectiveness of this practice for runoff and pollution
control is dependent upon the size and design of the
infiltration facility. If a basin is designed to collect and
infiltrate the storm runoff from the water quality design storm
over a given drainage area, the practice should be effective
for pollution abatement for storms up to and including the
design storm. The quality requirements of the New Jersey State
.Storm Water Management regulations will be satisfied as long as
the use of the infiltration practice results in zero runoff from
the site under conditions of the water quality design storm.

Construction Specifications
The construction of a 11 infiltration basins should comply %@ith
the criteria set forth in the standards and specifications for
Dual Purpose Detention Basins or subsequent revisions and the
additional criteria provided below.

Schedule

The sequence of various phases of basin construction shall be
coordinated with the overall project construction schedule. A
program should schedule rough excavation of the basin with the
rough grading phase of the project to permit use of the material
as fill in earthwork areas. The partially excavated basin
could serve as a sedimentation basin in order to assist in
erosion and sediment control during construction. However,
basins near final stages of excavation should never be used
prematurely for runoff disposal. Drainage from untreated,
freshly constructed slopes within the watershed area would load
the newly formed basin with a heavy concentration of fine
sediment. This could seriously impair the natural infiltration
characteristics of the basin floor. Final grade of an
infiltration basin shall not be attained until after its use as
a sediment control basin is completed.

Specifications for basin construction should state: (1).the
earliest point in progress when storm drainage may be directed
to the basin, and (2) the means by which.this delay in use is to
be accomplished. Due to the wide variety of conditions
encountered among proiects, each should be separately evaluated
@n order to postpone use as long as is reasonably possible.

Excavation

initial basin excavatioh should be carried to within 1 foot of
the final elevation of the basin floor. Pinal excavation to the
finished grade should be deferred until all disturbed areas on
the watershed have been stabilized or protected. The final
phase excavation should remove all accumulated sediment.
Relatively light tracked equipment is recommended for this
operation to avoid compaction of the basin floor. After the
final grading is completed, the basin floor should be deeply
tilled bv means of rotary tillers or disc harrows to provide a
well-aerated, highly porous surface texture.

Lining Material

Infiltration basins may be lined with a 6- to 12-inch layer of
.filter material such as coarse sand to help prevent the huilduP
of impervious deposits on the soil surface.. The filter layer
can be replaced or cleaned when it becomes clogged. When a 6-
inch layer of coarse organic material is specified for discing
(such as hulls, leaves, stems, etc.) or sDading into the besin
floor to increase the permeability of the soils, the basin floor
should be soaked or inundated for a brief period, then allowed
to dry subsequent to this operation. This induces the organic
material to decav rapidly, loosening the upper soil laver.

Establishing dense vegetation on the basin side slopes and floor
is recommended. A densevegetative stand will not onlv prevent
erosion and sloughing, but will also provide a natural means of
maintainincr relatively high infiltration rates. Erosion
protection of inflow points to the basin shall also be
provided. Removal of accumulated sediment is a problem only at
the basin floor. Little maintenance is normally required to
maintain the infiltration capacity of slope areas.

Selection of suitable vegetative materials for the side slope
and all other areas to be stabilized-with vegetation and
application of required fertilizer and mulches shall be done in
accordance with the New Jersey Standards and Specifications for
soil Erosion and Sediment Control. Local Extension Agencies
should also be consulted.

Visual Resource"Design

The visual design of basins in areas of high public visibility
shall be carefully considered for aesthetic quality. The
underlying criterion for all visual design is appropriateness.
The shape and form of a basin, excavated material, and planting
area to relate visually to their surroundings and t.P their
function.

The embankment mav be shaped to blend with the natural
topography. The edge of the basin may be shaped so it is
generally curvilinear rather than rectangular. Excavated
material can be shaped so that the final form is smooth, flowing,
and fitting to the adjacent landscape rather than angular
geometric mounds.

Maintenance

Inspection ScheOule

Drainage systems must be inspected on a routine basis to ensure
that thev are functioning properly. Inspections can be on a
semiannual basis but should always be conducted following malor
storms.

Sediment Control Effect on Vegetated Basins

Cleanout frequency of infiltration basins will depend on.whether
they are vegetated or nonvegetated and will be a function of
their storage capacity, recharge characteristics, volume of
inflow, and sediment load. Infiltration basins should be
inspected at least once a year. Sedimentation basins and traps
may require more frequent inspection'and cleanout.

Grass.bottoms on infiltration basins seldom need replacement
since grass serves as a good filter material. This is
particularly true of Kentucky 31 Tall Fescue, which is extremely
hardy and can withstand several days of submergence. If silty
water is allowed to trickle through the turf, most of the
suspended material is strained out within a few yards of surface
travel. Well established turf on a basin floor will grow up
through sediment deposits, f6rming a porous turf and preventing
the formation of an impermeable layer. Grass filtration would
work well with long, narrow, shoulder-type (swales, ditches,
etc.) depressions where'highway runoff flows down a grassy slope
between the roadway and the basin. Kentucky 31 Tall Fescue
demands very little attention and looks attractive when trimmed.
Grass planted on basin side slopes will also prevent erosion.

Sediment Removal From Nonveqetated Basin

(a) Technique. Remove sediment only when the basin floor is
completely dry, after the-silt laver has mud-cracked and
separated-from the basin floor. Eauipment maneuverability and
vrecise blade control are essential in small areas and can
greatly reduce the quantity of material to be removed.

(b) Frequency. All sediment must be removed prior to tilling
operations. As tilling is required periodically and at least
once annually, the frequency of sediment removal will be reduced
to small operations on a regular basis.

Tilling of Nonvegetated Basin Floor

In all cases, tilling must be preceded by thorough removal of
surface sediment as previously above.

(a) Purposes.- It is necessary to restore the natural
infiltration caiDacity by overcoming the effects of surface
compaction, and to control weed growth on the basin floor.

.(b) Techniaue. Rotary tillers or disc harrows normallv serve
this purpose. Light tractors should be employed for t@ese
operations. In the event that heavy equipment has caused deeper
than noVmal compaction of the surface, these operattions could be
preceded by deep plowinq. In its final condition after tilling,
the basin floor should be level, smooth, and free of ridges and
furrows to ease future removal of sediment and minimi7e the
material to be removed during future cleaning operations. A
levelling drag, towed behind the equipment on the last pass,
will accomplish this.

(c) Frequency. In the spring, the basin surface is usually
quite porous due to the effects of frost and subsequeht thawing.
The infiltration capacity diminishes rapidly thereafter. To
enhance infiltration capacity, tilling should be thorough once
each season, from late June through September. To control
vegetative growth, an additional light tillage may be advisable
during the growing season. Precautions must be observed,
however, to avoid any possibility of working sediment
accumulations into the basin floor as a part of light
cultivation for the purpose of weed control. It is therefore
stressed again that any cultivation or tilling operation be
preceded in all cases by careful sediment removal.

Side Slove Maintenance

(a) Pur2ose. To promote a 6ense turf with extensive
rootgrowth-, thereby enhancing infiltration through the slope
surface and prevent weeds from gradually taking over the slope
areas.

(b) Frequency. Grasses of the fescue family are recommended
for see8i g primarily due to their adaptability to dry sandy
soils, drought resistance, hardiness, and ability to withstand
brief inundations. The use of fescues will also permit long
intervals between mowings. This is important due to the
relatively steep slopes which make mowing difficult. Mowing
twice a year, once in June and again in September, is generally
satisfactory. Refertilization with 10-6-4 ratio fertilizer at a
rate of 500 lb per acre (11 lb per 1000 sq ft) may be required
the second vear after seeding.

References

1. B.C., M.L. Clar, R.R. Kautzman, Approaches to Storm
Water Management, prepared by Hittman Associates, Inc.,
for the Office of Water Research,.USDI, November, 1973.

2. Tourbier, J.T., R. Westmacott, A Handbook of Measures
to Protect Water Resources in Land Development,
Urban Land Institute, Washington, DC, 1981.

3. Anonymous, Controlling Storm Water Runoff in
Developing Areas: Selected Best Management 'Practices,
Metropolitan Washington Council of Governments, July, 1979.

4. Hannon, J.B., Underqround Disposal of Storm Water
Runoff, Design Guidelines Manual, prepared by the
California Department of Transportation-in cooperation.with
the Federal Highway Administration, U.S. Devartment. of.
Transportation, February, 1980.

5. Weaver, R.J., Recharge Basins for Disposal of Highway
Storm Drainage, Research Report 69-2, Engineering
Research and Development Bureau, New York State
Department of Transportation, Albany, NY, March, 1971.

6. Aronson, D.A. and G.E. Seaburn, Appraisal of
operating Efficiency of Recharge Basins on Long Island,
New York in 1969, U.S. Geological Survey Wat'er
Supply Paper 2001-D, 1974.

7. Harper, H.H.; Youself, Y.A.; Wanielista, M.P. Fate
of Heavy Metals in Storm Water Management Systems,
Universitv of Central Florida, Oct. 1983.

8. Wiqinqton, 'P.J.; Randall, C.W.; Griz*zard, T.J.;
Accumulation of selected Trace Metals in Soils of Urban
Runoff Detention Basins, Water Resources Bulletin, Vol. 19,
No. 5, Oct. 1983.

3.3 INFILTRATION TRENCH

(TT)

Description

An infiltration trench consists of a shallow excavated trpncb,
generally 2 to 10 feet in depth backfilled with a coarse stone
aggregate, allowing for temporary storage of storm runoff in the
voids between the aggregate material. Stored runoff then
gradually infiltrates into the surrounding soil. Ficrure 3-4
provides a typical section of infiltration trenches.

The surface of the trench will consist of stone, gabion, sand,
or a grassed covered area with a surface inlet.

Applicability

An infiltration trench will generally be used on relatively
small drainage areas. This practice can be used on residential
.lots, commercial areas, parking lots and open spade areas. A
trench may also be installed under a swale to increase the
storage of the infiltration system.

Plannina Considerations

Soil Perneability

The permeability or final infiltration rate of the various soil
textural classifications will be a limitinq factor i *n the
se lection of infiltration trenches bv itself. The infiltration
rate becomes a fa'ctor when combined with other considerations
such as 1) minimum construction depth, 2) maximum allowable
storage time, and 3) surface are requirements for a specified
level of control. Soil textural classes with infiltration rates
greater or equal to 0.27 inches per hour can be considered for
use of infiltration trenches.

Depth of Trench

An infiltration trench is projected to range from 2 to 10 feet
in stone reservoir depth. A trench with a grassed covered
surface will consist of at least one foot of overlying soil
above the aggregate reservoir. This 3 foot depth of agqreqate
is felt to represent the shallowest infiltration trench likel
to be built. In general, the design engineer will seek to make
the trench as deep as possible to minimize the surface area of
the trench.

Di ke

Runoff Runoff Runof

20' Minimum Vegetated Strip

j Pavement
Stone Surface Vegetat(.d A-rea
(For Filtering)

Filter Fabric 3' Mini-mum
Depth

Aggregate

Fi I ter Fabric

Width

Trenches
'v 4*

off Runo
t r@
'p
401$@

Figure 3-4' Typical Section of Infiltration
Modified after Frederick Co., MD. (1979)

The final infiltration rate of the soil below the ih!7iltration-
trench will dictatethe maximum allowable t:rench depth. The
maximum storage depth of the trench.

Water Table, Bedrock, anef Groundwater Conditions

The seasonally bigh ground water table as well as bedrock should
be located at least 2 to 4 -feet below the bottom of the trench
at all times. Therefore these two parameters may often
determine the maximum allowable depth for the trench. Also
infiltration trenches shall be located at least 100 feet
horizontally away from any water supply well.

Design Criteria

Design Storm

All hydrologic and hydraulic calculations shall be based on the-
design storm criteria pro-ided in the New Je'rsey SWM Regulations
N.J.A.C. 7:8.

Storage Time

All infiltration trer4ches shall be designed to-completely drain
stored runoff in 3 days. Thus the maximum allowable storage
time TS of 72 hours shall be used.

Backfill Material

The agqregate material for the infiltration trench shall
consist of a clean aggregate with a.maximum diameter of 3" and
a minimum diameter of I - 1/2". The aggregate should be graded
such that there will be few aggregates smaller than the
selected size. Void space for these aggregates are assumed to
bi? between the range of 30 to 40 percent. Quarry blends :and
road mixes shall not be used.

The aggregate fill material shall be completely surrounded as
shown in Figure 3-4 with an engineering filter fabric. In the
case of an aggregate surface, filter fabric should surround
all of the aggregate fill material except for the top one foot.

Runoff Filtering

At all times, greasef oil, floatable organic ma@terials, and'
settleable solids should be removed from-runoff water before
it enters the infiltration facility. These materials can
take up storage capacity and reduce infiltration rates. Runoff
filtering devices such as grass filter strips and sediment
traps can be used to remove objectionable materials.
The design and construction of vegetated filter strips is,

described in this chapter. All trenches with surface inlets
shall be designed to capture sediment before discharging into
the stone aggregates.

The use of infiltration trenches in combinati-on with swales with
check dams is recommended. In this situation the trench should
be constructed below the swale. The pool created by the check
dam will increase the volume of runoff infiltrated into the
trench.

Overflow Channel

In general, because of the small drainage areas controlled hv
the infiltration trench, an emergency spillway is not
necessary. In all cases, though, the overland flow path of
surface runoff exceeding the capacity of the trench should be
evaluated to preclude the development of uncontrolled, erosive
concentrated flow. A non-erosive overflow-channel leading to
a stabilized watercourse shall be provided for the runoff from
the larger design storms.

Seepage Analysis and Control

-An analysis shall be made to determine any possible adverse
effects of seepage zones when there are nearbv building
foundations, basements, roads, parking lots, or sloping sites.
Developments on sloping sites often require the use of extensive
cut and fill operations. The use.of infiltration trenches on
fill sites with steep slopes is not permitted. Fill areas can
be very susceptible to slope failure due to slippage along the
interface of the in-situ and fill material. This condition
could be further aggravated if the fill material is allowed to
saturate. The methods for seepage analysis and estimation of
infiltration rates usinq Darcv's law ana flow nets can be used
to conduct the seepage analysis.

When infiltration trenches are used in residential areas,
special care must be taken to prevent seepage from the trenches
causing wet basements. Infiltration trenches 3 or more feet
deep shall be located at least 10 feet down gradient from
foundation walls.

Hydrologic Desiqn Methods

A recommended hyeroloqic design method based on SCS procedure is
provided in Chapter 4.

Observation Well

An observat-ion well shall be installed in every infiltration
trench. The observation well will serve two primary

functions: 1) it will indicate how quickly the trench dewaters
following a storm and 2) it will provide a inethod of observing
how qui.cklv the trench fills up with sediments.

The observation well should consist of 'perforated PVC pipe, 4
to 6 inches in diameter. It should. be located in the center of
the'structure and be constructed flush with the ground
elevation of the trench as shown in Figure 3-5. The top of the
well shall be capped to discourage vandalism and tampering.

Water Quality

The effectiveness of this practice for runoff and pollution
control is dependent upon the size and design of the
infiltration facility. If a trench or a series of trenches are
designed to collect and infiltrate the total volume of runoff
for the water quality design storm.over a criven drainage area,
the practice will be effective both runoff control and pollution
abatement for storms up to and including the design storm. The
water quality reauirements of the New Jersey State Storm Water
Manaaement regulations will be satisfied as long as the
infiltration practices results in zero runoff from the site
.under conditions of the water quality design storm.

Construc-tion Specifications

Timing

An infiltra'tion trench shall not be constructed or placed in
service until-all of the contributing drainaqe area has been
stabilized and approved by the responsible inspector.

Trench Preparation

Excavate the trench to the design dimensions. Excavated
materials shall 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 shall be roughened where sheared and sealed
by heavy equipment.

Fabric Laydown

The filter fabri*c must be cut to the proper width prior to
installation. The cut width must include sufficient material to
conform to trench perimeter' irregularities and for a 6-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

Metal Cap with Lock

1111111111177777-

Topsoil or
Aggregate

Filter Fabric
Aggregate Backfill
0

4-6 inch, Perforated Undisturbed Material
PVC Pipe C
0

Foot Plate

Figure 3-5. Obser-ation Well Detail

trench opeh during windy periods. when overlaps are recuired
between rolls, the upstream roll should lap a minimum of 2 feet
over the downstream roll in order to provide a shinqlpd effect.
The overlap ensures fabric continuity or to ensure that the
fabric conforms to the excavation surface during aggregate
placement and compaction.

"Stone Aggregate'Placement and Compacti:on

The stone agaregate 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.

overlapping and Covering

Following the stone aggregate placement, the filter fabric shall
be folded over t'he stone aggregate to form a 6" minimum.
longitudinal lap. The desired fill soil or stone aggregate
shall be placed over the lap at sufficient intervals to
maintain the lap during subsequent backfilling.

Contamination

Care shall be exercised to prevent natural or fill soils from
intermixing with the stone aggregate. All contaminated stone
aggregate shall be removed and replaced with uncontaminated
stone aggregate.

Voids Behind Fabric

Voids can be created-between the fabric and excavation sides and
shall be avoided. 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 to ensure fabric conformity to the
excavation sides. Soil piping, fabric clogging, and possible
surface subsidence will be avoided by this remedial process.

bnstable ExcavAtion Sides

Verticaliv excavated walls may be difficult to maintain in areas
i-@here the soil-r-,oisture 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.

Vegetative Buffer

A veqetative buffer of at least 20 fe'et (wider, if possible)
shall be used to intercept surface runoff from all impervious
areas, as shown in Figure 3-4.

Traffic Control

Heavy equipment and.traffic shall be restricted from travelling
over the infiltration areas to minimize compaction of the soil.

Observation Well

An observation well, as described earlier and in Figure 3-5
shall be provided. The depth of the well at the time of
installation will be clearly marked on the well cap..

Maintenance

Infiltration trenches shall he desianed to minimize
maintenance. However, it is recognized that all infiltration
facilities are subject to clogging by sediment, oil, qrease,
grit and other debris. In addition, the performance and
longevity of these structures is not well documented.
Consequently, a monitoring observation well is required for all
infiltration structures.

The observation well shall be monitored periodically. For the
first year after completion of construction, the well should be
monitored on a quarterly basis and after every large'storm. It
is recommended that a log book be maintained indicating the
rate at which the -facility dewaters after large storms and the
depth of the well for each observation. Once the performance
characteristics of the structure have been verified, the
monitoring schedule can be reduced to an annual basis, unless
the performance data indicate that a more frequent schedule is
required.

Sediment build-up in the toc foot of stone aggregates or the
surface inlet should be monitored on the same schedule as the
observation well. A monitoring well in the top foot of stone
.aggregate will be required when the trench has a stone surface.
Sediment depo'sited sh'all not be allowed to build up to the
point where it wil-1 reduce the rate of infiltration into the
trench.

References

1. Stormwater Management design Manual for Frederick
Co., Maryland, 1979.

2. Anonymo us, Controlling Stormwater RUnoA'f in Developinq
Areas: Selected Best Manacrement Practices, Metropalitan
Washinqton Council of Governments, July, 1979.

3. Percolation Pits; Their Desiqn, Construction, Use
and Maintenance for Stormwater Disposal, Ground Water
Recharge, and Surface Water Ouality Protection in
Adams County, Colorado, Adams County Planninq
Department.

3.4. DRY WFLL

(DW)

Descrintion

A dry well.consists of a small excavated pit backfilled with
aggregate. The dry well is similar to the infiltration trench
previously described but differs in two ways. The dry well is
generally a much smaller structure particularly with respect to
the surface area dimensions. The deoth of the well will
generally range from 3 to 12 feet. @n important difference
between the dry well and the trench is the inflow mechanism.
Inflow to the dry well will occur by means of an inflow pipe
and through surface infiltration. As previously described, the
infiltration trench can only accept inflow through surface or
inlet inflow. A typical dry well cross section is presented in
Figure 3-6.

Applicability

A dry well will generally be used to capture and store runoff
from roof top areas of less than one acre in surface area. This
practice can be used to store runoff from residential.,
.commercial, industrial, and institutional roof tops. A
secondary application can be built as open-bottomed structures
to provide for infiltration of stormwater as shown in Figure 3-
7. This secondary application can he utilized for complying
with the watet quality requirements of N.J.A.C. 7:8.

For both areas of application, the site conditions must include
soils that are suitable for infiltration with sufficient depth
available between the bottom of the well and the top of the
groundwater table.

Planning Considerations

Soil Permeability

The permeability or final infiltration rate of the various soil
textural classifications will be a limiting factor in the
selection of dry wells. The infiltration rate becomes a factor
when combined with other considerations such as: 1) minimum
construction depth, 2) maximum allowable storage time, and 3)
surface area requirements for a specified level of control.
Soil textural classes with infiltration rates greater or equal
to 0.27 inches per hour are acceptable for use of dry wells.

Depth of Well

A dry well is expected to range from 3 to 12 feet in depth. In
all cases the top foot o@ the well will consist of a soil filter
medium. Thus, a 3 foot deep well would consist of 1-foot of
soil filter media and 2 feet of aggregate storage area. This 3
foot depth is felt to represent the shallowest dry well likely
.to be built. In general, the developer and design engineer will
seek to make the dry well as deep as possible to obtain the
maximum are of control possible. Table-3-2 indicates that a
silt loam soil texture with an f value of 0.27 inches per hour
can drain a 4-foot deep storage area in a 72-hour period.

The maximum depth of dry wells will generally be determined bv a
number of factors. These include: 1) the soil textural
characteristics and 2) the denth of the water table or bedrock.

Water Table, Bedrock, and Groundwater Conditions

The bottom of the dry well shall be located at least 2 to 4 feet
above the seasonally high groundwater table as well as bedrock.
Therefore, these two parameters will often determine the
.maximum allowable depth for the well. Also dry wells shall be
located at least 100 feet horizontally away from any water
supply well.

Desiqn Criteria

Design Storm

All hydrologic and hydraulic calculations shall be based on the
design storm criteria provided in the New Jersey STHY.
Regulations. N.J.A.C. 7:8

Storace Time

All dry wells shall be desianed to be empty within 3 days from
the beginning of the storm. Thus an allowable storacre time T S
of 72 hours shall be used.

Backfill Material

The agcrregate fill material for the infiltration trench shall
consist of a clean aggregate with a maximum diameter of 3" an 'd a
minimum diameter of 1-112". The aggregate should be poorlV
graded with a few stones smaller than the selected size. Void
space for these aggregates are assumed to be between the ranges
of 30 to 40 percent. Quarry blends or quarry process mixes
should be avoided.

Any stone aggregate shall be completely surrounded with an
engineering filter fabric as shown in Figure 3-6.

Runoff Filtering

At all times grease, oil, floatable organic material*s, and
settleable solids should be removed from runoff water before it
enters the dry well. These materials can-take up storage
capacity in addition.to reducing infilt5ration rates. Screens
should @e placed. at the top of the roof leader to prevent leaves
from entering the dry well.

When a runoff filtering system or structure is included in the
design, the maintenance requirements shall be included.

Outflow Structures

Other than overflow provisions outflow structurp-s are generally
not used with infiltration systems. The use of a positive
drain or discharge pipe from such structures converts the
infiltration structure into a detention structure, unless the
positive drain is located such that a volume of storage is
provided below the positive draim invert.

In all cases, however, the overland flow path'of surface runoff
exceeding the capacity of the well shall be evaluated to
preclude the development of uncontrolled,'erosive concentrated
flow. An overflow system leading to a stabilized channel or
watercourse including measures to provide non-erosive flow
conditions along its length and at the outfall shall be
provided. An overflow orifice shall be installed in the inflow
pipe above the dry well-surface area to allow drainage in
extreme events as shown in Figure 3-6.

Seepage Analysis and Control

A foundation analysis shall be made to determine any possible
adverse' effects of seepage zones on nearby building foundations,
roads, parking lots, and other structures. This is particularly
important on a steeply sloping site. Developments on sloping
sites often require the use of extensive cut and fill
operations. The use of dry wells on large or steeply sloping
fill sites is not recommended. Fill areas can be very
susceptible to slope failure due to slippage along the interface
of the in7situ and fill material. This condition could be
further,:a-ggravated if the fill material is saturated by using
infiltration practices. The methods for seepage analysis and
estima@tion of infiltration rates us*ina Darcy's law and flow nets
can be used to conduct the seepage analysis.

7@5-01' Leader

Surcharge Pipe

Splash Block ap with Lock

12q 40

a Aggregate
0 OlDservation well
Filter 0 4-6 inch, Perforated
Building Fabric a PVC Pipe
Foundation 0
0

mi rilmum
01

Foot Plate

Fiqurf- 3-6. Typical Dry Well Cross Section
@ng.

n o w

C)
'0 Filter-Fabric
0 o 0 0 0
0
o 0 o 0 C)
0 0 0 0
6 4 / 1 0 00
0 0 0
C@ 0 4
0 0 0 0
0
C V,
0 0 0 0
0 0 0 0
1 0,
0 0 0 0 0 0 elo V-,"-311 Aggregate
0
0 @9 Tl-.s ft.
0

6 ft..

(a)

Source: Modil"ied from Sullivan (1981)

Discharge-Outlet May Be Grates
Elevated to Increase Head Curb Inlet

Flow
V1Uq 11
rrm

Filter
Fabric 01'
30#1
Observation
Well
Aggregate

Foot Plate
(no scale)

Source: COG (1979)

Figure 3-7. Exarpples of Storm Drain Catch. Rasins Used as Dry
TATe s

When drv wells are used in residential areas, special care must
be taken to prevent seepage from the dry wells creating wet.
-basements. Dry wells should be lo'cated'at least a distance of
four times the well*de@th down gradient from foundation walls
(minimum of 10 feet).

Hydrologic DesignMethods

A recommended hydrologic design method based on SCS procedures
is provided in Chapter 4.

Observation Well

An observation well shall be installed in every dry well. The
observation well will serve two primary funcitons: 1) it will
indicate how quickly the trench'dewaters following a storm, and
2) it will provide a method of observinq how quickly the well
fills up with silt and thus requries maintenance clearout.

The observation well should consist of perforated PVC pipe, 4
inches in diameter. It should be located in the center of the
structure and be constructed flush with the ground elevation of
.the structure as shown in Figure 3-5. The top of the well shall
be capped to discourage vandalism and tampering. The depth of
the well at the time of installation should be clearly marked on
the well cap.

Water Quality'

The effectiveness of this practice for runoff and pollution
control is dependent upon the size and design of the structure.
If a dry well is designed to collect and inkiltrate the total
volume of runoff for a design storm over a given drainage area,
the practice theoretically should be effective for both runoff
control and pollution abatement*for storms up to and including
the design storm. The water quality requirements of the New.
Jersey State SWM regulations will be satisfied as long as the
use of the infiltration practice results in zero runoff from the
site under conditions of the water quality design storm.

Construction Specifications

Timing

A dry well shall not'be.6onstructed Qr placed in service until
all of the contributing drainage area has been stabilized and
approved by the responsible. inspector.

Drv Well Preparation

Ex cavate the dry well to the design dimensions. Excavated
materials shall be placed away from the excavated sides to
enhance wall stability. Large tree roots shall 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 shall be roughened where sheared and sealed by
heavy equipment.

Fabric Lavdown

The filter fabric roll shall be cut to the proper width prior to
installation. The cut width must include sufficient material to
conform to well perimeter irregularities and for a 6-inch
minimum top overlap. Place the fabric roll over the well and
unroll a sufficient length to allow placement o" the fabric down
into the well. Stones or other anchorina ob iects should be
placed on the fabric at the edge of the well to keep the lined
well open during* wind'y periods. When overlaps are required
between rolls, the upstream roll shall lap a minimum of 2 feet
over the downstream roll in order to provide a shingled
effect. The overlap ensures fabric continuity or the fabric
conforms to the excavation surface during aggreqate placement
and compaction.

Aggregate Placement and Compaction

Drainage aggregate shall 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 conformitv to the excavation sides, therehv
reducing the potential for soil piping and fabric clogging.

Overlapping and Covering

Following aggregate placement, the fabric previously weighted by
stones should be folded over the aggregate to form a 6" minimum
longitudinal lap. The desired fill soil should be placed over
the lap at sufficient intervals to maintain the lap during
subsequent back"illing.

Contamination

Care shall be exercised to prevent natural or fill soils from
intermixing with the drainage aggregate. All contaminated
aggregate shall be removed and replaced with uncontaminated
aggregate.

Voids Behind Fabric

Voids can be created between the fabric and excavation sides and
should be avoided. 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 to ensure fabric conformity to the
excavat-ion*sides. Soil pipina, fabric clogging, and possible
surface subsidence will be avoided by this remedial process.

Unstable Excavation Sides

Vertically excavated trench walls mav be difficult to mainta4n
in areas where the soil moisture is @iqh 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.

Foundation Protection

Dry wells 3 or more feet deep shall be located at least 10 feet
down gradient from foundation walls.

Observation Well

An observation wel-1, as described earlier and in Figure 3-5,
Will be provided. The depth of the well, at the time of
installation, will be clearly marked on the well. cap.

Maintenance

Dry wells shall he designed to minimize maintenance. However, it
is recognized that all infiltration facilities are subject to
clogging by sediment, oil, grease, grit and other debris. in
addition, the performance and longevity of these structures is
not well documented. Conseauently, a monitoring observation.
well is required for all infiltration structures.

The.observation well should be monitored periodically. For the
first year after completion of construction, the well should be
monitored on a quarterly basis and after every large storm. It.
is recommended that a log book be maintained indicating the
rate at which the facility dewaters after large storms and the
depth of the well for each observation. once the performance
characteristics of the structure have been verified, the
monitoring.schedule can be reduced to an annual basis, unless
the performance data indicate that a more frequent schedule is
required.

References

1. Becker, B.C., M.L.* Clar, and R.R. Kautzman, Approaches
to Stormwater Management, prepared by Hittman Associates,
Inc. for the Office of Water Resources Research, USDT,'
November, 1973.

2. SullivAn, P.J., editor,'Urban Stormwater Management,
Special Report No. 49, American Public works Association,
Chicago, Illinois, 1981.

3. Anonymous, Controlling Stormwater Runoff in Developina
Areas: Selected Rest Management Practices,
Metropolitan Washinqton Council of Governments, July,
1979.

4. Desian Guidelines for Subsurface Drainage Structures,
MIR.AFI, Inc., P.O. Box 240967, Charlotte, NC 28224.

3.5. VEGETATFD SWALES WITH CHECK DAMS

(VS)

Description

This practice consists of vegetated swales with check dams (see
Figure 4-27) to retard or impound concentrated runoff to induce
infiltration. This is achieved by directing,concentrated flows
of surface runoff through vegetated drainage swales or
channels where gentle channel slopes and dense vegetative
cover provide for a nonerosive flow velocity. The combination
of low velocities and vegetative cover also provide an
opportunity for nutrients and other pollutants to be filtered or
settle out. The check dams, creating small infiltration pools,
will generally consist of earthen fill 6 to 24 inches in height.

Applicability

Swales are most applicable in residential and institutional.
a.reas of low to moderate density where the percentage of
impervious cover is relatively small. This practice requires
that subdivision and site designs respect natural drainage
patterns in lieu of elaboratestorm. drain systems.

Swales are usually located in a drainage easement at the side or
back of residential lots as shown-in.Figure 3-8. Swales can
also be used along the edge of roadways as a substitute for
curb and gutter. This application, which is normally used in
rural right-of-way road section is illustrated in Figure 3-9.

Swales with check dams can be used in combination with
infiltration trenches when the trench is constructed below the
vegetate swales. The pool created by the check dam increases
the volume of runoff infiltrated into the trench, while the
vegetated swale helps to filter out suspended solids and other
runoff pollutants. The infiltration trench *in turn will
increase the opportunity for runoff to infiltrate into the
underlying soil.

Swales with check dams can also be used in conjunction with
detention basins. The s 'wales would be designed to control the
water quality storm, with overflows controlled by the basin.

Drainage Divide

Rear
Slopes
Protective Slopes to Side Swale or Channel
Drainage
Easement

Rear Drainage asement
Running to Proper Outfall

(A) Profile View

IL
Lot Grading Lot Grading
Type C Type D
Block Grading Type 4: Valley Along Rear Lot Lines

(B) Plan View

FIGURE 3-8. Typical Lot Grading P lan

601 Right of Wa>

St 41 61 24,
61 41 5,
<-@@ 64
Con S; t r. -S* Profile
Grade Line
6% 2% %
6%

Sta ili
ze
Shoulder

Fiqure 3-9. Typical Road Cross Section Usinq Veqetated Swales

Source: RPC 1980

-58

Planning Considerations

Soil Permeabilitv

The permeability or final. infiltration rate of the soil as
provided in Table 3-1, will limit the utilization of swales for
ponding and infiltration of stormwater runoff. The maximum
allowable pondincr time for swale pond is 24 hours. Table 3-2
relates the ponding time criterion to soil texture and maximum
allowable depth of ponding. Based on the data r)resented in
Table 3-2, it is recommenaed that impoundments not be
constructed in swales on soils with f values less than 0.27
inches per hour.

Swale Gradient

The use of small check dams to create impoundments in swales is
limited to gradients of 5 percent or less. Although check dams
can be constructed on steeper slopes, the volume of runoff
stored behind the check dams on steeper slopes will be too small
to be of much value. Therefore swales proposed on qradients
steeper than 5% shall not be used for stormwater manaq-ement.

Water Table, Bedrock and Groundwater Considerations

The seasonally high ground water table or bedrock should be at
leas't i to 2 @eet below th e bottom of the swale. Local
officials may require qreater depths to the qroundwater table
where proposed developments are expected to hal7e large
pollutant loads. It can be observed that one of the advantages
of swales is that they allow. the use of infiltration methods in
permeable soils with relatively high water tables.

Other Considerations

In planning future street drainage systems, officials should
weigh heavily the offsite advantages of using vegetated swales
instead of curb and gutter systems. The following
considerations should also be taken into account:

1. Vegetated swales are generally less expensive to install
than curb and gutter systems.

2. Roadside swales keep flow away from-the street surface
during rainstorms, reducing the potential for hydroplaning
of auto.tires and wet brake linings.

3. Vegetated swales require more maintenance than curb and
gutter systems (mowing, seeding, debris removal, etc.)

4. Roadside swales are 'subject to damage due to snow plowing
and off street parking.

5. Streets with swales may require more riqht-of-way and
are less compatible with sidewalk systems.

6. Roadside swales become less feasible as the number of
drivewav en*trances requiring culverts increases.

7. Roadside swales can be used in conjunction with
infiltration trenches and basins to further attenuate
runoff and improve its quality.

8. Swales are not usable in all cases where poorly drained.
soils, extreme slope conditions, or the lack of positive
outfalls exists.

Water Oualitv Control

Vegetative swales with check dames, as with all infiltration
practices, can satisfy the water quality requirements of the New
Jersey SWM regulations as long as they result in zero runoff
.from the site under conditions of the water quality design storm.

The diversion of storm water runoff throuch vegetative swale
systems without check dams has been demonstrated to provide for
the removal of significant amounts of water bourne pollutants
(8,9). Removal is due to filtration, absorption, adsorption,
volatilization and other means. Vegetative swales have also
been shown to dramatically increase ph of runoff. (9)

Desiqn Criteria

Design Storm

All hydrologic and hydraulic calculations shall be based on the..
design storm criteria provided in the New Jersey SWM Regulations.

Ponding Time

The maximum allowable ponding time T in swales is 24 hours.
p
Velocitv

Swales must be designed so that the velocity of flow expected
from the design storm will not exceed the permissible velocity
for the type of vegetative lininq used. Permissible velocities
for qrassilined channels are presented in Table 3-3.

TABLE 3-3. PERMISSIRLF VELOCITIES FOR VELOCITIES POP VFGETATED
CHANNFLS*

Permissible Velocity
Slope 2/
Cover Range
Erosion Re- Easily
(percent) sistant Soils Eroded Soils
(ft. per sec.) (ft. per sec.)
K=0.10 - 0.35 K=0.36 - 0.80

0-5 8 6
Bermudagrass 5-10 7 5
over 10 6 4

Kentucky bluegrass 0-5 7 5
Tall fescue 5-10 6 4
over 10 5 3

Grass mixtures 2/ 0-5 5 4
Reed canarygrass 5-10 4 3

Lespedeza Sericea
Weeping lovegrass
Redtop i 3/ 0-5 3.5 2.5
Red fescue

I/ Use velocities exceeding 5 feet per second only where good cover.and proper
maintenance can be obtained.

2/ Do not use on slopes steeper than 10 percent except for vegetated side
slopes in combination with a stone, concrete, or highly resistant vegetative
center section.

3/ Do not use on slopes steeper than 5 percent except for vegetated side slopes
in combination with a stone, concrete, or highly resistant vegetative
center section.

4/ K is the soil erodibility factor used in the Universal Soil Loss Equation.
(See Appendix Al)

Reference Soil Conservation Service Engineering Field Manual Chapter 7,
(1969)

Capacity
The swale. .must have sufficient capacity to' pass the peak
discharge rate of the design storm above the check dam.

The channel above the check dams shall be designed in
accordance with the Manning Formula, unless more detailed
design.procedures (backwater programs, for example) are
required by the approving agency. The Manninq Formula is:
Q 1.49_ AR 2/3 S1/2-
n

where: 0 = flow rate, in cubic feet per second (cfs)

n = rouqhness coefficient

R. = hydraulic radius (feet)

S = hydraulic gradient (ft/ft)

2
A = cross sectional area (ft

Side Slope

The.maximum side slopes shall be qoverned by the soil
condition, type of flow, amount of flow and channel lining.
In qeneral, the side slopes shall not exceed the following
criteria:

Seeded grasses 3 (horizontal) 1 (vertical)

Sod 3 : I

Riprap 2 : 1

Gabion baskets can be vertical

9" Gabion mats 1 1/2 : 1

Special designs to be stepped or placed at 1:6 for heights
over 5'.

Cross Sections

Channel cross sections may be vee-shaped, parabolic, or
trapezoidal. Properties of tvpical channel cross-sections are
shown in Figure 3-10.

0-6-4 -0-4 k. S@ C.--3.0-, Too wwts.
T A- 0.4.- 01,1

- - - - - - - - - - - - - - - - - - - --
bd 4 Z,. 6 - 2dz
64 + zd, 6 Zd VIZ -- '- T- 6 -202
-h

Ttwevawl Cv" Sct-@

T ZdZ
Z,d 2. VT'

Trbw C@. S@Iom

- - - - - - - - - - - - - - - - - - - - - - Od ,;r.
Id 1 30 0,
800-

Fiqure,3-10. Properties of Typical Channel Cross Sections

(a) Vee-Shaped (Triangular) Ditches. Vee-shaped ditches are
generally used where the quantity of water to be handled is
relatively small, such as along roadsides. A grass or sod
linincr will suffice where velocities in the ditch are low. For
steeper slopes where high velocities are encountered, a riprap
or gabion lining may be necessary.

(.b) Parabolic Channels. Parabolic channels are often used
where the quantity of water to be handled is larger and where
space is available for a wide, shallow channel with low velocity
flow. Riprap should be used where higher velocities are
expected and where some dissipation of energy (velocity) is
desired. Combinations of grass and riprap or gabion mats are
also useful where there is continuous low flow in the channel.

(c) Trapezoidal Channels. Trapezoidal channels are often used
where the quantity of water to be carried is large and
conditions require that it be carried at a relatively high
velocity.

A cross..-section of a 60-foot-side road riqht-of-way is shown in
Figure 3-9. This cross-section illustrates the common u'se of
center crown sloping at a rate of one-quarter inch per foot
.toward a swale on each side of the street. Sidewalks can be
placed along the swales' banks where hydraulic capacities
permit gentle slopes.

In ceneral, swales adjacert to a roadway should have a sect."Lon
with a side slope not steeper than 2:1 (horizontal to
vertical) and should preferably have a rounded bottom.

Channel Linings

(a) Gra ss. The grass type used sha 11 be appropriate for the
site conditions, i.e., drainage tolerance, shade tolerance,
maintenance requirements, etc. The veqetation should have@a.
dense root system and be water tolerant. Refer to Standards for
Soil Erosion and Sediment Control in New Jersey.

(b) Riprap. Riprap shall meet the design criteria set forth in
the Riprap Standard in the Standards for Soil Erosion and
Sediment Control in New Jersev.

Outlets

Every swale ok waterway shall have a stable outlet. This outlet
may be another waterwav, a stabilized open channel, etc. In all
cases, the outlet shall discharge in such a manner as not to
cause erosion. Outlets shall be constructed and stabilized
before operating the waterway.

Outlet conditions for all channels are 17ery important especially
at the transition from a man-made lining such as concrete
to a vegetatiVe lining. Appropriate measures shall be taken
to dissipate the energy of the flow to prevent scour of the
receiving channel. Refer to Outlet Protection Standard -in
the Standards for Soil Erosion and Sedimen@ Control-in New
Jersey.

Construction Specifications

The construction specifications provided in the Standards and
Specifications for Grassed Waterway of the Standards for Soil
Erosion and Sediment Control in New Jersey, shall be used
for this practice.

Maintenance

(a) Grass Lined Channels. During the initial establishment of
grass-lined channels, any repairs and grass establishment shall
be done immediately. After grass has become established, the
channel shall be checked periodically to determine if the
grass is staying in place. Any mowing of the channel should not
damage the grass. Permanent channels shall be mowed
periodically to maintain their capacity.

Grassed waterways shall be inspected periodically, especially
after large storms, to detemine whether there are erosion
problems that need to be controlled, to remove accumulated
debris, and to check the condition of the vegetation.

(b) Other Linings. Earth or ripra p lined chan'nels shall be
checked periodically to ensure that scouring is not occurring
at the soil surface or beneath the riprap layer. The channel
shall also be checked for slumping of the side slopes and to
determine if-any stones particularly in channel bends or
constrictions have been dislodged by the flow.

(c) Sediment Deposition. If the channel is below a high
sediment-producing area, sediment should be trapped usincr
vegetative buffers or sediment traps before it enters the
channel. If sediment is deposited in grass-lined channels, it
shall be removed promptly to prevent"damage to the grass.
Sediment deposited in riprap and earth channels shall be
removed when it reduces the capacity of the channel.

(d) Homeowner Responsibilities. In residential subdivisions,
swale maintenance, such as grass cutting and debris removal is
usually the responsibility of the homeowner. More education may
be necessary to inform the property owner of these
responsibilities and of the need to discourage the off-road
parking along swale areas, especially during wet periods when

the swale grasses and underlying soils are Most 17Ulnerable to
damage. Also education is necessary so that the homeowner does
not remove the check dams. Such activities can alter the
drainage configuration of the swale area and kill the vegetation
that stabilizes it and provides natural treatment of runoff.

References

1. Becker, B.C., M.L. CLar, R.R. Kautzman, "Approaches to
Stormwater Management," prepared by Hittman Associates,
Columbia, MD, for the office of Water Resources Search,
U.S. Department of the Interior, Washington, DC, 1973.

2. Post, Buckley, Schuh and Jernicran, Inc., "Evaluation of Two
Best Management Practices: A grassy Swale and a
Retention/Detention Pond," prepared by the Brevard
County Water Resources Department and County Board of
Commissioner, Brevard Countv, Florida, January 1982.

3. Stormwater Workshop: Best Management Practices,
Reqional Planning Council, Baltimore, MD, 1981.

.4. Anonvmous, "Controlling Stormwater Runoff in-
Deveioninq Areas: selected Best Management Practices,"
Metropolitan Washinqton Council of Governments, July 1979.

5. Searcy, J.K., Design of Roadside Drainage Channels,
Hvdraulic Desiqn Series No. 4, U.S. Bureau of Public.
Roads, U.S. Government Printing Office, Washington, DC
20402.

6. Stormwater Management Desiqn Manual, for Frederick
County, Maryland, 1979.

7. Standards for Soil Erosion and Sediment- Control in
New Jersey State Soil Conservation Committee September,
1974.

8. M.P. WaNielista, Y.A. Yousef and J.S. Taylor,
"Stormwater Management to Improve Lake Qualitv," EPA-60012-
82-048, USEPA, March .1982.

9. Oakland, Paul H., VIAn Evaluation of Urban Storm
Water Pollutant Removal throuah Grassed Swale
Treatment."; 1983 Internation symposium on Urban
Hydrology, Hydraulics and Sediment Control.

3.6. VEGETATIVE FILTER STRIP

(VFS)

Description

A vegetative filter strip or buffer area is an area of
vegetative C017pr through which runoff may flow before it
leaves the site or enters a storm water control, such as a
detention basin. As the water containing sediment and other.
pollutants flows through the filter strip, some of the
sediment and pollutants are removed by filtering,
absorption, and gravity sedimentation as the flow velocity is
reduced.

Applicability

Vegetative filter strips can be used by themselves or in
conjunction with other stormwater management measures. A
vegetative filter can provide the following benefits
(Clar, 1981):

1. Serves as an effective.method of reducing sediment yield
by protecting the soil from rainfall impact energy.

2. Reduces runoff by reducing overland flow velocities,
increasing the time of concentration and increasing
infiltration.

3. Removes suspended sediment in overland flow by
filterinq, absorption, and gravity sedimentation as the
flow velocity is reduced.

Vegetative filter strips should be considered for use in the.
following locations.

1. Surrounding storm water management infiltration
structures, to reduce the sediment load delivered to the
structure.

2. Adjacent to all watercourses (waterways, diversions) and
water bodies (streams, ponds and lakes).

3. Along the tops and toes of slopes.

4. Between parking lots and stormwatpr management structures,
where drainage is primarily sheet flow.

5. On certain small proiects, such as small commercial sites,
offices etc. filter strips can be used to.meet the water
quality requirements of the New Jersey Storm Water
Management Regulations.

Planning Consideration

Almost any stand of vegetative cover will remove some sediment
from water flowing throuqh it. These filter strips can occur
naturally or be man-made. The type of vegetation used can be
very broad. Best performance is associated with dense stands of
turf-forming grasses.

The most common, naturally-occurring filter strips are those
vegetative stands associated with floodplains or found adjacent
to natural swal.es and watercourses. In some cases, preservation
of these areas is all that is required for them to continue to
function as filter strips, As these filter strips are exnected
to perform for several months or more, a top dressing of
fertilizer is recommended to improve the stand. In areas where
natural vegetation is of poor quality or nonexistent, it is
possible to establish a man-made veqetative filter (Albrecht
and Barfield, 1981);

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 low points in parking.lots or grass areas, then a
level spreader or reverse bench should be used to establish
sheet flow. When water is discharged directly from a pipe, the
filter strip can not be considered an effective water quality
control.

Selecting the Type 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. The New Jersey Standards and
Specifications for Soil Erosion and Sediment Control should be
used as a guide in selecting the vegetation type.

Slope Characteristics

The effectiveness of vegetative filters as water quality control
devices decreases with increasinq slope. Their effectiveness
has not been established on slopes greater than 17 percent
(Albrecht and Barfield, 1981).

Runoff

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

2. Good drainage to ensure satisfactory performance.

3. A level spreader at the inlet to ensure uniform
distribution of flow.

4. Dry periods between flows to reestablish an aerobic soil
profile.

5. An adequate filter area and length of flow to provide
the desired treatment.

6. Slopes less than five percent are more effective;
steeper slopes recruire a greater area and length of flow to
achieve the same effectiveness.

7. Provisions for mowing and removing undesirable
vegetation to maintain the effectiveness of the filter
area.

Water Quality

Filter strips have been shown to be effective in removina
sediment and pollutant loads in urban storm water runoff. In
order to adequately address the water quality requirement of the
N.J. SWM regulations, filter strips must be designed to provide
at least 75% trap efficiency using Figure 3-13. Also, all
runoff must be overland sheet flow only. This can be usually
only be accomplished with runoff from small parking lots or
roads where depth of flow is less than the height of vegetation.
.Where catch basins and storm sewers are used to collect and
transport runoff, velocities and depths of flow usually preclude
the use of filter strips as effective water quality controls.

Design Criteria

Length of Filter Stri2

The minimum length of filter strip used in conjunction with all
other stormwater management infi-1tration structures shall be 20
feet.

Additional guidelines to assist the designer in calculating the
trap efficiency of an existing vegetative buffer strip, or the
length of vegetation filter required to provide a specific trap
efficiency are provided below.

Graphical Solution

A solution for computing the sediment trap efficiency of a
vegetative buffer strip can be represented graphically (Wong
.and McCuen, 1982). Figure 3-11 shows the relationship between
trap efficiency and the length and slope of the filter strip,
as well as the rouc'Thness coefficient of the vegetation. The
required length of a buffer strip is very sens-itive 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 buffer
length. The curves also suggest that a significant trap
efficiency (up to 75 percent) may be achieved at relativelv
s1hort buffer lengths. Figure 3-11 assumes a coarse silt
material.

The trap efficiency for other soil textures may also be
determined using Figure 3-11. The settling velocity of sediment
particles manifests the appropriate trap eificiencies that are
attainable usinq filter strips for a particular particle size.
In general, the greater the settling velocity, the higher the
trap efficiency per 1 'ength 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
3-11 should be multiplied by this ratio to obtain the filter
trip length for a fine silt. This would provide the same trap
efficiency indicated on Figure 3-11. The settling velocity
ratio of coarse silt to medium silt, fine sands, and mbdium
sands are 1.3, 0.02, and 0.005, respectively (Wong and McCuen,
1982).

Construction Specifications

Site Preparation

1. Install needed erosion and sediment control practic-es
such as silt fences, dikes, and contour ripping, erosion
stops, channel liners, sediment trap@s and sediment
basins.

2. If grading is required and tonsoil 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
the specifications. The depth of topsoil to be salvaged
shall be 6 inches unless the depth described as a
representative profile for that particular soil type as
described in the soil surve,,., is less than 6 inches, in
which case the lesser depth shall be removed.

1 3-

12-

1 1-

10-
9- n:0.80 - 1500

8- n:0.3 5
w - 1400 E-z
& 7- n:0.20
0
6-
to
- 1300
5-

4-
- 1200
3-

2---
-1100
TR: 9 9 %
0- TR:S 5 %
1000 C:)
(L
z
ul

-,4
Soo

AJ
cc
-800

.61 ul)
bo all
cc
Lu
700 ..1 Ln
U-
LA-
a
D 00
Soo a]

tu
:9 0 %
TR > P,
Soo

Lu
LL.
-400 LL.
Lu
T;k :8 5 %
300 a)
IR:80%
ZT1,
200

T;t 7 5 % 100

0.33 0.67 1.00 1.33
RUNOFF VELOCITY (ft/sec)

3. Grade as needed and feasible to permit the use of
conventional equipment for seedbed preparation, seeding,
mulch applicat ion, anchoringand maintenance.'

4. Liming: Where the subsoil is either highly acid or
composed of heavy clays, ground dolomite limestone shall be
spread at a rate of 2 tons per acre (100 pounds per 1,000
square feet). Lime shall be distributed uniformly over
designated area and worked into the soil in conjunction
with tillag operations as described in the following
procedures.

5. Tilling: After the are 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 or
by scarifying to a depth of at least 3 inches to permit
bonding of t@e topsoil to the subsoil. Pack by passing a
bulldozer vertically tracking over the entire surface area
of the slope to create horizontal erosion check slots to
prevent topsoil from sliding down the slope and rilling.

Soil Preparation and Amendments

.1. Materials: Topsoil shall be a 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 5 per'cent by
volume of cinders, stones, slaq, coarse fragment, gravel,
sticks, roots, trash or other extraneous materials laraer
than 1-1/2 inches in diameter. Topsoil must be free oi
plants or plant Darts*of bermudagrass, quackgrass, Johnson
grass, nutsedge, poison ivv, Canada thistle, or others as
specified. 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'orcranic content of not
less than 1.5 percent by weight is required. If 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 has elapsed to permit
dissipation-of toxic materials.

Note: Topsoil 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 4 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 irregqlarities in the surface
resulting from topsoiling or other operation shall be
corrected in order to prevent the formation of depressions
or water pockets. Topsoil shall not be placed while in a
*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 soil testing
laboratory.

4. In lieu of soil tests apply 1,000 pounds 10-10-10 or
equivalent per acre if ureaform. fertilizer is not used, and
600 pounds of 10-10-10 or equivalent per acre if ureaform.
fe*rtilizer is used. Apply the lime and fertili7er before
seeding and harrow or disc uniformly into the soil to a
minimum depth of 3 inches on slopes flatter than 3:1/ On
slopes steeper than 3:1 grade, the lime and fertilizer
shall be worked in as best 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 Nrery smooth
after discing. When the 600 pounds per acre rate of 10-10-
10 fertilizer rate is used, at the time of seeding, apply
400 pounds of a ureaform fertilizer of a grade of a least
30-0-0 per acre.

Note: The slow release'ureaform fertilizer will supply
nitrogen over a longer period of time and will result in a
healthier grass stand.

Seeding

1. Select a mixture from standards for Soil Erosion and
Sediment Control In New Jersev.

2. Apply seed uniformly with a cyclone seeder, dril-1,
cultipacker seeder or hydroseeder (slurry includes seed
and fertilizer) on a firm, moist seedbed. Maximum
seedincT depth should be 1/4 inch on clayey soils and 1/2
inch on sandy soils, when using other than hydroseeder
method of application.

Note: If hydroseeding is used ane the seed and fertilizer
is mixed, they will be mixed on site and the seeding shall
be immediae without interruption.

Mulchinq

Mulch materilas are listed in order of their effectiveness.
Mulch mattincTs are normally only used on critical areas such as
waterways or steep slopes.

1. Materials and Amounts

a. Mulch 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 sever. If the area is to be mowed, do
not use metal staples.

b. Straw: Straw shall be unrotted small grain applied
at the rate of 1-1/2 to 2 tons per acre, or 70 or 90
(two bales) pounds per 1,000 square foot. Mulch
materials shall be relatively free of all kinds of
weeds and shall be free of prohibited noxious weeds
such as: thistles, Johnsoncrrass-and quackgrass.

Spread uniformly by hand or mechanically. For
uniform distribution of hand spread mulch, divide
area into approximately 1,000 squarp feet section
and place 70-90 pounds of mulch in each section.

C. Wood chins: at the rate of approxim'atelv 6 tons
pier acre or 275 pound per 1,000 scruare feet may be
used when available and when feasible. These are
particularly well-suited for utility and road
rights-of-way. If wood chips are used, increase
the application rate of nitrogen fertilizer by 20
pounds (200 pounds 10-10-10 or 66 pounds 30-0-0).

d. Wood cellulose fiber: mulch at the rate of 1,500
-pound per acre or 35 pounds per 1,000 square foot
may be applied by hydroseeding.

2. -Mulch anchoring shall be accomplished immediately after
mulch placement to minimize loss by wind or water. This
may be done by one of the following methods, (listed by
preference) depending upon size of area, erosion hazard,
and cost. on slopping land, practice No. I below, should
be done on the contour whenever possible. Contouring of all
operations applies to all straw and to wood chip practices

TABLE 3.4
SOILS, SEED MIXTURES AND DATES
FOR PERMANENT SEEDINGS

RATE Lbs./1000 OPTIMUM SEEDING DATES
SOILS MIXTURES Lbs./Ac. Sq. Ft. North Jersey South J .e-rsey

A Droughty A-i KY-31 Tall fescue 15 .3/8
10 1/4 Before 5/1 Before 4/15
(sands, shallow, Crownvetch 8/10-9/10
steep, shaly, Creeping red fescue 10 1/4 8/1-9/1
gravelly) Chewings red fescue 10 1/4

A-2 Weeping lovegrass 2 1/16
Sericea lespedeza 20 112 3/1-7/1
Chewings red-fescue 10 1/4
creeping red fescue 10 1/4
A-3 Midland bermudagrass 8bu. 4/15-7/15
2-31 centers

A-4 Other hybrid bermuda-
grass 20 bu. 4/15-7/15
12" centers

A-5 Perennial ryegrass 20 1/2
Chewings red fescue 15 3/8 Before 5/1 Before 4/15
Creeping red fescue 15 3/8 8/1-10/15 8/15-11/1
B. Well drained; mixtures A-1 thru A-5. are
moderately well applicable
drained B-1 Ky-31 tall fescue 30 3/4 Before 6/1 Before 5/1
Chewings red fescue 15 3/8 18/1-10/15 8/15-11/1
creeping red fescue 15 3/8

B-2 Ky-31 tall fescue or 15 3/8
creeping red fescue 15 3/8 Before 6/1
Chewings red fescue 15 3/8 8/1-9/1
Birdsfoot trefoil 10 1/4

B-3 Perennial ryegrass 20 1/2
Chewings red fescue 15 3/8 Before 6/1 Before 5/1
-Creeping red fescue is 3/8 8/1-10/15 8/15-11/1
Kentucky bluegrass 15 3/8

d. 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.

e. Pet and Twine: drive 8- to 10-inch wooden pegs to
within 2 to 3-inches of the soil surface every 4
feet in all directions. Stakes may be driven before
.or after applving mulch. Secure mulch to soil surface
by stretching twine between pegs in a criss-cross
within a scTuare pattern. Secure twine around each
pea with two 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.

Irriaation

If soil moisture is deficient, supply new seedings with adequate
water for plant growth until they are firmlv established, if
feasible. This is esr)eciallv true when seedings-are made late
in the plantinq-season, in abnormally'dry or hot seasons, or on
adverse sites.

Maintenance

Maintenance.is a vital factor in maintaining an adequate
vegetative erbsion control cover. See Standards for Soil
Erosion and Sediment Control in New Jersey to obtain the
maintenance fertilization program for permanent seedings.

a. Irrigation: If soil moisture becomes deficient, irrigate
to prevent loss of stand of protective vegetation,.if
feasible.

b. Repairs: Inspect all seeded areas for failures and make
necessary repairs, replacements, and reseedings within the
planting season, if possible.

1. ..If stand is inadequate for erosion control, overseed
and fertilize usinq half of the rates originally
applied.

2. If stand is over 60 percent damaged, reestablish
following original lime, fertilizer, seedbed
preparation and seeding recommendations.

7 8

References

1. Clar, M.L., P. Das, J.J. Ferrandino, R.J. Barfield.
Handbook of Erosion and Sediment Control Measures for Coal
Mines, prepared by Hittman Associates, Inc. for the Office
of Surface Mining, USDI, June, 1981.

2. Albrecht, S.C., and B.J. Barfield. "Use of a
Vegetative Filter Zone to Control Fine-Grained Sediments
from Surface Mines," prepared bN7 Hittman Associates,
Inc. for the U.S. EPA, Cincinnati, Ohio, March 1981
(PB81-116110).

3. Tollner, E.W., B.J. Barfield, C.T. Haan and T.U.
Kao. pended Sediment Filtration Capacity of Rigid
Vegetation, Transactions ASAE19(4): 676-682, 1976.

4. Wong, S.L. and R.J. McCuen. "The Design of Vegetative
Buffer Strips for Runoff and Sediment Control," Appendix J
in Stormwater Management in Coastal Areas prepared by R.H.
McCuen, Department of Civil Engineering, University of
Maryland, for t.he Tidewater Administration, Department of
Natural Resources, Annapolis, Maryland, September 1982.

5. Fayes, J.C., B.J. Barfield, and R.I. Barnhisel. "The
Use of Grass Fillers for Sediment Control in Strip Mine
Drainage, Vol. III, Laboratory and Field Evaluations on
Real Grasses," Institute for Mining and minerals
Research, University of Kentucky, 1981.

6. 1983 Maryland Standards and Specifications for Soil
Erosion and Sediment Control. Published jointly by
the Water Resources Administration, Soil Conservation
Service and state Soil Conservation Committee, Annapolis,
Maryland, 1983.

7. Best Management Practices Handbook, Urban, Virginia
State Water Control Board, Planning Bulletin 321,
Richmond, Virginia, 1979.

8. Viessman, W. Jr., J.W. Knapp, G.L. Lewis, T.E.
Harbaugh, Introduction to Hydrology. . 2nd edition,
Harper and Row Publishers, New York, NY, 197-7.

9. Anonymous, "Controlling Storm Water Runoff in
Developing Areas: Selected Best Management Practices",
Metropolitan Washington Council of Governments, July 1979.

10. Standards for Soil Erosion and Sediment Control in
New Jersey, New Jersey State Soil Conservation
Committee, September 1974.

CHAPTER 4

HYDROLOGIC DESIGN METHODS, INFILTRATION

AND VOLUME CONTROLS

INTRODUCTION

Design procedures are presented in this chapter for four volume
control stormwater management methods: infiltration basins,
infiltration trenches, dry wells, and vegetated swales with
check dams.

As used herein, an infiltration ba-sin is an open, surface
storage area that has no primary hydraulic outlet. Outflow is
.assumed.to be infiltration through the underlying soil and thru
an emergency spillway.

The, use of these methods should be considered in the context of
general limitations outlined in Chapter 3.

Infiltration basins can be used in con@unction with detention
basins. This would usually'be accomplished by installing a
discharge structure with outlets rai-sed. The volume piovided
below the outlet should not exceed the infiltration capacity of
the basin. For dual purpose designs, this volume should be
equal to the runoff from the water quality design storm.

Infiltration trenches and drv wells depend on subsurface
storage; the-se two methods differ on the mechanism for input.
Input to an infiltration trench is through a highly porous stone
medium that overlavs the trench or an inlet. Input to a dry
well is through boih infiltration and pipe inflow.'

Estimating the Runoff Depth_Required for Control

The methods proposed herein are based on the control of
discharge rates either by storing the runoff depth due to
changes in land use or by reducing both the contributing area
and the runoff depth. New Jersey storm water management policy
requires the peak d-ischarge for selected return periods to be
not increased after development. However, if the increase in
peak discharge cannot be managed, the infiltration practice can
be designed to capture the water quality storm runoff.
Capturing the water quality design storm runoff will satisfy the
water quality requirements of the New Jersey storm water
management regulations.

Unlike rate controls, such as detention basins, volume controls
do not require detailed hydrograph routings. Infiltration
practices can be designed usina the Soil Conservation Service
Graphical Method of determining peak discharge.The before
development peak discharge (q b) can be determined by:
qb = q ub AQ b (4-1)
in which q ub is the unit peak discharge, in cubic feet per
second per square mile per inch of runoff (csm/in.), from Figure
4-1 based on the before development time of concentration
(t cb ) in hours., Qb is the before development depth of runoff
in inches, and A is the drainage area in square miles. Using a
subscript "a" to indicate "after development", the after
development peak discharge (q a ) is given by:
qa = qua AQ a (4-24)
While the total drainage area (A) will remain constant, both the
unit pea.k discharge (q U ) and the runoff depth (Q) will
probably be greater for the after development conditions. if
development causes a decrease in the time of concentration, then
the unit peak disnharge will increase. Similarly, an increase
in the-percent of imperviousness will cause an increase in the
volume of runoff. If the storm water management policy requires
qa to equal q b' then the policy could be met if a difference
in dept@ of runoff 0 was con@rolled; this As determined as
follows:

q ua A(Q a Q) = q ub AQb (4-3)
Therefore, solving for Q-yields:
0 Oa - (qub /q ua)Qb (4-4)

If there is no change in tc, then (q ub Iq equals 1.0,
and Q = Qa - Qb- Since the developmeunt will usually
decrease the time 0f concentration, q ub will usually be less
than q and Q will be greater than the difference in the
.runoffugepths (Q a - Qb).
in addition to controlling discharge rates bv adjustments in the
runoff depth, it is also feasible to limit discharge rates by
reducing both the areas and depth of the after development
runoff. Specifically, if development causes an increase in both
q and Q, then the rate of runoff can be reduced by decreasing
AUand Q a in Equation 4-2. If a part of the watershed, which
will be denoted by A c is modified so that it does not
contribute runoff to the watershed. outlet, then if- is possible
for the after development peak discharge to equal the before

1000

760 j,

Soo f Vj.... 11 wl

400

'00

00
200
171 1*:,a
@Mlrill
L
S. !I. I;!

r r
100 1 :,. , -:iII . '.
0.1 0.2 0.3 0.4 0.5 0.7 1.0 2.0 3.0 4.0 5.0
TIM OF coNaxiWaieN hourg

Figure 4 -1. Peak discharge in csm per inch of runof f
versus tim, of 6oncentration(TO for 24-hour, type Il
and type IH storm distribution.

peak discharge. Using Equations 4-1. and 4-2, the drainage area
section A would be given by:
c
qua (A Ac ).Qa qub AQb (4-5)
where 0 a is the total after development runoff depth minus the
equivalent runoff volume stored bv the infiltration
measures(%rW/A). Substituting the value of Q a in Equation 4-
5 and solving for A c yields:
Ac = A 1 q ubQb (4-6)
ua a w

If an area A is designed to be noncontributing, then the
c
after development peak discharge rate will equal the before
development rate even though the depth of runoff increases
and/or the time of concentration is reduced.

General Design Situations

There are two general types of situations in which the above
.methods can be applied. First, one may be interested in the
dimensions of the infiltration device that is required to
control the increase in peak discharge rates. second, site
conditions may dictate the layout and c *apacity of the
infiltration measures and one might be interested in determining
the level of control provided by such a layout. in the latter
case, control may not be sufficient and additional control,'
possibly using surface detention, may be required. It is
important to emphasize that the same principles apply to both
cases.

Design methodologies were developed for four methods:
infiltration basins, infiltration trenches, dry wells, and...
vegetated swales with check dams. The desiqn procedures are
based on either 1) total volume control of the area contributing
runoff or 2) partial control of the runoff volume from an area
(i.e., the difference in the before and after development runoff
depths). For the TR-55 graphical method (Equations 4-1 and 4-
2), it is assumed that the peak discharge could be controlled
through either Q or A and 0. Dry wells, which provide total
area and volume control, store all the runoff incident to the
contributing surface area. Infiltration measures that provide
partial volume control store.the difference in runoff volume
necessary to meet the peak control criteria. These include the
infiltration basin, infiltration trench, and swale storage. Any
of the infiltration measures can control the total contributing
area and volume of runoff if the after development design storm
depth is stored.

Applicabili ty

The use of infiltration practices to control large volumes of
runoff is often impractical. For example, the increased runoff
from the 10-year or 100-year storm is often much too large to
address throuah infiltration. The most useful application of
the infiltration
practices presented is to control the 1-ye@r
water quality and 2-year design storms. Infiltration practices
that capture all the runoff from the 1-year storm will usually
provide sufficient reductions in the 2-year peak to meet the
required predeveloped rate of flow. 10-year and 100-year flows
will usually require flood control techniques, such as detention
basins. The design methodologies presented are based on the Soil
Conservation Service Runoff Curve Number procedures. This was
done because it is the most widely used and accepted. However,
the New Jersey storm water management regulations do not dictate
its use in the design of storm water management controls.

Water Quality Design Storm Analysis

In the development of designs for water quality controls, such
as infiltration practices or the water quality portion of dual
detention basins, the use of the RCN procedure will often
.underestimate the runoff volume produced by small design storms.

When precipitation (P) is small, producinq less than 1.5 inches
4C
O.L runoff, the accura7cy of the RCN procedure is affected. when
analyzing the New Jersey water quality storm, 1 year 24 hour
stor@, the loss of accuracy can be critical. This is especially
true when the pervious area RCN is below 50. For the purposes
of watef quality design aspects of storm water basins, the water
quality storm analysis should include only impervious areas when
the pervious areas RCN is below 50. RCN values under 50 will
produce little or no runoff from the small water quality design
storm and by weighting the pervious and impervious RCN values
the SCS models will underestimate runoff volumes. RCN's under
50 are usually limited to areas with hydrologic soil type "All.

If the weighted RCN method is used to analyze the small rainfall
events normallv associated with water quality concerns, the
volume of runoff provided for in water quality controls may not
be sufficient.. The following example shows how runoff
determination can vary when using a weighted curve number versus
the use of impervious areas only.

Example 1

Area of site = 10.0 acres, "All soils throughout, land use is 50%
lawn (pasture in good condition) and 50% impervious. 1-year
rainfall 2.8" Determine 1-year runoff.

Weighted RCN Method

RCN (wt) .5(98) + .5(39) 6R.5
(P - (0.2S)2
Q ------------- (4-7)
P + .08 (S)

1000
where S ----- - 10 (4-8)
CN

S 4.598, use 4.60; Q = 0.545" 2
Total Volume of Runoff = 0.545" x 10 Acres x 43560 ft

------ -----
12"/ft Acre
= 19,783.5 ft. 3

Weighted Runoff Method

Impervious RCN = 98, S = 0.204, Q = 2.57"
-Pervious RCN = 39, S= 15.64, Q = 0"
Total Volume of Runoff = 2.57" x 5 Acres x 43560 ft2

12"/ft Acre

46,645.5 ft.

This example shows that the RCN procedure will underestimate the
runoff from a 1-year storm if the weighted runoff method is
used. This will always be true when the pervious area RCN is
small. To determine whether the pervious area RCN will produce
runoff the expression P - 0.2(S) should be used. If P - 0.2 (S).
> 0 there will be runoff. The amount is determined by equation..
4-7.

If P - 0.2 (S) < 0 there will not be any runoff. P, Q, and S
are as defined by the USDA - Soil Conservation Service.

The reason for the discrepancy between the.weighted curve number
and weighted runoff procedures can be best explained by
analyzing what is physically occurring in the catchment and how
the model (RCN procedure) simulates the hydrologic.response.
Specifically, one should review how the hydrologic soil-cover
complex models the runoff potential of the land surface when
there is a larce variation in RCN. Large variations in RCN are
typically found when development occurs on A or B soils.

.85

In the example the assumption was made that all the impervious
areas are connected. This means rooftop drains lead directly to
parking lots, driveways and/or drainage pipes and the rain that
falls on an imnervious surface has no opportunity for
in-filtration o@ transmission losFes until it is di'scharged at
the terminus of the onsite drainage conveyance system. With
these conditions, a 2.8" rain falling on the pervious areas
(lawns etc.) would produce no runoff as the rate of delivery of
the precipitation@ would not exceed the infiltration capacity
(field capacity) of the "A" soil with the land cover chosen. In
other words, the initial abstraction exceeds the precipitation
(or 0.2S > P) and there will be no rainfall excess. The portion
of rain falling on the connected impervious areas would produce
an equivalent runoff depth of 2.57" (see example). Assuming
transmission losses are negligible, the volume of runoff
produced by the rainfall event would eaual the depth of runoff
times the area of the connected impervious surfaces. This is
what is-modeled by the "Weighted Runoff Method".

The "Weighted RCN Method" assumes that the runoff from the
impervious area and pervious area can be modeled by combining
the infiltration/runoff potential of the two different
hydrologic soil cover complexes in a weighted fashion. This
indicates that if thp rainfall fell on a mix of the two areas,
the runoff produced can be modeled by using a weighted average
of the RCN's. This may he true if the 10 acres were laid out
with alternating strips of lawn and asphalt where the runoff
from the impervious areas would travel across the lawn areas.
This would allow for transmission loses through infiltration..
In essence, the runoff estimated in the example catchment is not
being produced by an area with a RCN of 68.5; it is being
produced by two separate areas, one with a RCN of 98 and one
with a RCN of 39. In this case, with a 2.8" rainfall, only the
area with the RCN of 98 produces runoff.

Therefore in the analysis of small rainfall depths, (associated-
with the States water quality design storm) in areas where there
is a large variation in RCN values, only the connected
impervious areas should be considered producing runoff.

4-.2 DESIGN OF INFILTRATION RASIN

(IB)

An infiltration basin is defined as an open area that ha.s no
outlet for direct runoff other than an emergency spillway.
The storm runoff, which includes both rainfall that falls
on the surface of the basin and direct runoff from the upland
area, will either infiltrate or evaporate. Since evaporation
is usually negligible during periods of heavy rainfall, the
design is to be based on water loss by way of infiltration only.

The required size of the basin will depend greatly on the
increase in runoff volume from developnent and the maximum
allowable ponding time. A maximum ponding time of 3 days is
allowed within the basin. The approving agency and the design
engineer may select other specific pondling times provided that
the maximum 3 davs is not exceeded. If the available area for
the basin is smaller than that required to meet the specific
peak control criteria, the basin can be designed to capture the
first flush of runoff. Capturing the runoff from the water
quality design storm will'remove many of the waterborne
pollutants.

Site Layout

For.pt@rposes of definition, the site will consist of two
areas: (1) the portion of watershed that contributes direct
runoff to the basin, which is denoted as A(u); and (2) the
portion of the watershed allocated to the basin, which is
denoted as A(b). The subscript u and b are used to indicate the
upland and basin drainage areas, respectively. Notations for
the site layout are given in Figure 4-2.

Feasibility Test

Before designing an infiltration basin, it is necessary to
determine the textural class of soils.underlying the basin such
that a feasible design is possible. Soils with infiltration
rates less than 0.27 inches-per hour should not be considered
for an infiltration basin. Those soil textural classes that
have slow infiltration rates (i.e. less than 0.27 inches per
hour) limit the flow 'of water through the soil. Additionally,
the allowable depth of storage may be too shallow to be
practical when constructing the structure, based on the maximum
ponding time of 3 days. Thus, the suitable textural class of
the soil underlying the basin shall be either a silt loam,
loam, sandy loam, loamy sand, or sand.

AQU Ab=LW

A =LBWLI
Z
At

(a) Plan View of Site (b) Cross Section of Basin

LB
-T
W13

Notation L L

Au upland drai-nacTe area (acres)
QU upland runoff depth (ft)
Ab top surface area of basin (ft 2
A bottom surface area of basin (ft 2

L top lenqth of basin (ft)

LB bottom length of basin (ft)

TAY top width of basin (ft)

WB bottom width of basin (ft)
db basin depth (ft)
z basin side slope ratio

Fiqure 4-2. Schematic of Tnfiltration Basin

The de,sign of the infiltration basin is also based upon the
maximum depth ofthe basin d a * The maximum allowable
depth shall meet the followiwgxcri.teria:
dmax = f x Tp (4-9)
where f is the final infiltration rate of the basin area
in inches per hour, and T is the maximum allowable ponding
time not to exceed 72 hourp- The maximum allowablp-.depth*s are
given in Table 3-2 for selected values of f and T P*
Design Method

The desiqn method is based on controlling the increased runoff
for a specific frequency storm event. The design return period
shall be the I year frequency storm event. If the discharge
associated.'with the 1 @,ear storm event cannot be managed, a
first flush event should be the minimum selected for design.

The basin is sized to accept the runoff volume that enters the'
basin during the design storm. The design volume of the basin
equals the upland runoff volume 0 A to the basin, plus the
.volume of rain that falls on the hurface area of the basin
PA bl minus the exfiltration volume f"'A b out of the bottom
of the basin. Based on the SCS hydrograph analysis, the
effective basin filling time (T) will generally be less than
two hours. The exfiltr 'ation volume is small during the two hour
period of time to fill the basin. Therefore, the volume of
water that enters the soil during the filling of the basin is
not significant compared to the much qreater upland runoff
volume to the basin and may be ignored with little loss of
accuracy. The volume of water that must be stored in the basin
Vw is defined as:

V A + PA
W U U . b

where QU is the upland 2runoff depth (ft), A U is the
upland runoff area (ft ), P is the design r infall event
(ft), and A is the basin surface area (ft The upland
runoff dept@ OU is determined by:
QU (P - 0.2S)2
P + 0.8s
(4-11)
CIP11, lisle and "CN" are defined in TR-55)

The volume.of rainfall and runoff entering the basin can be
defined in terms of the basin geometr@v. The geometry of the
basin will generally be in the shape of an excavated trapezoid

with specified side slopes,- shown in Fiqure 4-2. The volume of
a trapezoidal shaped basin can be approximated by:
(LW + L B WB) d b
V -----------------
W 2
(4-12)
where LW is' the top surface area of the basin 2(ft 2),
LRWB is the bottom surface area of the basin (ft ), and
d is the basin depth (ft). The bottom lenqth and width of
t@e basin can be defined in terms of the top length and width
as:

LB = L - 2Zd b

WS = W - 2Zd b (4-13)

where Z is a specified side slope ratio (h:v). By
setting Equations 4-10 and 4-12 equal and substituting
the above relationship for L B and V7B , the following equation
is derived for.the basin top length:
A + Zd b2 (W - 2?Zd b
L ------------------ 2 (4-14)
W(d b - P) - Zd b

The surface dimensions of the basin are determined by selectinq
a to p width (W) and side slope ratio M, and solving for the
top length (L) from Equation 4-14. Both the top width and
length must*be greater than 2Zd such that the bottom
dimensions are feasible. All t@e variables in Equation 4-14 are
in units of feet or scruare feet. The solution to Equation-4-
14 is provided in Figures 4-3 through 4-8 for P = 3 inches,
Z = 2 or 3, and d b = 3, 4, 5, 6, 8, or 10 feet. Equation 4-
14 may be used to solve for the top length (L) for values not
given in Figures 4-3 through 4-8. A maximum side slope
ratio of 3:1 for a vegetated basin and 4:1 for a non-vegetated
pit is recommended.

In the event that the basin is constructed with an embankment,
the basin area and volume will- be determined from the natural
ground contours. By planirnetering the area of the contours
above the base of the embankment, a relationship can be
developed for depth and surface area to obtain the volume of the
basin. The basin volume from the contour data shall be greater
than or equal to the volume determined by Equation 4-10.

Design Procedure.

(1) Determine the after development runoff volumes 0
from after development curve numbers from the Ses
TR-55 Manual. Eauation 4-11 is used to corrpute the
upland runoff voiume Q u:
QU = Qa
(2) Compute the maximum allowable basin depth (d max)
from the feasibility equation, d f T Select
the basin design depth d baseR"on the Bepth that is
at least two feet abov@ the seasonal high groundwater
table, or the depth less than or equal to d max.-
whichever results in the smaller depth.

(3) The basin surface area dimensions can be determined
from Figures 4-3 through 4-8 or directly by Equat on 4-14.
If the Fiaures are used, compute the value 0 uA Uft ,
select a top width (W) greater than 2Zd a side slope
ratio (Z), and determine the required @o'p length (L). if
the variables (0 A , Z d b' POI @,) are different
than those given Ynuthe'Figures, use the Figures only as a
first trial to find an approximate width and length
combination. The exact basin top length W is
determined from Equation 4-14:
QUAu + Zdb2 (W - 2Zdb
W(d b -P) - Zd b2
The basin top length (L) and width (W) must be greater than
2Z.dbfor a feasible solution. If L and W are not greater
than 2Zd b' the basin would have no bottom dimensions. In
this case, the basin depth d bshall be reduced for a feasible
solution.

The design procedure may also be used for controling an increase
in runoff (Qa - Q ) caused by development. In these cases,
the basin would b@ designed to capture the increased volume of
runoff and QU (Qa Qb).

300

275 -7

250

225
T'7..
2 do 4

175

LENGTH OF
BASIN
(IN FEET) 150 ooo

125

100 10,
le. db=3 FT
z=3
75 z=2

P=3.0 in.

25
MAW

0
0 20 40 60 80 100 120 140 160 180 200

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Figure 4-3. Determination of Infiltration Basin Length

300

2 75

250

A;/
225

200

175
LENGTH OF
loe
BASIN
(IN FEET) 150

01
.01
125 100, oor

.0ol" .01

lao

@ol
75 .4o,
db=4 FT
Z=3
50
Z=2

P=3. 0 in.
25 7--'

0
0 20 40 60 80 100 120 140 160 180 200

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Figure 4-4. Determination of Infiltra tion.Basin Length

275

2 5

225

200

175
LENGTH OF t
BASIN
(IN FEET) 150

125

100
db=5 FT
100,
75 Z=3
Z=2

P=3.0 in.

0
-4

0 20 40 60 so 100 120 140 160 180 :2

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Fiqure 4-5. Determination of infiltration Basin Lenqth

275

.61

250

225 IfN

200
OF

loll

175
LENGTH OF
BASIN
(IN FEET)
150 .01 '.001

125 -L

100
db=6 FT
z=3
75 t z=2

P=3.0 in.

25
L

0
0 20 40 60 80 10.0 120 140 160 180 200

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Figure 4-6. Determination of Infiltration Basin Lencrth

275

250

225

-200

LENGTH OF 175
BASIN
(IN FEET)
150

0-1
125

oe.

100

4
X
75
oe db=S FT
50 oe z=3
Z=2

P=3.0 in.
25

0
0 20 40 60 so 100 120 140 160 180 2 0, 0

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Figure 4-7. Determination of Infiltration Basin Length

300

275

250

225

200
10,

175
LENGTH OF
100, 016% 0.01
BASIN 4% @, -@o
(IN FEET) @.*:
150

100,

125
-.01

100

75
10
.01 Ob =10 FT
Z=3
50 Z=2

P=3.0 in.

0 L
0 20 40 60 80 100 1.10 140 160 180 200

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION BASIN
(IN THOUSANDS OF CUBIC FEET)

Figure 4-9. Determination of Infiltration Basin Lenqth

4.3. DESI(-,N OF INFILTRATION TRENCHFS

(TT)

An infiltration trench is defined as a subsurface trench that
is used to temporarily store runoff in a stone filled
reservoir and exfiltrate the runoff throught the surroundinq
soil media. The surface of the trench will consist of either a
stone covered area or a grass covered area with an inlet,
shown in Figure 4-9. If the trench surface area consists
entirely of stone, qabion mattresses or large size stone shall
be placed on the surface to prevent loss of stone. A grassed
trench surface will require an 'inlet so runoff can effectively
enter the stone filled reservoir storacre area. The inlet
shall be designed to remove heavier.sediments to prevent the
clogging of the stone void spaces, as shown in Figure 4-10. A
filter fabric material will be required at the inter4ace of the
soil and reservoir area or below the gabion mattress and
around the sides and bottom of the trench. A minimum 'In foot
vegetted buffer strip is required around the perimeter of
the trench to prevent sediments from entering the trench
and thus, increase the life of the trench.

Site Layout

The site for an infiltration trench consists of two areas: (1)
the portion of the watershed that contributes runoff*directly to
the trench, which is denoted as A ; and (2) the portion of
the watershed allocated to the treHch which is denoted as A
A schematic of an infiltration trench is given in Figure 4-@.*
The use of an infiltration trench may be in combination
with a grassed swale to provide additional storage below thp
swale.

Feasibility Test

Before designing an infiltration trench, it is necessary to
determine the textural class of the soils underlying the
trench such that a feasible design is possible. Soils with
infiltration rates less than 0.60 inches per hour should not be
considered for an infiltration trench. Those soil textural
classes that have slow infiltration rates (i.e. less than
0.60 inches per hour) limit the flow of water through the
soil. Additionally, the allowable depth of storaqe may be too
shallow to be practical when constructinq the structure,
based on a maximum storage time of 3 days. Thus, the
suitable textural class of the soil underlying the trench
should be either a silt loam, loam, sandy loam, loamy sand oi@

A. A

;&:ions

dt Filter Facric
inlet

f At
(a) trench with aggregate surface

Z*wer4ln; goal
-T ri:ter FabrIc
dt Vr
Filter Fabric

f
(b) trench with vegetated surface with inlet

Notation

P rainfall depth (feet)
QU upland incrPase in runoff depth (fpet,i
A upland drainaqe area (acres)
u
At tren ch surface area (acrL-s)
dt trench depth Ueet)

f final infiltration rate (feet/hour)

V void ratio of stone in trench
r
T Time during which the trench fills with.
water. Based on SCS hydrograph analysis,
T will qenerally be less than two hours.

Figure 4-9. Schematic of Infiltration Trench
Vr

2C' 20'

Grate

L@l kd N
vegetatic,-, Vegetation
0 0 0 0
a 0

0 0 0 0

a- 0 0 0 Perforated Inlet

0 0 0

0 0 0 0 0

0 0 0 0 0
Filter Fak-,rlc -2 0 Filter Fabric

Figure 4-10. Typical Inlet to Infiltration Trench

100

sand.- The soil characteristics shall be investigated and
recorded to a depth at least four feet below the bottom of the
infiltration trench. The-b6ttom of trench*shall be located at
least 2 to 4 feet above the seasonal high water table or any
impermeable stratum.

The design of an infiltration trench is also based upon the
maximum allowable depth of the trench d . The maximum
allowable depth should meet the following mcHiteria:
d max x s/Vr (4-15)

where f is the final infiltration rate of the trench area in
inches per hour, TS is the maximum allowable storage time of
72 hours, and V r is the void ratio of the stone reservoir.
The iraximum allowable depths are given in Table 3-2 for selected
values of f, T , and V
s r'
D sign Method

The design method is based on controlling the runoff for a
specific frequency storm event. The design return period
-shall be the 1 year frequency storm event. If the discharge
associated with the I year storm event.cannot be managed, a
first flush event should be the minimum selected for design.

An infiltration trench is designed based on the upland runoff
volume contributing to the trench during the'desiqn storm. The
design volume of the trench equals the upland volume of
a A contributing to the trench, plus the volume of rain
tHa@ falls on the surface area of the trench PA , minus the
exfiltration volume fTA out the bottom of the trench. For
designs based on the Soi@ Conservation Service Type III storm,
the trench filling time is less than a 2-hour duration. Thus, a
duration of 2 hours is.used for a value of T. The volume of
water exfiltrating during the filling period of the trench@may
be significant for permeable soils and cannot be ignored. The-
volume of water that must be stored in the trench V is
estimated as:. w

Vw = QU Au+ PA t -fTAt (4-16)

where units of cubic feet are used to represent the volume.
The upland runoff depth 0 for the upland contributing
area A is determined as: u
u
(P (0.2S )2
Qu= -------------- (4-17)
P + .08 (S)

101

The volume-of rainfall and runoff entering the trench ran be
defined in terms of the trench geometry. The gross volume of
the trench V is equal to the ratio of the volume of the
water that must be stored V to the void ratio V of the
W
stone reservoir in the trench; V is also equal to the product
of the depth d t and the surface. area A A t :
Vt = V w /V r = dtAt (4-18)
Combining Equations 4-16 and 4-18 yields the following
relationship:
d t At Vr = Ou A u+ PA t - fTAt (4-19)
Since both dimensions of the trench d and A At are unkown,
Equation 4-19 may be rearranged to determine the area of the
trench A if the value of d were set based on either the
location t of the water table t or the maximum allowable depth of
the trench d max
QUA u
----------------
At (Vrd t -P + fT) (4-20)
Additionally, if the width of a trench was set, Equation 4-20
would yield the required.trench length:

Q uAu
Lt (Vrd t -P +fT)W t (4-21)
Equations 4-20 and 4-21 are used to develop) a graphical
relationship for A and L with values of the other
variables. The solution tot Equations 4-20 and 4-21 are
provided.in Figures 4-11 through 4-20 for four different soil
types. All variables in Equations 4-20 and 4-21 must be in
units of feet.

Design Procedure

1. Determine the after development runoff volume Q from the
after development curve 'numbers, from the SCS TR-55
Manual. Equation 4-17 is used to determine the runoff
volume as 0a. = Qu
2. Compute the maximum allowable trench depth d from the
feasibility equation, d = f T /V r. Select the
trench design depth d based on the depth that is at
least two feet above the seasonal high groundwater table,
or a depth less than or equal -to d max whichever
results in the smaller depth.

102

3. Determine the trench surface area A from FicTures 4-11
through 4-15, for the particular tsoil type or compute
the trench area directly by Equation 4-20':

QUAu
At = ---
(Vrd t -P + fT)
If the variables () A Vr dtf, P, fT) are different
than those given in'@hg'Figures, use Equation 4-20 for the
exact solution.

4. If the width of the trench is set at either 3 or 6 feet,
Figures 4-16 through 4-20 may be used to determine the
trench length or computed directly from Equation 4-21:

Lt = - u u
(Vrd t - P+ fT)Wt
If the variables 0 A , V d , P, fT) are different
than those given A Ehe fitgures, use the Figures only as a
first trial to find an approximate width and length
combination.

..In the event that the side walls of the trenc@ must be
sloped for stability during construction, the surface
dimensions of the trench area should be based on the followinq
equation:
At = (L Zdt) (W - Zdt (4-22)
where L and W are the top length and width, and Z is the
trench side slope ratio. The design procedu,re would begin bY
selecting a top wi-dth (W) that is greater than 2Zd for a
t
specified side slope ratio (Z). The side slope ratio value
will depend on the soil type and depth of the trench. The
length (L) is then determined as:

L.= Zd t + At
--- (4-23)
(W ZDt

103

3, 00C

2, /50

2 5 C 0

5 t Loam
f=0.27in/hr
4,250 P=3.0 in.
v =0. 40
r
2,000 a,(;/

1,750
TRENCH
SURFACE
AREA 1,500
(IN 5-;?UARE
FEET)
47.
1,250

11000

750

Soo

250

G
0 11000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-11. Determination of Infiltration Trench Surface Area
tilly

/-// /, Zx

104-

3,000

2#750 -7- 7

2p500
Loam
f =C).
0. 52in/hr
2, 250 P=3.0 in.
Vr=O. 40

2,000

1,750
TRENCH
SURFACE
AREA 11500 9:
(IN SQUARE
FEET)
1,250

11000

750

500

250

0
0 1,000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-12. Determination of Infiltration Trench Surface. Area

105

3, 0@jC

2, 750

2 t 50e@

Sandy Loam
2,250 f=1.02in/hr
T-3-0 in.
r=0-40
2,000 71 1

1,750

TRF-NZ:H
SURFACE
AREA 1,500
(IN SQUARE
FEET)
1,250

1,000

750

500

250

0
0 11000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-13. Determination of Infiltration Trench Surface Area

.106

3,000

2,750

2, 500

2,250

Loamy Sand
f=2.4in/hr
2,000
P=3.0 in.
vr=O. 40

I , 750

TRENCH
SURFACE
AREA 1,500
(IN SQUARE
FEET)
1,250

1,000

750

500

250

0
0 11000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Fiqure 4-14. Determination of Infiltration Trench Surface Area

107

3,000

2,750

2,500

2,250

2,000

1,750
TRENCH Sand
SURFACE f=8.2?in/hr
AREA 1,500 P=3.0 in.
(IN SQUARE Vr=0.40
FEET) 1,250

11000

-7

750

500

250
;4-

-4
0
0 1,000 2,000 3,000 41000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-15. Determination of Infiltration Trench Surface Area

108

600

550 -4

500 40

450
Ji

400

350

LENGTH OF
TRENCH
300
(IN FEET)

250

t
200

150

10C Silt Loam
f=0.27in/@r
P=3.0 in.
V =0.40
50 r

0
0 11000 2,000 3,000 4,000 5,0rj C)

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-16. Determination of Infiltration Trench Surfacp Area

109

550

3b
450 b-
A

4 00

35C

LENGTH OF
TRENCH 300
(IN FEET.

250

ar
200

ISO

Luam
100 f=0.52in/hr---
P=3.0 in.
50 vr=0.40

C
0 11000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-17. Determination of Infiltration Trench Surface Area

110

600

550

500

450

400

350
e;
LENGTH OF
TRi NCH 300
(IN FEET)

250

200

150

Sandy Lca7

100
P=3.0 in.
r= 0.40
50

0
0 11000 2,000 3,000 4,000 5,0D0

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Figure 4-18. Determination of In-filtration Trench Surface Area

6 0, Z'

550

500
Loarnv Sand b"
40
5 f=2.41in/hr
P=3.0 in.
v 0.40
r

400

350

LENGTH OF I
TRENCH 300
(IN FEET)

250

200 . . ....

150

loc

0
0 1,000 2,000 3,000 4,000 5 , GOu

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Fiqure 4-19. Determination of Infiltration T rench Length

112

600

550

500

450

Sand
400 f=8.27in/hr
P=3.0 in.
Vr=O. 40

350

LENGTH OF
TRENCH 300
AIN FEET)

250

'Ole
200
0161

150

100

50 -7-

0
0 1,000 2,000 3,000 4,000 5,ODO

VOLUME OF RUNOFF CONTRIBUTING TO INFILTRATION TRENCH
(IN CUBIC FEET)

Fiaure
4-20. Determanation of Infiltrat-ion Trench Lenqth

113

4.4 DESIGN OF DRY WELLS

(DW)

Dry well storage is defined as subsurface storage in which
inflow is bv way of both pipe inlet(s) and infiltrating
water through the overlying soil. The volume of storage in the
dry well must accommodate runoff from a contributing area (A c
in Figure 4-21) as well as water that infiltrates through
the overlying soil (A in Figure 4-21).
w
While a dry well serves to control runoff using concepts similar
to that of infiltration trenches, the areal extent of the
runoff control methods serves as an important distinguishing
factor. As used herein, dry wells have small areal extents in
comparison to infiltration trenches. The time of concentration
of runoff from the contributing area is very small; in fact, it
mav be considered as instantaneous in comparison to both the
du@ation of the design storm and the smallest time increment for
routing through such systems. For example, the time required
for rooftop runoff to be disoharged from a downspout could
probably be measured in seconds rather than minutes.

Site Layout

Dry well storage is subsurface storage in which inflow is b 'y way
of pipe inlet(s) draining runoff from rooftop areas A C with
gutters leadin'g directly into the stone filled reservoir area
Aw below the overlying soil shown in Figure 4-21. In
special cases, the dry well may be desiqned with a surface inlet
to capture runoff from an upland drainage area, althouqh its
functioning life may be reduced due to sediments clogg--ncr the
stone reservoir. A filter fabric material shall be required
directly below the overlying soil surface and around the sides
and bottom of the dry well. Any surface inlet above the dry
well shall have a minimum 20 foot vegetated buffer strip on
each side of the inlet. The vegetated buffer strip will
prevent sediments from entering the well-and clogging the
stone voids and thus, increase life of the dry well.

Feasibility Test

Before designing a dry well, it is necessary to determine the
textural class of the soils underlying.the well such that a
I;oi
feasible design is possible. , ls with infiltration rates a
dry well. Those soil textural classes that have slow less than
0.27 inches per hour should not be considered for infiltration
rates (i.e. less than 0.27 inches per-hour) limit the flow of

114

@do
CN Q

CW
Building dW
Foundation
V
r

Notation

P rainfall depth (ft)
CNC CN for contributing area
Ac are@ contributing runoff to dry well
(ft

QC depth of runoff from contributing area
(ft)

2
Aw surface area of drv well (ft
Q runoff depth from overlying area Aw
(ft)

CN CN of overlying soil
Cw water capacity of overlying soil
(in/in)

d0 depth of soil overlying dry well (ft)
dw depth of dry well storage (ft)
Vr void ratio in dry well

f final infiltration rate below dry well
(ft/hr)

Figure 4-21. Schematic of a Dry Well

115

water through the soil..- Additionally, the allowable depth of
storage may be too shallow to be practical when constructing the
structure, based on the maximum storaqe time of.3 days. Thus,
the suitable textural class of the soil underlying the dry well
should be either a silt loam, loam, sandy loam, loamy sand, or
sand.

The design of a dry well is also based upon the maximum
allowable depth of the trench d max . The maximum allowable
depth should meet the following criteria:

d f x T /V (4-24)
max S r
where f is the final infiltration rate of the dry well area in
inches per hour, T is the maximum allowable storage time of
72 hours, and V r TS the void ratio of the stone reservoir.
The maximum allowable depths are given in Table 3-2 for
selected values of f, T S, and Vr*

Desiqn Method

The design method is based on controlling the runoff for a
snecific freauencv storm event. The design return period shall
.b@ the 1 year" fre-quency storm event. If the discharge
associated with the water quality design storm event cannot be
managed, a first flush event should be the minimum selected for
design.

The design method proposed here-in is based on the conceptual
framework of Figure 4-21. Rainfall produces runoff from both
the surface area of the drv well A w and the contributing area
A . Runoff from the contributing area QC passes to the dry
w9ll by way of a pipe. Rain that-falls onto the surface area
AW will run off (0) according to the curve number (CN) of the
dry well area; Q will be based on the overlying soil and
grass cover. That which does not run off (i.e., P-Q) willbe
available for infiltration through the overlying soil and'
into the dry well. The depth that infiltrates into the dry well
during the storm will also depend on the depth of the overlying
soil d and the water capacity of the overlying soil C W.
The fTow out of the base of the dry well will depend on the
infiltration rate (f), the area A , and the time that the
flow into the dry well exceeds thwe flow out of the dry well,
or the effective filling time M. Values of f should be
obtained from Table 3-1. For designs based on the SCS type TTI
storm, a time of one hour will be used for a value of -T
because, for storm volumes typically used for design, the
inflow rate will exceed the outflow rate during a time period
less than one hour.

116

The volume of'water that must be stored Vw is the sum of the
runoff from the contributing area (A c Q ), plus the volume
of water entering the dry well surface.@P - Q)A less the
volume of water stored in the overlying soil d . CwA
minus the volume of water exfiltrating out of ?he wdrw well
y
bottom (fTAw
Vw ACQC + (P - 0 - d0 Cw) Aw --fTAw (4-25)
in which (P Q d C ) must be greater than or equal to
0.0. 0 w

The volume of the dry well may be defined in terms of the well
qeometry. The gross volume of the dry well V is equal to the
product of the depth d and the surface area R w; vS also
equals the ratio of thwe volume of the water that must be stored
V to the void ratio V of the aggregate reservoir in the
w r
dry well:
VS Vw /V r =dw Aw (4-26)
Combining Equations 4-25 and 4-26 yields the following
relationship:
d wAw Vr = Acoc + (P - Q - d0 Cwj Aw - fTAw (4-27)
Since both dimensions of the'dr@, well (dW and A,) are
unknown, Equation 4-27 represents one equation with.two
unknowns. Thus, there are a number of combinations of d w and
A. that can he used. If the value of d were set based on
e@ther the location of the water table wor the maximum allowable
depth dmax , then the area Aw could be determined by
rearranging Equation 4-27.

A
.c c
---------------- ----------
w (4-28):
V d P + Q + d C' + fT
r r 0 w
Equation 4-28 is used to graph the relationship between A w
and dW for values of the other variables, shown in Figures 4-22
through 4-26. The overlying soil was assumed to be a 12 inch
deep loam textural class soil having a water capacity of 0.19
in/in and a runoff curve number of 61. All variables used in
Eauation 4-28 must be-in units of feet.

Design Procedure

1. From the selected design rainfall (P) and runoff curve
numbers, compute the runoff depths for the overlying soil
(Q, based on grass cover) and the contributing area (Q cil
curve number 98 for impervious surface).

117

2. Compute the maximum allowable dry well depth d from
the feasibility equati'on, d - @ fT /V SeleRxthe
a@ 0
dry well design depth dw baTe. . tRe Ee-pth that is at
least"two feet above the seasonal high groundwater table,
or a depth less than or equal to d max , whichever results
in.the smaller depth.
3. Determine the dry well surface,area A W from Figures
4-22 through 4-26 or compute the required surface area of
the dry well from Equation 4-28:

Oc A
A = ------ __S ----------------
w V d P + Q + d C + fT

C. and f are obtained.from Table 3-1 based on the
o@7erlying and underlying soil types of the dry well,
respectively'. Use a T value of one hour for design. If
the variables Q A I V d , P, 0, C d , fT
c r W.
are different tSan those given in t@e Pigures, use Equation
4-28 for the exact solution.

In the event that the side walls of the drv well must be sloned
-@or stability during ocnstruction, the sur'face dimensions of the
drv well area should be.based on the following equation:

Aw = (L-Zd w) (W - Zd-w)

where L and W are the top length and width, and Z is the dry
well side slope ratio. The design procedure would begin by
selecting a top width (W) that is greater than 2Zd w for a
specified side slope ratio (Z). The side slope ratio value
will depend on the soil type and depth of dry well. The top
length (L) is then determined as:

A
L Zd - --------- ------
w (W Zd W)

118

3,000

2,750

2,500

2,250

Silt Loam
f=0.271n/hr
21000 Cw=.19in/hr --------
P=3.0 in.
V =0. 40
1,750 r
DRY WELL
SURFACE
AREA 1,500
(IN SQUARE 2;
FEET)
1,250

1,000

750

500

250

0
0 1,000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO DRY WELL
(IN CUBIC FEET)

Figure 4-22. Determination of Dry Well Surface Area

119

310GG

2,750

2, 50C

2,250
Loan
f=0.52in/hr
2,000 Cw=.19in/i
P=3.0 in.
r=0.40
-.JL-

DRY WELL
SURFACE
A. R EA 1,500
(IN SQUARE
FEET)
1,250

7
1,00c --

750

500

250

0
0 11000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO DRY WELL
(IN CUBIC FEET)

Figure 4-23. Determination of Dry Well Surface Area

120

3,000

2,750

2,500

2t250
Sandy Loam
f=1.021n/hr
21000 CW=.19in/in
P=3.0 in.
Vr=0.40
1,750
DRY WEA-L
SuRrACE
AREA 11500
'IN S,2UARE lb
FEET)
1,250

500

250

0
0 11000 2,000 3,000 4,000 51000

VOLUME OF RUNOFF CONTRIBUTING TO DRY WELL
(IN CUBIC FEET)

Figure 4-24. Determination of Dry Well Surface Area
b

4@
121

2, 25G
Loamy Sand
f=1.41in/hr
2,000 c,=.191n/in
P=3.0 in.
Vr=C.40
1,75C)

DRY WELL
SURFACE
A F% --- A 1,500
(IN 5;--UAR-7
FEET)
1,250

1,000

750

50Q

250

C.
0 1,000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO DRY WELL
(IN CUBIC FEET)

Fiqure 4-25. Determination' of'Drv Well Surface Ark-a

122

3, 00o
1

2,750

2,500

2,250

2,000

Sand
f=8.27in/hr
1,750
DRY WELL
P=3.0 in.
SUR17ACE
AREA 1,500 Vr=0.40
(IN SQUAREE
FEET)
1,250

750

500

250

o
0 1,000 2,000 3,000 4,000 5,000

VOLUME OF RUNOFF CONTRIBUTING TO DRY WELL
(IN CUBIC FEET)

Figure 4-26. Determination of Dry Well Surface Area

123

4.5 DESIGN OF VEGETATED STATAT-ES

A vegetated swale with a check dam is a grassed channel with a
small earth check dam that is used to store and infiltrate
-storm runoff. The small check dams will generally be 6 to
24 inches in height to'create small infiltration pools. The
use of small check dams to create infiltration pools is limited
to swale slope gradients of 5 percent or less. Swale
gradients greater than 5 percent may cause erosion and will
limit the volume of runoff stored behind the check darns.
The volume of water stored by a swale will depend on the land
slope, the depth, and width of the swale at the top of the check
dam.

A vegetated swale without a check dam is encouraged over
conventional curbs and qutters to remove some nutrients
and pollutants from runoff and reduce the volume of runoff for
small storm events (less than a 2 year frequency storm
event). The design of swales without check dams can be
accomplished with the existing SCS design methods by the
appropriate modification of the flow length and the time of
concentration.,

Site Layout

-The site layout will consist of the portion of the watershed
that contributes direct runoff to the swale area or the
upland area, which is denoted as A(u); and the portion of the
watershed allocated for swale storage, which is denoted as
A(s). It is important to note that the upland area (A(u))
does not include the area allotted to the swale surface
.(A(s)). Swale locations are usually either on the side or
back of the property line or along the side of roadways.
They are probably most applicable in parking lots where they can
be used in between parking bays. This will break up the typical
"sea of asphalt" and can be used for landscaping purposes by
planting trees in the swales. installation of berms or check
dams at certain intervals along the length of the swale will
result in stcrage. Figure 4-27 shows the schematic of a
vegetated swale with a check dam.

Feasibility Test

Before designing a vegetated swale, it is necessary to
determine the textural class of soils underlying the swale
such that a feasible design is possible. 'Soils with
infiltration rates less than 0.27 inches per' hour should not be

124

-op of Swale

ds Ss

(a) Cross Section of Swale with Check Dam

Z
ds
ZI

(b) Dimensional View of Swale Impoundment Area

Notation

L length of swale impoundment area per
check dam (ft)

ds depth of check dam (ft)
Ss bottom slope of swale (ft/ft)
W top width of check dam (ft)

Wb bottom width of check dam (ft)
Z 1 & Z2 ratio of horizontal, to vertical change
in swale side slope (ft/ft)

Figure 4-27. Schematic of Vegetated Swale with Check Dam

125

considered for a vegetated swale. Those soil textural classes
that have slow infiltrati@m rates 4.P. less than 0.27
inches per hour) limit the flow of water through the soil.
Additionally, the allowable depth of storage may be too
shallow to be practical when constructing the structure,
based on the maximum ponding time of 24 hours. Thus, the
suitable textural class of the soil underlying the swale
should be either a silt loam, loam, sandy loam, loamy sand,
or sand.

The design of a vegetated swale is also based upon the
maximum depth of the check dam d max* The maximum allowable
depth should meet the following criteria:
dmax @-- f x TP (4-29)
where f is the final infiltration rate of the swale area in
inches per hour, and T is the maximum allowable ponding
time of 24 hours. The Riaximum allowable depths are given in
Table 3-2 for selected values of f and T
P
Design Method

The desiqn method is based on controlling the runoff for a
specific frequencv storm event. The increase in peak discharge
associated with the 2-year storm event can be managed, or the 1-
year water cruality storm can be selected for design.

The vegetated swale is sized to store the runoff volume that
contributes from the upland area for a selected design
storm. The desiqn volume of the swale V w equals the upland
runoff volume Q u A u to the swale, plus the volume of
rain that falls on the surface of the swale (PA ), minus the
exfiltration volume (fTA ) out of the bottom of Ehe swale.
Based on the SCS type IIT storm, the effective swale fillinq
time will generally be less than a two hour duration@ The.
effective filling time is the duration in which flow into
the swale exceeds the exfiltration out of the swale.
Thus, a duration of 2 hours is used for the value of T. The
volume of water that must be stored in the swale 17 is defined
as: W

V W* = Qu Au + PA s -fTA s (4-30)
where units of feet are used to get volumes in terms of cubic
feet. The upland runoff depth Q U for the upland contributing
area A is determined as:
u
(P - 0.2 S) 2

--------------------
Qu P + 0.8S

126

The volume of rainfall and runoff stored in the swale may be
defined in terms'-of the swale geometry. The total. volume
of swale storage is detprmined as:
ds (W + Wb )-L
V. = ----------------- (N (4-31)
W 4 s

where d is the depth of the swale check dam (ft), W is
the swale top width (ft), W b, is the swale bottom width (ft),
L is the length of each swale impoundment behind a check dam
(ft), and N is the number of check dams along the total
hydraulic Angth of the swale L The length (L) behind each
swale check dam may be determin@d* by:

d
L --- s--- (4-32)
S s
where S is the longitudinal slope (ft/ft) along the length
of the Iwale. The number of check dams N that mav be -
installed along the total length of swale t may be-determined
by:
L t
Ns (4-33)

By setting Equations 4-36 and 4-38 equal*, the total
hydraulic length of the swale needed to store the upland
runoff is given as:
Ou Au
Lt -------------------------- (4-34)
ds (W + W b) + W(fT P)

where W is the top width of the swale check dam and is
determined as:

W = Wb + dS (Z1 + Z2) (4-35)
where Z1and Z are the side slope ratios of the swale
cross secti6n2 area, horizontal to vertical. Frequently,
the side slope ratios will be the same on both sides of the
swale, given Z (ft/ft); substituting into Equation 4-35 yields:

w W + 2d Z (4-36)
,b S

127

Swale cross sections may also be vee-shaped or parabolic and
the top widths-for each of these shapes are given in Fiqure 3-
10.

When the site layout is such that it'restricts the swale
dimensions to a specific configuration, the level of control
provided, given swale-dimensions, may be determined as:
V
Qs (4-37)
A
U
where Qs is the depth of runoff controlled by the swale and
V is determined by Equation 4-31 for the given swale
dYmensions. All parameters in the equations must be converted
to feet.

Design Procedure

1. Determine the after development runoff volume (0 ) from
the SCS TR-55 Manual. Equation 4-11 is used to 8ompute the
upland runoff volume (Q U
2
(P - (0.2S)
Qu= - --------------
P + .08 (S)

2. Compute the maximum allowable swale depth d. from the
feasibility equation, d fT Select Hex swale
max p
design depth*d based on a Cepth that is at least two feet
above the seas6nal high groundwater table, or the depth
lessor equal to d max' whichever results in the smaller
depth.

3. The swale surface area dimensions can be determined from,
Equations 4-34 and 4-36. The bottom width W is
selected along with the side slope ratio ( Z) 11band depth of
check dam D The swale top width (W) and total
hydraulic lan*gth Lt may be computed as:..
W = W b + 2 dS Z

and:
L Qu Au
ds (W + W + W(fT - P)
-- s b
4

4. The number of check dams needed to impound and store the

128

increased runoff volume is determined as:
N Lt
s L

The maximum require spacing between check dams is
computed as:

d
L --S
S
s
if L-t is restricted by the site layout, the level of control
provided by the swales is determined by equation 4-37:

V
Qs A
where: u

ds (W + Wb )L
VW = ------------------- (N S)
4

The same desic[n procedure above may be used for rate control,
except that Q used above would be equal to the increase in
runoff volume Yor Q a Qb

129

APPFNDIX A

STORM WATER MANAGEMENT PEGULATIONS

Subchapter 1 GENERAL PROVISIONS

7:8-1.1 Purpose and Authority
7:8-1.2 Construction
7:871.3 Definitions
7:8-1.4 Applicability
7:8-1.5 Proqram Information
7:8-1.6- Severability
7:8-1.7 Relationship to other permittinq programs

Subchapter 2 - PROVISIONS FOR PREPARATION OF PT,ANS AND ORDINANCES

7:9-2.1 objectives
7:9-2.2 Schedule for Comp3ption and Submission of Plans and
ordinances
.7:9-2.3 County Review Process
7:9-2.4 Failure of County to Approve
7:9-2.5 Notification to the State
7:9-2.6 Exceptions
7:9-2.7 Enforcement
7:9-2.8 Periodic Reexamination
7:9-2.9 Technical Assistance

Subchapter 3- ELEMENTS OF PLAN AND ORDTNANCE

7:8-3.1 Planninq Phases
7:8-3.2 Flexibility of Approach
7:8-3.3 Plan Conformity
7:8-3.4 General Standards
1. Flood and Erosion Control
2. Water Quality Control
3. Detention Basins in Flood Plains
4. Alternatil7es to Detention Basins
5. Maintenance and Repair
6. Control Measures
.7. ProDaqation of insects
8. Aestp_tics
7:8-3.5 Variance or Exemption From the Standards
7:8-3.6 Stormwater Control Ordinance

130

STORM WATER MANAGEMENT REGULATTOMS

Subchapter I GENERAL PROVISIONS
7:8-1.1 Purpo;e and Authority

This chapter shall implement the provisions of the New Jersey
Storm Water Management Act, P.L. 1981, c. 32, which amends and
supplements the Municipal Land Use Law, N.J.S.A. 40:55D-1 et
seq. These Storm Water Manaqement Regulations establish minimum
requirements and controls to compensate for the differences in
the hydrologic res'ponse of the watershed frorn the undeveloped to
the developed condition. The Storm Water Management Act further
creates a State grant program, (however, no funds have been
appropriated for this purpose as of this time). Nothing in
these regulations shall change the assigned duties of counties
and municipalitie-s responsible for approval of storm water
management provisions, submitted as.part of site plans, and
subdivisions as established by the Municipal Land Use Law.

7:8-1.2 Construction

(a) This chapter shall be liberally construed to permit the
Departmen@ to discharge its statutory function under the
New Jersey Storm Water Management Act, P.L. 1981, c.32.

(b) The Commissioner may amend, repeal or rescind this chapter
from time to time in conformance with the Administrative
Procedure Act, N.J.S.A. 52:14B-1 et sea.

7:8-.1.3 Definitions

"Agricultural Development" means land uses normplly associated
with the production of food, fiber and livestock for sale. For
purposes of this chapter, such uses shall not include the
development of land for the processing or sale of food and the
manufacture of agriculturally related products.

"Commissioner" means the Commissioner of the Department of
Environmental Protection, or his appointed designee.

"Department" means the Department of Environmental Protection.

"Flood Fazard Areas" means the floodway and flood frinqe areas
determined by the Department under section 3 of the Flood Hazard
Areas Control Act (P.L. 1979, c. 359).

"Flood Plain" means the flood hazard areas of delineated streams
and areas inundated by the 100 year flood in non-delineated
areas.

"Floodway" means the channel of a natural stream and portions of
the flood hazard-areas adj`oininq the channel, which are
reasonably required to carry and discharge the flood water or
flood flow of any natural stream.

"Impervious Surface" means any natural or man-made surface which
does not permit infiltration of water and causes surface runoff.

"Major Development" means that, in addition to the definition of
development in the Municipal Land Use Law N.J.S.A. 40:55D-4; anv
activitv must satisfy 1 or 2 below:

1. Any site plan or subdivision plan that will ultimatelv
cover one or more acres of land with additional impervious
surfaces.

2. Anv construction of one or more of the following uses; (i)
feeding and holding areas that provide for more than 100
head of cattle or 15,000 hens, 500 swine, 40000 turkeys,
10,000 ducks; this section shall also apply to all other
equivalent numbers of animals units as determined by the
SCS Agricultural Waste Management Field Manual for
measuring BOD producing potential. (ii) pipelines,
storaae, or distribution systems for petroleum products or
chemicals; (iii) storage, distribution or treatment
facilities (excluding individual on-site sewacre disposal
systems) for liquid waste; (iv) solid waste storage,
disposition, incineratidn or landfill; (v) auarries, mines
or borrow pits; and (vi.) land application of sludge or
effluents; (17ii) storage, distribution or treatment
facilities for radioactive waste.

"MLUL" means the Municipal Land Use Law N.J.S.A. 40:66D1 et
seq.

Non-point Source Pollution" means pollution from any
confined and
source other than from any discerni
discrete conveyances, and shall include, but not be limited
to, pollutants from agricultural, silviculturalf mining,
construction, subsurface disposal and urban runoff sources.

"Ordinance" means the same as "development regulation"
under the MTjTTL.

"Recharge" means the replenishment of underaround water
reserves.

"Storm Water Runoff" means flow on the surface of the
qround, resulting from precipitation.

132

7:8-1.4 ApPliqability

(a) Any storm water management plans or ordinances hereafter
adopted in New Jersey shall comply with this chapter.

(b) Phase I is applicable only to new developments, and not to
the remedy of existing runoff pollution situations.
Specifically, as regards animal feeding and holding areas,
it applies only to new agricultural facilities or additions
to facilities involving additional animals in amounts
sufficient to constitute a ma-ior development.

7:8-1.5 Program Information

Unless otherwise specified, any questions concerning the
recruirements of this chapter shall be directed to the Water
Supply and Watershed Management Administration, Division of
Water Resources, New Jersev De *partment of Environmental
Protection, CN-029, Trenton, New Jersev 08625.

7:8-1.6 Severability

If the provisions of any article, section, subsection,
paragraph, subdiVision or clause of this chanter shall. he judged
invalid by a court of competent jurisdiction, such order or
ludgement shall not affect or invalidate the remainder of any
article, section, subsection, paragraph, subdivision or clause
of this chapter.

7:8-1.7 Relationship to other permitting programs

Nothing in this chapter shall be construed as limiting the
rights of other agencies or entities, such as the Pinelands
Commission, from imposing stricter standards or other
reauirements as allowed by statute.

Subchapter 2 - PROVISIONS FOR PREPARATION OF PLANS AND ORDINANCES

7:9-2.1 objectives

(a) A storm water management plan and it's implementing
ordinance or ordinances shall be designed:

1. To reduce artificial!-,,, induced flood dainage to public
health, life and property;

2. To minimize increased storn, water runoff from anv new
land develoDment where such runoff will increase flood
damage;

3. To maintain the adequacy of existing and proposeO.
culverts and bridges, dams and other s-tructures;

133

4. To induce water recharge where natural storage and
geologically favorable conditions exist where
practical;

5. To prevent, to the greatest extent feasible, an
increase in non-point source pollution;

6. To maintain the integrity of stream channels for their
biological functions, as well as for drainage and
other purposes;

7. To reduce the impact of development upon stream
erosion;

8. To reduce erosion from any development or construction
project;

9. To minimize the increase in runoff pollution due to
land development, which otherwise would degrade the
quality of water and may render it both unfit for
human consumption and detrimental to biological life;
and

10. To preserve and protect water supplv facilities and
water resources hy means of contrnliincT increased
flood discharges, stream erosion, and runoff pollution.

7:9-2.2 Schedule for Completion and Submission of Plans and
ordinances
(a) If a grant -11ror 90 percent of the costs for'the preparation
of the olan is provided hy the Department pursuant to
section 6 of the Act the storm water management plan shall
be completed by the municipality within one Year from.the
date of promulgation of storm water management regulations
by the Commissioner, or by the next reexamination of the
municipality's master plan required pursuant to N.J.S.A.
40:55-D-89 whichever is later. The storm water management
plan shall be an integral part of each municipal master
plan as provided by N.J.S.A. 40:55D-28. Each storm water
management ordinance or ordinances prepared under such a
grant shall be adopted.by the municipality within one year
of the completion of the storm water manaaement plan and
shall be revised thereafter as needed. Such a storm water
manaqem&nt plan, control ordinance or resolution prepared
by counties, municipalities or designated-regional agencies
shall be prepared in accordance with this Chapter.

134

'17 :9-2.3 County Review Process
(a) Each*rnunicipality shall submit its storm water management
plan and implementing ordinance adopted pursuant to this
Act to the county agency or county water resources
association, designated by the freeholders, for apDroval.
The implementing ordinance shall not take effect without
county approval.

(b) The agency or association shall approve, conditionally
approve, or disapprove said Plan and/or ordinance. It
shall review its compatibility with applicable municiDal,
county regional or State storm water management and flood
control plans. It shall consult the approDriate Soil
Conservation District and verify that the coordination by
the municipality and the District has been satisfactorily
accomplished, as specified in N.J.A.C. 7:8-3.3. No storm
water management plan or ordinance shall be api'Droved which
fails to meet the State storm water management standards,
established bv thi-s chapter. The agency or association
shall set forth in writing its reasons for disapproval of
any plan or ordinance, or in the case of the issuance of a
conditional aDproval, the agency or association shall
specify the necessary amendments to the plan or ordinance
to the municipality. Once conditions, if any, are met by
the municipality the plan and/or ordinance shall. be deemed
approved.

7:9-2.4 Failure of County to Approve

Where the agency or association fails to aDprove, conditionally
approve, or disapprove a plan or ordinance within sixtv (60)
days of receipt of the plan or ordinance, the plan or ordinance
shall he considered approved.

7:9-2.5 Notification to the State

Upon receipt of each completed municiDal storm water management
plan and ordinance, the designated county agency shall notify
the Department of its receipt and keep an up-to-date accounting
of its standing in the approval process. The county agency
shall submit copies of the approved plans and ordinances to the
Department and shall Provide access to all other relevant
records to Department personnel.

135

7:9-2.6 Exceptions

The Commissioner may upon apvlication.by any appropriate agency
grant an exception from anv of the ob-lectives listed in N.J.A-.C.
7
-2.1(a) 1 through 10 above, as provided for in Sections 3 and
:8 1
4 of the Act provided that the Commigsioner shall determine that
such exception will not materially increase flood damage, non-
point source.,pollution, or erosion within or without the
municipality. Any municipal request'for such exemptions shall
be accompanied by proof of notice to all affected municipalities
of such request and the request shall be submitted to the State
through the appropriate county planninq agency.

7:9-2.7 Enforcement

No building permit shall be issued in violation of an adopted
ordinance. Any such issuance shall. he in violation of the MLITT,
and subJect to the enforcement provisions thereunder.

7:9-2.8 Periodic Reexamination

in accordance with the MLTJL, storm water management plans and
storm water control ordinances shall be included in the re-
.examination of the master plan and development reaulations.

7:9-2.9 Technical Assistance

Counties, countv planning agencies and countv water resources
.associations are authorized and encouraged to provfde technical
assistance and planning grants to municipalities to assist in
the preparation and revision of municipal storm water management
plans and implementing ordinances.

Subchapter 3 - ELEMENTS OF PLAN AND ORDTNANCE

7:8-3.1 Planning Phases

(a) Planning for storm water management is designed in two
phases. The Phase I plan is tarcreted at preventive measure
to be applied to the site plan and subdivision review
process. it shall identify existing control requirements
and establish plans and ordinances in order to meet the
standards' in these regulations for at least the short
.term. The Phase IT plan shall provide for the lonq term
comprehpn.sive plannina of alternative preventive storm
water manaqement measures in con@unction with remedial
storm water management measures.

136.

Phase I

i. A Phase I storm water management plan shall
consist of the following elements:
1. A statement concerning how the plan will
achieve the qoals of the Act.

2. A delineation of jurisdictional authority
and responsibility in the Phase I plan
area. This may include a -fee schedule for
implementation.

3. an evaluation of existing county and local
storm water management plans and
ordinances. This evaluation shall examine
the consistency.of the existing ordinances
-with regard to the water quantity/quality
objectives and minimum standards discuss@d
in this chapter.

4. An evaluation of needs. This evaluation
shall consist of two parts: A) a general
assessment of those items necessary for the
county and/or local ordinances to achieve
full compliance with this chapter including
but not limited to soil surveys, natural
resource inventories and pertinent elements
of local and county master plans; and B) an
estimate of the technical (personnel and
physical resources) and institutional needs
necessary to undertake implementation of the
Phase I plan.

5. Develop a recommended storm water management
brdinance.

ii. Within one year following the completion of the
Phase I plan, municipalities shall adopt
ordinances which are to be consistent with the
policies and principles of the Phase I plan.
These ordinances shall be amended, as required,
followinq the adoption of the Phase IT plan.
Such ordinances must be adopted if the Deoartment
pro-ides a grant pursuant to Section 6 of the Act.

2. Phase II

i. A Phase II storm water manacrement Plan will he
based uPon a detailed analysis of alternative
storm water management approachp-s on an inteqrat-
ed or regional basis. T@e plan will consist of a-
system of non-structural and/or structural storm
water manaqemefit Programs to mi-tigate floodinq

i37

and non-point source pollution. The nPed for
master detention basi ns to supplement or replace
individual detention basins or other facilities
otherwise recruired at each site of development
shall be considered. The need for expanded
protection of environmentally critical areas
including flood plains and wetlands shall be
reviewed. Plans shall also be developed to
address appropriate remedial-storm water control.
measures. A survev of any institutional issues
involved and of the social, environmental anO.
economic implications of the proposed actions
shall be included.

7:8-3.2 FlexibilitY of Approach

Each storm water management plan shall be coanizant of the
unique character and limitation of-the environment in the
planning area. A main purpose is to distinguish those special
conditions where an exception to the standards detailed in this
document may he required to best manage storm water runoff.
Unless circumstances justify exception or variance, the
standards will be applicable to all development as specified in
the remaining sections of this chapter.
7:8-3.3 Plan Conformity

(a) Fach municipality shall coordinate storm water ma *naqement
olans and ordinance prepared under these regulations with
soil and water conservation plans and regulati"ons under the
New Jersey Soil Conservation Act of 1937 as amended,
N.J.S.A. 4:24-1 et sea., and with appropriate soil
conservation districts. Storm water management plans shall
refer to be in compliance with and not duplicate Soil
Conservation District requirements for control of soil
erosion. Additionally, such plans shall be coordinated
with any storm water management plans preparecl by the @
county and any other municipality in the basin, and in full
compliance with the Water Qualitv Planning Act, N.J.S.A.
58:11A-1 et seg., and with any areawide plans for water
quality relating to the river basi-ns in the municipality.
The storm water management plan and the storm water .
management ordinance or ordinances shall also be consistent
with relevant Federal and State statutes, rules and
regulations concerning storm water management, dam safety
and flbod control, and with the Water Supply Manaqement
Act, P.L. 1981, C.262, and the County. Environmental Fealth
Act, N.J.S.A 26:3A2-21 et secT.

7:8-3.4 General Standards

The following stanclards'are,spe0ifiedfor general use as
minimums to be applied to ma@or developments. Local. plans and
ordinances which require a greater degree of control or require
retention for a greater period of time, or apply to classes of

138

developments in addition to those specified herein, will be
acceptable as long as the ob 'Jectives are met. Plans and
ordinances expressed in different terms but which are considered
by the Department to achieve substantially the same objectives
will also be acceptable.

(1) Flood and Erosion Control

The flood and erosion control standard for detention will
require that volumes and rates be controlled s,o that after
development the site will generate no greater peak runoff
from the site than prior to development, for a 2-year, 10-
year, and 100-year storm considered individually. These
design storms shall be defined as either a 24-hour storm
using the rainfall. distribution recommended by the U.S.
Soil Conservation Service when using U.S. Soil Conservation
Service procedures (such as U.S. Soll Conservation Service,
"Urban Hydroloav for Small Watersheds," Technical Release
No. 5-q,) or as the estimated maximum rainfall for the
estimated time of concentration of runoff at the site when
using a design method as the Modified Pational Method.
Tabulations of estimated maximum rainfall are available
from the Department. For purposes of computing runoff, all
lands in the site shall be assumed, prior to development,
to be in good condition (if the lands are pastures, lawns
or parks), with good cover (if the lands are woods), or
reqardless of conditions existing at the time of @ith
cons'ervation treatment (if the land is cultivated),
computation.

i. Any major agricultural development as defined in this.
chapter shall be submitted to the local Soil
Conservation District for review and comment in
accordance with this chapter and any Soil Conservation
Dist.rict guidelines. An agency, may condition approval
of such storm water control measures upon a positive
recommendation of the appropriate Soil Conservation
District.

(2) Water Quality Control

i. The water quality requirement for detention will
require prolonged retention of a small design storm
which shall be either a one (1) year frequency 24-hour
storm using the rainfall distribution recommended for
New Jersev'by the U.S.'Soil Conservation'Service or a
storm of @ne and one-quarter (1-1/4) inches of
rainfall in two (2) hours. Provisions shall be made
for it to be retained and relpased so as to evacuate
ninety percent in approximately eighteen (18) hours in
the 'case of residential developments and thirty-six
(36) hours in the case of other developments. This is
usually accomplished by a smAll outlet at the lowest
level of detention storage, with a larger outlet or

139

outlets above the level sufficient to control the
smalldesign storm. If the above requirement would
result in a pipe smaller than three (3) inches in
diameter, the period of retention shall be. waived so
that three (3) inches will be the minimum pipe size
used.

ii. Where soils have sufficient permeability, the
production of zero runoff from the site under
conditions of the 1-1/4 inch water quality storm will
be considered sufficient to meet the water quality
requirement for residential developments, provided
that the seasonal high groundwater does not rise to
within two (2) feet of the bottom of the detention
basin. For other than residential developments,
approvals will be on a case-by-case basis after
technical review by the designated authority. The
object of this review will be to avoid pollution of
cTroundwater. Other technology may he substituted
pursuant to paragraph (a)4i, below.

(3) Detention Basins in Flood Plains

i. There will be no detention basins in the floodway
except for those on-stream.

ii. New development, including construction of detention
basins, should be avoided in flood plains, but where
this is unavoidable, the plan and the ord,inance must
require a special examination to determine adeauacv of
proposed detention measures durinq the 1n0-year
flood. one acceptable method is to apply the 100-year
design storm to both the site and to the entire
watershed contributing to the flood plain, assuming
that the two peak simultaneously at the point in
question. The time of concentration assumed for the
entire watershed should be that appropriate to the
larger area, rather than the shortpr period applicable
to the site.

iii. In addition such development must be in compliance
with all applicable regulations under the Flood Hazard
Area Control Act, N.J.S.A 58:16A-50 et seg.

iv. In default of an analysis such as described above,
detention storage provided hv construction of dikes or
embankments below the elevation of the 100-year flood
(either specially calculated or taken from an official
flood plain delineation map) will be credited as
effective storage at a reduced proportion as indicated
in the table below:

140

TABLE 1

Allowable proportion of storage to be assumed usable
in detention basins created by the construction of
dikes and embankments of various sizes in drainaqe
basins.

DRAINAGE BASIN AREA AT SITE

Less than 5-100 Over 100
5 Sq. Mi. Sq. Mi. Sq. Mi.

Elevation of storage provided
below 100-year flood level

Less than 2 ft. 40 % 65 % 90 %
2 - 4 ft. 25 % 50 % 75 %
over 4 ft. 10 % 25 % 50 %

V. This effective detention storacre plus any other
supplementary measures, will be reauired to provide
for storm waier detention, in accordance with
establish standards. However, the aross storaqe
considered for this evaluation will not exceed that
which would be filled bv runoff of a 100-year storm at
the site.

vi. In making computations the volume of not fill added to
the flood hazard area portion of the project site will
be subtracted from the capacity of effectiVe detention
storage provided. Net fill is defined as the total
amoun@_ of fill created by the P*role&t less the amount
of material excavated during the construction of the
project, both measured below the elevation of the Ino-
year flood but abcve the elevation of low water in the
stream. There fore, net storage provided by
excavation in the-flood plain above the seasonal high
water table will be credited 100% towards effective
detention storage.

(4) Alternatives to Detention Basins

i. It is not necessary that basic requirements for water
quantity and quality control he satisfied by means of
detention basins. measures including but not limited
to rooftop storage, tanks, infiltrati 'on, pits, porous
navement, dry wells, cra@rel layers underneath paving,
or sheet flow throuqh irecretated areas may he used for
the purpose, with appropriate consieeration for length
of life ard feasibility of contlinued maintenance in
accordance with technical guidance from the
Department.@ Vacuum street sweeping may be substituted
for the water quality requirement, in cases in which
continuity of the service can be assured, and where
the pollution in auestion originates on the pavement.

141

ii. Non-structural management practices, including but not.
limited to cluster land use development, open space
acquisition, stream encroachment and flood hazard
controls, protection of wetlands, steep slones and
veqetation should be coordinated with detention
recruirement.s. Chanqes in land use can often reduce
the scope and cost of detpnt-ion provisions reauired by
means of appropriate changes in runoff coefficients.

iii. Municipalities should consider waivinq or amending
local requirements for extensive impervious pavement,
curbinq and storm sewers where small pavement areas
could be used or where grassed swales could be
substituted.

(5) Maintenance and Repair

i. Maintenance of detention basins and infiltration
means, or of other alternatives, is a very important
aspect of a storm water manaqement program. Control
measures shall be designed so as to provide for
mechanical maintenance operations. Responsibility for
operation and maintenance of storm water management of
storm water management facilities, including periodic
removal and disposal of accumulated parti-culate
material and debris, unless assumed by a aovernmental
agency, @hall remain with the property owner and shall.
pass to any successor or owner. In t@e case of
developments where lots are to be sold, permanent
arrangements, satisfactory to the approving agency
shall be made to insure continued performance of these
obligations.

ii. A schedule of maintenance inspections shall be
incorporated into the local ordinance. ordinances
shall also provide that in.cases where maintenance or,
repair is neglected, the municipality or the coun .ty
has the authority to perform the wori and to back-
charge the owner.

(6) Control Measures

ordinances and plans shall be desi.qned to allow for
flexibility in tho development of control measures. In
Phase I! planning, the use of regional basin or watershed
systems and the consideration of economies of scale shall
be investigated wherever practical. In addition, the
economic advantage of non-structural measures (i.e. changes
in land use, densities, site configuration, and use of
natural topography) should be.considered. Combinations of
remedial measures for existing systems and preventative
measures for new developments shall be investigated.

142

(7) Propagation of insects

Municipal plans should be prepared so as to minimize
propagation of insects, particularly mosquitos.

(8) Aestetics

Detention facilities should be de-sic.rned in a harmonious and
attractive manner.

7:8-3.5 Variance or Exemption Prom the Standards

If a municipality grants a variance or exemption from the
standards set forth in their storm water management plan and
control ordinance, a written report shall be made to the county
detailina the nature of the variance, the change(s) requested,
and an e.xplanation of the decision. An exemption from the
county ordinance or regulations or resolution shall be reported
in the same fashion to the Administrator,.Water Supply and
Watershed Management Administration, Division of Water
Resources, Department of Environmental Protection.

7:8-3.6 Stormwater Control Ordinance

The storm water con'trol ordinance is required to be adopted by
the municipality within one year of the completion of a.storm
water management plan funded pursuapt to Section 6 of this Act.
It is an implementation document for the plan. The ordinance
shall conform with all requirements of this chapter. Upon
adoption of this chapter, the Department will supply each
municipality with a Model Storm Water Control ordinance as a
guide for municipalities to prepare their own ordinances.

143

Apioendix B

A Draft Stormwater management-ordinance for municipalities

Division of Water Resources

New Jersey Department of Environmental Protection

This ordinance has been prepared by the New Jersey Department of
Environmental Protection, Division of Water Resources as a guide
for participants in the State's Storm Water Management Program.
In particular, this ordinance is designed to comply with
requirements of N.J.A.C. 7:8-3.1(a) 1. The intent of the
ordinance is to provide preventive measures to be applied to the
site plan review process. The general standards of this
ordinance, Section V, shall be considered as minimums and any
municipal ordinance, providing equal or greater protection as
defined by N.J.A.C. 7:8-2.1, and which are considered by the
Department to achieve substantially the same objectives will be
acceptable.

It should be noted that this ordinance is designed for general
application, and may be modified. Prior to adoption, however,
any modified ordinance must be reviewed and approved by the
appropriate County agency as required by 7:8-2.3 or by the
State's Bureau of Flood Plain Management.

STORMWATER MANAGEMENT ORDINANCE

AN ORDINANCE ESTABLISHING REQUIREMENTS FOR THF MANAGFMENT OF

STORMWATER WITHIN THE OF

AND THE STATE OF NEW JERSEY.

144

SECTION 1. SHORT TITLE

This ordinance shall be known and may be cited as "The Stormwater
Management Ordinance" of the of
of

SECTION II. STATEMENT OF PURPOSE

It is hereby determined that the lakes and waterways within the
of are at times
subjected to floodina; that such flooding is a dancrer to the
lives and property o@ the public; that such flooding is also a
danger to the natural resources of the of
county and State; that development tends
to accentuate such flooding by increasing stormwater runoff, due
to alteration of the hydroiogic response of the watershed in
changing from the undeveloped to the developed condition; that
such increased flooding produced by the development of real
property contributes increased quantities of water-borne
.pollutants, and tends to increase channel erosion; that such
increased flooding, increased erosion, and increased pollution
constitutes deterioration of the water resources of the
of , the county and the
State; and that such increased flooding, inc-reased erosion and
increased pollution can be controlled to some extent by the
regulation of stormwater runoff from such development. It is
therefore-determined that it is in the public interest to
regulate the development of real property and to establi.sh
standards to regulate the additional discharge of stormwater
runoff from such developments as provided in this ordinance.

SECTION III. APPLICABILITY

The provisions of this ordinance shall be applicable to each of
the types of development named below.

1. All-site plans and subdivision plans that will add one or
more acres of impervious surface (except as provided in
subparagraph 3 below).

145

2. Any construction of one or more of the following uses: (a)
confined feeding anO*holding areas that provide for-more than 100
head of cattle, 15,000 head of poultry, 500 swine, 4,000 turkeys
or 10,000 ducks; this secti@n shall also appl-yr to all other
equivalent numbers o4 animal units as determined by the SCS
procedure for-measuring BOD producing potential. (b) pipelinesi
storage, or distribution sy6tems for petroleum products or
chemicals; (c) storage, distribution or treatment facilities
(excludina onsite sewage disposal systems) for liquid waste; (d)
solid waste storage, disposition, incineration or landfill; (e)
quarries, mines or borrow pits; (f) land application of sludge or
effluents; and (g) storage, distribution or treatment facilities
for radioactive wastes. Except where permitted and subject to a
New Jersey Pollutant Discharge Elimination System (NTPDES) permit
or a approved DPCC Plan.

3. In the event that control of storm water runoff is mandated
in certain areas for construction covering less than one acre-of
ground, such lesser developments shall come under provisions of
this ordinance.

.SECTION IV. PROCEDURE (Recommended)

A. Burden of Proof

Whenever an applicant seeks a municipal approval of a development
to which this ordinance is apDlicable-from any board or official
of the municipality, that applicant shall be required to
demonstrate that his project meets the standards set forth in
this ordinance.

B. Submission Materials Due

The applicant shall submit materials-, as required by Sectio.n VI
her'eof, to the municipal board or official from whi.ch he seeks
municipal approval prior to or at the same time he submits his
application for the municipal approval.

C; Review

The applicant's project shall be reviewed by the municipal board
.or. official from which he seeks his municipal approval. That
municipal board or official shall consult with the municipal
engineer to determine if the project meets the standards set
forth in this ordinance.

146

D. Time for Decision

The municipal board or official shall promptly determine if the
project meets the standards set forth in this ordinance. The
time for that determination should be the time permitted to
review and act on the applicant's application for a municipal
approval.

E. Failure to Comply

Failure of the applicant to demonstrate that the pro@ect meets.
the standards set forth in this ordinance is reason to deny the
applicant's underlying application for a municipal approval.

F. Variance

For good reason, the municipality may grant a waiver of the
standards given in section V below. In each such case, the
municipality must make a report within 30 days to the county,
planning board, giving a full explanation of the nature of the
variance, and the reasons why it-was grantec.

SECTION V. STANDARDS

Each proposed project not exempted from the operation of this
ordinance shall meet the fo-1lowing storm drainage standards:

A. General Standards

The project plans submitted s-hall demo'nstrate careful
consideration of the general and specific concerns, values-and
standards of the municipal Master Plan and applicable county,
regional and state storm drainage control programs, anv County
Mosquito Commission control standards and shall be based on'
environmentally sound site planning, engineering and
architectural techniques.

B. Alternatives to Detention Basins

I.- It is not necessary that basic requirements be satisfied by
means of detention basins. Rooftop storage, tanks, infiltration
pits, dry wells, or gravel layers underneath paving, may be used
for the purpose, with appropriate consideration for length of
life and feasibility of-continued maintenance.

147

2. Non-structural management practices, such as'clust*er land use
development, op.en'space accTuisition, stream encroachment and
flood hazard controls should be coordinated with detention
requirements. Changes in land use can often reduce the scope and
cost of detention provisions required by means of appropriate
changes in runoff coefficients.

C. Specific Standards

(a) Flood and Erosion Control

A detention facility must accommodate site runoff generated from
2 year, 10 year and 100 vear 24 hour storms considered
individually (in each case a type III rainfall as defined in Soil
Conservation Service Publications). Runoff greater than that
occurring from the 100 year 24 hour storm will be passed over an
emergency spillway. Detention will be provided such that after
development neither the peak rate of flow from the site, nor the
total flow during the hour of maximum releases will exceed the
corresponding flows which would have been created by similar
storms prior to development. For purposes of computing runoff,
all lands in the site shall be assumed, prior-to development, to
be in good condition (if the lands are pastures, lawns or parks),
with good cover (if the lands are woods),. or with conservation
treatment (if the land is cultivated), regardless of conditions
existing at the time of computation.

(b) Water Ouality Control

In order to enhance water quality of storm water runoff, all
storm water management plans mus@ PrON7ide for the control of a
water qualitv desiqn storm. The water quality design storm shall
be defined as the one year frequency SCS Type 111 24 hour storm
or a 1-1/4 inch two hour rainfall.

The water quality design storm shall be controlled by one o f the
following practices.

1. In "dry" detention basins, provisions shall be made to insure
that the runoff from the water auality design storm is retained
such that not more than 90% will be evacuated prior to 36 hours
for all non-residential prolects or 18 hours for all residential
projects. The retention time shall be considered a brim-drawdown
time, and therefore shall begin at the time of peak storage. The
retention time shall'be reduced in any case which would require
an outlet size diameter of 3" or less4 Therefore, 3" diameter
orifices shall be the minimum allowed.

148

2. In permanent ponds or "wet" basins, the water-quality
requirements of this ordinance-shall be satisfied where the
volume of permanent water is at least three times the volume of
runoff produced by the water quality design storm.

3. Infiltration practices such as dry wells, infiltration
basins,-infiltration trenches, buffer strips, etc., may be used
to satisfy this requireinent provided they produce zero runoff
from the water quality design storm and allow for complete
infiltration within 72 hours.

(Note: Any municipality that wishes to allow for water quality
control techniques in addition to those described above shall
submit evidence of the effectiveness of the control techniaue to
the Department for review and approval prior to adoption of the
ordinance.]

(c) In all cases, multiple level outlets or other fully automatic
outlets shall be designed so that discharge rates from the
development for the design storms will not be increased from what
would occur if the development were-not constructed. Outlet
waters shall be discharged from the development at such locations
and velocities as not to cause additional erosion or cause
addition.'il channels downstream'of the development.

(d) Where the project coi@sist of two phases, (a) new construction
which requires provisions of storm drainage under the terms of
this ordinance and (b) repair or rehabilitation of structures and
surfaces which does not result in increasing the extent of
impervious areas or in rendering existing surfaces less pervious,
the detention requirements may be computed on the basis of phase
(a) exclusively.

(e) If detention basins or other detention facilities are
provided through which water passes at times other than following
rainfall, the municipal engineer should be consulted concerning
design criteria. It will be necessary for detention requirements
to be met, despite the necessity of passing certain low flows.
This applies to all on stream or on line detention basins.

(f) Outlets from detention facilities shall.be designed to
function without manual, electric, or mechanical controls.

149

(q) The retention of site runoff as required by this ordinance
will result in the accumulation in the detention basin of
sediment, including particulate.polluting substances, silt, and
debris. Provision must be made for periodic removal of
accumulated solid materials. ComDutations for storage canacitv
shall include estimates for one year's accumulation of solid
materials.

(h) Dams - Anv stormwater basin that impounds water through the
use of an artificial dike, levee or other barrier and raises the
water level five feet or more above the usual, mean low water
height when measured from the downstream toe-of-dam. to the
emergency spillway crest is classified as a dam and subject to
N.J.A.C. 7:20 the New Jersev Dam Safety Standards. All such
dams must be designed, constructed, operated and maintained in
compliance with the rules of N.J.A.C. 7:20.

M In many instances, the provisions of separate detention
facilities for a number of single sites may be more expensive and
more difficult to maintain than provisions of joint facilities
for a number of sites. in such cases, the municipality will be
willing to consider provisions of joint detention facilities
which will fulfill the requirements of this regulation. In such
cases, a properlv-planned staged program of detention facilities
may be approved by the municipality in which compliance with some
requirements may be postponed at early stages while.preliminary
phases are being undertaken and construction funds accumulated.
The necessary planning to facilitate such arrangements mav bp
accomplished by Phase !I planning under 'Provisions of N.J.A.C.
7:8.

D. Regional Storm Water Planning Areas

All proposed projects located in a designated regional planning-
area will be required to comply with the provisions of this
section.

(a) The proposed project shall include adequate onsite storm
water management controls to satisfy the requirements of section
C (b) - Water Ouality Control and must ac "comodate the site runoff
generated from the 2-year 24 hour storm such that the maximum
rate of runoff will not increase as a result of the proposed
developments..

150

(h) In lieu of providinq onsite control of the 10-year and
100-year storms, the proposed prolect shall contribute the
required fee towards the implementation the proposed regional
storm water control facilities. Such fee shall be based on the
increased volume of runoff resulting from the proposed
development under conditions of the 100-year 24 your Soil
Conservation Service Type III storm. The increased runoff shall
be determined by the Soil Conservation Service runoff -curve
number procedure utilizing the appropriate curve numbers from
Table A. The fee shall equal S per acre foot of increased
runoff volume.

(For example, if the predeveloped runoff volume produced by the
100-year storm was determined as 1.0 acre foot and the developed
runoff volume was determined as 3.0 acre feet, the fee will be
based on the change in volume, or 2.0 acre feet.)

The fee shall be paid to the designated regional storm water
planning group upon final planning board approval of the proposed
project.

E. Detention Facilities in Flood Hazard Areas

1. whenever practicable, develonments and their stormwater
detention facilities should be beyond the extent of the flood
hazard area of a stream. When that is not possible and detention
facilities are proposed to be located partially or wholly within
the flood hazard area (as defined by the New Jersey Division of
Tolater Resources, Bureau of Flood*Plain Management)-, or other
areas which are frequently flooded, some storm conditions will
make the facility ineffec@ive at providing retention of site
runoff. This will happen if the stream is already overflowing
its banks and the detention basin, causing the basin to.be filled
prior to the time it is needed. In such cases the standards-
established in these regulations will be modified in order to
give only partial credit to detention capacities located within a
flood hazard area. The credit will vary in a ratio intended to
reflect the probability that storage in a detention basin will be
available at the time a storm occurs at the site.

2. Detention storage provided below the elevation of the edge of
the 100 year flood plain will be credited as effective storage at
a reduced propo rtion as indicated in the table below:

151

TABLE A

Runof f Curve Nunibers for Selected Tand Use Descriptions
Hydrologic
Soil Groups

Industrial/Ccnitercial or Residential Categocies- A R r n

10% jg22rvious 90% lawnsingood condition 45 65 76 R9

20% to 80% 11 of It of 51 68 79 84
30% It 70% to is to 57 72 81 85
40% is 60% 63 76 84 87

50% 44 50%, It is 69 80 86 89

60% 4o% 74 R3 RR 91

-70% 30% It An A7 Q1 Q'A

80% 20%. 86 91 93 94
90% 10% 92 94 96 9
10% 90% woodland in good condition 32 59 73 79
20% It 80% to It it 40 64 76 81
30% It 70% to It of to 47 68 78 83
40% to 60% to 11 is is 54 72 81 85
.50% 50% $1 It Is to 62 77 88
.60%. Is 40% to '11 It of 69 81 87 90

70% Is 30% If It is so 76 85 90 92

to of Is of to
- 80% 20% 83 1 89 92 1 94
1-90% 10% of to II Is 191 94 96
To obtain CN values for other percentages use ar) arithmetic interpolation.

Si7e of Drainage Area*
Greater th&n
Elevation Less than Sq. Mi. 5-100 Sq. Mi. 100 Sq. Mi.

Less than 2' below 40% 65% 90%
Between 2' and 4' below 25% 50% 75%
Over 41 below 10% 25% 50%

* Area contributing floodwaters to the flood hazard area at the
site in question.

This effective detention storage will be required to provide for
drainage of the developed land in accordance with the criteria
already established in these regulations. However, the aross
storage considered for crediting will not exceed that which would
be filled bY runoff of a 100 year storm from the site.

3. As an alternative to approach 2 abovel- if the developer can
demonstrate that the detention provided would be effective,
during runoff from the 100 year 24 hour Type III storm, peaking
simultaneously at the site and. on the flood hazard area, his plan
will be accepted as complying with provisions of paraqraDh 2
above.

4. In making computations under paragraph 2 or 3 above, the
.volume of'net fill added to the flood hazard area portion,6f the
project's site will be subtracted from the capacitv of effeCtiVe
detention storage provided. (Net fill is defined as the total
amount of fill created incidental to the completion of the
prolect less the amount of excavated material removed during the
completion of the project, both measured below the elevation of
the edge of the flood hazard*area.)

5. Where detention basins are proposed to bQ located in areas
which are frequent*ly flooded but have not been mapped as flood
hazard areas, the provisions of either paragraph 2 or 3 will be
applied, utilizing the elevation of a computed 100 year flood.

6. Developers are also required to show compliance with the
Flood Hazard Areas regulations of the Department of Environmental
Protection.

153

F. Standards for Stream Corridor Protection (Recommended)

To the.extent practicable and consistent with other site planning
criteria, and with appropriate beneficial use of the site as a
whole, it is recommended that no alteration of the natural
terrain should occur and no impervious surfaces should be
located, within a stream corridor. The corridor should include
all flood plain areas, adjacent slopes of 1.1% or greater, and
contiauous areas where the depth of the seasonal high water table.
is one foot or less.

SECTION VI. SUBMISSIONS (Recommended)

The following submissions shall be required for each proposed
project subject to review under this ordinance. The applicant is
free to combine exhibits or otherwise consolidate the required
information, so long as all required information is clearly
presenteO.

A. Topographic Base Map

Topographic base map of the site, and extending a minimum of 200'
bevond the limits of the proposed development at a scale of I"
200' or greater, showing 2' contour intervals. The map shall
indicate at least the following: existing surface water
drainage, marshlands, outlines of woodland cover, existing
man-made structures, roads, utilities, bearing and distances of
property lines, and significant natural and man-made features not
otherwise shown.

R. Vicinity Map

Applicants must prepare a map at a scale of I" = 4001 or greater
on a paper print of the latest air photographs available, updated
in the field to reflect current conditions, showing the
relationship of the proposed development to significant features
in the general surroundings. The map must indicate at least the
following: roads, pedestrian ways, access to the site, adjacent
land uses, existing open space, public facilities, landmarks,
.places of architectural and historic significance, utilities,
drainage (including, specifically, streams and other surface
water shown on U.S.G.S. and soils maps), and other significant
features not otherwise shown.

154.

C. Environmental Site Analysis

A written and graphic description of the natural and man-made
features of the site and its environs. This description should
include a discussion of soil conditions, slopes, wetland!@, ..
vegetation and animal life on the site. Particular attention
should be given to unique, unusual, or environmentally sensitive
features and to those that provide particular opportunities or
constraints for development.

D. Project Description and Site Plan(s)

A mar) (or maps) at the scale of the topographical base map
indicating the location of proposed buildings, roads, parking
areas, utilities, structural facilities for detaining or
recharqincr stormwater and sediment control, and other permanent
structures. The map(s) shall also clearly show areas where
alterations in the natural terrain, cover, and grade are
proposed, and changes in natural cover, including lawns and other
landscaping. A written description of the site plan and
justification of proposed changes in natural conditions may also
.be provided,.

E. Water.Detention Facilities Map

The following information-, illustrated on a map of the same scale
as the topographic base map, shall be included:

(a) Total area to be paved or built upon, estimated land area to
be occupied by water detention facilities and the type of
vegetation thereon, and details of the proposed plan to contro-1
and dispose of surface water..

(b) Details of all water detention plans, durincr and'after
construction, including discharge provisions, discharge capacity.
for each outlet at different levels of detention and emergency
spillway provisions with maximum discharge capacity of each
spillway.

(c) Maximum discharge and total volume of runoff which would
occur from the project area before and after development for the
following storms:

(1) One and a auarter' inch of rainfall occurring within two
hours, or a one year frequency Type 111 24 hour storm.

(2) The specified design storms. (2 year, 10 year, and 100 year
24 hour SCS Type III.)

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The municipal official or board reviewing an application under
this ordinance may, in consultation with the municipal engineer,
waive submission -of anv of the above requirements when the
information requested 'is impossible to obtain or when it would
create a hardsliip on the applicant to obtain and where its
absence will not ma-teriall affect the review process-.
y

SECTION VII. MAINTENANCE AND REPAIR

Responsibility for operation and maintenance of detention
facilities, including periodic removal and disposal of
accumulated particulate material and debris, shall remain with
the owner or owners of the property with permanent arrangements
that it shall pass to any successive owner, unless assumed by a
government agency. If portions of the land are to he sold,
legally binding arrangements shall be made to pass the basic
responsibility to successors in title. These arrangements shall
designate for each project the propertv owner, governmental
agency, or other legally established entity to be permanentlv.
responsible for maintenance, hereinafter in this section referred
to as the responsible person.

Prior to'granting approval to any project subject to review under
this ordinance, the applicant shall enter into an agreement with
the municipality to ensure the continued operation and
maintenance of the detention facility. This aqreement shall be
in a form satisfactory to the municipal attorney, and may
include, but may not necessarily be limited to, personal
'guarantees, deed restrictions, covenants, and bonds. In cases
where property is subdivided and sold separatelv, a homeowner's
association or similar permanent entity should @e established as
the responsible entity, absent an aqreement by a governmental
agencV to assume responsibility.

In the event that the detention facility becomes a dancrer to'
public safety or public health, or if it is in need of
maintenance, the municipality shall so notify in writinq the
responsible person. From that notice, the responsible person
shall have fourteen (14) days to affect such maintenance and
repair of the facility in a manner that is approved by the
municipal engineer or his designee. If the responsible person
fails or refuses to perform such maintenance and repair, the
municipality may immediately proceed to do so and shall bill the
cost thereof to the responsible person.

156

SECTION IX. FEES (Pecommended)

In addition -to any'fee due to the municipality as a result of the
applicaAt's underlying application for a municipal approval,
there shall be due to the municipality at.the time of submission
of materials in support of this application-a fee as follows:

(a) for each 10,000 square feet to be graded or developed as part
of the project.

(b) This fee is an approximation of the estimated cost to the
municipality to have its professional staff and consultants
review the proposed project.

-SECTION X.. SEVE RABILITY

Should any section or provision of this ordinance be declared
invalid by a court of competent jurisdiction, such a declaration
shall not affect the remaining sections or provisions of th@is
ordinance which are herebv declared to be severable.

SECTION XI. EFFECTIVE DATF

This ordinance shall take effect upon final passage and approval
by the county plannina agency or water resources association as
appropriate or sixty (60) days after submission to said agency if
they fail to act.

157

DATE DUE
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GAYLORD No. 2333 1PRINTED IN U S.A.

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