[From the U.S. Government Printing Office, www.gpo.gov]
Municipality of Anchorauqu C @j ASTA L Z - - - INFORMATIONr C--,---,T,-4 FURROW CREEK R'hABBIT t CREEK DRAINAGE '11@@' T", TD 225 .A4 S F8 LL 1983 -J URS ENGINEERS Anchorage, Alaska 74 February 1983 FURROW CREEK-RABBIT CREEK DRAINAGE STUDY for Municipality of Anchorage Department of Public Works URS ENGINEERS The preparation of this report was financed in part by funds from the Of fice of Coastal Zone Management, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, administered by the Division of Community P 1 a n n i n g ,Department of Community and Regional Affairs. FURROW CREEK-RABBIT CREEK DRAINAGE STUDY PAGE NO. SUMMARY, CONCLUSIONS, & RECOMMENDATIONS CHAPTER ONE - INTRODUCTION Authorization . . . . . . . . . . . . . . . . . . 1-2 Scope of Services . . . . . . . . . . . . . . . . 1-2 Objective . . . . . . . . . . . . . . . . . . . . 1-4 Previous Studies . . . . . . . . . . . . . . . . 1-4 CHAPTER TWO - DESCRIPTION OF STUDY AREA Location & Boundaries . . . . . . . . . . . . . . 2-1 Climate . . . . . . . . . . . . . . . . . . . . . 2-1 Anchorage Bowl . . . . . . . . . . . . . . . 2-1 Furrow Creek-Rabbit Creek Area . . . . . . . 2-5 Geology . . . . . . . . . . . . . . . . . . . . . 2-6 Anchorage Bowl . . . . . . . . . . . . . . . 2-6 Furrow Creek-Rabbit Creek Area . . . . . . . 2-7 Physical Features/ Topography . . . . . . . . . . . 2-8 Groundwater . . . . . . . . . . . . . . . . . . . 2-9 Subbasin Delineation . . . . . . . . . . . . . . 2-10 Land Use . . . . . . . . . . . . . . . . . . . . 2-11 Existing Drainage Patterns . . . . . . . . . . . 2-2-11 General . . . . . . . . . . . . . . . . . . 2-211 East of New Seward Highway . . . . . . . . . 2-23 PAGE NO. New Seward Highway to Alaska Railroad . . . 2-27 West of the Alaska Railroad . . . . . . . . 2-29 CHAPTER THREE - PLANNING CRITERIA Introduction . . . . . . . . . . . . . . . . . . 3-1 Goals . . . . . . . . . . . . . . . . . . . . . . 3-2 Overview . . . . . . . . . . . . . . . . . . . 31 -2 Area-wide Goals . . . . . . . . . . . . . . 3-4 Specific Goals . . . . . . . . . . . . . . . 3-4 Street Drainage . . . . . . . . . . . . . . . . . . 11-9 Design Storm . . . . . . . . . . . . . . . . . . 3-11 Quantity . . . . . . . . . . . . . . . . . . . IZ-11 Quality . . . . . . . . . . . . . . . . . . 11-17 Water Quality . . . . . . . . . . . . . . . . . . . 3-25 208 Water Quality Management Plan . . . . . . 15-25 Alaska State Standards . . . . . . . . . . . 3-29 CHAPTER FOUR - EVALUATION OF EXISTING SYSTEM General . . . . . . . . . . . . . . . . . . . . . 4-1 Quantity . . . . . . . . . . . . . . . . . . . . 4-1 General . . . . . . . . . . . . . . . . . . 4-1 Subbasins A-M . . . . . . . . . . . . . . . 4-17 Quality . . . . . . . . . . . . . . . . . . . . . 4-31 General . . . . . . . . . . . . . . . . . . 4-31 Estimated Pollutant Loads . . . . . . . . . 4-33 Water Quality Effects and Co'ntrol Measures . 4-49 PAGE NO. CHAPIE1 FIVE - PRESENTATION OF ALTERNATIVES Introduction . . . . . . . . . . . . . . . . . . . 5-1 Description of General Alternatives . . . . . . . 5-1 General . . . . . . . . . . . . . . . . . . 5-1 Alternatives 1-6 . . . . . . . . . . . . . . 5-3 Overview of Recommended Alternatives . . . . . . 5-7 Individual Subbasins A-M . . . . . . . . . . . . 5-21 Implementation of Alternatives . . . . . . . . . 5-101 APPENDIX A - COMPUTER ANALYSIS . . . . . . . . . . . A-1 APPENDIX B - DISCUSSION OF ICINGS IN DRAINAGE SYSTEMS B-1 APPENDIX C - CAPITAL IMPROVEMENT COST ANALYSIS . . . . C-1 APPENDIX D - GLOSSARY OF TERMS . . . . . . . . . . . . D-1 REFERENCES . . . . . . . . . . . . . . . . . . . . . . E-1 LIST OF TABLES TABLE TITLE PAGE NO. II-1 SUBBASIN ACREAGE . . . . . . . . . . . . . . 2-11 11-2 LAND USE CLASSIFICATION CORRELATION . . . . 2-19 11-3 EXISTING LAND USE . . . . . . . . . . . . . 2-21 11-4 FUTURE LAND USE . . . . . . . . . . . . . . 2-22 III-1 STATE OF ALASKA WATER QUALITY STANDARDS . . 3-31 111-2 SURFACE WATER ANALYSIS . . . . . . . . . . . 3-37 IV-1 CAPACITY OF EXISTING SYSTEMS . . . . . . . . 4-3 IV-2 RECEIVING WATERS OF FURROW CREEK-RABBIT CREEK . . . . . . . . . . . . . . . . . . . 4-32 IV-3 POLLUTANT BUILDUP MATRIX . . . . . . . . . . 4-34 IV-4 POLLUTANT WASHOFF LOADS . . . . . . . . . . 4-37 IV-5 ESTIMATED POLLUTANT LOADING . . . . . . . . 4-39 1963 (Average) Summer, Existing Land Use . 4-39 1963 (Average) Summer, Future Land Use . . 4-41 1967 (Wet) Summer, Future Land Use . . . . 4-43 1969 (Dry) Summer, Future Land Use . . . . 4-45 V-1 SUBBASIN ALTERNATIVE EVALUATION . . . . . . 5-13 V-2 REGIONAL DETENTION VOLUME . . . . . . . . . 5-36 V-3 DESIGN PARAMETERS . . . . . . . . . . . . . 5-93 A-1 INFORMATION REQUIRED FOR SAM MODEL . . . . . A-2 A-2 SAM INPUT DATA FILE . . . . . . . . . . . . A-6 A-3a LAND USE DATA FOR STORM MODEL INPUT . . . . A-9 (EXISTING SYSTEM) A-3b LAND USE DATA FOR STORM MODEL INPUT . . . . A-10 (FUTURE SYSTEM ) A-4 STORM INPUT DATA FILES . . . . . . . . . . . A-12 TABLE TITLE PAGE NO. C-1 ALLIED PROJECT COSTS AS A PERCENT OF CONSTRUCTION COST . . . . . . . . . . . . C-2 C-2 UFC-Sl COST ESTIMATE . . . . . . . . . . . . C-6 C-3 UFC-S2 COST ESTIMATE . . . . . . . . . . . . C-7 C-4 UFC-S3 COST ESTIMATE . . . . . . . . . . . . C-8 C-5 UFC-S4 COST ESTIMATE . . . . . . . . . . . . C-8 C-6 UFC-Nl COST ESTIMATE . . . . . . . . . . . . C-9 C-7 UFC-N2 COST ESTIMATE ... . . . . . . . . . . C-10 C-8 UFC-N3 COST ESTIMATE . . . . . . . . . . . . C-11 C-9 MFC COST ESTIMATE . . . . . . . . . . . . . C-12 C-10 LFC COST ESTIMATE . . . . . . . . . . . . . C-14 C-11 LFC-Nl COST ESTIMATE . . . . . . . . . . . . C-14 C-12 SUBBASIN M OUTFALLS COST ESTIMATE . . . . . C-15 C-13 COST ESTIMATE SUMMARY . . . . . . . . . . . C-16 LIST OF FIGURES FIGURE NO. TITLE PAGE NO. ii-i VICINITY MAP . . . . . . . . . . . . 2-3 11-2 EXISTING LAND USE . . . . . . . . . . 2-13 11-3 FUTURE LAND USE . . . . . . . . . . . 2-15 11-4 FUTURE RESIDENTIAL INTENSITY . . . . 2-17 11-5 FLOW PATTERNS/SUBBASIN DELINEATION 2-25 III-1 RAINFALL INTENSITY/DURATION /FREQUENCY CURVES . . . . . . . . . 3-15 111-2 HISTORICAL LARGE EVENT STORM . . . . 3-19 111-3 DESIGN STORM . . . . . . . . . . . . 3-21 111-4 DESIGN STORM . . . . . . . . . . . . 3-23 IV-1 COMPARISON OF HISTORICAL SUMMER PRECIPITATION EVENTS . . . . . . . . 4-47 V-1 - V-13 SUBBASIN MAPS A THRU M CHAPTER 5 COMPOSITE SUBBASIN MAP BOUND IN BACK : i II I I I I I I I I I I 0 1 1 1 SUMMARY, CONCLUSIONS, & RECOMMENDATIONS w I I I i SUMMARY, CONCLUSIONS AND RECOMMENDATIONS The Municipality of Anchorage Department of Public Works, recognizing the need to improve and expand the existing storm- water drainage-related facilities in the south Anchorage areas of Furrow Creek and Rabbit Creek, authorized URS Company to prepare a comprehensive drainage study to meet the demands for the projected ultimate development of this area. The following paragraphs are a condensed review summarizing the major features of this drainage study, as well as stating specific conclusions and recommendations based upon the findings reached during the course of this study. SUMMARY The study area boundaries were established by the Munici- pality of Anchorage Department of Public Works. Topography was evaluated from existing Municipal contour maps. Existing land use patterns were determined from aerial photography. Existing drainage patterns within the study area resulted from a review of record drawings for subdivision plats, road improvement projects, storm drainage improvement projects and from site inspection. The existing storm drainage network was evaluated from a quality and quantity point of view through the review of various resource documents, computer simulation, public input, and field investi- gations. i An essential factor in the evaluation of existing and projected future systems in this storm drainage study is the prediction of land use patterns. Land use patterns used for the present were obtained from aerial photographs. The future land use was as published by the Municipality of Anchorage Planning Department. The existing and future land use patterns were used in the computation of runoff from the study area. Any deviation from this land use plan will de- crease the value of this study in direct proportion to the magnitude of those changes. Six alternative drainage/water quality control measures were formulated to meet the objectives of the defined goals. These alternatives were evaluated for each of the individual subcatchments within the thirteen subbasins. The criteria used to evaluate the alternatives and to size the proposed storm drainage networks were based upon future land use pat- terns, and storm drainage and water quality related goals of the Municipality of Anchorage. Population densities were based upon present and future population forecasts as published by the Municipal Planning Department. Planning criteria with respect to water quality objectives were as identified in the Anchorage 208 Water Quality Management Plan. Design rainfall storms were as identified from existing rainfall data information. A re- commended alternative (or alternatives) is presented for each of the subcatchments which best fits the qoals of the Municipality of Anchorage. Compiling the recommended alternatives, a comprehensive storm drainage plan resulted. Cost estimates were developed for the major revisions or expansions to the existing and for proposed major storm drainage facilities. CONCLUSIONS 1. The study area has a number of minor stormwater drainage collection facilities and in some cases, major stormwater drainage trunk systems which have been installed individually without considerations for an overall stormwater drainage network. 2. Presently, no water quality problems have been identified within the study area. Future stormwater runoff quality will not present any serious danger to the beneficial uses within the study area with minor exceptions to that of recreation and aesthetics in the Lower and Middle Furrow Creek segments. 3. The high percentage of localized flooding, inconvenience, safety hazards and maintenance problems associated with the storm drainage systems within the study area are the result of inadequate attention paid to the formation of ice and damage caused by ice within drainage structures. 4. Design, construction, and maintenance of storm drainage facilities within the study area have historically been directly associated with the design and construction of roadways and associated roadway improvements. 5. The areas which drain directly to Rabbit Creek and the areas flowing north out of the study area have minor and isolated stormwater quantity related problems but present no existing major stormW2ter/water quality related problems nor are any major quality problems projected for the future. 6. Areas within the Sunset Manor/Turnaoain Park subdivisions (Subbasin G) which drain directly to Turnagain Arm have isolated cases of localized ponding of stormwater runoff. These problems are related to the undersizing of outfall structures and associated embankment erosion. 7. The Upper Furrow Creek drainage area, areas east of the New Seward Highway, is a developing area which needs immediate attention to an overall storm drainage network in order for development to proceed at a reasonable rate. 8. The Upper Furrow Creek segment does not have an adequate trunk system nor associated collection systems to convey existing and future stormwater runoff to the Middle Furrow Creek segment. The major cause of the overloading of the existing structures in the Middle and Lower Furrow Creek drainage networks is the result of the increased urbanization pressures in the Upper Furrow Creek segement. 9. The Middle Furrow Creek segment, the area between the New Seward Highway and the Alaska Railroad, has inadequate capacity for present or future flows for its entire length. iv 10. The Lower Furrow Creek segment, the area west of the Alaska Railroad to Turnagain Arm, exhibits isolated cases where inadequate capacity exists for both present and future runoff volumes. These areas are generally associated with railroad and street crossings along the creek corridor. 11. The maintenance of future wetland areas in their natural state is important to meet the goals and objectives of this study. RECOMMENDATIONS It is recommended that the following steps be taken to improve the existing storm drainage network within the study a r e a 1. Development of a stormwater drainage ordinance within the Municipality of Anchorage. This ordinance should identify the drainage criteria, construction, and inspection and operation of drainage structures. Implementation of this ordinance should be the responsibility of the Department of Public Works. 2. Areas presently not covered by road service districts should be either formed into loca-'l road improvement service districts or annexed into the Anchorage Road Service Area. These service areas will allow the legal mechanism to improve roadway conditions and stormwater networks for the area. v 3. The construction of future storm drainage systems should be within public streets or right-of-ways or easements in con- junction with the expansion of the roadway network in the area. 4. A comprehensive stormwater drainage network should be designed and implemented per the recommendations for the individual subbasins and subcatchments covered by this report which would intertie all the isolated small trunk and collection systems and expand these systems into a comprehensive storm drainage network for the study area. 5. For areas of land identification for use as regional detention basins, the Municipality of Anchorage should purchase the land from the present land owner, thus ensuring its use as a detention basin. 6. The use of Level II control strategies as identified in the 208 Water Quality Management Plan should be implemented for the management storm drainage related facilities with respect to water quality. 7. The installation of two precipitation gauges within the study area. One gauge should be located west of the Old Seward Highway and one gauge should be located east of the New Seward Highway. 8. In Subbasins L, M and portions of G which drain directly to Turnagain Arm, the existing outfall pipes should be removed vi and installed with new outfall pipes with non-separating, non- leaking joints and designed to carry the identified capacity to avoid erosion of the bluff area. 9. In the Upper Furrow Creek segment, local depression and regional detention ponds should be incorporated into the drainage system throught the use of existing small depression areas and wetlands. As development pressures increase in Subbasins D, E, and F, a major stormwater trunk system network should be con- structed to convey the collective stormwater runoff to the Middle Furrow Creek segment. These trunk systems should follow the existing natural corridors and street patterns to the maximum extent possible. 10. A study should be initiated to evaluate the various methods of increasing capacity of existing storm drainage net- work for the entire Middle Furrow Creek corridor. The study should be initiated immediately as severe constrictions exist for both present and future projected flows. Also, because of the potential for development at the west side of the Old Seward Highway and the intersection of Huffman, a method to transfer the collected upstream portions of water through the Alaska Railroad track foundations should be actively investigated and implemented. 11. In the Lower Furrow Creek segment, the existing road and stream crossings should be increased to carry the flows identified. Also, portions of the Lower Furrow Creek segment immediately west of the Alaska Railroad should be increased to carry projected vii flows and alleviate a potential for severe embankment erosion for the Oceanview subdivision area. 12. All future designs and construction of stormwater drainage facilities within the study area should have methods for the control of ice formation and practical methods for maintenance crews to remove icing conditions at major street crossings. 13. In the design of future facilities, a preliminary feasibiity analysis should be performed by the developer and should be reviewed and approved by the Municipality. viii I -1 i I I I I I I I I I I I I I I CHAPTER ONE -1 INTRODUCTION w t CHAPTER 1 INTRODUCTION Anchorage is presently experiencing a high rate of growth. Areas which were previously undeveloped or contained limited development are now being impacted by the high growth rate resulting in rain and snowmelt Flooding, glaciation and erosion. These problems will continue to increase in severity as growth continues, unless a comprehensive storm drainage plan is imple- mented. This study analyzes the existing drainage system for current problem areas and predicts future system requirements within the study area of Furrow Creek and Lower Rabbit Creek. Information that was gathered includes existing and future land use, local hydrologic conditions, existing drainage facilities, sails and topography. This information was coded into the System Analysis Model (SAM) computer program to evaluate the hydrological and hydraulic response. The SAM output was then used in conjunction with information on future growth to establish drainage alternatives. Cost estimates were prepared for each chosen alternative. Pollutant load data was coded into the Storage Treatment Overflow Runoff Model (STORM) to compute the pollutant washoff loads and water quality effec- tiveness of each alternative. This report sets forth the results, conclusions and recom- mendations from the storm drainage analysis for the study area. Technical appendices have been included discussing the computer analysis and the problems and possible solutions of icing in culverts. AUTHORIZATION Recognizing the need to have a comprehensive storm drainage plan for the Furrow Creek-Rabbit Creek area, the Municipality of Anchorage authorized URS to conduct a storm drainage study for the Furrow Creek-Rabbit Creek area using the SAM and STORM computer models. This analysis has been completed in accordance with the terms of the Contract for Engineering Services by and between the Municipality of Anchorage, Alaska and URS Company, dated November 6, 1981. SCOPE OF SERVICES The scope of this study is as follows: 0 Collect, with the assistance of the Municipality of Anchorage, the existing data necessary to complete the study. Review existing storm drainage and water quality planning and design requirements and modify to the study conditions. Establish the hydrologic boundaries within the approximate boundaries of the study area and delineate and identify major subbasins and their receiving waters. 1-2 Establish subcatchments within each major subbasin. Select two single event design storms and one continuous period of average precipitation from spring break-up to winter freeze-up, using published available precipitation data. Develop, whenever possible, two to three conceptual drainage plans for each of the major subbasins. Simulate and evaluate the hydrologic and hydraulic response of the alternative plans to the selected design storms for each major subbasin using the Systems Analysis Model (SAM). Compute the pollutant wash-off loads and water quality effectiveness of the alternate plans over the selected continuous precipitation period. For each major subbasin use the Storage Treatment Overflow Runoff Model (STORM). Develop cost estimates for recommended plan. Provide the Municipality of Anchorage with input data for STORM and SAM computer models with an abstract outlining the procedure for the updating data file. Provide the Municipality of Anchorage with final computer output for each subbasin. Provide the Municipality of Anchorage with 1" 1000' scale maps, to be incorporated into the final report. 1-3 Arrange and conduct two community meetings with area residents to review alternatives and the final plan. OBJECTIVE it is the objective of this study to develop a comprehensive stormwater drainage and water quality plan for the Furrow Creek- Rabbit Creek area. This study will provide a basis by which the Municipality of Anchorage Department of Public.Works can make management decisions with respect to stormwater and water quality control measures for the area. As the study area grows in development, this plan can be used by both the public and private sector to implement an orderly development plan for the area. PREVIOUS STUDIES During the course of this study many resource documents were referred to for information. The following is a description of the information gathered from these documents. A summary list of references is located at the end of this report. 0 Comprehensive Land Use Plan - published by the Municipal Planning Department in September 1981. In March 1982 additional land use and residential intensity map revisions became effective. The March 1982 land use was used as the basis for future land use within the study area. 1-4 Anchorage Wetlands Management Plan - published by the Municipal Planning Department in October 1981. Supple- mental revisions were added in February 1982. In April 1982 the plan was passed by the Assembly. The plan identifies the permitted use of wetlands within the study area. Land use within this report conforms to the Wetlands Management Plan. Hillside Wastewater Management Plan published by Arctic Environmental Engineers and the Municipal Physical Planning Division in January 1982. The plan was passed by the Assembly in May 1982. The information contained in the plan was incorporated into the land use data of this study. Interim Snow Disposal Study - published by the Municipal Planning Department in January 1981. This study was used to identify the location of the existing snow disposal site within the study area as well as to gain information on the potential future snow disposal practices. Title 21 of the Anchorage Municipal Code - Land Use Regulation - became effective January 1, 1982. This document was used in identifying the local regulations for development. 1-5 0 208 Areawide Water Quality Management Plan, Anchorage, Alaska published in August 1979. The 208 Plan provided information on water quality within the Anchorage area. Metropolitan Anchorage Urban Study, Volume 7, Anchorage Area Soil Survey, prepared by the Army Corps of Engineers in 1979. Soils information for this study area was obtained from this document. 0 Hydrology for Land Use Planning: the Hillside Arej. Anchorage, Alaska, Open File Report 75-105 - prepared by the Department of the Interior, 1975. Provided information on the Hillside area hydrology. 0 1995 Employment Population and Land Use Forecasts prepared by the Municipal Planning Department in March 1977. Forecasts aided in establishing estimates of future conditions. The following documents were referred to in using the SAM computer model: Campbell Creek Drainage Basin, Task Memorandum Number Seven, Methodology Manual, CH2M-Hill, January 1979. 0 Wastewater Collection System Analysis Model (SAM), User's Manual, CH2M-Hill, June 1978. 1-6 Storage Treatment Overflow Runoff Model (STORM), User's Manual, U.S. Army Corps of Engineers, 1976. 0 Weather tape for 1963-1979, National Oceanic and Atmospheric Administration. Reports which were referenced as general information on stormwater studies included: Drainage Management Plan for Homer, Alaska, CH2M-Hill, August 1979. 0 Juanita Creek Drainage Plan, URS Engineers, 1977. 0 Stormwater Drainage Study for the City of Soldotna, Ted Forsi & Associates, December 1979. 0 Sand Lake Drainage and Water Quality Management Study, Quadra Engineering, August 1981. 0 Urban 5tormwater Management Special Report No. 49 - published by the American Public Works Association in 1981. A number of researched reports were used in preparing the appendix on icing in storm drainage facilities. These reports included: 0 "Solving Problems of Ice-Blocked Drainage Facilities", 1-7 in Special Report 77-25, August 1977, K. L. Carey, Cold Regions Research and Engineering Laboratory. a Soil Erosion and Sediment Control for Anchorage, Municipal Department of Public Works, December 1978. 0 "Insulated Roadway Subdrains in the Subarctic for the Prevention of Spring Icings", H. Livingston and Eric Johnson. 0 "Storm Drainage Design Considerations in Cold Regions", ASCE Conference Proceedings on Applied Techniques for Cold Environments, May 1978. 0 "Icings Developed from Surface Water and Ground Water", CRREL Monograph 111-D3, Kevin Carey, May 1973. "Hydrologic Effects on Frozen Ground", CRREL Special Report 218, S. L. Dingman, March 1975. Background information for this study was also obtained from: aerial photography 0 record drawings 0 flood plain insurance documents 0 field investigation 0 topographic maps public input 1-8 Alaska Department of Environmental Conservation - water quality regulations Alaska Department of Fish and Game fish and wildlife information 1-9 I i I I I I I I I I I I I 0 I I I CHAPTER TWO DESCRIPTION OF 15@@ L i .W k 3 I l I STUDY AREA Is CHAPTER 2 DESCRIPTION OF STUDY AREA LOCATION AND BOUNDARIES The study area is located in the southwesterly portion of Anchorage, Alaska and is comprised of approximately 3825 acres. The area is bounded as follows: to the north by Klatt and O'Malley Roads to the east more or less by Cange and Elmore Roads 0 to the south by Rabbit Creek Road a to the west by unnamed wetlands and Turnagain Arm The study area location and boundaries are identified in Figure II-1. It should be noted that while Rabbit Creek is a major stream which carries water from outside of the study area, only the subbasins which drain to Rabbit Creek within the study area boundaries are included in this study as per the contractual agreement. CLIMATE Anchorage Bowl Within the Municipality of Anchorage, the "Anchorage Bowl", is that area bounded by the Glenn Highway to the north, Potter Marsh to the south, the Chugach Mountains to the east and Turnagain Arm to the west. 2-1 le Lake Elmendorf Air Force Base Knik Arm Glenn H 3rd Ave. U) Point Woronzof 15th Ave. I- Northern Lights Blvd. Lake Hood Lake Spenard Tudor Rd. International Airport International irport Rd. Raspberry Rd. Kincaid Rd. cf) 3: (L 0 z cc 0 Dimond Abbott Rd. Campbell Lake O'Malley Rd. ................... Klatt Rd. ....... T ............... Huffman Rd. ................. 2 ................ ................ ....... @ .................................. :HHHEMMF ............. ... DeArmoun Rd. ......... .... ............ LEGEND R 01, 1 GrK. Rd N Turnagain Arm MILE r U 0 1/2 1 STUDY AREA WATER URS Engineers VICINITY MAP ka@' Alaska February 1983 Figure The relatively moderate climate in the Anchorage bowl is due to the surrounding mountains. These mountain barriers shield the region from the temperature extremes of the Alaskan interior and the heavy precipitation of regions along the Gulf of Alaska. Winters are not extremely cold with an average temperature from December through February of 140F. The summer growing season averages 124 days with an average temperature from June through August of 560F. The mean monthly temperature is about 3 5 a F . The mean annual precipitation in the region is 14.9 inches and 46 percent of the annual precipitation falls from July through September. The year's greatest monthly rainfall is 2.50 inches during September. The annual precipitation includes a mean snowfall of 70.2 inches and the greatest monthly snowfall is 15.1 inches during December. Prevailing winds in the Anchorage area are from the northeast and normally light. This phenomenon results from the fact that air movement is normally from the cold ice-capped mountains to the warmer Cook Inlet waters and that strong outside winds are blocked by the mountains. Furrow Creek-Rabbit Creek Area Within the study area temperature extremes are probably a few degrees greater and precipitation volumes somewhat higher 2-5 than the values presented for the Anchorage Bowl. The weather station for the Anchorage Bowl is located at the Anchorage International Airport which is about six miles from the study area. Weather at the airport station appears to be affected more by marine influences. Therefore, temperatures there are slightly warmer than those of the Furrow Creek-Rabbit Creek area. The proximity of the Chugach Mountains and their effects, such as strong winds, may have some impact upon the local weather in the study area. These conditions are the primary reason for the variance in climate between the local study area and the airport weather stations according to the National Weather Service. Although it was acknowledged during this study that the climate in the Furrow Creek-Rabbit Creek area is not the same as that of the airport weather station, no adjustments were made to the airport station data. The reason for this decision was that there was no longterm, reliable data available upon which to base an adjustment factor. Therefore, the climatological data used for this study is the data available from the Anchorage Inter- national Airport station. GEOLOGY Anchorage Bowl Within the Municipality of Anchorage, the "Bowl" is defined as the area bounded generally by the Glenn Highway to the north, Potter Marsh to the south, the Chugach Mountains to the east, and Turnagain Arm to the west. 2-6 Although glacial activity formed the physical features of the Anchorage Bowl area, the Cook Inlet Basin has been an area of low relief subjected to marine and continental deposition since Tertiary times. According to physical evidence, the basin was subjected to five distinct glacial movements during the Pleistocence times. Physical features formed during these times include the outwash plain, upon which most of Anchorage is located, the morainal hills, melt-water channels, and lakes and swamps. Soils in the Anchorage Bowl area vary from free-draining sands and gravels to highly impervious silts and clays. Vege- tation varies accordingly. Well-drained tracts are forested with evergreen and deciduous trees. Poorly-drained marsh lands are covered by mosses, sedges, grasses, and other marsh plants. Swamps deposited with water-saturated peat are located extensively throuohout the lowlands, The water-table depth in the planning area is relatively shallow and generally lies within 30 feet of the land surface. Furrow Creek-Rabbit Creek Area The study area is generally comprised of three types of geological surfaces. 1. Poorly sorted material deposited by glaciers 2-7 Almost half of the study area includes this kind of deposit which is marked by long ridges once signifying the margins of former glaciers. 2. Sand and gravel Sand and gravel deposited mainly by streams (particularly along Rabbit Creek) and along the stream channels cover at least one third of the study area. The sand and gravel are generally well-stratified and well-sorted, referring to the grain size similarity. The portion of the study area west of the railroad is also mainly sand and gravel. 3. Silt and clay Silt and clay deposited in former lakes and ponds in the lowlands (between lower Huffman Road and lower O'Malley Road) comprise a small portion of the study area. PHYSICAL FEATURES/TOPOGRAPHY The eastern limits of the study area lie in the foothills of the Chugach Mountains.- Along this eastern boundary the highest elevation is approximately 400 feet. From the eastern limits, the land slopes westward to Turnagain Arm. Two major collectors of the foothills runoff water are Furrow Creek and Rabbit Creek. Rabbit Creek, lying in the southern portion of the study area, commences east of the study area boundary and flows into Turnagain Arm. Furrow Creek also has its origin in the Chugach foothills. However, in the vicinity of the New Seward 2-8 Highway, Old Seward Highway and the Alaskan Railroad, the flow in Furrow Creek is interrupted by manmade obstructions. Downstream of the Alaskan Railroad the land again begins to drain to a central location and Furrow Creek is again recognizable. As with Rabbit Creek, Furrow Creek empties into Turnagain Arm. Mud flats extend along the entire length of the shoreline of the study area. West of Timberlane Road and south of Klatt Road the land becomes very flat until it attains a boggy quality. This bog, called the Klatt Bog, is identified in the map for Subbasin L, Figure V-12. GROUNDWATER Groundwater is primarily rain water and snow melt which has seeped into pores in the soil, rock and sediment, and includes all the water below the water table. Two principal groundwater systems exist for the Anchorage Bowl. An upper unconfined system (water table) is separated from a lower confined system by a continuous layer of clay. This separat ion is less distinct in South Anchorage (Furrow Creek-Rabbit Creek area) because of the glacial deposits and various levels of clay, sand and gravel. Information an groundwater and aquifers is readily available for Anchorage and its more densely populated areas but for the outlying parts of the Municipality, the data is more scattered and less complete. The following are some general observations about Anchorage 2-9 groundwater characteristics which were abstracted from several water study reports: 0 The chemical quality of groundwater in Anchorage is good to excellent. The temperature of confined and unconfined groundwater averages 370F between the surface and to a depth of 300 feet. 0 The estimated annual yield of groundwater in the Bowl is approximately 17 - 28 mgd. The hydraulic gradient of the groundwater closely con- forms with the regional topographic gradient. 0 The summer base flow of Furrow Creek is dependent for a large part on groundwater ex-filtration. SUBBASIN DELINEATION Using topographic maps as a basis the study area was divided into areas in which each area had a central point, or node, to which it drained. The boundaries of these basins were then modified to comply with existing drainage facilities such as ditches and culverts. Boundaries were also adjusted where physical features such as roads provided a barrier to the flow path; the barrier thus becoming a boundary. By this method the area was divided into thirteen subbasins, labeled A through M. Figure 11-5 identifies subbasin boundaries. Subbasins A, B, a'nd C drain to Rabbit Creek. The tributary area 2-10 to Furrow Creek within the study area is defined by lubbasins E, F, G, H and K. Subbasins I and J drain stormwater to the north, out of the study area. Subbasins L and M drain directly to Turnagain Arm. The acreage associated with each subbasin is listed in Table II-1. TABLE II-1 SUBBASIN ACREAGE Subbasin Area (acres) A 27 B 351 C 168 D 198 E 592 F 767 G 526 H 138 1 131 J 81 K 385 L 314 M 148 Total 3826 LANO USE Land use was identified for the present and future cases. The existing land use was determined from aerial photography and field observations. The projected ultimate land use is as published in the revised land use and residential intensity maps produced by the Municipality of Anchorage Planning Oepart- ment, dated March 1982. The existing and future land use is graphically depicted in Figures 11-2, 11-3, and 11-4. 2-11 OVALLEY R I ISO MEN F-1 RONNIE KLATT RD NEWER WOMEN., nompri MONTI t.1161h f103JAMER @@L N mom iimmiiimmmi MMMOMMOM, wommmosm- SOMEONE=! pt OR limossommuni Ir AMMER ON on In jA HUFFMAN R AME I !@v mom .... 0. 0 MOMMMI10"MME - "Z smog Milk, IS Mo Mo k IN NORTH .loon mom 100000- 1,AV 0 MILES 1/2 LOWLAND FOREST LOW DENSITY HIGH DENSITY CgF-F-K COMMERCIAL GRAVEL PIT STUDY AREA BOUNDARY w IS CLEARED PERVIOUS UJI cr_ BOGS AND MARSHES 0 _j UJI MULTI-FAMILY UPLAND FOREST SUBBASIN BOUNDARY INDUSTRIAL A-M DESIGNATED SUBBASIN AREAS EXISTING LAND URS Engineers Per aerial photographs USE MAP Anchorage, Alaska September 10, 1980 February 1983 Figurell-2 0WALLEY R ........... .............. ............ ........... ..... . ............ ............... . ........... ........ ....... ............. ......... ........... ........ KLATT RD ........-....... .......................... ........ w @.... ....... ........ F:::: 4 4 4 4 4 4 4 ...... 4 4 4 4 4 4 4 ................... ........... .......... ............ fifty ........ ...... ............. 4 4 4 ...... .... < 1 ...... ....... ...... A 4 .......... .............. ........ .. ........... L ................ :j@ ...... .......-......... ..................... I INK:11 ............ 4p.*.,::::::::::::::::::::::::::: ............................ 1:1410 .......... ...... ............... -HUFFMAN RD . ......... ...... ... ....... ........... .......... ........ ............ ........ 0w..: ........................ C R E* K:: ... .... - .... ...... ..... ::: ... -:: .... ........ .......... :: ::: ............ ...... .............. . ...................-...... .... ......................I......I............ .... ................. ............ ............. ...... ...... . ............. ...... ............ ........... ..................... NORTH ................... ......... .............. ...... ...... ................................. :::- :::a: *:::::: ........ ....I..... ::::: : *::::: :::: ..... -- ........ ............ -... . . ........ .. ..... .... .... ::: ..... : ... ............ ...... 0 MILES .............. .. . ........... .................... .. .... .................... IVIG :::: `E .......... .14V ... .................... .... .......... ...... ........ .......... @::::: .0 ..... ........ .... ..... ......... ........... . ............. *::::: ::::: ............... :.... ::: ............... ...... gt, it's .. .... #,of ...... ................. ............ .......... ........... .......... *if RESIDENTIAL ..... . . . ...... ........... ............ .... ................ ........... .... ............ .......... ...... ........... .. .. ....... PARKS/OPEN SPACE ......... ....... _EK ....... ....... ........ CIRF ......... ...... ...... M-M" STUDY AREA BOUN DARY .......... ... ........... PUBLIC LANDWINSTITUTIONS .... ....I........... ..... . .............. ................ ........... ............. FEIITJ-IIA ........ ............. .......... COMMERCIAL LU ........ ... ....... .... ....... ......... 0 .......... .......... COMM ERCIALIIN DUSTRIAL. -j LU INDUSTRIAL SUBBASIN BOUNDARY MARGINAL LANDS A-M DESIGNATED SUBBASIN AREAS URS Engineers As published by the FUTURE LAND X7 Municipality of USE MAP Anchorarme, Alaska Anchorage 3-82 February 1983 Figure 11-3 0WALLEY RD ........... .............. . . ......... ....... ............ KLATT RD ............... .... . A, .......... 10 ... ......* looligillis 911411 HUFFMAN . . .......... ....... ........ ..... .................... ................ .......... .............. ............ .............. ................ V4 ............. . ........................... ... . ...... .. ...... NORTH ::::::*@ ......... ........ ....... .......... ....... ....... .............. ........... ................. ........ ............ ....... ....... .... .. .. .............................. .. .............................. .. ..... ............................... .. ........... ............ ... .......... aw ......... ................. 0 MILES . . . . .. ................ DWELLING UNITS PER ACRE I"7=77= LESS THAN 1 STUDY AREA BOUNDARY 3 TO 6 0,i In these areas, densities to 10 DUAC may be allowed under controlled development requiring clustering of LLJ 7 TO 10 structures, internal circulation, water and sewerage cr availability, transition and buffering design, and site 0 plan review. U-1 TO 20 Special zoning limitation for transitioning and buffer. Ing design required. 21 TO 30 SUBBASIN BOUNDARY A-M DESIGNATED SUBBASIN AREAS As Published by the FUTURE U R S Engineers Municipality of RESIDENTIAL Anchorage, Alaska Anchorage 3-82 INTENSITY MAP February 1983 Figure 11-4 Land use classifications for the computer analysis required modification from those identified in Figure 11-3. The correlation between the land use classification for the computer and those of the Planning Department is presented in Table 11-2. TABLE 11-2 LAND USE CLASSIFICATION CORRELATION Computer Computer Planning Department Classification Code Land Use (Future) Comments Low density single family Less than 1 dwelling residential LD unit/acre High density 1-2 dwelling single family 3-6 dwelling units/ units/ac are residential HD acre also included (Planning Dept. does not address this density. Multi-family Greater than 6 It was assumed residential MF dwelling units/ that a single acre family lot would not contain less than 7000 sq. ft. per lot. Industrial IN Industrial plus 50'*0' of Industrial/ Commercial Commercial Co Commercial plus 50*10' of Industrial/ Commercial Lowland LF Public Lands/ For existing land Forest Institutions and use, the New Parks Seward Hwy was generally used as the division between upland and lowland forest with LF west of the New Seward Hwy. 2-19 TABLE 11-2 (continued) Computer Computer Planning Department Classification Code Land Use (Future) Comments Bogs and Marshes BM Marginal Lands Tide flats were not included in the computer analysis. Cleared Pervious UP Used solely for existing land use, indicating ground which has been cleared of vege- tative cover and does not have man- made structure an it. Upland Forest UF Used only for existing land use, generally identi- fied as east of New Seward Hwy. Gravel Pit GP Used only for existing land use to identify exist- ing gravel pits. Using the classifications given in the above table, the sub- basins were assigned values for the area in computer classification. Summaries of the land use in each subbasin for both the existing and future cases are presented in Tables 11-3 and 11-4. 2-20 TABLE 11-3 EXISTING LAND USE Land Use Classification (acres) Sub- Total basin LD HD MF IN CO LF BM UF UP GP acres A 27 27 B 306 45 351 C 139 8 21 168 D 27 131 40 198 E 280 287 25 592 F 350 21 50 71 191 84 767 G 279 101 130 4 12 526 H 62 20 56 138 1 37 49 45 131 3 7 25 39 10 81 K 71 65 12 ill 23 103 385 L 46 265 3 144 m 134 14 148 1594 477 217 36 105 190 341 584 195 87 3826 2-21 TABLE 11-4 FUTURE LAND USE Land Use Classification (acres) Sub- Total basin LD HD MF IN CO LF BM UF UP GP acres A 27 27 B 346 5 351- C 97 34 37 168 D 34 151 13 198 E 291 301 592 F 127 387 45 208 767 G 103 265 75 58 25 526 H 2 50 86 138 1 37 45 49 131 1 66 3 12 81 K 256 41 30 58 385 L 240 10 64 314 m 141 7 148 1025 1707 339 128 193 370 64 3826 2-22 EXISTING DRAINAGE PATTERNS General For simplicity in the following discussion of drainage patterns, the study area has been divided into three areas: east of the New Seward Highway, between the New Seward Highway and the Alaska Railroad, and west of the Alaska Railroad. These boundaries are the result of manmade structures (the highway and the railroad) which impede the flow of stormwater. Figure 11-5 depicts graphically the present general flow path of stormwater. East of New Seward Highway The portion of the study area lying east of the New Seward Highway can be divided into three parts based on drainage patterns. The approximate respective north/south limits of these three drainage areas are O'Malley and Huffman, Huffman and DeArmoun, and DeArmoun and Rabbit Creek Road. Between Huffman Road and O'Malley Road stormwater drains via overland flow and roads west and south to the north-east corner of the intersection of Huffman and the New Seward High- way. At present, water is detained at this location. However, the intersection of Huffman Road and the New Seward Highway is being upgraded. A culvert is being extended to the northeast 2-23 Q'MALLEY RD KLATT RD F -S/ L 10 K ORION HUFFMAN 10 A* 14ROW C 100 .0 10 .0 IIIIIIII tons IIIIIIII fill III go I0 Aile 1 111116 /019, 0 op G111V 0.0 10 * all# SO logo 1`10# *411 A DE ARL10i i Ry LLLLL v CgEEK C RP'Be@-v STUDY AREA BOUNDARY Lu BIT @@*,R D uj SUBBASIN BOUNDARY A-M DESIGNATED SUBBASIN AREAS FLOW PATTERNSi ;mv@-ID f URS Engineers SUBBASIN Anchorage, Alaska DELINEATION February 1983 Figure 11-5 corner of the intersection, 11 is anticipated that during the present construction the currently detained water at this location will be allowed to cross the New Seward Highway and continue its flow downstream. Stormwater flow between the approximate limits of Huffman and DeArmoun travels via road ditches and culverts and overland flow. In Turnagain View subdivision, a storm drainage system has been included in the construction of the subdivision. The travel path of Furrow Creek is not well defined east of the New Seward Highway. Flow from this area arrives at the southeast corner of the intersection of Huffman and New Seward Highway, and presently, does not traverse the New Seward Highway; rather, it is detained between the Frontage Road and the highway. With the completion of the new Huffman/New Seward Highway interchange flow will cross beneath the New Seward Highway and proceed west. South of DeArmoun stormwater flow is west and south to Rabbit Creek via overland flow in most cases, Rabbit Creek crosses the Old Seward in a set of twin 72-inch culverts. New Seward Highway to Alaska Railroad In the area lying between the New Seward Highway and the Alaska Railroad, the natural flow path is interrupted by manmade structures (roads, culverts, and ditches). This area is approx- imately 301% developed north of Huffman and about 90"0' south of Huffman. 2-27 Between Klatt Road and O'Malley Road the flow is to the west to the railroad via roads, and ditch/culvert systems. At the railroad flow diverts to the north crossing the railroad tracks outside of the study area north of O'Malley Road. For the area between Klatt Road and Huffman Road flow is west and south, traversing the Old Seward Highway and the Alaska Railroad approximately at Huffman. Flow is overland as well as via roads. Approximately 50% of the land area has been developed . Downstream of Huffman the flow path varies. Between Kruge and Huffman, the flow is generally west and north, crossing the Old Seward Highway and the Alaska Railroad near Huffman. The flow from this area drains to Furrow Creek. From Karen to Kruge and east*of the Old Seward Highway flow is west. Only in the vicinities of Kruge and Huffman do culverts exist for allowing an east to west flow path traversing the Old Seward Highway. Along the strip of area between the Old Seward Highway and the Alaska Railroad, flow is to the west, ponding at the foundations of the railroad tracks. Between George Bell Circle and the intersection of the Old and New Seward Highways, a seepage drainage system has been constructed along the railroad tracks, allowing drainage of the ponded areas on the upstream side of the tracks. Drainage from this seepage system is via outfalls into Turnagain Arm. 2-28 West of the Alaska Railroad North of Langnes Court and north of the portion of Klatt Road west of Toy Street stormwater drainage is to the northp exiting from the study area at Klatt Road. This land area is presently about 20"0' developed, with the undeveloped land being boggy. Bounded by Timberlane Road and Alaska Railroad to the west and east, respectively, the Langnes Court and Galleon/Lighthouse Streets to the north and south, respectively, this area constitutes the downstream tributary area of Furrow Creek. Development of the land area is approximately 50,05, with Furrow Creek being routed via culverts and greenbelts through the Oceanview subdivision. Furrow Creek exists at the southwest corner of this area to Turnagain Arm at Johns Park. South of Galleon/Lighthouse Streets and bounded by Timberlane Road and the Alaska Railroad to the west and east, respectively, the land area is 100"0' developed. This area drains via roads and underground drainage system to Turnagain Arm via outfalls. West of Timberlane Road the area is very boggy. Approximately one-third of this area has been designated as part of the Klatt Bog, and has the wetlands classification of "conservation- development with special considerations" which will limit its development. Flow is basically overland with two large drainage cuts existing to provide channels for discharge to Turnagain Arm. 2-29 m I I I I I I I I I I I I I m I m 0 1 w I I CHAPTER THREE PLANNING CRITERIA oz@@, a@ it I 0 CHAPTER 3 PLANNING CRITERIA INTRODUCTION This chapter contains topics which can be classified into two categories of criteria: conceptual and quantitative. The conceptual topics, planning goals and criteria for proper street drainage are presented first, followed by the quantitative topics of design storms and water quality criteria. The section on planning goals presents the fundamentals which formed the framework for decision making in the course of this study. Street drainage criteria presented herein was used in the critique of the existing system. Both sections, goals and street drainage provide criteria which will be useful in the design and construction phases of future drainage facilities within the study area. In order to proceed with the quantitative analyses in this study, it was necessary to establish the design storms used for the hydrologic investigation and the water quality parameters used as a basis for the evaluation of water quality. This information was used as input into the operation of the SAM and STORM computer models. 3-1 GOALS Overview The following paragraphs summarize the goals to be achieved through this planning process. This summary is a brief overview of the specific goals as outlined in the following paragraphs. Some of the more technical terms used in this section are dis- cussed and defined in the appendicies of the report. The study area as outlined in the report has been divided into thirteen subbasins labeled A through M. These subbasins are grouped in four general categories: 1) Subbasins A, B & C which drain to Rabbit Creek. 2) Subbasins I & J which drain out of the study area into the south Anchroage storm drainage study area. 3) Subbasins D, Eq F9 G (northern portion), H, and K which comprise the drainage network of Furrow Creek. 4) Subbasin G (southern portion), L, and M which drain directly to Turnagain Arm. De- pending an the natural topography, existing and future land use patterns, present manmade structures of each of the various subbasins and their drainage patterns, the goals for each of the subbasins will vary as to individual requirements and needs of each subbasin. It is the overall objective of this study to identify various alternatives for storm drainage control, both quantity and quality, and to recommend the best suited alternative for each particular subbasin/subcatchment to the goals and requirements of the Municipality of Anchorage Department of Public Works. 3-2 Of main concern is the identification of goals as they relate to the subbasins which comprise Furrow Creek. Furrow Creek has three different and distinct drainage patterns and land use patterns. These three distinct areas are: The Upper Furrow Creek segment, comprised of Subbasins D, E, and F, which is primarily the Furrow Creek subbasin east of the New Seward Highway; The Middle Furrow Creek segment, comprised of subbasins G and H, which is the area along Huffman Road between portions of the New Seward Highway and the Alaska Railroad tracks; and The Lower Furrow Creek segment, comprised of subbasin K, which is the area west of the Alaska Railroad through Johns Park and the outlet into Turnagain Arm. A similarity exists between the various segments of Furrow Creek and the other subbasins within the study area which are not a part of the Furrow Creek drainage. Subbasins A and B, which drain to Rabbit Creek, are very similar to the Upper Furrow Creek segment; Subbasins I and J, which drain north, exiting from the study area and entering the South Anchorage Drainage Study area, are very similar to the middle segment of Furrow Creek; Subbasins L and M, which drain into Turnagain Arm, and the southerly portions of subbasin G, which also drains to Turnagain Arm, are very similar in nature to that of Lower Furrow Creek. In the identification of goals for this study area, it was concluded that goals set for Furrow Creek could be common to the overall study area because of the various distinctions within Furrow Creek itself. Therefore, goals defined below for Furrow Creek are also 3-3 applicable to other portions of the study area similar to the respective segments of Furrow Creek. Area-Wide Goals Furrow Creek is to be preserved and enhanced as a valuable natural resource serving as arunoff corridor for stormwater. This creek serves as a natural drainage channel and is valuable open/greenbelt in an increasingly urbanized community. If maintained properly the creek increases the recreational and aesthetic values of the surrounding community. It also is a habitat providing food and shelter for numerous birds and small animals, and a few occasional moose. Specific Goals 1. THE FURROW CREEK CORRIDOR SHOULD BE PROTECTED TO THE GREATEST EXTENT POSSIBLE. The creek corridor is a strip of land of variable width on either side of Furrow Creek including the channel itself. it M contains those sensitive areas which if altered could seriously degrade the stream and/or cause nuisance or monetary damage to surrounding businesses and homes. Furrow Creek is recognized as a valuable natural resource that performs drainage and aesthetic functions, and provides a habitat for a variety of wildlife. New developments adjacent to the creek should leave a buffer strip on both sides of the channel adequate to preserve the drainage, habitat and aesthetic 3 @-4 function of the creek. This buffer strip should be wide enough to include the following elements when they are directly associated with the creek channel and if their disruption would degrade the creek: vegetation along the channel, wetlands, slopes over 15"0', or hiqhly erosive soils. If the creek corridor must be disturbed, special precautions must be taken to preserve or replant vegetation adjacent to the stream, prevent erosion and the transport of sediment into the creek channel, maintain water quality, maintain bank stability and free-flowing open channel. When new development or re-development occurs adjacent to the segments of the creek that have been previously degraded by land clearing and these modifications have created water auality or quantity problems at the impacted area or else- where in the system, creek channel and/or bank rehabilitation should be a condition of the development permit where feasible. 2. MAINTAIN THE NATURAL FUNCTIONS OF ALL ELEMENTS OF FURROW CREEK AND OTHER NATURAL DRAINAGE SYSTEMS WHERE POSSIBLE. Natural stream channels convey and store water as well as permit infiltration of surface water to groundwater reservoirs. These channels slow the rate.of flow of stormwater and delay flood peaks because of the high resistance of flow to rocky, grassy channels. All open channels should remain in use except for where road crossings are required to develop property . Such road crossings should be accomplished with a bridge or culvert of 3-5 adequate width and depth to permit free-flowing conditions during the spring runoff when portions of the culvert may be plugged due to icing . I Wetlands, ponds, and lakes store water and purify runoff water through the settling and biological action as well as provide groundwater recharge and wildlife habitat. Various wetlands in the basin as identified in the Wetlands Management Plan provide significant storage capacity for stormwater. The storage and recharge capacity of these wetlands should not be reduced through filling as the result of development. This policy is in accordance with the classification of "conservation" assigned in the Wetlands Management Plan. Presently, large portions of the study area are undeveloped. Existing topography is such that many small depression areas exist. These depressions become collection points for storm- water runoff and, as a result, decrease peak runoff quantities. The Municipality should encourage local developers to detain peak volumes of runoff throught the use of local detention methods to achieve runoff rates similar to natural conditions. Aquifer recharge areas are areas with porous soils where the underlying geology absorb, store and purify vast amounts of pr.ecipitation as ground water. The stored water is later released and fed into the creek from springs during dry weather when runoff no longer contributes to stream flow. Development in the portions of the basin which act as recharge areas, should 3-6 minimize impervious surfaces. Recognition of recharge basins as required for on-site detention facilities through the use of classified wetlands should be encouraged. 1, CONTROL MAXIMUM STREAM FLOWS WHILE MAINTAINING MINIMUM FLOWS DURING DRY WEATHER. Stream flow quantites and velocities in response to storms are to be moderated through the use of on-site controls and coordinated with in-stream measures. Stream controls to be provided will minimize flooding and private and public property damage, and will limit stream bank and bed erosion to non-destructive levels. Groundwater recharge is to be used where feasible to limit runoff and assure a source of flow during dry weather. 4. MAXIMIZE THE USE OF EXISTING RUNOFF CAPACITIES OF THE EXISTING STORM DRAINAGE NETWORK WHILE USING A COMPREHENSIVE TRUNK SYSTEM TO CARRY EXCESSIVE STOkMWATER FLOWS. Capacities of existing channels and/or pipes should be ident- ified. If necessary, these systems should be augmented with 2 comprehensive trunk system to carry peak flood flows. A comprehensive planned development of existing subbasins in conjunction with the proposed ultimate land use of the area should be required. Development should minimize the effect on surrounding 3 1-7 land owners both from a land use constraint and associated flooding problems. The layout of the trunk stormwater drainage paths should be along naturally low spots in topography or in conjunction with road improvements. The existing culverts, roadside ditches and pipes should be utilized as the local collection system to convey drainage from all the land within the subcatchment to the trunk system. If these alignments prove to be inconsistent with future development plans for the areas indicated, the alignment of the trunk system may be adjusted in the future to reflect a properly designed and constructed alternative. 5. MAINTENANCE OF WATER QUALITY OBJECTIVES AS DEFINED IN THE MUNICIPALITY OF ANCHORAGE 208 AREA-WIDE WATER QUALITY MANAGEMENT PLAN. The Anchorage 208 Water Quality Management Plan identifies the creeks and lakes within the Anchorage area as valuable recreational resources. These recreational resources can be impaired or even eliminated by poor water quality. The major causes of water quality degradation in the Anchorage Bowl are from: urban runoff, erosion which is primarily resulting from construction activities, runoff and percolation from snow disposal sites. In the evaluation of alternative solutions to drainage problems, the ability of the various alternatives to meet the 208 Water Quality Management Plan goals shall be included. 3-8 Wherever possible, non-structural methods shall be used to control water quality problems that have been identified, these include: land use controls, urban cleanliness, soil erosion and sediment control plans, and comprehensive snow disposal programs. STREET DRAINAGE Both State and Municipal roads exist within the study area. Therefore, design storm frequency criteria used by the State of Alaska Department of Transportation and Public Facilities (DOT/ PF) and the Municipality of Anchorage Department of Public Works were researched. The Alaska DOT/PF uses the following design storm frequencies: Type of structures Design storm frequency (years) Culverts or primary highways 50 Culverts on secondary highways 10 Storm sewers 10 Roadway ditches, stormwater inlets, gutter flow 10 The Municipal Department of Public Works uses a twelve-year design storm frequency (in conjunction with the ILLUDAS computer model) for subdivision work. For the large area involved in this study, the Municipality has chosen to use 10- and 100-year storm frequencies. The flows calculated using the 10-year storm frequency are to be identified throughout the study area. The flows generated using the 100-year storm frequency are to be identified at major road crossings and other important locations. 3-9 These storm frequencies coordinate well with the State DOT/PF storm frequencies detailed above. Once the design storm has been selected for particular type of street, it becomes necessary to identify the amount of ponding which is allowable without causing danger to public safety or damage to surrounding property owners due to flooding conditions. Presently, neither the Municipality of Anchorage Public Works Department nor the Alaska Department of Transportation and Public Facilities have definitive criteria with respect to the amount of allowable stormwater accumulation during times of initial stormwater runoff. The location and size of inlets is based upon the allowable stormwater spread or depth of flow in the streets. To properly identify the location of inlets for stormwater drainage in streets, the following is a synopsis of the criteria presently used by the Department of Transportation and Public Facilities: 0 at low spots and/or changes of grade a at all other points where necessary 0 ahead (upstream) of street intersections 0 at both sides of a street where water would flow towards the intersection 0 ponding of stormwater on curbed streets shall be limited to 1/2 of the outer lane As a guide, it is suggested that the design engineer selecting the location, size and capacity of gutter inlets, use an allowable depth of ponding between 6 to 8 inches. This has been identified as the most frequently used criteria by the American Public Works 3-10 Association (APWA) via a survey conducted in 1980 with respect to the stormwater management practice in the U.S. and Canada (Urban Stormwater Management Special Report No. 49, 1981). DESIGN STORM The design storms used in this study are not identical for the quantity and quality analyses. Therefore, in the following paragraphs the design storms are discussed for the separate cases of quantity and quality. Quantity Urban drainage facilities are designed to be capable of handling a storm runoff event of a certain recurrence frequency. Normally, for lack of long-term flow measurements, urban runoff events cannot be statistically defined. Therefore, in engineer- ing calculations of runoff, it is assumed that the frequency of occurrence of a rainfall event is identical to the frequency of occurrence of the resulting runoff. It is important to know that this assumption is not entirely correct, since a given storm may produce runoffs of various magnitudes and frequencies depending on the antecedent characteristics of the catchment. Ideally, the selection of the proper design frequency for drainage facilities is a compromise between periodic incon- veniences, and damages due to flooding and the cost of preventing this flooding. However, as the drainage system in this project area is not very sophisticated, it does not warrant a detailed 3-11 analysis of the relationship between the cost of flood protection and flood damage. Consequently, design periods as specified by the Alaska Department of Transportation and Public Facilities as well as by the trend set by other drainage basin planning studies are used. The desion event chosen for this project is a 10-year storm (i.e., a rainfall event of 10 years recurrence frequency) in conjunction with a snowmelt event. The subsurface condition during the design event is therefore considered to be frozen. For information purposes, the 100-year storm event Was also simulated. This data is presented in this report as information; it was not used herein for estimating capacities of facilities. It is desirable that the design storm event is to be derived from historical precipitation records. The nearest weather station which has long-term precipitation records is located at the Anchorage International Airport, about six miles northwest of the project basin. Precipitation date from this station were used to derive the design storm event. The International Airport station precipitation data were used as it is long-term. However, records from the Little Rabbit Creek weather station, located about 10 miles southeast of the study area at an elevation of 380 feet, show higher precipitation throughout the entire year than at the International Airport station. The cause of this variance in precipitation could be due to orographic effects of the Chugach Mountains. As a result 3-12 of the difference in precipitation data between the Little Rabbit Creek and International Airport stations, the precipitation data to be used for the study area possibly could be higher than the International Airport data. However, resulting from the quantity of data available from the Little Rabbit Creek station, it was not possible to accurately interpolate between the data for the two stations. It is recommended that precipitation gauges be installed in the study area to obtain site-specific data. In performing the runoff calculation, the SAM model requires the design storm be represented for input into the model by a storm hyetograph in which rainfall intensity varies with time as observed in nature. This design storm hyetograph was synthe- sized by distributing with respect to time the total volume of the design storm event to the entire storm duration. Profiles of the observed precipitation events as well as the intensity- duration relationships given by the intensity-duration-frequency (IDF) curves were used as references for establishing the design storm hyetograph. The IDF curves for the International Airport have been developed as part of the South Anchorage Drainage Study (Ref: CH2 M memo to Lee Browning, February 19, 1982). The curves, Figure III-1 were derived from 1953-1980 precipitation records and also included contributions from snowmelt during precipitation events in early spring. Six-hour duration of storm was chosen which is considered 3-13 1.6 1.5 1.4 1.3 1.2 1.1 1.0 .9 .8 .7 ECORRgNCI@ IN@ER .6 R VALS (years) .5 .4 1)0 -5 1 - 4t .3 --2 5 .2 @2 0 2 3 4 5 6 DATA BASE 1953-1980 REFERENCE: SOUTH ANCHORAGE DRAINAGE STUDY RAINFALL INTENSITY/ DURATION 15 @12 URS Engineers /FREQUENCY CURVES for the Anchorage International Anchorage, Alaska Airport Figure to be sufficiently long to ensure that the entire drainaqe basin contributes to the flow in the drainage system. Using a six-hour duration, 10-year storm, the IN curves give a constant intensity of 0.18 inches per hour. The total volume of the desiqn store is, therefore, 0.18 x 6 = 1.08 inches. Figure 11-2 shows the magnitude and the time distribution of several historical large storm events. Figure 111-3 shows the hourly distribution of the 10-year design storm and Figure 111-4 shows the 10-year and 100-year design storm hyetograph as they are distributed in a 10-minute interval for a total of six hour duration. QUALITY The section on design storms for the quantity analysis dealt exclusively with runoff quantities. When water quality aspects are to be considered, a special analysis of the precipitation data may be required. The frequency of occurrence of pollutant loads of a given magnitude differs significantly from the fre- quency of occurrence of the corresponding storm. The reason for this is that pollutant load produced by an event depends not only on the event itself, but also on the length of the antecedent dry weather period. Consequently, the design storm approach is rarely used in quality-oriented drainage design. Instead, a continuous simulation of runoff quality and the associated costs of quality control are more frequently used and these provide a good basis for selecting a cost-effective means of runoff quality control. 1-17 AUGUST 18-19, 1966 MAY 27, 1969 JULY 4, 1979 .30 Total Precipitation - .81 in Total Precipitation - .64 in Total Precipitation - .68 in .30 .20-- ...20 .10-- .10 '6 *2 4 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 0 2' 3 4 5 6 7 8 9 1b 0' 6 7 8 TIME (hours) HISTORICAL LARGE t URS Engineers EVENT STORM Anchorage, Alaska February 1983 Figure 111-2 10 YEAR STORM .4 DESIGN STORM AVERAGED FOR THE HOUR .30-- 0 U) C .2 0-- z ILI 0 2 3 4 5 6 TIME (hours) URS Engineers DESIGN STORM Anchorage, Alaska L February 1983 Figure 111-3 1.60 1.60 10 YEAR STORM 100 YEAR STORM 1001 1.10 1.10 .60 .6 .5 .32 .30 .28 .30 .28 .3 .27 .27 .27 .27 .26 .25 .25 .21 .23 .21 .21 .22 .23 .24 .24 .24 .24 22 20 .20 20 .20 .20 .20 .20 2-- .19 .19 .19 19 .2 .17 .16 .16 .16 .17 .18 .17 .18 .18 .15 .15 .15 .15 .14 .14 .14 .14 .12 .13 .13 .12 .10 .10 0 0 0 0 2 3 5 6 0 2 4 5 6 TIME (hours) W 27 27 28 23 24 @24 22 21 21 19 @O 20 20 1 17 1 7@ 1. 1 1@5 14 14 13 13 11 'l @12 '0 URS Engineers DESIGN STORMS Anchorage, Alaska February 1983 Figure 111-4 T The same snowmelt event selected for the quantity analysis was used to represent the spring break-up event for the quality analysis. This event assumes a roughly five months (150 days) of pollutant loading before wash-off. For the project area, two periods of time are critical to water quality. One is at the time of the spring break-up and the other is the period from spring break-up to winter freeze-up. Analysis of water quality was performed for each of the two critical periods. Three five-month historical events (May to Sept.), repre- senting three hydrological conditions - wet, average, and dry, were selected from the weather records to represent a variety of storm and pollutant wash-off events. These historical events are graphically presented in Figure IV-1. Hourly precipitation data from these events were inputed to STORM Model for continuous simulation of runoffs and resulting wash-off loads. The resulting statistics of pollutant loads and concentrations of the water quality constituents were used to determine the return periods and corresponding control measures. WATER QUALITY 208 Water Quality Management Plan The Anchorage 208 Water Quality Management Plan, mandated by Public Law 92-500 (the Clean Water Act) in conjunction with the State of Alaska's Water Quality Standards, sets forth the standards for water quality in the State of Alaska. It further 3 1-25 identified the management strategies needed to achieve or protect the water quality uses in the study area. The current Alaska Water Quality Standards considers seven beneficial uses of surface waters. Of these seven, only Class C (water contact recreation) and Class D (growth and propagation of fish and other aquatic life) are applicable to the Furrow Creek-Rabbit Creek study area. It should.be noted however, that the current water quality standards of the State of Alaska are undergoing review and revisions by the State. As a result of these revisions, a new list of freshwater uses will be developed for the Anchoraqe area, although the Municipality of Anchorage has not requested revisions to either Furrow Creek or Rabbit Creek water quality standards at this time. The Anchorage 208 Management Plan identifies three levels of control strategies which can be used to meet water quality standards. As taken from the 208 Management Plan the three levels of control strategies are: 0 Level 1 control strategy is the use of the existing programs and control measures. Through the use of existing comprehensive plans, zoning ordinances, sub- division regulations, and the review of various permit applications, the detrimental effect to water quality can be minimized. Level 1 controls will not meet the legal requirements established by the State of Alaska for water quality standards and will not protect beneficial uses. 0 Level 2 control strategy is a set of control measures 3-26 based upon existing municipal practices designed to reduce the four problem areas as identified in the 208 Plan: non-point pollution from urban runoff; erosion from construction sites; runoff from snow disposal site per- colation, and failure of on-site wastewater disposal systems. Level 2 control measures will result in water quality levels sufficient for all existing uses but will not satisfy all requirements of the State water quality standards. 0 Level 3 control strategy is a program which implements a system of interceptor storm sewers parallel to creeks and drainage swales and provides for the diversion of all stormwater runoff into Cook Inlet thereby decreasing the pollutant load in local receiving waters and meeting the State water quality standards. The .208 Management Plan recommends that, whenever possible, the Level 2 control strategy be used to meet water quality standards for the beneficial use of the surface water in the area. In reviewing the options available and the planning criteria designated to meet water quality standards, the follow- ing items outlined in the 208 Management Plan and updated to meet present needs have been identified as being most applicable for the Furrow Creek-Rabbit Creek study area. 0 Existing design criteria for stormwater controls should 3-27 be amended to include more emphasis on stormwater de- tention and water quality protection. Stormwater controls may include sedimentation-type de- tention ponds, infiltration ponds, drywells, and multi-use areas among others. The present stream corridor protection program should be continued with the following additions: - All inwater construction work should be discouraoed. That which is unavoidable should be conducted be- tween June 1 and July 15 to avoid conflict with spawning salmon (Rabbit Creek Only). - Any planned road crossing in the vicinity of salmon spawning areas should be accomplished by bridge wherever possible (Rabbit Creek Only). - Disturbed stream banks should be returned to a slope no greater than two horizontal to one vertical with replacement of natural vegetation. - Flood plain regulations should be amended so that new developments should utilize swales and ditches to the extent possible through provisions provided by subdivision regulations and design criteria and improvement standards. 3-28 Control of erosion and sediments at construction sites is recommended. Control of runoff from snow disposal sites should continue as outlined in the January 1981 disposal site study report. Alaska State Standards The two major uses of the water within the study area are fish and wildlife habitat and for recreational purposes. For these two uses of freshwater in the State of Alaska, the State standards for water quality criteria are as shown in Table III-1. Relevent water auality constituents include: dissolved oxygen (DO), pH, turbidity, dissolved inorganic substance, and toxic substances. Although there is little Water quality data ava.ilable in the study area, the existing and future land use with the majority of which is low density residential, would suggest little probability for the presence of toxic substances in the receiving waters. The only water quality recording station in the study area is at the mouth of Rabbit Creek. The water quality data collected from this station is presented in Table 111-2. Comparing the information in Table 111-2 to the standards in Table III-1 shows that the water at the recording station is meeting the water quality criteria. 3-29 Aw TABLE III-1 STATE OF ALASKA WATER QUALITY STANDARDS Fresh Water Growth and Propagation of Fish, Fresh Water Fresh Water Shellfish, Other Aquatic Life and Wildlife Parameter Contact Recreation Secondary Recreation including Waterfowl and Furbearers 1(B) W l(B) (ii) i(C) Fecal Coliform Bacteria Mean Value (1) 20 FC/100 ml 200 FC/100 ml 200 FC/100 ml 90 Percentile Value (1) 40 FC/100 ml 400 FC/100 ml 400 FC/100 ml Dissolved Oxygen (DO) Average Minimum (1) 4 mg/1 4 mg/1 4 mg/1 10 Percentile Minimum (1) 3 mg/1 Minimum For resident fish spawning waters, minimum DO 7 mg/1. Interstitial waters of the gravel bed minimum DO 5 mg/1. pH 6.5 to 8.5 5.0 to 9.0 6.5 to 9.0 Shall not vary more than 0.1 pH units from natural conditions. Turbidity Maximum Increase 5 NTU 10 NTU Maximum Secchi disk Depth Reduction When natural tur- 10/001. bidity is less than 50 NTU When natural tur- 10'10' or max. 20*'0' or max. Maximum turbidity increase. bidity is more 25 NTU in- 50 NTU in- 25 BTU above natural conditions. than 50 NTU crease crease (1) Based on a minimum of five samples taken over a period of 30 days. M as so so No '00 as am Am so on, -an so tim am aw go no TABLE III-1 (Continued) Fresh Water Growth and Propagation of Fish, Fresh Water Fresh Water Shellfish, Other Aquatic Life and Wildlife Parameter Contact Recreation Secondary Recreation including Waterfowl and Furbearers. 1(B) W 1(B) (ii) i(C) Temperature Maximum 30-C (86-F) Not applicable Natural +20C (3.60F) Max Rate 0.5*C (0.90F)/hr Dissolved Inorganic Substances Not applicable Not applicable TDS Max = Natural conditions +33"0' or 1500 mg/1 whichever is lower Sediment No measurable No measurable Sediment 4.0 mm max. increase increase increase 51% Max = 30"0' Toxic, deleterious Alaska Drinking Concentrations 0.1 96 hr LC50 value lowest measured Substances, including Water Standards shall not pose value for most sensitive biologically pesticides, related or EPA's Criteria hazards to important species. organic or inorganic For Water immediate contact material Not to exceed concentrations which im- part undesirable taste and order. Dissolved gas limit 110%' saturation. Color 15 units - true Free of substance In combination with durbidity limit color producing objection- reduction of compensation point for able color photosynthetic activity depth by 10/10' and Secchi disk depth by 10*% 0.01 96 hr LC50 value Petroleum Hydrocarbons No visible sheen No visible sheen Prefer continuous flow test - static animal fats and vegetable test acceptable. No deleterious oils chronic effects. Virtually free from floating oils. @fmlsw 800M@ 4MMOMM64mew mew aw @000000 TABLE III-1 (Continued) Fresh Water Growth and Propagation of Fish, Fresh Water Fresh Water Shellfish, Other Aquatic Life and Wildlife Parameter Contact Recreation Secondary Recreation including Waterfowl and Furbearers. 1(B) W 1(B) (ii) i(C) Radioactivity Shall not exceed Shall not exceed Shall not exceed limits in: limits in: limits in: * Alaska Drinking . Alaska Drinking Alaska Drinking Water Standards Water Standards Water Standards * 10 CFR 20 Federal . 10 CFR 20 Federal 10 CFR 20 Federal Regulations Regulations Regulations * National Bureau . National Bureau National Bureau of Standards of Standards of Standards Handbook 69 Handbook 69 Handbook 69 Total Residual Not applicable Not applicable Salmonoid Fish 2.0 ug/1 Chlorine 10 ug/1. M 406 an am- 4M -so M 4M 4w TABLE 111-2 SURFACE WATER ANALYSIS FOR RABBIT CREEK AT OLD SEWARD HIGHWAY Stream Temper- Color Pla ti- Specific Con- Total Date Discharge ature num-Cobalt ductance (micro- Dissolved Hardness Sampled (Ft3/s) (*C) Units PH m1ios at 25*) Solids (as CaCO 3 07-28-61 10 7.6 72 47 10-31-66 2.0 7.4 63 45 33 05-26-67 09-10-71 24.8 6.0 0 7.6 03 53 38 04-06-72 4.8 .0 0 7.0 100 60 44 04-10-73 a 5.0 .5 7 7.4 103 68 48 w Cal- Mag- Sodium Potas- Bicar- Chloride Sulfate Nitrate Fluo- Sit- Cad- Copper Iron Lead Manga- cium nesium sium bonate NO ride ica mium nese Ca Mg Na K HCO 3 C1 S04 as 3N F SID 2 Cd Cu Fe Pb Mn 07-28-61 11 1.2 1.1 .2 31 1.0 8.0 .7 .0 8.4 50u Du 10-31-66 10 1.9 .8 .0 30 .7 7.2 .9 .1 6.7 16OU Ou 05-26-67 15 3.0 2.5 .3 16 56 40 0 15 09-10-71 12 2.0 1.6 .3 37 .5 9.2 1.3 .1 7.7 10d 10d 04-06-72 13 2.5 2.3 .1 41 .7 12 1.3 .1 8.5 50t Ot 04-10-73 15 2.5 2.1 .4 47 3.0 11 1.9 .1 8.3 0 4 30d 3 10d SOURCE: USCS Open-file Report 75-105, Hydrology for Land-Use Planning: The Hillside Area, Anchorage, Alaska, Appendix A-5. e: estimated CHAPTER FOUR EVALUATION OF EXISTING SYSTEM CHAPTER 4 EVALUATION OF EXISTING SYSTEM GENERAL Information on the quantity and quality control of flow within the drainaqe system was gathered from resource documents, computer simulation, public input and field investigation. This information was compiled to obtain a clear understanding of the existing system and to make sound recommendations for the future system. This chapter presents the existing drainage system in terms of water quality and quantity. QUANTITY General In the first half of this chapter the quantity of flow will be addressed by Subbasin. Land use as discussed in this chapter is as presented in Chapter 2. The capacities identified for the existing system are based on Manning's formula using "n" values of 0.04 for greenbelts, 0.03 for ditches, and 0.024 for CMP culverts. Flow values for both the existing and future system are based on the 10 year storm described in Appendix A, Computer Analysis. It is possible to subdivide the study area into large areas which presently contain the same general type of drainage facility. The land lying north of Huffman Road consists generally of a ditch system. Between Huffman and DeArmoun Roads, the area is mainly curb and gutter west of Pintail Street, and a ditch system east of Pintail Street. South of DeArmoun Road ditches serve as 4-1 the drainage system. It should be noted that one of the basic problems of the existing system is the lack of planned regional drainage facilities. The majority of the present systems are isolated and sized for local use and do not interlink to form a trunk system. In the analysis of the existing system, a general trend was found regarding the culvert versus ditch size. Culverts were found to be of a significantly lower capacity than the adjoining ditch. At high flows, it is feasible that the ditch flow would surcharge the culvert and cause a culvert washout as well as local flooding. Recommendations for correction of t"his situation at key locations are presented in Chapter 5. Another observation made in the field while gathering existing system information was the present state of culvert maintenance. Culverts were found to be up to one-third full of debris and rocks, especially on the upstream end of the culvert. If such lack of maintenance is to continue, the resulting reduction in pipe capacity must be taken into consideration during the final design of future facilities. Throughout the quantity evaluation section of this chapter, reference is made to Table IV-1, "Capacity of Existing Systems", which is on the following pages. 4-2 mom TABLE IV-1 CAPACITY OF EXISTING SYSTEMS APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (ft/ft) -(ft) (cfs) TYPE A Rabbit Ck Rd Elmore Old Sew. Hwy 0.065 * 340 70 Ditch B Old Sew. Hwy North South Twin 72 0.05 * 500 500 each CMP @ Rabbit Ck Rd 1100 Combined Culvert C Frontage Rd E. 144th Porcupine 0.033 * 1350 50 Ditch New Sew. Hwy E. 144th Porcupine 0.033 * 1350 450 Ditch Frontage Rd Porcupine Rabbit Ck 0.037 * 1700 30 Ditch E. New Swd. Hwy Porcupine Study Bdry. 0.058 * 1950 600 Ditch De Armoun East of Staysail New Sew. Hwy 0.032 * 2200 50 Ditch Westwind Capstan De Armoun 15 0.0052 550 3 CMP and 0.012 Frontage Rd 144th De Armoun 0.002 * 1350 15 Ditch New Sew. Hwy De Armoun 144th 0.004 * 1350 150 Ditch Frontage Rd Tradewind De Armoun 0.006 * 1900 20 Ditch New Sew. Hwy Tradewind De Armoun 0.032 1900 450 Ditch (APPROX) an @ so @ M @ @ @ @ TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (ft/ft) (ft) (cfs) TYPE D South of Seawind Pintail Spinnaker 0.013 * 800 50 Greenbelt Spinnaker South North 18 0.004 60 4 CMP South of Seawind Spinnaker Westwind 0.017 * 1550 60 Greenbelt Westwind East West 18 0.006 60 4 CMP Westwind Spinnaker Greenbelt 18 0.004 500 4 Subdrain CMP West of Westwind Westwind Tradewind 0.026 700 950 Greenbelt Tradewind Greenbelt Frontage Rd 24 0.006 and 1100 10 CMP 0.012 Frontage Rd Tradewind North 24 0.006 220 10 CMP Frontage Rd CMP Steeple 0.008 1800 30 Ditch New Sew. Hwy Tradewind Steeple 0.014 1800 300 Ditch Legacy Vern Frontage Rd 10,15 0.05 160 4 CMP Hamilton Park Sub. Sue's Way Frontage Rd 15 0.02 850 4 CMP E Legacy Biscayne Lake Otis 18 0.013 750 1-0 CMP Lake Otis Westwind Legacy 18 0.023 150 10 CMP Lake Otis Legacy Nancy 18,21 0.003 and 550 5 CMP 0.007 North of Sue's Nancy Steeple 21 0.015 550 10 CMP Way Huffman Cir. Steeple Back of Cir. 21,24 0.007 and 450 20 CMP 0.002 Frontage Rd Steeple North of Steeple 0.017 1000 40 Ditch Frontage Rd North of Steeple South of Huffman 0.012 600 30 Ditch Frontage Rd South of Huffman Huffman 0.011 600 30 Ditch New Sew. Hwy South of Huffman Steeple 0.013 1700 300 New Sew. Hwy South of Huffman Huffman 0.019 700 350 (APPROX) TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (ft/ft) (ft) (cfs) TYPE F Huffman Bragaw Gander 0.037 * 4200 60 Ditch Huffman Gander Lake Otis Pkwy 0.023 * 1300 40 Ditch Huffman Lake Otis Pkwy Gregory 0.032 * 2000 50 Ditch Huffman Gregory E. New Sew. Hwy 30 0.003 500 10 CMP Huffman Northeast E. New Sew. Hwy 8 0.02 150 2 Perf. Pipe Huffman E. New Sew. Hwy W. New Sew. Hwy 30 0.013 240 25 CMP Huffman Northeast W. New Sew. Hwy 30 0.012 350 25 CMP Frontage Rd Chelea O'Malley 0.032 1800 50 Ditch Frontage Rd South of Chelea Chelea 0.038 200 60 Ditch Frontage Rd South of Chelea North of Klatt 0.016 400 40 Ditch Frontage Rd South of Klatt North of Klatt 0.010 and 1200 50 Ditch 0.06 Frontage Rd Midpoint between South of Klatt 0.06 900 70 Ditch Klatt & Huffman Frontage Rd North of Huffman Huffman 0.002 400 15 Ditch New Sew. Hwy Chelea O'Malley 0.026 * 1950 400 Ditch .New Sew. Hwy Chelea South of Klatt 0.025 * 1000 400 Ditch New Sew. Hwy South of Klatt North of Hu f fman 0.039 500 500 Ditch New Sew. Hwy North of Huffman Huffman 0.008 800 200 Ditch O'Malley Cange Reader 0.045 * 4000 60 Ditch O'Malley Tract A Reader 0.068 * 1300 90 Ditch O'Malley Tract A New Sew. Hwy 0.095 * 1300 90 Ditch (APPROX) @@ @ @ @@ M @@M TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM_ TO (inches) (ft/ft) (ft) (cfs) TYPE G Jarvus Karen Old Sew. Hwy 15 0.004 * 1300 2 CMP Hace Kruge Harding 12,15 0.003 and 800 2 CMP 0.004 Harding Hace Old Sew. Hwy 15 0.015 500 4 CMP South of Kruge Troy Hace 15 0.004 350 2 CMP Old Sew. Hwy Karen New Sew. Hwy 0.019 * 3150 90 Ditch Old Sew. Hwy Karen North of Dare 0.020 * 1900 90 Ditch Old Sew. Hwy West East 24 0.017 150 20 CMP @ Dare Old Sew. Hwy Dare Harding 0.009 1800 60 Ditch Old Sew. Hwy East West 24 0.007 150 10 CMP @ Harding Old Sew. Hwy East West 24 0.007 * 200 10 CMP @ Harding Old Sew. Hwy North of Harding Harding 0.004 * 1300 40 Ditch Old Sew. Hwy North of Harding Huffman 0.009 * 1100 60 Ditch Old Sew. Hwy Two Below-Huffman 24 0.007 * 150 10 CMP Alaska R. R. Hancock Rabbit Ck Rd 12 0.01 * 5100 2 Subdrain Alaska R. R. Hancock Rabbit Ck Rd 12,18,36 0.01 * 15 Crossdrain Jarvi @ Gwenn 12 0.003 200 2 Outfall DIP Alaska R. R. 36 0.01 * 200 40 CMP South of Huffman Huffman New Sew. Hwy Hace 36 0.006 500 30 CMP Huffman Hace East of Old 42,48 0.005 and 1500 60 CMP Sew. Hwy 0.008 Huffman East of Old West of Old 48 0.002 250 30 CMP Sew. Hwy Sew. Hwy (APPROX) TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (ft/ft) (ft) (cfs) TYPE H, North of Huffman East of New West of Old 24 0.010 * 300 20 CMP Sew. Hwy Sew. Hwy Old Sew. Hwy Klatt Huffman 0.016 * 2650 80 Ditch I Old Sew. Hwy O'Malley 112th 0.003 1000 30 Ditch Old Sew. Hwy Klatt 112th 0.008 1600 60 Ditch O'Malley Old Sew. Hwy New Sew. Hwy 0.012 1100 30 Ditch O'Malley Old Sew. Hwy East of Old 0.040 600 60 Ditch Sew. Hwy O'Malley Alaska R. R. East of Old 0.029 350 50 Ditch Sew. Hwy J Johns Rd @ Two 0.01 * 60 2 Crossdrain Deerfield 12 CMP Klatt Alaska R. R. East of Ellen 0.014 * 3100 30 Ditch Klatt Mary East of Ellen 0.032 * 500 50 Ditch Klatt Mary Timberlane 0.032 * 500 50 Ditch (APPROX) @m M M M M M M TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (rt/ft) (ft) (cfs) TYPE K Johns Rd Klatt Huffman 0.019 2600 200 Ditch Johns Rd Langnes Huffman three 0.01 * 60 2 each Crossdrain 12 CMP Johns Rd Huffman Galleon 0.024 1050 200 Ditch Johns Rd Huffman Galleon 12 0.01 * 60 2 each Crossdrain CMP West of Alaska R. R. and South 0.005 * 300 150 Flow from pond of Huffman East of Beach- 0.01 * 300 250 Flow to Huffman comber @ Huffman CMP Huffman East of West of 36 0.003 120 20 CMP Beachcomber Beachcomber Beachcomber Huffman Breakwater 12 0.001 500 1 CMP Clipper Ship Ct. North South 36 0.005 85 30 CMP Mariner Beachcomber Clipper Ship 18 0.006 280 4 CMP Johns Rd East West 24 0.026 40 20 Crossdrain CMP North of Galleon East of Clipper Clipper Ship Ct. Mariner 0.013 5.30 550 Greenbelt Ship Ct. South of Mariner Mariner Johns Rd 0.006 950 400 Greenbelt (APPROX) M M M MM mmmmm TABLE IV-1 CAPACITY OF EXISTING SYSTEMS (CONTINUED) APPROX. ESTIMATED Sub- SIZE SLOPE LENGTH. CAPACITY SYSTEM BASIN STREET FROM TO (inches) (ft/ft) (ft) (cfs) TYPE L Klatt Timberlane Toy 0.018 * 2600 40 Ditch Klatt Toy Victor 0.006 * 3300 20 Ditch Victor Klatt Turnegain Arm 0.011 * 1300 100 Drainage Cut West of Victor North of Klatt Turnagain Arm 0.010 1250 100 Drainage Cut M Johns Rd @ 18 0.01 * 300 5 Outfall CMP High View Oceanview @ North South 18 0.01 * 300 5 Outfall CMP Admiralty Oceanview @ North South 18 0.01 * 300 5 Outfall CMP Gulf (APPROX) SUBBASIN A Subbasin A (Figure V-1) has a southwesterly slope of 5-7,'0' to its outlet at Rabbit Creek. Drainage within the subbasin is overland with ditch Flow along roads. Along the north side of Rabbit Creek Road, the ditch capacity is approximately 70 cFs, as shown in Table IV-1, while the estimated design Flow From Table V-3 is much lower, indicating that the ditch is of adequate capacity. The existing and Future land use classifications are both low density single family residential (LD). As a result of the land use classification and topography of the land it.is antici- pated that property loss due to flooding would be minimal in this subbasin. SUBBASIN B Subbasin 6 (Figure V-2) slopes southward at a rate of 4-7.'a' to Rabbit Creek, indicating good overland drainage. Through a number of minor roads with ditches exist within the subbasin, the roadway drainage ditch system is for local collection and is not intertied to form a trunk system. Land use for the present and future as identified in Tables 11-2 and 11-3 is almost 100"Q low density residential (LD), indicating that property loss in the case of flooding during a large storm would be minimal. Care must be exercised during construction not to encroach upon the flood plain of Rabbit Creek. 4-17 SUBBASIN C Subbasin C (Figure V-3) flows westward to the New Seward Highway. The collected flow at the highway then travels south along the Frontage Road and the New Seward Highway to Potter Marsh. Capacity of these ditches along New Seward Highway and the Frontage Road are given in Table IV-1. Present drainage facilities in Subbasin C are ditches, pipes and overland flow. Because of the existing low levels of development in the subbasin, the drainage systems in use are local, and not inter- connected to form a trunk system at present. As a result of the localized drainage systems, ponding is prevalent within the subbasin. In the Turnagain View Subdivision a drainage problem exists in the vicinity of the intersection of Westwind Drive and Capstan Drive. During periods of high runoff, water flowing overland to the north, reaching the houses on the south side of Capstan Drive, traverses the land and enters th6 drainage system on Westwind. During this study the manhole on Westwind immediately south of Capstan has been observed surcharging. Capacity of the Westwind storm drain was estimated at 3 cfs (Table IV-1), much less then the design flow presented in Table V-3. Land use within the subbasin is presently low density single family residential or undeveloped. Future land use indicates high density single family residences will comprise approximately 20'10 of the land use within the subbasin with the remainder continuing 4-18 in the present land use. SUBBASIN D Subbasin D (Figure V-4) flow is westward to the New Seward Highway and Frontage Road, at which point the flow changes its direction and travels north. Within the subbasin the storm runoff paths are: overland, via curb and gutter, pipes, and greenbelt. In subcatchment 45, the most easterly subcatchment, the land is presently undeveloped or contains low density residential. However, future land use indicates that the entire area will be developed with half of the area in low density single family residential classification and the other half of the area in high density single family residential classification. Because of this impact the present overland flow cannot continue. Addition- ally, downstream facilities must be sized to handle the routed flow. Subcatchments 46 and 47 are high density single family residential, and have a storm drainage network consisting of 1811 cross drains under Spinnaker and Westwind and a greenbelt traversing the subcatchments. At Tradewind the greenbelt ends and a 2411 underground storm drain system extending from east to west on Tradewind allows storm flow to travel to the Frontage Road. Flow collected on the streets travels via curb and gutter to the cross drains at Westwind and Spinnaker. 4-19 The 18" cross drains at Spinnaker and Westwind have a estimated capacity of 4 cfs each. However, to handle the design flows identified in Table V-3, a much larger capacity is needed. The 24" underground storm drain on Tradewind is also of insufficient volume for projected future flows. Table IV-1 indicates a capacity of 10 cfs, much less than the design flow from Table V-3. In subcatchments 48, 49, and 50, flow travels via curb and gutter ditch and pipes, arriving at the Frontage Road. At the Frontage Road, ponding occurs both on the east and west side of the road. Ponding also occurs on the east and west sides of the Frontage Road in subcatchment 47. The estimated Frontage Road ditch capacity ranges between 10 and 40 cfs and portions of this ditch are unable to handle the design flows of Table V-3. Within Subbasin D, the overall need is to provide an inte- grated drainage system with the capability to handle regional needs and to avoid local ponding problems. The area will continue to develop, ultimately planned for approximately 75'/0' high density single family residential and 25'/0' low density 'single family residential. Much property damage could result from flooding during major storms, if no action is taken for future require- ments. SUBBASIN E Stormwater runoff through Subbasin E (Figure V-5) flows 4-20 from e@st to west in two drainageways which converge just north of Huffman Circle. The most northerly of the two branches is considered to be Furrow Creek. From this location the flow is northwest to the intersection of the New Seward Highway and Huffman. Drainage through the subbasin is accomplished via overland flow, curb and gutter, roadway ditch, pipes, channelized flow and greenbelt. In the southeast portion of the subbasin are subcatchments 60 and 66. At present this area is undeveloped and flow is overland. However, future land use projects that the area will become developed, approximately 60*10' as high density single family residential and the remainder as low density single family residential (LD). It is therefore necessary in the design of such future developments to include an adequate storm drainage network. Subcatchments 61 and 62 are in the northeast portion of the subbasin. In this area the present land use is low density single family residential and the future planned land use is the same. Flow through this area is overland and ditch. In the southcentral portion of Subbasin E are subcatch- ments 67 and 68 which form the northernly portion of Turnagain View Subdivision. This area is developed and flow travels primarily via curb and gutter and overland. Flow is presently primarily overland in subcatchments 64 and 65. However, this area will be developed in the future for 4-21 low and high density single family residential. Therefore, a drainage network for this area must be addressed during the design phase of development for the area. This network must also handle the upstream contribution from subcatchments 60 and 66. Flow from subcatchments 61 and 62 is to subcatchment 63. Downstream of subcatchment 63 is subcatchment 70 and 71. All three subcatchments (63, 70, and 71) are presently classified as low density single family residential. In the future it is planned that development will increase the density to high density single family residential. In this area flow travels by ditch, channelized flow and overland flow. Furrow Creek traverses these three subcatchments. The curb and gutter system of subcatchment 69 routes the flow to the underground pipe system which ranges in size from 1B" to 24". This pipe system has a present estimated capacity of 5-10 cfs (Table IV-1) which is much less than projected future flows as identified in Table V-3, indicating that the present p.ipe is inadequate in size. In the northwest portion of the subbasin is located sub- catchment 72. In-this area the flow is overland and channelized as well as routed via ditches. This subcatchment is presently undeveloped. However, future land use planning is for high density single family residential. During development of the area, drainage facilities able to handle the flows of Table V-3 must be constructed. 4-22 In summary, for Subbasin E, attention must be given to providing a regional drainage system of adequate capacity to handle the flows of Table V-3. Within Turnagain View Subdivision, inadequately sized facilities must be augmented to provide sufficient capacity. SUBBASIN F Within Subbasin F (Figure V-6) flow is generally west and south. The area is presently used for single family residential housing with approximately 10%' of the land area in use as a gravel pit, and approximately 25"0' of the land area in use as wetlands. Storm water flow at present through Subbasin F is overland and channelized flow in ditches along roads. Subcatchments 82 and 83 drain west to the wetland. At present the area is low density single Family residential. About one-half of the area is projected to become high density single Family residential. The wetland in the western portion of subcatchment 83 is to be conserved (Wetlands Management Plan, 1982). Flow through this area is presently via ditches and overland flow. No existing problems are apparent in this area. In the northeentral portion of the subbasin lie subcatch- ments 76 and 85. At present this area is a gravel pit in the eastern portion, and low density single family residential in the western portion. In the future it is planned that the area will become approximately 90"0 developed as low and high density residential. During this development a drainage system should be installed, draining to the wetland. 4-23 Subcatchments 75, 77, 7B, and 79, are presently drained by overland flow, roadway ditches and channelized flow. Existing land use is undeveloped or low density single family residential. As the projected higher residential density development occurs, adequate drainage should be provided. Two known problems exist within the area of subcatchments 75, 77, 78, and 79. On Cange Street south of the airstrip, the natural drainage has eroded the road. At the low spot in the road, approximately at Cleo Avenue, the flow traverses the width of Cange Road, forming a-deep wash in the road. Another problem is local flooding in the vicinity of Northern Raven Drive and Wilma Avenue. During snowmelt this spring cars were found stranded in this flooded area. Steps should be taken to alleviate this problem. The wetland in the western portion of Subbasin F contains all or part of the following subcatchments: 80, 81. 83, 84, 85, and 86. According to the future land use plan of the Municipality, this wetland is to be preserved. The volume of this wetland is computed in Table V-2. In summary, most of Subbasin F is projected to be generally developed as high density single family housing. As development occurs, a storm drainage network must be incorporated, possibly using the wetland as a detention area. 4-24 SUBBASIN G Stormwater within Subbasin G (Figure V-7) flows in three directions. Subcatchments 105, 107, 109, and 110 drain north. Draining west to the Alaska Railroad are subcatchments 101, 102, 1031, 106, 108, and 111. Subcatchments 100 and 104 drain south along the western edge of the New Seward Highway. In the southern portion of the subbasin, subcatchment 104 drains directly to the western edge of the New Seward Highway. Subcatchment 100 drains to the Old Seward Highway and transfers to the new Seward Highway where the two highways intersect. In these subcatchments, the present method of drainage is via roadway ditches and overland flow. No known problems exist or are anticipated. The present land use classification is low density single family residential. The Municipality la-nd use plan indicates that the area is projected to be classified ultimately as high density single family residential. The subcatchments which drain to the Alaska Railroad (101, 102, 103, 106, 108, and 111) exhibit no drainage problems at this time, nor are any problems anticipated in the future. The present drainage systems are overland flow, curb and gutter, roadway ditches, and pipe systems. At present, the land use is low density single family residential, except in subcatchment 111, and no future increases are anticipated. For subcatchment ill, the present land use is 6090' high density single family residential, and the remaining 40"0' is land which has been cleared 4-25 In summary, subbasin G has generally adequate drainage facilities for both present and future needs with the exception of the storm drain along Huffman Road and 36" cross culvert beneath the Alaska Railroad. SUBBASIN H Presently drainage in Subbasin H (Figure V-8) is handled with pipes, ditches, overland flow and channelized flow. At this time no problems are known to exist, and the capacities of the existing facilities are anticipated to handle the present and future estimated flows of Table V-3. However, an integrated trunk system should be incorporated in the design of future development in the subbasin. SUBBASIN I The present method of draining Subbasin I (Figure V-9) to the South Anchorage drainage study area is via ditches and overland flow. These systems have adequate estimated capacity for both present and future flows. The present system in Sub- basin I is not well interti ed within the subbasin for transfer of water ultimately to the subbasin outlet. It would be beneficial as the area continues to develop into an area projected to have industrial/commercial/ multi-family residential usage, that an intertied system within the subbasin be incorporated. 4-26 SUBBASIN J Subbasin J (Figure V-10) drains north, out of this study area into the South Anchorage drainage study. The area presently is drained via overland flow and roadway ditches. No problems are known to exist of this subbasin. However, as development continues to its planned ultimate level (high density single family residential), attention should be given to creating an integrated drainage system. SUBBASIN K The most downstream portion of Furrow Creek is located in Subbasin K (Figure V-11). The area is approximately one-half developed at this time, with the majority of land being used as high density single family residential. Ultimately the subbasin is planned to have approximately 90"0' developed with high density single family residential and multi- family housing. Beginning at the Alaska Railroad 36" cross culvert, upstream flow from Subbasin G travels to the Huffman Road right-of-way . At the intersection of Beachcomber and Huffman the flow is routed through a 36" cross culvert beneath Beachcomber. Flow continues along the south edge of the Huffman right-of-way to the west of Division Street where it is diverted south across Clipper Ship Court and enters the greenbelt system of Oceanview Subdivision. 4-27 Prior to reaching Beachcomber, the flow of Furrow Creek is impeded by a recent fill on the south side of the stream. This fill encroaches upon Furrow Creek and has inadequate side slope for soil stabilization. This fill presents an immediate problem to the flow of Furrow Creek. The greenbelt through Oceanview Subdivision extends from Clipper Ship Court to Johns Road. The capacity of the greenbelt itself is estimated to be adequate for the flows in Table V-3. However, the 18" to 3611 culvert crossings at Clipper Ship, Mariner and Johns Road have estimated capacities (Table IV-1) less than the estimated design flow as shown in Table V-3. As Furrow Cr eek continues downstream of Johns Road, the creek enters Johns Park. The portion of Johns Park lying within subcatchments 165 contains a channel of inadequate capacity (Table IV-1) for handling the estimated flow of Table V-3. However, in subcatchment 166, the capacity of the drainage channel improves and becomes capable of handling the estimated flow of Table V-3. In the northeast portion of the subbasin is subcatchment 167. This area is approximately one-third developed at this time. Ultimately 80% of the land area is projected for develop- .ment as residential and industrial usage. Present methods of handling drainage is overland flowand roadway ditches. No problems are known to exist presently in this area. As develop- ment progresses, an integrated drainage system should be incorpo- rated to drain approximately at Division Street to Furrow Creek. 4-28 Subcatchments 168 and 169 in the northcentral portion of Subbasin K drain to Johns Road. These subcatchments are presently being developed, using overland flow and road ditches as drainage methods. Attention should be given to an area-wide drainage network during development. The capacity of the Johns Road system (Table IV-1), which includes both cross culverts under Johns Road and a ditch system along Johns Road are adequate for the estimated flows as shown in Table V-3. In the northwest portion of the subbasin are subcatchments 170 and 185. These subcatchments drain to Timberlane Road and flow travels overland to the outlet of Furrow Creek into Turn- again Arm. The land area presently is approximately 25"0' developed. It is anticipated that the area will become almost entirely developed with high density single family residential housing. Presently the area is drained by overland flow and roadway ditches. As development progresses, an integrated drainage system should be implemented. In summary, Subbasin K has present problems west of the Alaska Railroad, at the landfill east of Beachcomber, and at road crossings along the greenbelt. As development occurs north of Huffman, an area-wide drainage network should be implemented. SUBBASIN L Subbasin L (Figure V-12) drains to Turnagain Arm. The topography of the land is quite flat, with wetlands in the 4-29 western portion. The present level of development is approx- imately 20"0'. It is anticipated that development will ultimately be approximately 80'10' of the land being used as high density single family residential. In the Wetlands Management Plan the Klatt Bog area of Subbasin L has been assigned "conservation status; i.e., development can occur on the fringes of the wetland but the natural character of the wetland must be returned to the greatest extent possible. The present method of draining stormwater is via overland flow, roadway ditches and channelized flow. No known problems exist in the subbasin at this time. However, during future development proper consideration must be made to management of the Klatt Bog per the Wetlands Management Plan. Also, in areas designated in the land use plan for development, an integrated storm drainage system should be incorporated into the design. SUBBASIN M Subbasin M (Figure V-13) comprises a portion of the Oceanview Subdivision and is presently about 90"0' developed as high density single family residential. Storm runoff is routed via curb and gutter, overland flow and pipe systems to three 18" outfalls along the bluff north of the mudflats. At present the three outfall pipes are exhibiting structural problems and the bluff is eroding away as a result. Estimated design fl. ows of Table V-3 indicate that the outfall pipes have 4-30 insufficient capacity (Table IV-1) to handle the projected future f lows. No other problems are known to exist. QUALITY General At the present, there are no water quality problems identified in the project basin. Storm runoffs do not present any serious danger to the beneficial uses in the basin with the exception of recreation and aesthetics. The Furrow Creek-Rabbit Creek basin consists largely of low residential lands and a small percentage of commercial and industrial lands, most of which are located in subbasins I and J. Runoff from subbasins I and J drain to the South Anchorage drainage basin study area. Table IV-2 shows the receiving water of subbasin runoffs in the Furrow Creek-Rabbit Creek basins. Channelized stream flow in the Upper and Middle Furrow Creek segments is difficult, if not impossible, to locate. Flow in this area is generally overland. The large wetland in subbasin F, Klatt Bog, and Turnagain Arm are receiving water bodies have no water quality concerns at present nor are problems anticipated in the future. The Alaska Department of Fish and Game maintains Dolly Varden and a small run of pink salmon in the Lower Rabbit Creek. Potter Marsh, the final outlet of the Rabbit Creek, provides important habitat for fish and wildlife. Lower Furrow Creek, however, has no recorded fish 4-31 TABLE IV-2 RECEIVING WATERS OF FURROW CREEK-RABBIT CREEK STORM MODEL RECEIVING BASIN NAME WATER AB Lower Rabbit Creek C Potter Marsh D Upper Furrow Creek El Upper Furrow Creek E2 Upper Furrow Creek F1 Wetland F2 Upper Furrow Creek G1 Potter Marsh G2 Turnagain Arm G3 Middle Furrow Creek H Middle Furrow Creek I South Anchorage Study Area i South Anchorage Study Area K1 Lower Furrow Creek K2 Lower Furrow Creek K3 Lower Furrow Creek Ll Turnagain Arm L2 Klatt Bog, Turnagain Arm M Turnagain Arm 4-32 ,opulalion but its surrounding park selling makes Furrow Creek important in the aspect of recreation and aesthetics. The portion of the study area which is tributary to Lower Rabbit Creek and Iolter Marsh constitute a small percentage (7'/0') of the total contributing area. The area is mostly of low density residential land use. The water quality effects of storm runoff from the area tributary to Lower Rabbit Creek should be insignificant on the receiving water, as indicated by the low estimated pollutant loads in Table 111-2. The Lower Furrow Creek, due to presently observed land erosion and potential large quantity of flow from upstream basins, would be the only critical water quality area in the study area. Estimated Pollutant Loads For water quality ev aluation purposes, pollutant loads from storm washoff were estimated using SAM and STORM models. (For details of the computer analysis - see Appendix A). The SAM model provides estimates of the pollutant loads at spring break- up. The STORM model computes the pollutant loadings for the period between spring break-up and winter freeze-up. Six water quality parameters, total dissolved solids, suspended solids, 5 day biological oxygen demand (BOD 5), grease and oil, fecal coliform and ammonia, which are critical to the beneficial uses were modeled. Both the SAM and the STORM used the pollution 4-33 TABLE IV-3 POLLUTANT BUILDUP MATRIX FE-COL TDS SUS-SOL BOD5 GRS-OIL Billion AMMONIA lbs/Ac/ lbs/Ac/ lbs/Ac/ lbs/Ac/ MPN/Ac/ lbs/Ac/ Land Use ID Day Day - Day - Day Day Day SUMMER/WINTER Commercial CO 1.3/0.8 5.0/4.9 .43/.33 .23/.20 .15/.009 .010/.005 Industrial IN 2.0/1.0 7.5/7.5 .60/.35 .34/.30 .01/.002 .010/.005 Multiple- Family Residential MF 0.40/1.3 .50/1.6 .12/.34 .06/.10 .007/.0008 .005/.005 High Density Residential HD 0.3/0.8 .20/1.0 .10/.49 .04/.03 .002/.002 .002/.016 Low Density Residential LD 0.25/0.3 .15/0.7 .05/.10 .02/.Ol .0015/.001 .002/.005 Cleared Pervious UP 0.2/0.3 .10/0.5 .02/.05 .002/.001 .001/.001 .002/.0005 Bogs and Marshes BM 0.1/0.2 0.0/0.0 .01/.02 .0001/0.0 .001/.0001 .0002/.0001 Lowland Forest LF 0.1/0.1 0.0/0.0 .01/.Ol .0001/0.0 .001/.0001 .0002/.0001 Upland Forest UF 0.1/0.1 0.0/0.0 .01/.Ol .0001/0.0 .001/.0001 .0002/.0001 Natural Pervious GP 0.1/0.1 0.0/0.0 .01/.Ol .0001/0.0 .001/.0001 .0002/.0001 ReFerence: Campbell Creek Drainage Study Task memo No. 7, 1979) 4-34 buildup factors in Table 11-1 which were derived from previous water quality studies for the greater Anchorag e area (208 Water Quality Management Plan, 1979). Usino the SAM model it is possible to generate the total pollutant washoff loads at the spring break-up and the associated peak concentrations at the mouth of the Lower Furrow Creek under future drainage conditions. The pollutant washoff loads for Lower Furrow Creek are presented in Table IV-4. Under the existing drainage condition, numerous upstream pondings would result in decreasing pollutant loads and concentration at the mouth of the Lower Furrow Creek. The results of the SAM model in Table IV-4 are for the recommended system. The STORM model computer runs provided long-term pollutant washoff loads which constituted numerous runoff events. Because the model has no routing feature, the pollutant washoff loads and corresponding average pollutant concentrations were generated on a subbasin basis. The first two runs performed used an average hydrologic year (1963) and compared long-term pollutant loads associated with existing and future land use. The two following runs made using the STORM model were to simulate the long-term pollutant loads and average pollutant concentrations under future land use were generated for two extreme hydrologic years (1967 and 1969), wet and dry. The results of the four computer runs are presented in Table IV-5. Figure IV-1 graphically depicts the monthly precipitation for the three summers (1963, 1967, 1969) chosen for use in this analysis. 4 - 315 TABLE IV-4 POLLUTANT WASHOFF LOADS Water Quality Total Pounds* Peak Parameter of Washoff Load Concentration (mg/L) Total Solids 5560 1580 Suspended Solids 4798 1355 BOD 50 325 92 Ammonia 5 1.5 Oil/Grease 196 55 Fecal Coliform 7.86** 0.52*** Sum of the pollutants from time of 0 to 8 hr. 20 min. MPN in Billion 10 3 MPN/L Note: The values shown are for total pollutant washoff loads and corresponding peak concentration at the spring break-up at the mouth of the Lower Furrow Creek which have been generated for the design storm event - based on the recommended drainage system. 4-37 TABLE IV-5 ESTIMATED POLLUTANT LOADING (1963 Summer (ave. summer), existing land use) TOTAL POUNDS OF WASHOFF FROM WATERSHED CONCENTRATION OF POLLUTANTS TO RECEIVING WATERS (Mg/1) BASIN Total Suspended Oil/ Fecal Total Suspended Oil/ Fecal ** ID Solids Solids @BOD Ammonia Grease Coliform Solids Solids BOD Ammonia Grease Coliform AB 4941 2979 2256 236 789 38 75 47 25 3 8 1.24 C 1968 1156 1068 88 366 15 65 40 25 3 8 1.06 D 3113 1931 1897 145 660 22 72 47 31 3 10 1.19 El 1671 955 804 66 260 11 64 38 25 3 6 1.19 E2 1442 609 735 52 148 27 38 17 12 1 2 0.72 F1 F2 38420 34193 10121 1885 3713 124 908 818 182 44 57 4.16 Gi 1125 692 579 40 198 2 71 45 27 3 9 0.96 G2 11006 9017 3945 543 1408 442 162 135 47 8 16 10.71 G3 8702 6720 3476 449 1371 115 115 91 37 6 14 2.69 H 29164 26139 7109 1433 2507 969 452 408 96 22 31 26.00 1 39635 35684 9578 1945 3429 920 531 481 114 26 38 22.00 j 2720 2115 1173 122 425 5 134 107 46 6 16 1.56 K1 4820 4276 1243 239 441 9 593 533 126 29 39 2.69 K2 1752 1035 1057 78 340 12 66 41 29 3 9 1.45 K3 950 485 452 37 104 11 53 28 18 2 4 1.27 Ll 3957 2479 2663 179 965 32 65 42 33 3 11 1.25 L2 470 234 363 11 108 4 43 23 24 2 7 1.43 M 2645 1645 1768 110 636 19 65 42 34 3 12 1.31 MPN in Billion J03 MPN/l TABLE IV-5 ESTIMATED POLLUTANT LOADING (1963 Summer (ave. summer), future land use) TOTAL POUNDS OF WASHOFF FROM WATERSHED CONCENTRATION OF POLLUTANTS TO RECEIVING WATERS (Mg/1) BASIN Total Suspended Oil/ Fecal Total Suspended Oil/ Fecal ** ID Solids Solids BOD Ammonia Grease Coliform Solids Solids BOD Ammonia Grease Coliform AB 4941 2979 2256 236 789 38 75 47 25 3 8 1.24 C 1968 1156 1-068 88 366 15 65 40 25 3 8 1.06 D 3291 2043 2111 1,47 757 25 67 44 31 3 10 1.11 El 2650 1610 1334 119 475 16 72 46 26 3 9 1.19 E2 6033 3754 3723 286 1345 55 67 44 31 3 11 1.19 F1 F2 4924 3012 3248 226 1154 54 64 41 31 3 11 1.25 G1 1125 692 579 40 198 2 71 45 27 3 9 0.96 G2 11006 9017 3945 543 1408 442 162 135 47 8 16 11.00 G3 16296 13788 5336 817 1952 673 186 159 51 9 17 13.16 H 52494 47530 12403 2581 4383 1507 575 523 123 28 41 30.63 1 39635 35684 9578 1945 3429 920 531 481 114 26 38 22.37 1 2720 2115 1173 122 425 5 134 107 46 6 16 1.56 K1 14367 12906 3545 706 1296 33 716 648 152 35 51 4.00 K2 3163 2085 1931 148 720 33 70 48 33 3 12 1.63 K3 2377 1453 1645 103 588 1B 62 40 32 3 11 1.16 Ll 3957 2479 2663 179 965 32 65 42 33 3 11 1.25 L2 470 234 363 11 108 4 43 23 24 2 7 1.43 M 2645 1645 1768 110 636 19 65 42 34 3 12 1.31 MPN in Billion 103 MPN/l TABLE IV-5 ESTIMATED POLLUTANT LOADING (1967 Summer (wet summer), future land use) BASIN TOTAL POUNDS OF WASHOFF FROM WATERSHED CONCENTRATION OF POLLUTANTS TO RECEIVING WATERS (Mg/1) Total Suspended Oil/ Fecal Total Suspended Oil/ Fecal ** ID Solids Solids BOD Ammonia Grease Coliform Solids Solids BOD Ammonia Grease Coliform AB 5733 3397 2435 261 838 39 70 44 26 3 9 1.36 C 2235 1323 1176 92 393 8 63 39 30 3 10 1.39 D 3762 2308 2287 158 817 20 63 41 33 3 11 1.28 El 3049 1834 1449 118 506 8 67 43 28 3 10 1.13 E2 6966 4276 4016 311 1434 52 63 41 31 3 11 1.24 F1 F2 5595 3406 3545 255 1252 54 60 39 34 3 12 1.44 G1 1325 798 624 40 203 1 67 43 28 3 10 1.14 G2 12473 10168 4312 628 1517 470 156 130 47 8 16 10.60 G3 18671 15684 5818 920 2103 724 180 154 49 9 17 12.48 H 59954 54084 13609 2917 4714 1608 564 513 117 27 38 28.52 1 45273 40595 10503 2206 3691 981 521 417 109 25 36 20.93 1 3076 2385 1287 127 455 5 131 105 48 6 17 1.50 K1 16290 14617 3910 795 1416 32 709 643 153 34 52 4.03 K2 3595 2352 2092 161 767 29 68 46 34 3 12 1.70 K3 2700 1653 1789 106 639 14 60 39 35 3 12 1.35 Ll 4549 2808 2886 203 1041 31 62 40 34 3 12 1.30 L2 472 250 410 10 115 4 40 22 32 1 10 1.51 M 3023 1867 1916 114 689 10 62 40 34 3 12 1.18 MPN in Billion 103MPN/l mmm mmmmmmm mmmm mmmm am TABLE IV-5 ESTIMATED POLLUTANT LOADING (1969 Summer (dry summer), future land use) TOTAL POUNDS OF WASHOFF FROM WATERSHED CONCENTRATION OF POLLUTANTS TO RECEIVING WATERS (Mg/1) BASIN Total Suspended Oil/ Fecal Total Suspended Oil/ Fecal ** ID Solids Solids BOD Ammonia Grease Coliform Solids Solids BOD Ammonia Grease Coliform AB 2978 1934 1967 176 714 45 84 55 44 4 15 2.40 C 1150 737 906 65 320 22 72 47 48 4 17 2.51 D 1940 1295 1818 103 675 32 75 51 58 4 21 2.37 El 1593 1044 1174 87 430 21 81 54 48 4 17 2.18 E2 3650 2433 3287 205 1223 61 78 53 58 4 21 2.41 F1 F2 2869 1892 2748 159 1006 59 71 48 59 4 21 2.62 G1 677 445 505 35 183 9 81 54 49 4 17 2.42 G2 6713 5689 3252 360 1235 391 197 168 84 10 31 21.40 G3 10273 8934 4498 555 1764 621 237 207 91 12 34 26.40 H 33459 30813 10038 1708 3947 1376 772 714 214 39 81 62.04 1 25234 23130 7777 1293 3090 844 705 649 198 36 76 45.33 j 1629 1324 973 82 368 14 160 131 85 8 31 3.03 K1 8899 8154 2778 453 1123 39 896 823 262 45 103 8.19 K2 1869 1311 1647 102 631 38 83 59 64 4 24 3.35 K3 1383 916 1389 74 511 25 71 48 64 4 23 2.79 Ll 2352 1575 2315 131 860 41 76 52 63 4 23 2.54 L2 269 148 296 13 95 9 47 26 39 2 12 2.72 M 1564 1048 1534 84 572 27 77 52 63 4 23 2.53 MPN in Billion 103MPN/l 1963 SUMMER (AVERAGE YEAR) H13TORICAL MEAN 3.0 2.75 2.80 3.0 2.50 2.20 U) 2.0 1.93 1.82 2.0 z z 0 0 1.0 .14 tt .98 LU LU cc .57 a. CL 0 0 May June July Aug Sept May June July Aug Sept 1967 SUMMER (WET YEAR) 19r9 SUMMER (DRY YEAR) 3.0 2.96 2.86 3.0 2.47 2.0 2.0- 2.14 1.44 z z 0 1.07 0 1.0 < 1.0 .86 .78 Q w LU w 1= - .33 0 0 May June July Aug Sept May 1june July Aug Sept COMPARISON OF URS Engineers SOURCE: HISTORICAL SUMMER Anchorage, Alaska National Oceanic and Atmospheric PRECIPITATION EVENTS Administration Asherville. N.C. February 1983 Figure IV-1 Water Quality Effects and Control Measures Water quality effects are evaluated in this study with respect to the beneficial uses. The beneficial use of Lower Furrow Creek is basically recreation. Key water quality parameters for Lower Furrow Creek are solids, oil/grease, and fecal coliform. At the spring break-up, the concentrations of these parameters are quite high in the creek. However, it is unlikely that much water recreation would occur on the creek during that period. During the summer, the height of the recreation season, the concentration of these parameters from each subbasin runoff are potentially high on the account of four basins, designated in the STORM model as F2, H, I, and K1. These basins all contain commercial and/or industrial land uses. It is important to note that the entirety of basins I and J drain to the South Anchorage storm drainage study area. Best management practices, such as improving road pavement, frequent catchbasin cleaning, regrading of disturbed areas and control of erosion would be sufficient to reduce, and potentially control, the pollutant loads. The beneficial use of the Lower Rabbit Creek and Potter Marsh is for fisheries. Key water quality paramaters are: DO, fecal coliform, oil/grease, and solids. The total pollutant loads and average concentrations from the tributary basin designated in the STORM model as AB, C, and G1, are not high. The majority (at least 83"0') of Rabbit Creek flow is from outside of this project area and should have better water quality than 4-49 the tributary study area as the result of very little develop- ment. The low pollutant level in the tributary outside the project area should dilute the pollutants from the project area substantially. Specific water quality control measures are not deemed necessary in the basins AB, C, and G1. 4-50 i I I, @l I I I I I I I I I 1 0 1 I CHAPTER FIVE I PRESENTATION OF i ALTERNATIVES -Al CHAPTER 5 PRESENTATION OF ALTERNATIVES IN TRODUC T ION Six general alternatives were identified for stormwater planning within the study area. Each of the six alternatives are described in detail in this chapter. These general alternatives were applied to the study area in conjunction with the goals and planning criteria for the area, in order to identify the best alternative for each subcatchment. An overview of the recommended alternatives for the study area is presented in this chapter. Following the overview, is a discussion of the alternatives for each subcatchment and their evaluation. The alternatives evaluated, as well as the recom- mended one(s) are summarized by subcatchment in Table V-1. Also included in this chapter is a section which outlines the use of the plan for developers and design engineers. DESCRIPTION OF GENERAL ALTERNATIVES General Historically, Furrow Creek-Rabbit Creek drainage area has 5-1 experienced minimal urbanization pressures. The development pressures that have existed have been in small, concentrated, isolated location. These include: strip development along Old Seward Highway; Huffman Road; O'Malley Road; Oceanview Sub- division; and the Turnagain View Subdivision area. In the past, each developing area provided for their own stormwater drainage needs and associated control measures. But as urbanization pressures increased and the concentration of developments increased, the problems associated with the lack of a compre- hensive stormwater management plan became apparent. In the Furrow Creek-Rabbit Creek urbanized setting, there are several stormwater runoff problems that require attention. These include: flooding, soil erosion, sedimentation, maintenance of drainage ways, water quality considerations, visual impact, and impedence to development. In developing alternatives for this study, it was concluded that the major problem associated with stormwater runoff was the lack of a comprehensive storm drainage network. As a result, alternatives for controlling stormwater runoff within the study area for present and future conditions were placed into six general categories. These categories are consistent with the goals and criteria set forth in this chapter. The six alterna- tives are: no modifying action; local detention ponds and trunk system; trunk system; regional detention ponds; corridor/Qreenbelt systems; and diversion to other subcatchments. As the thirteen subbasins include varying storm drainage conditions which require 5-2 different methods, no one alternative could be used throughout the entire study area. From the six alternatives listed above, it was possible to tailor modifications to existing facilities and to provide a comprehensive plan for future drainage networks. The alternatives presented are for comprehensive storm drainage/water quality management plan for each subbasin in the study area. There are no recommendations for minor collection facilities which should be designed to fit specific needs. ALTERNATIVE #1 - No Modifying Action Under this alternative, no modifications will be made to the existing stormwater drainage system. This alternative is recommended in subcatchments where the quantity of runoff assoc- iated with the design storm can be handled adequately by the existing stormwater drainage network, and no water quality problems are apparent. ALTERNATIVE #2 - Local Detention Ponds and Trunk System Stormwater flows would be routed in this alternative through a trunk system which would be either an open system of roadside ditches and culverts, or a closed system of underground culverts, or an open channel or a combination of all of these systems, all of which would maximize the use of existing drainage networks or planned roadways and easements. Peak flow rates would be dampened by the use of local detention ponds. Under this alternative, the 5-3 volume of stormwater runoff to be detained would equal the volume of stormwater necessary to keep runoff rates equal to pre-develop- ment rates. In this alternative development would not increase stormwater runoff rates, but might increase overall volume of stormwater runoff. This alternative is best suited to areas which are not completely developed. Such areas still have the ability to incorporate into their development pattern the necessary space for detailing stormwater runoff. This alternative has the potential of avoiding enlargment of existing facilities in downstream developed areas, as a result of upstream urbanization. ALTERNATIVE #3 - Trunk System Stormwater flows would be routed in this alternative through a trunk system which would be either an open system of roadside ditches and culverts, a closed system of underground culverts, or an open channel system, or a combination of all of these, all of which would maximize the use of existing drainage networks, or planned roadways and easements. This trunk system would require the sizing of all ditches and pipes to capacities which would be large enough to handle the anticipated peak flow rates. This alternative is best suited in fully developed areas where land is at a premium and easements are mandatory. ALTERNATIVE #4 - Regional Detention Ponds By detaining stormwater runoff generated from large areas 5-4 in man-made pondsp natural depressions, or existing identified wetlands, and controlling the release of the detained water, drainage facilities downstream will require smaller carrying capacities. These detention ponds also aid in the recharging of ground water levels and offer possible recreational value. However, the ability to incorporate a regional detention pond is limited to areas with low density levels, and areas containing existing wetlands or natural depressions. Additionally, it is necessary to incorporate adequate land in these areas for the detention pond system. ALTERNATIVE #5 - Corridor/Greenbelt System Under this alternative the natural drainageway would be preserved through the use of corridors and greenbelts. These facilities would be designed to carry the average runoff flows and also would have the capacity to handle peak flow rates in an open channel system. The corridor/greenbelt system is aesthetically pleasing and provides a parkway appearance along the drainageway path. Since land must be reserved for this alternative, its use is limited to areas in which a corridor/greenbelt system can be incorporated into the development pattern for the area. It is mandatory to protect the corridor/greenbelt after its establish- ment from develpment encroachment through the use of zoning regulations. 5-5 ALTERNATIVE #6 - Diversion Where inadequate downstream capacity exists to handle the peak runoff rates, it may be possible to divert all of or a portion of the upstream flow to another subbasin or subcatchment where the facilities are capable of handling the additional flow. This alternative of diversion is not an independent alternative, but rather a partial alternative to be used in conjunction with another alternative as presented previously. 5-.6 1111VIEW OF IEIOMMENDED A11EINAIIIES In recent years, this study area has experienced a rapid increase in urbanization. As a result of this urbanization, a number of minor stormwater drainage collection systems, and, in some cases major stormwater drainage trunk systems, have been installed to alleviate individual, isolated stormwater runoff problems. This study is the first composite analysis of the study area which analyzes these individual minor collection and/or major trunk systems on a comprehensive level. There are two major problem areas which should be addressed in the Furrow Creek-Rabbit Creek basins: 1) the lack of a compre- hensive storm drainage trunk network which would tie together all of the small collection or isolated trunk systems for the areas; and, 2) the Middle and Lower Furrow Creek segments in critical areas do not have adequate capacity for present and future stormwater flows. The areas which drain directly to Rabbit Creek (subbasins A9 B, C and portions of G) and the areas flowing north out of the study area to the South Anchorage Drainage Study area (subbasins I and J,) have minor and isolated stormwater related problems at present but no major stormwater related problems presently exist or are foreseen in the future. For these subbasins it was generally recommended that the alternative of a trunk system be implemented for situations where future flows would exceed capacities of existing systems or for areas where no starmwater 5-7 systems presently exist. Because the drainage area for subcatch- ments within these subbasins is small, this trunk system will be a minor drainage system, such as a collection system, in most cases. In subbasins L, M, and portions of G, which drain directly to the Turnagain Arm, only isolated cases of existing or projected future stormwater runoff related problems exist. These problem areas are located in subbasin M where the existing outfall pipes from the collection systems are undersized for present and future flows, and it is recommended that the capacity of these outfalls be increased. In the remaining portions of these subbasins, it is recommended that either no modifying action be implemented, because of the lack of existing or future stormwater runoff pro- blems, or that a trunk system be implemented where no stormwater system presently exists and future land use indicates increased development. The drainage area of these subbasins are such that this trunk system will be a minor drainage facility, such as a local collection system discharging to an outfall pipe to Turnagain Arm. The remaining subbasins are the three segments of Furrow Creek: 1) the Upper Furrow Creek segment east of the New Seward Highway; 2) the Middle Furrow Creek segment between the New Seward Highway and the Alaska railroad; 3) The Lower Furrow Creek segment from the Alaska Railroad to Turnagain Arm. In the evaluation of these three segments of Furrow Creek it was concluded that the main requirement for the Upper Furrow Creek 5-8 segment for the future will be the installation of an adequate trunk and associated collection system to convey stormwater runoff to the Middle Furrow Creek segment. The main deficiency of the Middle Furrow Creek segment is inadequate capacity for present or future flows for its complete length. In the Lower Furrow Creek segment the major deficiencies are the isolated cases where inadequate capacity exists for both present and future runoff volumes, generally associated with streets and railroad crossings. One of the major problem areas identified in this study, is the overloading of the Middle and Lower Furrow Creek segments as a result of increased urbanization pressures in the Upper Furrow Creek segments. In the evaluation of alternatives, it was deter- mined that stormwater management techniques which would reduce the peak design stormwater flows from Upper Furrow to Middle Lower Furrow Creek segments should be implemented. To minimize this overloading condition alternatives which maximize the use of local depressions and regional detention ponds by using existing small depression areas or wetlands are recommended. These alternatives are recommended only in areas of single-family and multi-family dwellings or in identified wetland/open space areas, so as not to affect future development patterns. However, the projected peak flow rates are such that the Middle and Lower Furrow Creek segments will be overloaded. It is therefore recommended that the Municipality of Anchorage give consideration to additional regional detention ponds in areas where future land use patterns would be altered, particularly in subcatchments 72 5-9 and 71 (located in subbasin E) in order to further minimize peak runoff in those areas. In addition to these detention facilities, a major trunk system layout has been recommended for subbasins D, E, and F. These trunk systems will convey collected stormwater runoff to the middle segment of Furrow Creek located approximately at the intersection of Huffman Road and the New Seward Highway. These trunk systems follow the existing natural drainage corridors or existing streets to the maximum extent possible. For the upper reaches of these subbasins, alternatives recommended are for a localized trunk/collection facilities which will control storm- water runoff. The areas east of subbasins D, E, and F where overland flows cross the study area boundary, it is recommended that the overland flows be diverted away from the Furrow Creek drainage either to the Rabbit Creek drainage to the south or the Campbell Creek drainage to the north in order not to overload the recom- mended trunk systems as presented in this study. Irrespective of which detention methodology is used in the Upper Furrow Creek segment, there exists a severe problem in the Middle Furrow Creek segment for both present and projected flows. The Middle Furrow Creek segment, which is the northern portion of subbasin G and subbasin H between the New Seward 5-10 14 Highway and the Alaska Railroad, contains an existing stormwater drainage system. This system is inadequate for existing and projected stormwater runoff flows. Recommendations are presented in this chapter to alleviate this situation. Because of the close proximity of major highway corridors to this drainage system and existing business development in this segment, it is recommended that action be taken as soon as possible to alleviate this problem area. The Lower Furrow Creek segment, or Subbasin K, presently is a corridor/greenbelt system in which flow constrictions exist which will cause localized ponding and flooding during times of peak runoff. Recommend ations are presented for this subbasin which will alleviate these local constrictions by allowing an increase in the carrying capacity of the creek, while still maintaining its corridor/greenbelt nature for future projected flows. In summary, all of the areas which drain directly to Turnagain Arm, out of the study area, or to Rabbit Creek, have minimal existing stormwater related problems, and by implementing the recommendations of this study, the potential future storm- water problems will be alleviated at minimal expense. The Furrow Creek drainage has no trunk system in the upper segment and has existing problems in the middle and lower segments. These situations can also be alleviated with the use of the alter- natives as presented in this chapter. Refer to Table V-1 for an overview of the alternative evaluation for this study area. 5-11 TABLE V-1 SUBBASIN ALTERNATIVE EVALUATION EXISTING COLLECTION SYSTEM ALTERNATIVES (RECOMMENDED ALTERNATIVE(S) CIRCLED c 0 E -4 4) 41 tP 41 0 44 0 0) w 0 >1 4j W 41 -4 4j 03 w r -@ 4j 4j 0 44 41 :14 44 AJ -4 -4 0 14 10 0 41 4j .0, 0 >1 -M 0 41 '0 Co '* 0 0 0 r. -4 $4 E -4 41 w 41 0 41 $4 14 54 z 0 0 w 4) 4) 0 w @4 0 FUTURE 0 .1 E@ M a 0.0 Q > DEPRESSION TRUNK SYSTEM SUBBASIN SUBCATCHMENT 0 L) 9 P. U 0 AND/OR WETLAND PROBLEMS -I.D. A 1 30 RC Rd Q B 11 20 RC 0 12 30 RC (D 13 20 RC Q 14 .)o RC 15 2) 7 RC 0 16 30 OSH-2 17 20 RC C 25 0 ponding NSH-E 26 20 ponding NSH-E 27 30 ponding NSH-E 28 20 ponding NSH-E 29 30 ponding NSH-E D 45 15 UFC-Sl 46 . * 0 UFC-Sl 47 * * 0 ponding UFC-Sl 48 * * 0 ponding UFC-Sl 49 * * 0 ponding UFC-Sl 50 * * 0 ponding UFC-Sl Trunk system I.D. corresponds to the trunk systems as shown on Figures V-1 through V-13. @ m @ @ "m so an TABLE V- I SUBBASIN ALTERNATIVE EVALUATION EXISTING COLLECTION SYSTEM ALTERNATIVES .,RECOMMENDED ALTERNATIVE(S) ejgCI.Fp r 0 E r .,1 0 4) 01 V 4j 4) c U) 4) w rz 4) >, 4j 01 4) V U) " 41 44 41 -.4 4) 0 44 d) >1 -4 0 w 0 0 -4 -0 w w -0 .10 0 >, -4 -4 V C: 0 c 4j V to u) 0) 0 0 0 r -,1 54 E *A -0 w r @4 0 w 41 4j u 4j 0 m4j w -4 0 0 0 w a) 4) 0 4) FUTURE z go A C) > 0 0 -4 w DEPRESSION TRUNK SYSTEM SUBBASIN SUBCATCH14ENT 0 0 9 93. 0 AND/OR WETLAND PROBLEMS I.D. E 60 10 UFC-S3 61 20 UFC-S4 0 62 15 UFC-S4 63 16 UFC-S4 64 12 UFC-S3 65 0 UFC-S3 66 0 UFC-S3 67 0 UFC-S3 68 0 UFC-S3 69 0 UFC-S3 70 40 UFC-S4 G 71 0 ponding UFC-S4 72 0 UFC-S2 0 P 75 12 Cange St. ponding UFC-N3 (D E) 76 5 UFC-N2 (D 77 0 flooding UFC-N3 0 78 0 UFC-N3 79 4 ponding UFC-N3 80 25 81 UFC-N3 82 85 regional ponding Wetlands 9 UFC-Nl 83 30 UFC-Nl 84 30 Wetlands 85 10 regional ponding Wetlands 15 regional ponding Wetlands 86 aim m m an am TABLE v-1 SUBBASIN ALTERNATIVE EVALUATION EXISTING ALTERNATIVES COLLECTION-SYSTEM (RECOMMENDED ALTERN r 0 rz r .,I a) a) v 41 a c to 4) W' E w >1 4j 01 a) 4j LO U) v -, 4j r 43 41 44 4) >1 4, -4 0 w U) .0 41 -.4 4) -H Aj 10 0 >1 0 r r V V U) to 0 w 0 r -H $4 r. A 4) w 41 u Aj :3 a, 41 $4 -4 0 401 0 $4 0 0) 0 4) A r 4) 54 90 0, Od 41 FUTURE I z to .4 a I- go L).Q a :3 0 4 .4 14 0 U g P. ri 0 DEPRE TRUNK SYSTEM SUBCATCHMENT AN 0 SS=ND PROBLEMS I.D. w SUBBASIN D/ R WET G 100 15 OSH-2 101 4 AK RR 102 10 AX RR 103 0 AX RR 104 0 NSH-W 105 0 OSH-1 106 0 AK RR 107 0 OSH-1 108 0 AK RR 109 0 OSH-1 110 0 MFC ill 0 AK RR H 125 0 MFC 126 0 MFC 1 135 20 N. of study area 136 20 N. of study area 145 5 N. of study area 146 0 N. of study area 147 0 N. of study area meow awswumm M so @Sw4wftw@ TABLE V- I SUBBASIN ALTERNATIVE EVALUATION EXISTING COLLECTION SYSTEM ALTERNATIVES RECOfjfj TVF. fq] rlRrT.Pn r XO 0 9 9: A w 0) @4 a, 41 .,j e 14 44 r r w 4) 0 u -4 4) >1 41 a, a) 41 V E -a >1 41 to ta "1 41 r 44 4J 4 0 0 44 0) >1 0 $4 W 0 41 " 41 13.W M M_q 0 >1 _4 -,4 r 0 13 9 . 41 .0 0 to >, @4 0 0 '1 0 x 0 r -H k 4d a to to 4) E -4 (d $4 � -H 4) $4 43 -4 9 r 41 u 41 0141 $@ @ k c 0 0 0 0 w 4) 4) 0 4) -H a w 10 "1 0 41 FUTURE z la .4 .4 E@ Q@ 0 L) A a > :1 0 . .2 W 0 U C4 0. U 0 DEPRESSION TRUNK SYSTEM SUBBASIN SUBCATCH14ENT AND/OR WETLAND PROBLEMS I.D. @4 K 160 0 LFC 161 0 LFC 162 0 LFC 163 0 LFC Q 164 5 LFC (D 165 12 LFC (D 166 14 LFC 167 5 LFC 168 8 Johns Rd. 169 0 Johns Rd. 170 0 LFC 185 0 LFC L 184 0 Turnagain Arm 186 0 Turnagain Arm 187 80 Turnagain Arm 188 40 Turnagain Arm 189 16 Turnagain Arm M 200 0 Turnagain Arm 201 0 Turnagain Arm 202 2 Turnagain Arm IUBBAIIN A Subbasin A drains to Rabbit Creek. The various applicable alternatives evaluated are: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; and Alternative #3, trunk system. As shown on Table IV-1, the existing capacity of the ditch on the north side of Rabbit Creek is approximately 70 cfs. Both the existing and future flows projected for sub- basin A are 5.9 cfs, which is much less than the ditch capacity. It is recommended that Alternative #3, a trunk system be implemented throughout this subbasin. This system will be a minor drainage system and will resemble a local collection system. This system should be a series of roadside ditches and culverts designed to carry flows proportional to the area collected as identified on Table V-3 and ultimately discharging to Rabbit Creek via the ditch along the north side of Rabbit Creek Road. These collection systems should be constructed in conjunction with the expansion of the existing roadway network in the future to serve the ultimate growth of the area. There are no new major drainage structures recommended for this subbasin, as such the following map indicates only existing drainage systems and project flows from the area. 5-21 LEGEND 12;.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed NODE- 123 123A -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM OVERLAND FLOW SUBCATCHMENT BOUNDARY wo, FE - -------- . . ........- 106 -4- Ti? t 14- -- - - - - -. ..... . ...... .. 0 ji - - - ------- Sc t 31 16 9 10 7 5.9 . . . . . ........ . .... .. C.-ARD-- A- -4@ [email protected] C." ROAD 2 3 )6 CFS TO RABBIT CREEK'VIA RABBIT CREEKROA&\ /Y - 21N 21 3 1 'a 6, Oil 9 Q 4J 2 fiESZEN V@ 1 500' 6 2@1 0 URS Engineers SUBBASIN A Anchorage, Alaska February 1983 Figure V-1 IUBBAIIN B Subbasin B drains to Rabbit Creek. As shown on Table V-1, the various applicable alternatives evaluated in this subbasin are: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; and Alternative #3, a trunk system. It is recommended that Alternative #3, a trunk system, be implemented throughout the subbasin. This system will be a minor drainage system and will resemble a series of local collection systems which individually, except for subcatchment #15, drain to Rabbit Creek and should be a series of roadside ditches and culverts. These collection systems should be constructed in conjunction with the expansion of the roadway network in the future and be designed to carry the flow associated with the area from which it is collected as identified in Table V-3. There are no new major drainage structures recommended for this subbasin, as such the following map indicates only existing drainage systems and project flows from the area. The area to the east of the study area boundary, which pre- sently drains into subbasin B, should be diverted away from subbasin 8 and into Rabbit Creek as a storm drainage network is developed in that area so as to not overload the proposed trunk/collection system. 5-25 LEGEND Jilk > Existing Proposed SIC 14. 1 ;.4 ROUTED FLOW IN CFS 100 YR SC 11 NODE- 123 -ROUTED FLOW IN CFS 10 YR 1234 DITCH, CURB & GUTTER 7 3_3 SUBBASIN BOUNDARY STORM DRAIN SYSTEM OVERLAND FLOW 10 1 9 8 4 3 41 2 SUBCATCHMENT BOUNDARY -30 31 17 18 20 21 22 1 2 4 25 2 1 2 8 9 41 =rLd= - m F_ V\" LJ X_ 4R @C 15 C" 4 3- 1 33 D1' c@r) zi II @' 46 2 4 \X Cl) m 56 51 5 5 ko j 1 61 Dc 62 RA13 63 6,4 L 4@i 4 . . .... E 144th t44- a, 1, 7 1 72 9 6 65 0 6 4 Uj G) ir 0,- 1 92 3 94 1 95 96 9 o B 82 1 > 2.5 V f2.0 L 15 f- - - -@ -- - - 7-, I _T 107 - 0 -9 106 100 99 112 4- L LLJ 10.2 6 12 7 8 \1; 3 114 5 1 !17 1 1 1 P 121 12 21 23 12 b 14 SC 16--, .8 7 142 14 1 140 130- 4 136 13, 136 134 !32 t30 'IJ --A 7 N b '73 1 56 1 1-8 @)q !6f b49 !50 15, 155 SIC 17 SC 13r- iC 9 168 67 162 174 1 3 !72 66 165 64 <6 3 30,5.2 184 I'T 183 i85 189 190 186 188 !78, SCALE V= 500' 16 RABBIT CREEK ROA D 7 _F____ 7 @123, URS Engineers SUBBASIN B Anchorage, Alaska February 1983 FigureV-2 SUBBASIN C Subbasi-n C drains to Potter Marsh. As shown on Table V-1, the various applicable alternatives evaluated in this subbasin are*. Alternative #2, local detention and trunk system; Alter- native #3, trunk system; and Alternative #6, diversion. This subbasin is presently experiencing problems with localized ponding, and as a result, the no modifying action alternative was not evaluated. In the northeast and southwest areas of the subbasin, diversion of stormwater out of subbasin C into subbasin G was evaluated in order to decrease downstream stormwater flows. However, it was determined that storm drainage networks in this subbasin would not be overly impacted by existing or future flows. As such, no diversions were deemed necessary. It is recommended that Alternative #3, a trunk system, be implemented throughout this subbasin. This trunk system should be either a series of roadside ditches and culverts or closed underground storm drainage networks, depending on the development pattern of the five subcatchments and should be designed to carry the flows as identified in Table V-3. The major trunk system for this subbasin is the existing stormwater drainage network along the Frontage Road and the drainage system of the New Seward Highway. Both of these trunk systems have existing capacity (Table IV-1) capable of handling the estimated future flows as shown on Table V-3. In conjunction with this trunk system, it is 5-29 recommended that the upper portions of the subbasin implement a collection system designed to carry the proportionate amount of flows for the area to be served, as identified on Table V-3. This collection system should be constructed in conjunction with the expansion of the roadway network in the future and should consist of either a series of open ditches and culverts or closed underground storm drainage systems, depending on the ultimate development pattern of the area. The State Department of Transportation and Public Facilities (DOT/PF) is presently designing improvements to the New Seward Highway for construction in the immediate future. It is recom- mended that DOT/PF be contacted to in sure that adequate capacity in the inlet points are planned in the improvements to the New Seward Highway. The flows identified are for a 10-year design storm, and as such, DOT/PF should be contacted to identify what design storm is to be used for these systems, the 10-year or the 50-year design event. 5-30 17111 LEGEND 1 ;.4 ROUTED FLOW IN CFS 100 yR Existing Proposed 10 C; NODE- 123 -ROUTED FLOW IN CFS 10 YR DITCH, CUR ... ... ... 123.4 B & GUTTER A SUBBASIN BOUNDARY > OVERLAND FLOW v SUBCATCHMENT BOUNDARY STORM DRAIN SYSTEM J 7 If 17 r Uj j oy v Ov N) Jo)/ x, J If In 25 Z) 9. ,v 4 IV Z) IIf 77 V/ __j 0 -j Q J/ Q/ 0 62.5 I'----NSH-E 7_ 14.6 41. 6.1 J/ -co -4 a_ 1@@ If ii n,, 44. Al 4b 29 Ji Z v -TOLPOT-TER MAR! 4.8 C @S [I A Iv SCA@@ @_X NEW SEWARD HWY URS Engineers V @123. X@ 1z, 17 Anchorage, Alaska SUBBASIN C February 1983 FigureV.3 T SUBBASIN D Subbasin D forms the most southerly portions of the Upper Furrow Creek segment. As shown on Table V-1, the applicable alternatives evaluated in this subbasin were: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; Alternative #4, regional detention; Alternative #5, a corridor/greenbelt system; and Alternative #6, diversion. It is recommended that a combination of Alternative #2, local detention and trunk system, and Alternative #3, trunk system, be implemented in subcatchments 45, 46, 47, and 48. The combination of these alternatives will form the upper portion of the trunk system UFC-S1. In subcatchments 49 and 50, Alternative #3, trunk system, and Alternative #4, regional detention, should be implemented. This trunk system is the middle portion of trunk system UFC-S1. The following paragraphs discuss each subcatch- ment in more detail. In the easterly portion of the subbasin in subcatchment 45, the area east of the end of the Starboard Lane and Capstan Court, it is recommended that Alternative #2, local detention and trunk system, be implemented. This system will resemble a local collection system because of the drainage area involved and should consist of a series of roadside ditches and culverts or closed underground storm drainage network, depending on the ultimate development pattern of this subcatchment design to carry 5-33 flows identified on Table V-3. This system will discharge to the UFC-S1 trunk and should be constructed in conjunction with the future expansion of the roadway network. For the areas south of Legacy and west of DeArmoun Sub- division Addition No. 2, subcatchments 46, 47, and 48, it is recommended that Alternative #3, a trunk system, be implemented. This trunk system is identified as UFC-51. The majority of this trunk system is already in place via the existing street and gutter system, pipe system, and open channel/greenbelt network. As identified on the subbasin map, there are portions of the existing system which are to become a part of the UFC-S1 trunk system which have inadequate capacity for the estimated future flows. These areas should be upgraded in the immediate future. In particular, along Spinnaker Drive, and Westwind Drive, where the greenbelt flows crosses the roadway through existing 1811 culverts, it is recommended that these culverts be upsized in accordance with the flows as identified on Table V-3. Also, where the existing greenbelt discharges into a 2411 closed culvert system along Tradewind Drive, there is inadequate capacity for future flows and this system should be increased either through the removal of the existing culvert and the installation of a culvert system designed for flows identified on Table V-3, or by paralleling the existing system with a second system designed to carry the additional capacity. In the lower reaches of the subbasin, in subcatchments 49 and 50 between the Frontage Road and the New Seward Highway, 5-34 it is recommended that Alternative #3, a trunk system, and Alternative #4, a regional detention pond, be implemented. This regional detention pond will dampen out the peak flows from Subbasin D prior to their confluence with the middle segment of Furrow Creek. This regional detention pond should be located between the Frontage Road and the New Seward Highway and be approximately sized in accordance with Table V-2. The Frontage Road along the New Seward Highway will be used as the lower UFC-Sl trunk system. The existing open ditch system does not have adequate capacity for estimated future flows as identified on Table V-3, and it is recommended the Frontage Road ditches cross-sectional area be increased to the carrying capacities as identified on Table V-3. 5-35 TABLE V-2 REGIONAL DETENTION VOLUME 1. Estimates for Subbasin D. Subcatchment Width Length Depth Volume No. (ft) (ft) (ft) (ac-ft) 49 10 750 2.5 0.43 50 10 750 2.5 0.43 Subbasin D Total Volume 0.86 2. Estimates for Subbasin F. Subcatchment Width Length Depth Volume No. (ft) (ft) (ft) (ac-ft) 80 400 2600 2.5 59.75 81 450 2600 2.5 67.25 83 500 1850 2.5 53.00 84 450 1600 2.5 41.25 85 300 1220 2.5 21.00 86 500 1400 2.5 40.25 Subbasin F Total Volume 282.50 5-36 LEGEND UL 123.4 - R TED FLOW IN CFS 100 yR Existing Proposed NODE-(@@ OU 3 123.4 ROUT ED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM --------- -----_--- --- ----- ------ @7 --- - ----- )OROSH OVERLAND FLOW -------- SUBCATCHMENT BOUNDARY 6 aG- ;,Lkji UY@@ .9- 5 01 4&8 16! ix 9 UG \N_ REG\1ONAL TF M DETE,NTIM, tw' k AREA\ E' 10 43'@ 4 Jo P Oil E 110 AC;lf 'SC 48 ;Ile SC 46 -S PTff OA: -10 5 TR I TH SC 45 7.0-, :E .4 I L3 P K 6 4Pf 4 4 41.1 A 6 zo, P 4 j -PEE 4 I - " ------------- P4Br 45 20.7 9 r_ HOOL T 31- 'P 6 -,3 A RV G-S -- -- --- Avg_ 4 J P @ @9_tta@ SCALE 1 5'"" TION OF EXISTING SYST M P TP P 4 WHICH MUSH BE ENLARGED /00@" UA FOR FUTURE WS (TYP) J L -F!_9 S T' 47 T P -4 @"' @2T'1191 '' " I TR TRAf F7 URS Engineers @10 Anchorage, Alaska SUBBASIN D --------- j February 1983 Figure V-4 SUBBASIN E Subbasin E forms the middle portion of the Upper Furrow Creek segment. As shown on Table V-1, the various applicable alternatives evaluated were: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; Alternative #4, regional detention; and Alternative #5, a corridor/greenbelt system. This subbasin is the largest contributor of flow to the Middle Furrow Creek segment. In this subbasin, Furrow Creek begins to fan out and as a result, it is recommended that four new main trunk/corridor systems should be implemented. These systems are: 1. The UFC-S4 trunk system in the northerly part of the subbasin. This trunk system drains the north portion of the subbasin, more or less paralleling Merganser Avenue. Because of the existing development pattern of this area, it is recommended that this trunk system be kept natural as an open greenbelt corridor as much as possible, following the natural path of this portion of the Upper Furrow Creek drainage. 2. The UFC-53 trunk system which serves the existing subdivision in the south of the drainage basin and provides for a trunk system for the southeast corner of the developing basin. 5-39 This trunk system follows the existing storm drainage network to the greatest extent possible, but as a result of upstream development, the existing drainage network will require upgrading in the near future to handle the projected future flows. The two above mentioned trunk systems, (UFC-S3 and UFC-S4), join in the northwest corner of the subbasin to form the UFC-S2 trunk system. This trunk system should be left in a open corridor/greenbelt state wherever possible and follow the natural drainage path of Furrow Creek. 4. The last trunk system in this subbasin is UFC-S1, which is a trunk system paralleling the Frontage Road draining from south to north, and originates in subbasin D. The following paragraphs discuss each of the subbasins and their particular requirements. It is recommended in the southeasterly portions of the subbasin in subcatchments 60 and 66, the area bounded on the north by Leyden Drive, on the west by the extention of Pintail Street, on the east by the study area boundary, and on the south by Subbasin D, that Alternative #2, local detention and trunk system, be implemented. This system should be a trunk/collection system feeding trunk system UFC-S3, and designed to carry the 5-40 flows as identified in Table V-3. This system should be con- structed in conjunction with the expansion of the roadway network in the area and should consist of roadside ditches and culverts or closed underground storm drainage network, depending on the ultimate development pattern of this area. It is recommended that in the middle portions of the sub- basin, in subcatchments 64 and 65, the area bounded on the north by Flyway Avenue, on the south by Leyden Drive, on the east by Pintail Drive, and on the west by Lake Otis Parkway, that Altern- ative #2, a local detention and trunk/collection system feeding trunk system UFC-S3, should be implemented. This system should be constructed in conjunction with the expansion of the roadway network in the future and should be sized to accommodate the flows as identified in Table V-3. For the middle segment of the northern portions of the subbasin in subcatchments 70 and 71, the areas east of Silver Spruce Drive, north of Flyway Avenue, west of the Gander Street, and south of Huffman Road, it is recommended that Alternatives #3, trunk system, and Alternative #5, corridor/greenbelt system, be implemented. This trunk system is identified as trunk UFC-S4, and should be designed to carry the flows identified on Table V-3. The routing of this system should follow the natural drainage path of Furrow creek to the maximum extent possible in an open channel/greenbelt type system which maximizes the recreational/aesthetic potential of Furrow Creek. In this portion of the subbasin is the lower part of trunk system, 5-41 UFC-S3, which originates in subcatchment 69, and it is recom- mended that this trunk be a open-channel/greenbelt system also and join UFC-S4 in subcatchment 71. The combination of these two trunk systems becomes the trunk system UFC-S2. In the northwesterly portion of this subbasin in subcatch- ment 72, the area bounded by Huffman Road on the north, Flyway Avenue on the south, Spruce Drive on the east and the New Seward Highway on the west, it is recommended that Alternative #3, a trunk system and Alternative #5, a corridor/greenbelt system, be implemented. There are two trunk systems in this subcatchment, UFC-Sl and UFC-S2. The trunk system identified as UFC-S2, should be designed to carry the flows as identified in Table V-3, and should follow the natural drainage path of Furrow Creek to the maximum extent possible in an open channel/greenbelt system which maximizes the recreational/aesthetic potential of the creek. Also in this subcatchment is the lower portion of trunk system UFC-S1, which originates in Subbasin D. This system (UFC-S1) should parallel the Frontage Road as an open ditch and join the UFC-S2 system at the northwest corner of subcatchment 72. Presently, the carrying capacity of UFC-Sl in subcatchment 72 is inadequate for the projected flows orginating in Subbasin D, and it is recommended at the time of improvements to the Frontage Road, that modifications to this open ditch be made in order to increase the capacities to those identified on Table V-3. 5-42 LEGEND 11 ;.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed NODE- 123 W DITCH, CURB & GUTTER 123.4 -ROUTED FLOW IN CFS 10 YR SUBBASIN BOUNDARY mimisim STORM DRAIN SYSTEM OVERLAND FLOW SUBCATCHMENT BOUNDARY TP /5 @I 6.8 CFS TO SUBBASIN G SIC 72 S C 7 1 //SC 70 SC 63 34.0 H4F-&;4A @4- -All Ot Bc__ 2 a ReA 7E)- 76.8 20 \J 72 134.0 TP 4 4j 01 01\ 4*0 SA assists (1663 41-5 J@@fD \3 sells%' of to 4- DA C.r \477 '-CHAP E L D, PORTIO EX STIN@ SYSTEM (t\ 13 ENL@ARGE 7 FOR FUTURE FLOWS 12 WH[q_O@M !sT BE 0 %F M SC 65 '45.8 CF (P SUBBASI' D "b 4 A@ TR,!@c C -7LCDI 3 T 14. 0 is @kC #%1. B-2 if ),3 - v TI? R C_ 72@ UPC P \N,,rR Cr-- On LE 'iLE \1 500, 2 C 6 TR CA 35. C IG PA P URS Engineers @123. _@_44 @@J Anchorage, Alaska LEGEND Existing Proposed 1 .4 - ROUTED FLOW IN CFS 100 YR NODE- 123 123.4 -ROUTED FLOW IN CFS 10 YR > DITCH, CURB & GUTTER STORM DRAIN SYSTEM SUBBASIN BOUNDARY SUBCATCHMENT BOUNDARY OVERLAND FLOW _--e7- 62 15.0 Sc 63..'4_@ 777fl @to 61 L .... . .... . . ...... . ...... - - - -- ------ F @_Y W a v e Ij 7 A tu F N ve 61 R,6 31 _94; TH z m "'X Sc 64,- f SCALE 1 500' 10 '17-P TP E c --E EFE R P! MREACH Of -@P_INNAR_E_@@ TR TR 10 fo EA-S--- i'- L P's . ..... . . .... 0--D236 /I _. --- ( I Sp 6 W 1) PA C, F I D 4/ Tly Lcj@ 7- TP 4 1 /0'L P@ IP, :7P V .34 c @123. U0, Z.6 C URS Engineers Anchorageq Alaska SUBBASIN F Subbasin F drains the northerly portion of the Upper Furrow Creek segment. As shown on Table V-1, the applicable alternatives evaluated were: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; Alternative #4, regional de- tention; alternative #5, corridor/greenbelt and Alternative #6, diversion. In Subbasin F, there exists a large wetland area along the westerly side of the subbasin, more or less paralleling the New Seward Highway. All of the subcatchments in Subbasin F drain to this wetland except subcatchments 75, 77, 78, and 79 and portions of 80. The Wetlands Management Plan classifies this wetlands area as being under "conservation" status. Under this classiFi- cation the area is to be managed in such a way as "to conserve the natural function and values to the maximum extent practicable while permitting uses to occur on wetland fringes and less critical wetlands areas". In the development of the fringes and less critical areas, it is paramount that enough of the wetland be preserved to handle the storm runoff. Table V-2 lists the runoff volume required. There are three major trunk systems identified which are the future storm drainage network in Subbasin F. These trunk systems are: 1) the UFC-NI trunk system which drains subcatch- ments 82 and 83 to the wetlands area; 2) the UFC-N2 trunk system 5-47 which drains subcatchments 76 and 85 to the wetland area; and 3) the UFC-N3 trunk system which drains subcatchment 75, 77, 78, 79 and portions of 80 to Huffman Road. This trunk system ultimately discharges to the Middle Furrow Creek segment at the southwest corner of the subbasin. In the northeast portions of the subbasin are subcatchments 82 and 83 which comprise the area bounded on the north by O'Malley, on the east by the study area boundary, on the south by approxi- mately 112th Avenue, and on the west by wetlands. It is recom- mended that Alternative #3, a trunk system, be implemented throughout both subcatchments with the exception of the westerly portion of subcatchment 83 where it is recommended that Alternative #4, regional detention, be implemented. This trunk system is identified on the map as UFC-N1 and should be de- signed for the flows as shown on Table V-3. This system should be constructed in conjunction with the expansion of roadway network in the future and should consist of roadside ditches and culverts or closed underground storm drainage networks, depending upon the ultimate development pattern of this area. The ultimate discharge of UFC-N1 will be to the wetland. It has been estimated that the volume of flows associated with UFC-N1 will be absorbed into the wetland area (See Table V-2) and there will be no ultimate discharge from this trunk system into the Middle Furrow Creek segment. In the north central portions of the subbasin are located sub- catchments 76 and 85 which comprise the area bounded on the south 5-48 by Klall Road, on the north by an extension of 1121h Avenue, on the east by the study area boundaries and on the west by a wetland area. It is recommended that Alternative #3, a trunk system, be implemented. This trunk system is identified as UFC-N2 and should be designed for the flows as identified on Table V-3, ultimately discharging into the wetland area. It is estimated that the volume of flows associated with UFC-N2 will be absorbed into the wetland area (See Table V-2) and there will be no ultimate discharge from this trunk system into the Middle Furrow Creek segment. This trunk system should be either a series of roadside ditches and culverts or a closed underground storm drainage system paralleling Klatt Road, depending upon the development pattern in the future. It is also recommended that in the westerly portions of subcatchment 85, that Alternative #4, regional detention pond, be implemented in the wetland area. In the southerly portions of the subbasin are subcatchments 75, 77, 78, and 79, which are bounded on the east by the subbasin boundaries, on the north by Klatt Road, on the south by Huffman Road, and on the west by a wetland area. It is recommended that in the easterly sections of this area (subcatchment 75) that Alternative #2, local detention and trunk system, be implemented which consists of a collection system designed for the flows as identified in Table V-3, discharging to the upper portions of the UFC-N3 trunk system. For the remaining portions of the subbasin, (subcatchments 77, 78, and 79), it is recommended that Alternative #3, a trunk system be implemented. This trunk system should be designed to carry the flows as identified on Table V-3. 5-49 Though there presently exists a local collection system along the UFC-N3 proposed trunk routing, this system does not have the carrying capacity for the estimated future flows of the area. However, it is recommended that no action be taken at this time with respect to the replacement of the identified undersized pipes until the projected flow resulting from increased trunk/ collection systems in the contributing area exceeds the existing capacity, or at the time of street improvements to Gregory and Rainbow Avenues, which ever occurs first. In the routing of this trunk system which is identified as UFC-N3 on the subbasin map, for the areas east of Rainbow Avenue and west of Gertrude Street, more or less following Cleo Avenue to the east, there is a potential conflict. On the subbasin map, a suggested routing for this trunk system (UFC-N3) is shown but the selected route must be verified during a final design procedure. This potential conflict results from existing development in the area which is located on the proposed Cleo Avenue. It is suggested that this trunk system be a series of roadside ditches and culverts or underground storm drainage network, depending on the ultimate development pattern of the land. In the westerly section of this subbasin, there exists a large wetland area, approximately paralleling the New Seward Highway between Huffman Road and O'Malley Road. Within this area exist portions of subcatchments 80, 81, 83, 84, 85, and 86. It is recommended for these areas that Alternative #4, regional detention, be implemented as shown on the subbasin map. For areas outside of the identified wetland area but within these 5-50 subcatchment boundaries, a localized collection system consist- ing of roadside ditches should be implemented for the flows identified on Table V-3 draining to this wetland area. The volume of runoff discharging into this wetland area from these subcatchments (80, 81t 84, 85, and 86) and from trunk systems UFC-N1 and UFC-N2 is 24.2 Ac-ft and is shown on Table V-2. As such, the outlet point (node 81) as identified on the subbasin map has no contribution to the middle segment of Furrow Creek. All storm water flow will be assimulated and will either be lost through evapotranspiration or local ground water recharge. 5-51 LEGEND FLOW IN CFS 100 YR Existing Proposed NODE--( @123-4 ROUTED 123.4 _ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM SUBCATCHMENT BOUNDARY OVERLAND FLOW C 83 SC 82@ O'MALLEY 82 22.8 17 18 JC_E_F_ j 44 WI-0 N 4 A UFC- N2 7 19.6 '3D6-8 76 "@Im- .., - - 1161 t*@A' 5 MATCH LINE BB \SC86 SC 8@5 N 7P 4- 4 w 4 1 >1 I< L -WILMA-- Ave 500' SCALE lff URS Engineers Anchorage, Alaska LEGEND 0 11 ;.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed jEFFCi NODE 123 r- @ @. -_-, I 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER 6 SUBBASIN BOUNDARY mmmem STORM DRAIN SYSTEM D, OVERLAND FLOW SUBCATCHMENT BOUNDARY ui \01 L -1,10 N A Ave /SC 77 SC 8'1 SC 80\ M ATifIH- LINE- J, K LIA R &A-D -4- @REGIONAL DETENTION AREA TR 4 TP 3 r- --T U) TP 6 TR \A Fd' N3 2 75 33-5 77 ymill 403 'Q'L@ VAR&Pe-Ate_77@-@ / I @ , 19. 1 0.9 T R 9 48.4 - 71 TR 8 F (@5_33 0 1'7 13- A 12 FC-N3+--_ ---scAlt 1 500' 79 NtAN*- - P F - I 71.3 CFS TO S 78 SUBBASIN G S C79 1w, T@-2 4 LL) Dr T 1<1 zf @u 0.3 URS Engineers Anchorage, Alaska SUBBASIN G Subbasin G drains to the Middle Furrow Creek segment in the north, to Turnagain Arm in the westerly portions and to the south along the Old Seward Highway (OSH). As shown on Table V-1, the various alternative evaluated in this subbasin are: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; and Alternative #3, trunk system. It is recommended that for all but the northerly portions of Subbasin G (subcatchments 100-109 and 111), that Alternative #1, no modifying action be implemented. The present system, which consists of a street and gutter drainage network, roadway ditches, and a pipe system along the major highway corridors, collector streets and the Alaska Railroad tracks are adequate for the projected storm water Flows. It is recommended that in the area along Huffman Road between the New Seward and Old Seward Highways in subcatchment 110, that Alternative #3, a trunk system, be implemented. The projected Flows from the Upper Furrow Creek drainage system indicate that the present 3611 to 48" closed underground storm drainage system along Huffman Road between the Old and New Seward Highways is inadequate for both existing and future flows. It is recommended that a preliminary engineering report be initiated to identify methods to carry the flows as identified in Table V-3. Three routing options which should be looked at in this analysis are: 1) using open channels along Huffman Road; 2) using the now 5-57 non-existant old drainage channel of Furrow Creek located approx- im ate ly 300 to 500 f t . sou th o f Hu f fman Ro ad ; and 3 ) an under- ground storm drainage system similar to the present system. In the analysis of capacity for these alternatives it is suggested that both a complete new system and a system which will carry the excess flows during times of high stormwater runoff be analyzed. The flows from the Middle Furrow Creek segment are dis- charged into the Lower Furrow Creek segment through an existing 3611 culvert located under the Alaska Railroad (ARR) tracks approximately 15,0 ft. south-southwest of the intersection of Huffman Road and the Old Seward Highway. Presently this land is undeveloped and in times of high runoff a large pond exists between the intersection and this culvert. In the future, however, development pressures will increase and this parcel will be filled and developed. The present carrying capacity of the 36" ARR cross culvert is estimated to be 40 cfs, as shown in Table IV-1, versus the estimated required carrying capacity of 317 cfs as shown in Table V-3. It is recommended that at the time this parcel of property is developed that a storm drainage network capable of handling the projected flows be constructed from the storm drainage system located immediately south of the intersection of Huffman and the Old Seward Highway to a series of culverts located beneath the ARR tracks. Because of the critical nature of this drainage system, it is recommended that special precautions for the prevention of icings (as discussed in Appendix B) be implemented during the design process to insure that the culverts under the ARR are capable of allowing runoff to be discharged to the Lower Furrow Creek segment at all times. 5-58 LEGEND 12;.4 ROUTED FLOW IN CFS 100 YR Existing Proposed NODE 123 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM > - cy OVERLAND FLOW SUBCATCHMENT BOUNDARY Q/ Li 71.3 CFS FROM ROM SUBBASIN E SUBBASIN F [email protected] CFS F 7 246.8 <- E Vv S E @vV4 R own H!GH W4 NSH PORTION'OF EXISTING SYSTEM @_77W WHICH MUST BE ENLARGED FOR FUTURE FLOWS Y' 4 c i r F, (0- 18.8 CF@S -FROM 4-N S-- SUSBASIN H [email protected] G, < 0 40 A L@j 6.5 CFS 62.0 FROM SU BASIN H 8g.4 OSH-1 10 4 107 9OL6 17.7 47.7 1@2 n,/ 311.7 CFS n TO SUBBASIN K W-@ AL_ 13.0 ru 7 nv sc"I'll, to /41-1 S'C/ lit" N11 r! / v SC108 41.1 ny 4, L A K 4 LE 500" 1 /,= 41.1 CFS TO TURNAGAIN AR@@2 CFS @123. URS Engineers Anchorage, Alaska LEGEND Existing Proposed @123.4) ROUTED FLOW IN CFS 100 YR NODE_Q 23 .4 ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM SUBCATCHMENT BOUNDARY OVERLAND FLOW 'S ------------- 7.1 CFS TO POTTIiR-1*1kRSf! VIA NEW SEWARD HW,(. SC I 1,60 w scap uj Al 100 . ...... ... . OSH-2 K lor, .-A - ----- -@Ay - - - -71 35.8 C, lot 4 X C TUR S 0 40 -c al 0 R 35.8 CFS TO 0 TURNAGAIN ARM sy, RAIL9,04D SCALE 1 URS Engineers 5 Anchorage, Alaska t SUBBASIN H Subbasin H drains the northerly part of the Middle Furrow Creek segment to Furrow Creek along Huffman Road. The alter- natives evaluated for this subbasin as shown on Table V-1 are: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk sytem; and Alternative #6, diversion. It is recommended for this subbasin that Alternative #3, a trunk system, be implemented. This system, because of the drainage area involved and the existing roadway ditch system, will be a minor drainage system and will resemble a local collection system and should be a series of roadside ditches and culverts designed to carry the flows identified in Table V-3. These collection systems should be constructed in conjunction with the expansion of the existing roadway network in the future to serve the ultimate growth of the area. There are no new major drainage structures recommended for this subbasin; as such the following map indicates only existing drainage systems and projected flows from the area. 5-63 LEGEND Existing Proposed 123.4 - ROUTED FLOW IN CFS 100 YR NODE- 123 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM SUBCATCHMENT BOUNDARY OVERLAND FLOW BROADDUS St J lui SC'126 < 0 9 . . . ...... . . .. . ... ....... . .... .. L3 /P 4 TP 8 S C 125 HIG.'-ILAND r- Avo SEC TP 8 TP F A- T 9,41 St B@Elf4fR T P F TP S@ TR P sl@ TP C -26 TP 4 T P, @7 3,44 126 FG . .... .. N s8@t @SC7ALE 1 500' IL CF$ TO BBASIN G CFS . .. ... C" SU48 _A$1 N LLJ Lij 15 \TP O@3123 URS Engineers SUBBASIN H Anchorage, Alaska February 1983 FigureV-8 SUBBASIN I Subbasin I drains north, out of the study area. As shown in Table V-1 the various alternatives evaluated in this subbasin were: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; and Alternative #4, regional detention. It is recommended that Alternative #3, a trunk system, be implemented throughout the subbasin. This system, because of the drainage area involved, will be a minor drainage system and will resemble a local collection system and should be a series of roadside ditches and culverts designed to carry the flows identified in Table V-3. These collection systems should be constructed in conjunction with the expansion of the existing roadway network in the future to serve the ultimate growth of the ,area. The South Anchorage Drainage Study should be consulted prior to design on any storm drainage facilities in this area to determine the proper routing of flows from this subbasin into the South Anchorage study area. 5-67 LEGEND ROUTED FLOW IN CFS 100 yR Existing Proposed NODE-(g@123-4 I@ 23.4 _ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM SUBCATCHMENT BOUNDARY VVLKLAND FLOW 0 SC 136 SC 135 it .5-C -M, TO 30! - 71 THE SOUTH ANCHPRAGE STUDY@-AREA :x) 0 T-R C N 0 [email protected] 3Q5 17.1 135 SCALE 1 0 . .. ....... t7lnet- ---------- TP 4 . . . . ....... p -P Ij URS Engineers SUBBASIN I Anchorage, Alaska February 1983 Figure V-9 SUBBAIIN I Subbasin J drains north, out of the study area. As shown in Table V-1 the various alternatives evaluated in this subbasin were2 Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; and Alternative #4, regional detention. It is recommended that Alternative #3, a trunk system, be implemented throughout the subbasin. This system, because of the drainage area involved, will be a minor drainage system and will resemble a local collection system and should be a series of roadside ditches and culverts designed to carry the flows identified on Table V-3. These collection systems should be constructed in conjunction with the expansion of the roadway network and urbanization in the area to serve the ultimate growth of the area. There are no new major drainage structures recommended for this subbasin. As such the following map indicates only existing drainage systems and projected flows from the area. The South Anchorage Drainage Study should be consulted prior to design on any storm drainage facilities in this area to determine the proper routing of flows from this subbasin into the South Anchorage study area. 5-71 LEGEND 12;.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed NODE- 123 1234 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM > OVERLAND FLOW SUBCATCHMENT BOUNDARY 0 -ij _j R_ S ETC T-1, w _j _j IN, 12.3 CFS TO THE SOUTH ANCHORAGE 9.2 CFS TO THE SOUTH "ANCHORAGE STUDY -9, 8.4 CF$ AR STUDY STUDY AREA EA 1--Rood-- 16.1 A 14 "O@ 9.2 kL A 0 a d L A @!,j 1\, Efs, Ct it: Lane Dr I CE' .Lu SIC 147/ !n_ SC 146 7)1 0 ANe E L L Ef@ _j Place I @\ LQ U_j 0 121s! _;7 Ave 12 1 st Ave TAIi, 67 4 Lu Wo'' SCALE 1 "=fWO' cc al _@_ AN Ct@ Lu < C0 @ocd I L LTO 2 9-2 n123. La n URS Engineers Anchorage, Alaska IUBBAIIN I Subbasin K forms t-he Lower Furrow Creek drainage segment. As shown in Table V-1, the various alternatives evaluated in this subbasin are: Alternative #1, no modifying action; Altern- ative #2, local detention and trunk system; Alternative #3, trunk system; Alternative #4, regional detention; and Altern- ative #5, a corridor/greenbelt system. This subbasin is the most complex basin from a stormwater management/plan point of view in this study. There are basically three types of systems within this subbasin: 1) the existing system of overland flow in undeveloped areas, 2) the existing storm drainage trunk and collection system which discharges to the Lower Furrow Creek segment, and, 3) the Lower Furrow Creek segment. In general, the recommendations for this subbasin are: 1. In areas of existing overland flow, a trunk or collection system should be implemented to collect and discharge the flows to the Lower Furrow Creek segment. 2. In areas where there is an existing trunk/collection systems discharging to the Lower Furrow Creek segment, no modifying action shall be taken, as these systems are adequate to carry the projected flows. 5-75 3. An enlargement of the corridor/greenbelt system to increase the channel carrying capacity of Lower Furrow Creek for the projected flows and the installation of suitable structures at major intersection crossings along Lower Furrow Creek capable of handling the projected flows. The following paragraphs discuss each of the subbasins and their par- ticu lar requirements. The main corridor of the Lower Furrow Creek segment is contained in subcatchments 160, 162, 164, and 166. It is re- commended that in these subcatchments that Alternative #3, a trunk system, and Alternative #5, a corridor/greenbelt system, be implemented. The majority of this system is already in place. Presently, a corridor/greenbelt for Lower Furrow Creek exists from the outlet of Furrow Creek to the Turnagain Arm located in Johns Park to approximately the crossing of the creek at Clipper Ship Court located in subcatchment 160. It is recommended for this portion of the Lower Furrow Creek segment, that the existing channel capacity be expanded to carry the flows as identified in Table V-3, for areas identified on the subbasin map with in- adequate capacity for future flows. Particular critical areas requiring upgrading are the Furrow Creek crossings of Johns Road, Mariner Drive and Clipper Ship Court. Modifications to these creek crossings should be implemented to increase their carrying capacity to the flows identified in Table V-3. Pre- sently these crossings consist of 18" - 3611 diameter corrugated metal pipe. Because of th e nature of this corridor/greenbelt 5-76 it is suggested that either a bridge or a plate arch culvert be constructed to increase the carrying capacity while simul- taneously maximizing the aesthetic potential of this creek corridor. In addition with the use of these types of stream crossings, there will be minimal requirements for maintenance as related to icings, and there will be less potential for property damage resulting from flooding at these intersections during times of high volume runoff in the spring. In the upper reaches of Lower Furrow Creek from the Alaska Railroad to approximately Clipper Ship Court, located in sub- catchment 160, the existing channel of Furrow Creek does not have adequate capacity for existing or future flows, and the carrying capacity of the corridor/greenbelt for this area needs to be increased. Additionally, there is a recent fill on the south side of this portion of the stream segment which is encroaching upon Furrow Creek without adequate side slope soil stabilli- zation. It is recommended that the side slopes of this fill area be stabilized immediately and that considerations of how this fill will affect the corridor/greenbelt nature of this creek segment be analyzed when improvements to the creek corridor are made. In conjunction with the channel modifications to Furrow Creek, it is recommended that design procedures be used which will allow for the base flow of Furrow Creek to be channelized. Also within these subcatchments, in particular subcatchments 164 and 166, there exists a large Future open space/park area, 5-77 and for these areas it is recommended that Alternative #1, no modifying action, be implemented in order to maintain the natural characteristics of these areas to the maximum extent possible. In the southeast section of this subbasin is subcatchment 167 which is bounded on the west by approximately Gregg Lane, on the north by the study area boundary and on the west by the Alaska Railroad, it is recommended that Alternative #3, a trunk system, be implemented for subcatchment 167. This system, because of the drainage area involved, will be sized as a local collection system and should be a series of roadside ditches and culverts designed to carry the flows identified in Table. V-3. This system should be constructed in conjunction with the expansion of the existing roadway network in the future to serve the ultimate growth of the area, the system should discharge to Furrow Creek in the vicinity of Division Street. In the northcentral portions of this subbasin, are subcatch- ments 168 and 169 which are bounded approximately on the east by Gregg Lane, on the south by HufFman Road, on the north by the study area boundary and on the west by Ellen Avenue. Altern- ative #1, no modifying action, is recommended for this area. Presently, these two subcatchments are in a developing state, and the existing trunk system for these sub-catchments is an open ditch/ culvert system along Johns Road. As shown in Table V-3 and Table IV-1, the capacity of this system is adequate to handle projected flows. 5-78 In the northwest portions of the subbasin, the boundaries of Ellen Street on the east, Hilltop Drive on the west and south, and the study area boundary on the north identify sub- catchments 170 and 185. It is recommended that Alternative #3, a trunk system, be implemented for this area. This system, because of the drainage area involved, will resemble a local collection system and should be a series of roadside ditches and culverts designed to carry the flows as identified in Table V-3. This system should be constructed in conjunction with the expansion of the existing roadway network in the future to serve the ultimate growth of the area and discharge to trunk system LFC-N1 at approximately the intersection of Timberlane Drive and Huffman Road as shown on Figure V-II. The LFC-N1 trunk system should be a closed, underground pipe corridor to its confluence with Furrow Creek, located approximately where Furrow Creek discharges to Turnagain Arm in subcatchment 166. To preserve the park/open space area located in 166 which the LFC trunk system traverses, it is recommended that no inlet points to this trunk system be planned. Approximately in the center of the subbasin is subcatchment 165. For this subcatchment it is recommended that Alternative #1, no modifying action, be implemented. The discharge from subcatchments 168 and 169 flow through subcatchment 165 along Johns Road and the existing storm drainage network along Johns Road has adequate carrying capacity for the future volume of flows as indicated in Table V-3. 5-79 In the southeast portions of this subbasin are subcatch- ments 161 and 163. It is recommended that Alternative #1, no modifying action, be implemented in this area. Presently, the existing storm drainage network in these two subcatchments can handle the estimated future flows for the area involved. 5-80 LEGEND 100 yR Existing Proposed ROUTED FLOW IN CFS NODE-(jjjj@ 123.4@ _ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM 0 OVERLAND FLOW SUBCATCHMENT BOUNDARY -irs SC 17,0\ /Sc 185 "IS K, Fq RO d LU m ... ...... . no -12 01 C) 24@4-i -ecAv a L, -42 a ve 4 . . .... d, 4 S-0 4. P-A 14*-& Y- A*@ X @K Pood 9.2 -f- P, 1 5 32.3' 2 J-2 3x d 6.3 1 9 68 12.8 THO@IjASSON Drivc 7 @C;l .10 SC 162 -v 40 ,Eli C 165 2 0 11.4 166 165 SCALE 1 500' LFC 36 16 2.5 43/ 369. j G@ 392.0 CFS TO' SIC 166 TU RNAGAIN A RM PO RT ION OF EXISTING SYST M WHICH MUSH BE ENLARGED Avnnuc 6 FOR FUTURE FLOWS (TYP) SC-164 10 Ave V N URS Engineers Anchorage, Alaska SUBBASIN L Subbasin L drains to Turnagain Arm. As shown in Table V-1, the various alternatives evaluated are: Alternative #1, no modifying action; Alternative #2, local detention and trunk system; Alternative #3, trunk system; and Alternative #4, regional detention. It is recommended that in the westerly portions of the subbasin (subcatchments 187, 188 and 189), that Alternative #1, no modifying action, and Alternative #3, a trunk system, be implemented. In portions of these subcatchments there are major wetlands designated to be conserved. Development should be such that it does not alter the drainage of the area. There- fore, in these areas no modifying action should be implemented. For the remaining area located in these three subcatchments, Alternative #3, a trunk system, should be implemented. This trunk system will be a minor drainage system and will resemble a collection system because of the drainage area involved. These collection systems should be sized for the flows as identified in Table V-3. Also in this area, there are two major open channel drainage corridors, which presently drain portions of Klatt Bog located north of Klatt Road. An analysis of these contributing flows should be conducted and these flows should become additive to the flows identified in Table V-3, in the design of future systems for these subcatchments. The flows in Table V-3 for those subcatchments which contain the designated wetland area, do not contain any flow routed from the wetland. Also, the design 5-83 criteria of future drainage systems for the areas located in the near proximity of the wetland should contain considerations which ,insure these wetlands are not drained, thus insuring the future of this ecosystem. It is recommended that Alternative #3, a trunk system, be implemented for the remaining subcatchments, subcatchments 184 and 186. This system will resemble a series of local collection facilities which drain to Turnagain Arm and should be a series of roadside ditches and culverts or a closed stormwater drainage network, depending upon the development patterns of the sub- catchments. This system should be constructed in conjunction with the expansion of the roadway network in the future and should be designed to carry the flows as identified in Table V-3. The Wetlands Management Plan assigns the classification of "conservation" to the wetlands in Subbasin L. Under this designation the wetlands would be managed in such a way as to conserve the natural function and values to the maximum extent practicable while permitting uses to occur on wetland fringes and less critical wetland areas. It is imperative that in the potential development of part of this wetland, that the drainage cha racteristics of the wetland be retained, and not compromised or eliminated. 5-84 LEGEND Existing Proposed 123.4 - ROUTED FLOW IN CFS 100 YR NODE- 123 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM C UVEHLAND FLOW SUBCATCHMENT BOUNDARY KLATT BOG SC 189 SIC 188\ SC 187 SC 186-\ 189 11.2 j L V U 11. 2 CFS TO TURNAGAIN ARM' 188 6.6 616 CFS TO 2.6 TURNAGAIN ARM 187 2.6 CFS TO JURNAGAIN ARM TP 4 SCALE 1 500' @123. URS Engineers Anchorage, Alaska LEGEND 12J.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed NODE- 123 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER SUBBASIN BOUNDARY STORM DRAIN SYSTEM ENT BOUNDARY OVERLAND FLOW SUBCATCHM ROad T ER ------------ -1.anq Y" > 0@ C WINF L Cir m 0 \_Z > 121SI Is-- ve 12 t ,-rA TPA C T 4 LU 4/'1 _j 77 / NI 4j 12@ "7 , 1 2 Ile 7 UJ 123rd Z' L -7 MI @'Y1_ v T I&NI, S 96VI HUFrW4N T 'y E, 7p C SIC 184- N 184 21.7 21. 7 C FS TO TURNAGAIN ARM T_ SCALE V= 500't n123. URS Engineers SUBBASIN L Anchorage, Alaska 2 of 2 February 1983 Figure V-12 SUBBASIN M Subbasin M drains to Turnagain Arm. As shown in Table V-1, the various alternatives evaluated in this subbasin were: Alternative #1, no modifying action; and Alternative #3, a trunk system. There are two different types of drainage systems within this subbasin: 1) a collection system which consists primarily of street and gutter flow; and, 2) corrugated metal pipe out- falls from the collected storm drainage network which discharge to Turnagain Arm. The present collection system is adequate to handle the projected stormwater Flows. But e,ach of the three outfalls, one located in each of the subcatchments, 200, 201, 202, are inadequate to carry the combined discharge from the respective collected- area. As shown in Table IV-1, each of the outfall pipes have an estimated capacity of approximately 6 cfs, whereas the range of required capacity for these outfall pipes for projected flows, is between 11.5 and 34.5 cfs. It is recommended that each of the individual outfall pipes be removed and replaced with a storm drainage pipe of sufficient size to handle the capacity shown in Table V-3. It is also recommended, that because of the steep slope of the outFall pipe to Turnagain Arm over the bluff in this area, that design considerations be implemented to minimize the velocity of the runoff water and associated erosion of the bluff. 5-89 LEGEND 12;.4 - ROUTED FLOW IN CFS 100 YR Existing Proposed NODE- 123 123.4 -ROUTED FLOW IN CFS 10 YR DITCH, CURB & GUTTER 19 SUBBASIN BOUNDARY STORM DRAIN SYSTEM J 1--,- '0( BRINY cir'; SUBCATCHMENT BOUNDARY OVERLAND FLOW 15 1 19 J@a-, A:-, VYAT E .7, C 17 J, S PORTION OFEXIP H A R 8 0R SYSTEM WHICI-y UST BE ENLARGED FOR FUTURE-FLOWS (TYP)@ 17 SIC 15 ON GALI-r i i n ,4 T H n i J@ 12 onuG C v 10 e Q) 13 GQ BOON 202 4.5 SIC 202 34.5CFS TOTURNAGAIN ARM 201 11. 5/ 4.1 20 24.6 11.5CFS TO TURN- AGAIN ARM - 24.7 CFS TO TURN- AGAIN ARM SCALE 1 500' n123, URS Engineers Anchorage, Alaska TABLE V-3 DESIGN PARAMETERS SUBCATCHMENT DESIGN TRUNK SYSTEM ROUTING TRUNK SYSTEM REGIONAL DETENTION SUBCATCHMENT FLOW (cfs) SUBCATCHMENT (NODE) DESIGN FLOW (cfs) POND VOLUME SUBBASIN AODE 10-Yr 100-Yr From To 10-Yr 100-Yr (Ac-ft) A 1 5.9 10.7 1 Rabbit Creek 5.9 10.7 B 11 12.0 21.6 11 Rabbit Creek 12.0 21.6 12 11.4 20.6 12 Rabbit Creek 11.4 20.6 13 11.8 21.3 13 Rabbit Creek 11.8 21.3 14 10.2 18.4 14 Rabbit Creek 10.2 18.4 15 2.5 4.5 2.5 4.5 16 5.9 10.7 15/16 16/Rabbit Creek 8.4 15.2 17 5.2 9.4 17 Rabbit Creek 5.2 9.4 C 25 9.5 17.4 9.5 17.4 26 11.8 21.2 25 26 34.6 62.5 27 9.1 16.5 27 26 14.6 26.2 28 6.1 10.8 28 27 6.1 10.8 29 15.0 26.7 26/29 29/NSH (1) 44.8 80.6 D 45 20.7 36.9 20.7 36.9 46 12.9 23.1 45 46 31.2 55.7 47 4.8 8.8 46 47 41.1 73.5 48 7.0 12.4 7.0 12.4 49 4.2 7.4 47/48 49 43.2 77.1 0.43 50 3.3 6.0 49/50 50/(722) 45.8 (2) 81.9 (2) 0.43 (1) New Seward Highway (2) Not Detained Flow Flow/with Detention 35.5 cfs, 10 Yr. mmmmmm mm mm mmmimmm mmm TABLE V-3 DESIGN PARAMETERS (CONTINUED) SUBCATCHMENT DESIGN TRUNK SYSTEM ROUTING TRUNK SYSTEM REGIONAL DETENTION SUBCATCHMENT FLOW (cfs) SUBCATCHMENT (NODE) DESIGN FLOW (cfs) POND VOLUME SUBBASIN /NODE 10-Yr 100-Yr From To 10-Yr 100-Yr (Ac-ft) E 60 23.6 42.5 23.6 42.5 61 12.6 22.9 12.6 22.9 62 15.0 27.5 15.0 27.5 63 17.3 31.4 61/62 63 41.5 75.6 64 14.0 24.8 14.0 24.8 65 13.0 23.2 64 65 57.6 99.2 66 9.8 17.5 60 66 30.9 55.5 67 5.2 9.5 66 67 35.9 64.6 68 5.5 10.0 67/65 68 62.9 108.9 69 7.2 13.1 68 69 67.7 117.8 70 8.0 14.3 63 70 48.0 87.2 71 14.3 25.5 70/69 71 125.1 225.4 72 (720) 8.8 16.1 71/72/50 72/(720)/(720) 134.0 241.5 (176.8) (318.0) F 75 33.5 60.4 33.5 60.4 76 19.6 35.4 19.6 35.4 77 8.5 15.4 75 77 40.3 72.7 78 (771) 10.5 19.0 77/78 (771) 10.5 (48.4) (87.4) 79 9.5 16.0 80/79 79(722) 71.3 127.6 (246.8) (943.3) 80 26.1 47.1 (771) 80 65.3 117.9 59.75 81 0.9 1.6 81 Wetlands 0.9 1.6 67.25 82 22.8 40.7 22.8 40.7 83 20.0 35.5 82 83 39.6 70.5 53.00 84 13.1 23.4 83 Wetlands 13.1 23.4 41.25 85 24.1 36.0 76 85 36.8 60.3 21.00 86 18.3 32.2 86 Wetlands 18.3 32.2 40.25 (1) New Seward Highway TABLE V-3 DESIGN PARAMETERS (CONTINUED) SUBCATCHMENT DESIGN TRUNK SYSTEM ROUTING TRUNK SYSTEM REGIONAL DETENTION SUBCATCHMENT FLOW (cfs) SUBCATCHMENT (NODE) DESIGN FLOW (cfs) POND VOLUME SUBBASIN /NODE 10-Yr 100-Yr From To- 10-Yr 100-Yr (Ac-ft) G 100 13.1 23.6 100 NSH (1) 13.1 23.6 101 35.8 64.6 101 Inlet 35.8 64.6 102 41.3 74.5 102 Inlet 41.3 74.5 103 46.2 83.3 103 Inlet 46.2 83.3 104 7.1 12.8 104 NSH (1) 7.1 12.8 105 9.4 17.0 9.4 17.0 106 16.2 29.2 106 Inlet 16.2 29.2 107 (1071) 8.5 15.4 105 107 20.6 37.3 (7.7) (13.9) 108 28.1 50.7 111/108 108/Inlet 41.1 84.2 109 17.2 31.1 107 109 37.0 67.0 110 (128) 14.4 26.1 (722)/ (128)/ 47.7 86.4 109/110 110/(128) (317.7) (571.4) ill 13.0 23.5 11 108 13.0 23.5 H 125 18.8 34.1 125 (128) 18.8 34.1 126 6.5 11.4 126 (128) 6.5 11.4 1 135 17.1 '30.9 136 SA (3) 17.1 30.9 136 16.0 28.4 135 136 30.5 54.6 1 145 8.4 14.9 145 SA (3) 8.4 14.9 146 16.7 16.1 146 SA (3) 9.2 16.1 147 27.8 22.1 147 SA (3) 12.3 22.1 (1) New Seward Highway (3) South Anchorage Study Area mmm mmm am TABLE V-3 DESIGN PARAMETERS (CONTINUED) SUBCATCHMENT DESIGN TRUNK SYSTEM ROUTING TRUNK SYSTEM REGIONAL DETENTION SUBCATCHMENT FLOW (cfs) SUBCATCHMENT (NODE) DESIGN FLOW (cfs) POND VOLUME SUBBASIN /NODE 10-Yr 100-Yr From To 10-Yr 100-Yr (Ac-ft) K 160 (159) 11.7 21.6 (128)/ (159)/ 342.7 616.9 167/(159) (159)/160 (300.8) (595.0) 161 3.4 6.1 3.4 6.1 162 6.1 11.0 169/168/163 162 362.5 651.7 163 3.5 6.1 3.5 6.1 164 3.6 6.5 162 164 369.8 664.9 165 11.4 20.1 11.4 20.1 166 11.1 19.4 170/164 166 392.0 704.2 167 17.6 31.7 166 Inlet 17.6 31.7 168 12.8 23.0 12.8 23.0 169 6.3 10.7 6.3 10.7 170 9.9 16.7 18.0 32.3 185 9.2 16.6 185 170 9.2 16.6 L 184 21.7 38.3 184 Inlet 21.7 38.3 186 37.7 69.2 186 Inlet 37.7 69.2 187 2.6 3.5 187 Inlet 2.6 3.5 188 6.6 8.6 188 Inlet 6.6 8.6 189 11.2 19.4 189 Inlet 11.2 19.4 M 200 24.6 34.4 200 Inlet 24.6 34.4 201 11.5 20.8 201 Inlet 11.5 20.8 202 34.5 62.4 202 Inlet 34.5 62.4 IMPLEMENTATION OF ALTERNATIVES Existing and future drainage standards will be met by using the identified alternatives per each subbasin/subcatch- ment as presented in this chapter (summarized in Table V-1). The Municipality of Anchorage, in the implementation of their capital improvement program and in conjunction with private developers, should provide the major trunk systems necessary to convey stormwater runoff. For each of the recommendations contained in this study the developer should conduct a pre- liminary engineering feasibility analysis to: 1) identify the most acceptable type of drainage structure and system in accordance with the alternatives recommended herein, and, 2) identify the most feasible route alignment as per the land use pattern at the time of implementation. The results of this analysis will be subject to Municipal review and approval. The design engineer prior to designing any of these systems for the study area, should use the following procedure as a guideline, but still rely on professional judgement where applicable. Use the composite map bound in the back of the report to identify the subbasins, subcatchments, flow paths, and major existing and proposed drainage ways for the areas involved. 5-101 Review the alternatives and associated recommendations of each subbasin and/or subcatchment to be served, and update accordingly for the present land use patterns. From the appropriate subbasin map identify flow rates for subcatchments involved (particularly when identifying future collection systems) , identify that portion of the subcatchment to be served, and compute the proportionate amount of contributing flow for each segment of the system to be designed. Identify the type of storm drainage system to be used. This system should meet the criteria as presented in Chapter 3 for this study, as well as the zoning regulations in effect at the time the design is performed. Use accepted Municipality of Anchorage Department of Public Works design criteria. Review Appendix B of this report (Discussion of Icings in Drainage Systems), use of this section will minimize the amount of icing associated with drainage structures. A deficiency of many of the existing drainage systems within this study area are the result of inadequate storm drainage design, as related to icings. With proper design considerations for icing conditions, operational and maintenance costs associated with icinq conditions for these drainage structures can be minimized. 5-102 I "t I I J, .1 i I i I I i I 1 0 I I APPENDIX A COMPUTER ANALYSIS ox/@\\\,\ i a 9 APPENDIX A COMPUTER ANALYSIS Two computer programs (mathematical models) were used in this study to perform the hydrologic/hydraulic and pollutant loading analysis. System Analysis Model (SAM), was used to compute 1) the runoff hydrographs and pollutographs from each subcatchment, 2) to route the hydrographs and pollutographs through the drainage system, and, 3) to size the required capacities for the drainage facilities, from a given design storm event. Storage Treatment Overflow Runoff Model (STORM) was used to compute the pollutant washoff loads in each sub- catchment and the resulting pollutant concentrations in the runoff, from several long-term historical events (May to Sept.). The usages of both models are well documented in the respective user manuals (SAM: O'Neel et al, 1979; STORM: U.S. Army Corps of Engineers, 1976). The following sections describe the compilation of the input data files for each model, the assumptions made, and how these files can be used in further analysis for the study area. Analysis using SAM Model Table A-1 lists the SAM input data requirements, and the data sources and remarks for compiling the input data files for the study area. A-1 TABLE A-1 INFORMATION REOUIRED FOR SAM MODEL Input Data Requirements Data Sources Remarks 1. A hyetograph for the Precipitation records from A design event of 10-year'return event being simulated Anchorage Airport (1953 to frequency was used (see Design 1980) Storm, Chapter III) 2. Subsurface flow accounting Table 4 subsurface values, The surface runoff is not parameters for previous SAM methodology manual sensitive to the variability areas; e.g. soil moisture (Task memorandom No. 7), of these parameters for the zone storages, evapotrans- Campbell Creek Drainage design condition of the soil piration indices, and ground Study is assumed to be freezing. water characteristics 3. Computation time increment 10 minutes 4. Pan evaporation rate Weather data, Anchorage An evaporation rate typically Airport, U.S. Weather for the time of the design Service event (April). 5. Interception and de- Field survey for the Used 0.10 in. which is an pression storage for project area average value for the impervious area impervious area. 6. Fraction of land use Existing land use - For the design event when the area which is impervious Municipal Planning Dept. freezing ground condition was 1980 aerial photograph assumed that 85 to 90"0' of the and field investigation; impervious area was treated Future land use - Munici- to be impervious in the model. pal Comp. Land Use Plan 7. Hydraulic characteristics Table B-1 "Land Use The hydraulic characteristics of pipe, gutter, and Classifications", of closed conduits and natural channel cross section Campbell Ck.Drain. channels are represented by Study, Task Memo. 7, dimensionsless curves so that and section@-data -61 one set of coefficients can be and D2 applied to all cross sections of similar shape. An ON 'MI as am am am, am Ow ow an 1@ 1@ am an "am, M M No M Sm M 1@ 4M TABLE A-1 (Continued) Input Data Requirements Data Sources Remarks 8. Daily rates of pollutant Table 5, "Land Use Build- Data for winter months were buildup in pounds per up Matrix", Campbell Ck. used. acre Drain. Study, Task Memo. 7 Pipe and gutter inverts, Record drawings for sub- Data for winter months were Lengths and other hydraulic divisions, and highway, and used. characteristics field investigations 10. Drainage basin area, slopes Municipal Planning Dept. Scale used was 1" = 500'. and overland flow length topographic maps and field investigation 11. Local retention (depression) Municipal Planning Dept. Modeled as the fraction of storage area topographic maps and field the subcatchment area that investigation contributes no runoff. Much of the input data for the SAM model, such as the subsurface flow parameters, percent pervious in each land use type, and pollutant buildup rates, were calibrated during the investigation of Campbell Creek drainage basin. For lack of streamflow data in the Furrow Creek-Rabbit Creek study area, these constants were not able to be calibrated specifically for the study area. However, because of the similar hydrologic and land use settings between the Campbell Creek drainage basin and the study area, the model constants should represent the study area reasonably well. As local stream data for Furrow Creek and Rabbit Creek become available, the validity of these constants as applied to the study area should be evaluated. The local depression storage areas were modeled as the portion of the subcatchment area that contributes no runoff. The extent of the existing depression storage area is measured from the basin aerial maps and later substantiated by field investi- gations. The possible future depression area in the basin is extrapolated from the future land use information. For example, the existing depression area will assume to be lost if the land use for a parcel of land is changed from a low density residential or undeveloped to high density residential, commercial or in- dustrial land use classifications. To execute the program, the input data files were organized in three groups: 1. Data base file - consists of all the data listed in Table A-1. A-4 2. Analysis command file - instructs the model to compute overland flow, then the flow routing (using Kinematic wave method) and pollutant washoff. 3. Output command file - selects the portion of the output results to be printed; e.g., inlet and routed hydrographs and pollutographs. For the convenience of basin analysis, the input data files for the following subbasins were grouped together: Subbasin Subbasin is tributary to: A, B Rabbit Creek C, G (southern portion) Potter Marsh & Turnagain Arm D, E, F, G (remaining), H, K Furrow Creek I, J South Anchorage Study Area L, M Turnagain Arm The SAM model was run on the Boeing Computer Service (BCS) CTS system. Table A-2 lists the SAM input data files for this project. These files are stored on magnetic tape which is located in the office of the Municipality of Anchorage Department of Public Works. SAM input files can be easily retrieved and modified for future applications. The subcatchment (NODE) and drainage system (LINE) data files along with the design storm (HYETOGRAPH) data file will probably require revisions with time to simulate changes occuring in the future. These files, NODE, LINE, and HYETOGRAPH, are contained in the subbasin data files. A-5 TABLE A-2 SAM INPUT DATA FILES Subbasin Data Base File* Analysis Command File Output Command File 1. A, B (existing land use) ABEX FT12FOOl Analyze FT12F0Ol OUTPT CNTL*** (future land use) ABFU FT12FOOl AB 2. C, G (existing land use) CGEX FT12FOOl CG NODEOUT (future land use) CGFU FT12FOOl 3. D, E, F, G H, K DEFGHK CNTL DEFGHK NODEOUT (existing land use) DKEX SUBCAT (future land use) DKFU SUBCAT 4. I, J (existing land use) IJEX FT12FOOl 13 NODEOUT (future land use) IJFU FT12FOOl L9 M (existing land use) LMEX FT12FOOl LM NODEOUT LMFU FT12FOOl These data files are set up for running the quantity portion of the analysis; to conduc quality runs, merge all files with POLLUT DATA. ANALYZE FT12FOOl is a common file for all subbasins with the exception of the first car specifies the unque basin name, e.g. AB, CG, DEFGHK, IJ and LM. OUTPUT CNTL is a common file for all subbasins, it needs to merge with the accompanying e.g. AB NODEOUT to form an output command file. The output of run using the SAM model typically contains inlet and routed hydrographs at selected locations and a table summarizing the drainage system, e.g. the peak flow, surcharge conditions, the existing and required capacity. 100-year Storm: Runoff and Routing The computation of the runoff and routed flow quantities was performed in a somewhat different manner than that used for the 10-year storm. The following methods presents the method of calculation used for the 100-year storm event. The computer run performed using the SAM model to generate the 100-year storm runoff was based on the assumption that all existing depression area will be destroyed in the future when the land is developed. However, later this assumption was modified such that a portion of the depression area will be re- tained in the future. Instead of performing another computer run based on this new assumption, a correction factor was applied to the design flow obtained in the computer run in order to account for the loss of runoff due to the depression area. To produce the adjusted 100-year storm runoff values the flow ratio of the total non-depression area to total area was calculated for those subcatchments that have a significant amount of depression area. The corresponding subcatchment design flow was then adjusted by multiplying by the appropriate flow ratio. A-7 The trunk system design flow was calculated using the peaking factor for the 10-year storm event and applying it to the 100-year storm event. For example: Flow from node 1 is routed to node 2. The peak flows for node 1 and node 2 inlet hydrographs are 10 and 20 cfs, respectively. The resultant routed hydrograph of node 2 from the model is 27 cfs. Then the peaking factor is 27 0.9. 10 + 20 Analysis using STORM Model The STORM Model was used to compute the pollutant washoff loads from each subbasin. The Boeing Computer Service (BCS) EKS System was used to run the STORM model. To better characterize the pollutant loads, several of the subbasins defined for the use of the SAM model were further divided into several subbasins. The land use data used with the SAM model were used with the, STORM model. Table A-3a and A-3b give the existing and future land use data for STORM model input. Table IV-7, the pollutant buildup matrix shown in the main text of this report, was used to represent the summer pollutant loading rates for the model. The STORM model does not have the capability to route either the flow or the pollutant through the drainage system; so that the drainage data developed for the SAM model were not used. A-8 TABLE A-3@ IAM USE DATA FX)R S70M MCDFL INPUT MSTING SYSTEM) STOW TOM LAND USE OF TCTAL BASIN AREA DEPRESSIGN RUNOFF SUB- AREA Tc DEPTH BASIN CATCHMENTS (AC) LD HD MF IN CO LF BM UP LF EE (Hr) (in) AB 1,11,12,13,14 333 98 2 0.49 .100 15,16,17 C 25,26,27,28,29 168 58 20 22 0.64 .155 D 45,46,47,48, 49,50 198 14 66 20 1.28 .140 El 61,62,63,70 252 70 30 1.39 .160 E2 60,64,65,66,67, 68,69,71,72 340 20 72 8 72 4.72 .260 Pi 76,81,82,83,84, 85,86 423 54 11 17 18 1.27 .188 F2 75,77,78,79,80 344 35 7 32 26 2.08 .229 GI 100,104 71 75 25 0.59 .100 G2 101,102.103, 106,108,1081, ill 234 21 55 5 10 9 0.13 .123 G3 105,107,109, 110,1071 221 22 17 59 2 0.33 .100 H 125,126 138 45 15 40 0.45 .100 1 136,137 131 28 34 38 0.28 .100 1 145,146,147 81 81 4 15 0.12 .138 KI 167 60 20 30 50 1.02 .275 K2 160,161,162, 163,168,169 163 14 40 20 1 25 0.76 .203 K3 164,165,166, 170,185 162 30 37 14 19 1.67 .270 Ll 184,186 223 95 5 0.56 .113 L2 187,188,189 91 30 70 0.61 .310 m 200,201,202 148 95 5 0.14 .113 'TC, time concentration A-9 TABU A- 3b LAND USE D%TA FOR STORM MODEL INPLyr (FUTURE SYSTEM) smm TDTAL LAND USE OF T0TAL BASIN AREA) DFPRESSIGN RLMFF SLlB- AREA Tc* DEPTH BASIN CAICHMENTS (AQ LD HD MF IN CO LF BM UP UF GP (Hr) (in) AS 1,11,12,13,14 333 98 2 0.49 .100 15,16,17 C 25,26,27,28,29 168 58 20 22 0.64 .155 D 45,46,47,48, 49,50 198 17 76 7 1.11 .118 El 61,62,63,70 252 78 22 1.39 .100 E2 60,64,65,66,67, 68,69,71,72 340 28 72 1.48 .100 FI 76,81,82,83,84, 85,86 423 IS 37 11 34 .90 .185 F2 75,77,78,79,80 344 14 68 18 1.39 .145 GI 100,104 71 75 25 0.59 .100 G2 101,102,103, 106,108,1081, 111 234 21 56 5 9 9 0.17 .123 G3 105,107,109, 110,1071 221 53 29 16 2 0.28 .105 H 125,126 138 2 36 62 0.31 .100 1 135,136 131 28 34 38 0.28 .100 1 145,146,147 al 81 4 15 0.12 .138 KI 167 60 30 50 20 0.32 .150 K2 160,161,162, 163,168,169 163 76 14 10 0.52 .125 K3 164,165,166, 170,185 162 81 19 0.56 .148 LI 184,186 223 96 4 0.49 .110 L2 187,188,189 91 30.. 70 0.54 .310 M 200,201,202 148 96 4 0.14 .110 *Tc, time amcentraticn A-10 The major difference between SAM and STORM model input is the storm data. SAM uses a storm event which may cover several hours with a 10-minutes or less time steps. One can repeat the analysis using the SAM model For many storm events, but this effort is constrained by the cost of the computer runs. The STORM model uses a coarser time step (an hour) than the SAM model, therefore it is more suited for analyzing the runoff for continuous period such as the entire summer season. The hourly precipitation data for the STORM model were read directly from a tape (Format TD-9654) which was purchased from the National Climactic Center located in North Carolina. The data in the tape contains the most recent precipitation records (from 1963-79), measured at the Anchorage International Airport. Table A-4 summarizes the input data files used for the simulation. The model output gives results of both quantity and quality analysis. It also gives the number of storm events occured during the modeling period. The storm event for this project as defined to be, 1) it consists no more than 6 hours of dry period, and, 2) it produces a runoff of more than 0.01 in/hr. A-11 TABLE A-4 STORM INPUT DATA FILES The input file for each run consists of a combination of three separate files: 1. CNTL Control date file 2. WDATA67 Precipitation data, May-Sept., 1967 3. FUTULU Future land use and pollutant loading data Other relevant data files: WDATA63 Precipication data, May-Sept., 1963 WDATA69 Precipitation data, May-Sept., 1969 ExistLU Existing land use and pollutant loading data Following is a typical job file, called JSTORM, for the STORM model run: TEST, CM205OOO,PO2,T10 USER* ATTACH, STORM GET, CNTL, WDATA67, FUTULU. COPYEI, CNTL, TANG. COPYEI, WDATA67, TANG.** COPYEI, FUTULU, TANG. REWIND, TANG STORM, PL=20000, TANG, TOUT1.*** EXIT, U. COST, LO=F DAYFILE(TOUT1) REPLACE(TOUT1) User no. and password Input file Output file A@-12 I i I I i I I I I I I i 1 0 I I I I I i APPENDIX B DISCUSSION OF ICINGS I IN DRAINAGE SYSTEMS f APPENDIX B DISCUSSION OF ICINGS IN DRAINAGE SYSTEMS The purpose of this chapter is to supply information re- garding the icing phenomenon and its related problems and to present methods of control and prevention. These methods were taken into consideration in the formulation of alternatives presented in this report. The basic sources of icings are springs, streams and general seepage. Within the study area springs do not constitute a significant portion of the water supply sources (Hydrology for Land Use Planning: The Hillside Area, 1975, pg.15). Thus, streams and general seepage are the sources of icings within the study area. The results of icing can be hazardous. In the case of complete blockage of the drainage facility, water may become ponded in areas or diverted to areas such as roadways, that were meant to be kept drained. Other potential hazards are soil erosion, increases in water levels in streams and channels, raised water tables, saturated fills and embankments and washouts. B-1 EXAMPLES OF BLOCKAGE PROBLEMS Two major factors involved in the formation of icings are heat loss and depth of flow. Heat loss is a function of the air temperature and exposure. As the air temperature decreases, the rate of ice formation increases. And as the area surrounding the water becomes less of a barrier to heat loss, the rate of heat loss increases. Depth of flow is important in relation to the thickness of ice that can be formed for any particular heat loss condition. The problems of ice blockage occur when the depth of flow is the same or smaller than the thickness of ice that can form. T he flow freezes solid, reducing the drainage facility cross section and forcing the flow to spread out on top of the already formed ice and become frozen itself. Ice formation inside a culvert reduces its cross section and its cpacity to carry flow. Depending on hydraulic character- istics, such as slope, flow rate and inlet and outlet conditions, ice may build up uniformly throughout the length of a culvert, or form primarily at the entrance or exit of the culvert. In some cases, ice may begin forming in the upstream or downstream channel and progress to the culvert. A very common location of ice build-up is at the end of a culvert designed for free fall. A second common location is within a culvert having low flow where the water freezes in sheets gra'dually-reducing the culvert cross section. B-2 Icing in ditches can lead to ponding and overflow. Ice can form in ditch bottoms and progress upward, or may enter from the side of the ditch as a result of freezing backslope seepage. Debris in the ditch can become a contributor to icing problems. Snow, an insulator, can be a contributor if water seeps below the snow and freezes at the base of the snow, causing blockage in the ditch. Subsurface drains which collect subsurface seepage and ground water can become blocked in two ways. First, if the frost level falls to the depth of the drain, freezing will occur. Secondly, and more commonly, is the occurance of ice blockage at the outlet of the subsurface drain where the drainage water first encounters low air temperatures. The second case essentially forms a plug, backing up the entire drain with water, which in turn may become frozen. PROBLEM SOLUTIONS The solutions to problems of ice-blocked drainage facili- ties fall into two categories: ice control and ice prevention. Each will be discussed in detail in the folowing paragraphs. Methods of Icing Avoidance and Control For drainage facilities where elimination of icings is not possible, a number of methods are available as control measures. B-3 1 Transfer of location One of the most elementary yet most costly methods of solving icing problems is to relocate the drainage facility. This a "last resort" measure. 2. Raising grade By raising the grade of the structure, the seasonal encroachment of icings can be postponed. This measure must be used with caution as it may lead to washouts from ice blocked facilities as well as undesirable seepage effects. 3. Numerous and large drainage structures Providing more drainage facilities than might other- wise be required is based on the likelihood that icings will divert meltwatrer runoff to points nor- mally dry. This method is used to protect against washouts. 4. Storage space By excavating an area for icings to form and grow, the icings will present no hazard to the area of concern. 5. Dams, dikes or barriers Facilities, such as ice fences, are used to limit the horizontal dimensions of icings. They may be temporary or permanent. B-4 6. Culvert closures In situations where the storage space for ice up- stream from the culvert is large, and where the flow of water throughout the winter is very small and intermittent, it is possible that closures placed at the ends of a culvert in the autumn may facilitate opening the culvert to accept runoff in the spring.- In this way, the culvert is prevented from becoming filled with snow and ice, and the maintenance effort to remove the closures may be less than the effort that would otherwise be required to remove ice from within the culvert. 7. Staggered culverts Two or more culverts are used for one stream, one at the base of the roadway fill, and the other(s) at a higher elevation in the fill. The higher culvert is normally dry except during the spring when, because the lower culvert is blocked by the accumulated icing, the higher one carries the initial spring runoff over the icing. The lower culvert becomes cleared of ice as the spring thaw progresses, and eventually accommo- dates the entire flow, leaving the higher culvert dry again. The higher culvert may be placed to one side of the lower culvert. Thus, less vertical distance would be required for the installation, so that the initial amount of water blocked up during B-5 the spring is at a minimum. This method is most applicable where the topography permits or requires deep fills. The icing accumulation area must be large enough to store an entire winter's ice without having the icing reach the upper culvert or the elevation of the area being protected. 8. Filtration dikes The filtration dike is an embankment composed of very coarse granular material. These dikes have been used in Russia with success. However, in the United States they have not achieved widespread use. 9. Heat Icings are commonly controlled by the application of heat, the objective being not to prevent icings but to establish and maintain thawed channels through them to minimize their growth and to pass spring runoff. 10@ Steam This method is used to thaw culvert openings and to thaw channel into icings for collecting icing feed water or early spring runoff. 11. Fuel oil heaters Known more commonly as "firepots", fuel oil heaters are widely used. Use of firepots is declining because B-6 of high maintenance requirements, energy inefficiency and the difficulty in preventing theft of fuel. 12. Electrical heating Where electrical power is available, the use of in- sulated heating cables has proved successful. It requires less maintenance than steam thawing but may not be cost-effective if electricity costs are high. 13. Breaking and removing accumulated ice This measure should be limited to use in emergencies. 14. Blasting The process of blasting has two beneficial effects. First, blasting aids in the physical removal of ice. Secondly, fractures created by blasting provide paths for water flow deep within the icing where, protected from the atmosphere, it may not refreeze. 15. Chemicals Chemicals such as sodium or calcium chloride are some times used to prevent refreezing of the drainage facility once it has been freed of ice by other means. Negative aspects of using these chemicals are the corrosive effects on metal piping and the detrimental effects of fish and wildlife, vegetation and other downstream water uses. B-7 16. Solar heating In recent years, tests have been undertaken to deter- mine the practicality of using solar energy to thaw culverts (The Northern Engineer, Volume 13 No. 3, Pg 39). Results prove this method to be viable, especially as the technique becomes more refined. Methods of Icing Prevention A number of measures are available to avoid icings from forming including those described below. 1. Channel modifications Straightening and deepening a channel can prevent icings, although frequent maintenance is usually re- quired to counteract the stream's tendency to resume natural configuration by erosion and.deposition. Rock-fill gabions have been used to create a deep, narrow channel for low winter discharges. Such deepened channels permit information of ice cover to normal thickness while providing adequate space beneath for flow. Deepening at riffles, rapids or drop structures is especially important as icings are most apt to form in these shallow areas. 2. Insulation of critical sections River icings may be prevented by insulating critical B-8 sections of the river where high heat losses cause an excessive thickening of the normal ice cover, leading to complete blockage of the flow and subsequent icing formation. These sections may be located under a bridge or at riffles and rapids. Insulating covers are generally expensive and time consuming. 3. Frost belts Also known as a permafrost belt, the frost belt is basically a ditch or cleared strip of land located upstream or upslope from the icing problem area. The area is cleared of vegetation and snow is removed during the first half of winter. This enables deep seasonal frost to act as a dam to the water seeping through the ground forcing it to surface where it will form an icing upstream or upslope from the belt. When used in a drainage channel situation, a belt is formed by period- ically cutting transversely into the ice to cause the bottom of the ice cover to lower and merge with the bed. The icing is therefore induced to form away from the bridge or culvert entrance being protected. 4. Surface drainage In the region of icing development of the soil mantle can be drained by a network of drainage ditches. The ditches are made deep and narrow to expose only a small B-9 surface area to the atmosphere. Some ditches have insulated covers. 5. Subsurface Drainage Though better defense against icings than surface drains, subsurface drains cannot be used in permafrost areas. The water collected is transferred to a point away from the area being protected. However, the drain outlet will still experience icing problems. 6. Insulation of the ground In areas where deep seasonal frost penetration forms a dam to groundwater flow, icings have been avoided in some cases by insulating the ground. Caution must be exercised with this method to avoid the icing to simply be shifted to another problem area. 7. Earth embankments and impervious barriers The earth embankment used in conjunction with the barrier impervious to groundwater flow is another technique for preventing the formation of ground icings. The embankment and barrier are placed well away from the area being protected. This technique functions in a manner similar to frost belts in that they dam seepage flow through the soil and induce an icing to form where it is harmless. B-10 I "I I i I I i i i i i /I I "0 I I APPENDIX C I CAPITAL I IMPROVEMENT COST ANALYSIS APPENDIX C CAPITAL IMPROVEMENT COST ANALYSIS INTRODUCTION In the main text of this report a comprehensive trunk system is described. The complete trunk system is shown on the map bound in the back of this report, with 2 color code designating whether the segment is existing and of adequate capacity, existing but requires upgrading, or a proposed new trunk line. Cost estimates are presented by trunk system. The cost figures reflect recommended improvements to existing trunk systems or construction of recommended new trunk systems. All dollar amounts are for the year 1982. The costs shown are total project costs. Projected operation and'maintenance costs are also presented. The type of structure used in the cost estimates for each trunk improvement is as recommended in Chapter 5 of this report. It must be stressed that the choice of structure in each case is an engineering judgement based on experience and present conditions. The final decision 2S to the type of structure will be made by the Department of Public Works who will reevaluate the situation as time progresses and as community needs change. PROJECT COSTS The cost estimates presented in this appendix and summarized in Table C-13 located at the end of the appendix, are based on 1982 prices and represent total project costs. Project costs C - 1 include construction cost plus a contingency of 10 percent as well as allied costs. Allied costs include services and costs such as project administration, engineering, Municipal admini- strative costs. These allied costs have been computed as 30 percent of the construction cost. Other costs such as easement acquisition, assessment roll, and bond and legal counsel may also be an allied cost, depending upon the project. Should these items arise, they must be added to the allied costs. TABLE C-1 ALLIED PROJECT COSTS AS A PERCENT OF CONSTRUCTION COST Item Administration 1.0 Engineering Design 8.5 Construction Engineering 10.0 Bond Discount, Interest during Construction, Financial fees, etc. 10.5 TOTAL 3 0 '00' OPERATION AND MAINTENANCE COSTS/FACILITY SERVICE LIFE Proper maintenance of storm drainage facilities is imperative for such facilities to function correctly as well as to extend the service life of the facilities for as long as possible. Maintenance measures which have been included in the estimate of maintenance costs are: C-2 Closed pipe system: Clean out entrances to pipe at inlets and manholes. Keep clear of debris and ice buildup. This is to be performed at regular intervals (estimated at two times per year). Culvert/ditch system: Seed every summer to keep channels lined with grass. Clean out entrances to culverts; keep entrances clear of debris and ice buildup. The clean-out process is estimated at three times per year: twice in spring/ summer and onceduring a winter freeze/thaw cycle. Every other year one of the clean- up processes is to be replaced with retrenching of the ditches. Greenbelt: Operation and maintenance procedures com- parable to the culvert/ditch system. However, clean-out process is estimated at two times per year, and retrenching every three years. Outfall: Repair gaskets and brackets on a yearly basis. Annually repair the area downstream of pipe which functions as an energy dissipator. C-3 Service life for the closed pipe and outfall systems are estimated at twenty years. Ditch and culvert systems and green- belts are also estimated to have a twenty year service life, with some retrenching required every two or three years, respectively, to maintain flow capacity . Annual operation and maintenance costs for each system, based on a twenty-year life cycle are as follows: Annual 0 & M Cost in 1982 dollars (per LF) System (20 year life cycle) Closed pipe $2.00 Culvert/ditch $1.50 Greenbelt $1.50 Outfall $1.00 CONSTRUCTION MATERIALS All construction shall be performed per the Municipality of Anchorage Standard Specifications (MASS). For cost estimating purposes, all storm drain pipe was assumed to be corrugated metal pipe (CMP) and Class C bedding material. Ditches were assumed to have side slopes in the ratio of 2 (horizontal) to 1 ( vertical) . Ditch material was assumed to be the insitu material with seeding being performed on surface of ditch to prevent erosion. Costs include appurtenances along trunk route such as catch basins and inlets. Road repair is included where applicable. C-4 Where facilities are to be included in the development of an area, no dollar allotment has been made for road construction. TRUNK SYSTEM IMPROVEMENTS Cost estimates, as stated in the "Introduction" section of this appendix, are for the type of structures recommended in Chapter 5. Final determination of the facility type to be used for a given case will be made by the Department of Public WorKs. Cost estimates are presented in the following paragraphs by trunk system. A summary of the total cost for each trunk is presented at the end of this appendix in Table C-13. Trunks Requiring No Improvements The following trunk systems have no recommended improvements associated with them: Rabbit Creek Road (RC Rd), Rabbit Creek (RC), New Seward Highway - East and West (NSH-E, NSH,W), Old Seward Highway - 1 and 2 (OSH-1, OSH-2), Johns Road (Johns Rd), and the Alaska Railroad (Ak RR). Upper Furrow Creek - South 1 (UFC-Sl) The upstream section of this trunk is presently a pipe/ greenbelt system of adequate capacity except at two street crossings (Spinnaker and Westwind). For cost estimating it is C-5 assumed that the existing 18" culverts will be removed and 42" culverts inserted. The downstream segment of UFC-Sl consists presently of pipe, culvert, and ditch systems. Along Tradewind the existing 2411 pipe is slightly under the design capacity. It is recom- mended that the 24" pipe continue to be used unless severe drainage problems result. If such problems result, it is recommended that a 3011 pipe be used to replace the 24" line. Along the Frontage Road the culvert and ditch system presently in use is of inadequate size. It is assumed for cost estimating purposes that the present system will be enlarg ed in cross sectional area to be of the equivalent capacity of a 42" CMP pipe. TABLE C-2 UFC-Sl COST ESTIMATE Upgrade two (2) 1811 cross culverts to 42" (50 LF each) $ 12,000 Expand Frontage Road ditch (5000 LF) 66,900 Upgrade Tradewind pipe from 24" to 3011 (if required) (1100 LF) 117,700 CONSTRUCTION COST $196,600 10,10' Contingency 19,700 30,10' Allied costs 59,000 TOTAL PROJECT COST $275,300 0 & M per year $ 9,800 C-6 Upper Furrow Creek - South 2 (UFC-2) This trunk system is presently the Upper Furrow Creek channel. A well defined greenbelt is recommended and is used herein for cost estimating purposes. The capacity of the green- belt is assumed to be equivalent to a 54" pipe. TABLE C-3 UFC-S2 COST ESTIMATE Constuct Greenbelt (2300 LF) $60,900 CONSTRUCTION COST $60,900 10"0' Contingency 6,100 30"0' Allied costs 18,300 TOTAL PROJECT COST $85,300 0 & M per year $ 32500 Upper Furrow Creek - South 3 (UFC-S3) The upstream segment of this trunk is located in an area which will soon be developed. The trunk segment is proposed and costs are based upon the installation of a 30" pipe. The downstream segment of UFC-S3 is presently 21" and 24" pipe. This approximately one quarter of the required design capacity. For cost estimating purposes, it is assumed that the present pipe will be replaced with 48" pipe. C-7 TABLE C-4 UFC-S3 COST ESTIMATE Install 30" pipe (17000 LF) $248,300 Upgrading existing 21" - 24" pipe to 48" pipe (1900 LF) 308,600 CONSTRUCTION COST $556,900 1090' Contingency 55,700 110,10' Allied costs 167,100 TOTAL PROJECT COST $779,700 0 & M per year $ 99800 Upper Furrow Creek - South 4 (UFC-S4) The UFC-S4 trunk is a proposed system. The cost estimate is based upon an open greenbelt corridor following the channel of Upper Furrow Creek. The capacity of the greenbelt is assumed to be the equivalent of a 3611 pipe. TABLE C-5 UFC-S4 COST ESTIMATE Construct greenbelt (2500 LF) $381400 CONSTRUCTION COST $38,400 1090' Contingency 3,800 30.10' Allied costs 11,500 TOTAL PROJECT COST $53,700 0 & M per year $ 3,800 C-8 Upper Furrow Creek North 1 (UFC-Nl) The UFC-N1 trunk is a proposed sytem. It is recommended to be either a system of roadside ditches and culverts or a pipe system, depending upon the ultimate development pattern. TABLE C-6 UFC-N1 COST ESTIMATE Option 1: Construct 2111 pipe (1200 LF) $105,800 CONSTRUCTION COST $105,800 101% Contingency 10,600 30*'0' Allied costs 31,700 TOTAL PROJECT COST $148,100 0 & M per year $ 2,400 Option 2: Construct ditch/culvert system (1200 LF) $ 259000 CONSTRUCTION COST $ 25,000 10"0' Contingency 2,500 30,00' Allied costs 7,500 TOTAL PROJECT COST $ 359000 0 & M per year $ 1,800 C-9 Upper Furrow Creek - North 2 (UFC-N2) The UFC-N2 trunk is a proposed system. In Chapter 5, the UFC-N2 trunk was recommended to be an open ditch/culvert system or a closed pipe system, depending upon the ultimate development pattern. TABLE C-7 UFC-N2 COST ESTIMATE option 1: Construct 3611 pipe (2700 LF) $349,600 CONSTRUCTION COST $349,600 10*,D' Contingency 35,000 30,00' Allied costs 104,900 TOTAL PROJECT COST $489,500 0 & M per year $ 5,400 Option 2: Construct ditch/culvert system (2700 LF) $ 56,200 CONSTRUCTION COST $ 56,200 10"G' Contingency 5,600 10"0' Allied costs 16,900 TOTAL PROJECT COST $ 78,700 0 & M per year $ 4,100 C- 10 Upper Furrow Creek - North 3 (UFC-N3) The UFC-N3 trunk is a proposed system which is recommended to be either a series of roadside ditches and culverts or a pipe system. The cost estimates are based on, 48" pipe system. There is the potential of a routing problem during layout of the trunk. For this preliminary cost estimate the trunk location was assumed to follow the route delineated on the comprehensive map bound at the back of this report. TABLE C-8 UFC-N3 COST ESTIMATE Option 1: Construct 4811 pipe (4200 LF) $672,900 CONSTRUCTION COST $672,900 1090' Contingency 67,300 30,00' Allied costs 201,900 TOTAL PROJECT COST $942,100 0 & M per year $ 8,400 option 2: Construct ditch/culvert system (4200 LF) $ 96,900 CONSTRUCTION COST $ 96,900 100% Contingency 9,700 30"0' Allied costs 29,100 TOTAL PROJECT COST $135,700 0 & M per year $ 6,300 C-11 Middle Furrow Creek (MFC) The MFC trunk was recently upgraded to pipe ranging in size from 36" to 48.". However, the design capacity necessary to handle the flows calculated in this study is 78". As described in Chapter 5, it is recommended that a preliminary engineering report be initiated to analyze in depth methods of handling the additional flow. However, for purposes of this estimate the costs are based on a 3011 parallel pipe system. Also included in the cost estimate for this trunk is dollar amount associated with jack and bore crossings at the New and Old Seward Highways and the Alaska Railroad using ductile iron pipe. TABLE C-9 MFC COST ESTIMATE Construct 30" Parallel Pipe System (2500 LF) $4179100 Crossings at New Seward Highway, Old Seward Highway, and the Alaska kailroad (DIP pipe) (540 LF Total) 288,400 CONSTRUCTION COST $705,500 10,10' Contingency 70,600 30.10' Allied costs 211 700 TOTAL PROJECT COST $987,800 0 & M per year $ 5,300 C-12 Lower Furrow Creek (LFC) The LFC trunk follows the Furrow Creek channel. A number of segments of this trunk are of insufficient capacity and require upgrading . The most upstream segment of the LFC trunk is presently a channel system which is of insufficient capacity. It is recommended that the corridor be enlarged and better defined. The enlarged greenbelt/corridor is recomm ended to have the equivalent capacity of an 84" pipe. When Furrow Creek crosses Clipper Ship Court, Johns Road and Mariner Drive, the existing cross culverts are grossly undersized. A bridge or plate arch pipe is recommended. This cost estimate is based upon the use of a plate pipe arch for each location with a span of 8'7" and a rise of 5'11". The segment of the LFC trunk immediately downstream of Johns Road requires upgrading as the present stream corridor is of inadequate size. It is recommended that the stream corridor be enlarged so that its capacity is equivalent to a 7811 pipe. C- 13 TABLE C-10 LFC C05T ESTIMATE Upgrade Greenbelt Corridor (2100. L F ) $ 88,000 Upgrade Cross Culverts at Clipper Ship, Mariner, and Johns Road (60 LF each) 52,000 Upgrade Creek Corridor (1000 LF) 42,000 CONSTRUCTION COST $182,000 10"0' Contingency 18,200 3090' Allied costs 54,600 TOTAL PROJECT COST $254,800 0 & M per year $ 4,900 Lower Furrow Creek - North 1 (LFC-Nl) The proposed LFC-N1 trunk is recommended to be a pipe system. Costs are based upon a 21" pipe for the entire length of the trunk route. TABLE C-11 LFC-N1 COST ESTIMATE Construct 2111 pipe system (1400 LF) $152,700 CONSTRUCTION COST $152,700 10*,D' Contingency 15,300 301% Allied costs 45,800 TOTAL PROJECT COST $2117,800 0 & M per year $ 2,800 C- 14 lubbasin M Oulfalls The three 18" outfall pipes in Subbasin M are recommended to be removed and replaced. Two of the outfalls are recommended to be 3611, while the third outfall is recommended to be 48". Design features to minimize flow velocity and bluff erosion were incorporated in the cost estimate. TABLE C-12 5UBBA5IN M OUTFALLS COST ESTIMATE - Replace existing outfalls: 2 - 3611 outfalls (500 LF each) $ 61,000 1 - 48" outfalls (500 LF each) 41,600 CONSTRUCTION COST $102,600 10"0' Contingency 10,300 30"0' Allied costs 30,800 TOTAL PROJECT COST $143,700 0 & M per year $ 1,500 COST ESTIMATE SUMMARY In Table C-13 the total project cost is tabulated by trunk system. It should again be emphasized that the type of system (e.g. pipe, ditch) used in the cost estimate was as recommended in this report. Final determination of system type will be made by the Department of Public Works. C-15 TABLE C-13 COST ESTIMATE SUMMARY TOTAL PROJECT COST 0 & M PER YEAR TRUNK (1982 DOLLARS) (1982 DOLLARS) UFC-Sl $275,300 $ 9,800 UFC-S2 85,300 3,500 UFC-S3 779,700 9,800 UFC-S4 53,700 3,800 UFC-N1 Pipe 148,100 2,400 Ditch/Culvert 35,700 1,800 UFC-N2 Pipe 489,500 5,400 Ditch/Culvert 78,700 4,100 UFC-N1 Pipe 942,100 8,400 Ditch/Culvert 135,700 6,300 MFC 987,800 5,300 LFC 254,800 4,900 LFC-N1 213,800 2,800 OUTFALLS (M) 143,700 1,500 Total w/pipe option 4,373,800 57,600 Total w/ditch/culvert option 3,043,500 53,600 C-16 I 4 t-, , tl I i :1 i t i m I i I 0 @@ i I APPENDIX D GLOSSARY OF TERMS it APPENDIX D GLOSSARY OF TERMS Depression storage - The fraction of precipitation that is trapped in depressions on the surface of the ground. Design Criteria - Guidelines upon which planning and engineering decisions and judgments are based. Design Storm - A precipitation event that, statistically, has a specified probability of occurring in any given year (ex- pressed either in years or as a percentage). Detention - Temporary storage of surface runof f-either on, below or above the ground surface-accompanied by controlled release of the stored water. Detention Basin - A stormwater detention facility, natural or artificial, which normally drains completely between space runoff events; e.g., parking lot, rooftop, athletic field, dry well, oversized storm drain pipe. Detention Pond - A stormwater detention facility natural or artifical, which maintains a fixed minimum water elevation between runoff events except for the lowering resulting from losses of water due to infiltration or evaporation. Drainage - Interception, collection and removal of excess storm- water from an area into another area or into a receiving water body. Drainage Area - The area from which flow of stormwater at a given point is derived. Since water flows downhill, water from a given drainage area will collect and flow through an outlet point. Drainage basins are subdivided into subbasins and further divided into subcatchments. Easement - A right to control or use the property of another for designated purposes. Event - An individual occurrence of precipitation or snowmelt. Excess Runoff - Direct surface runoff that cannot be accommodated satisfactorily by the existing or planned drainage system. Flow Routing Path of travel of runoff through the drainage area. Flood Control Preventing the entry of stormwater into an area from another area, or from a stream or other water body. Grade - The slope of a road, channel, or natural ground. D-1 Hydrograph - A graph of runoff rate, inflow rate or discharge rate, versus time. Hyetograph - A graph of intensity of a precipitation event versus time. Infiltration - The process whereby water enters the surface of the soil and moves downward toward the water table. Institutional Problems - Social, economic and political problems existing within or between agencies, organizations or groups-either public or private. Intensity - The rate at which a precipitation event occurs, expressed as a depth of water per unit of time, such as inches of rain per hour. Interception - Rainfall that is caught by foliage, branches, leaves, and other above-ground objects. Invert - The lowest part of the internal cross section of a channel or conduit. Lag - The time int-erval from the center of mass of excess rain- fall to the peak rate of runoff. Local Detention - Temporary storage of runoff on the same land development site where the runoff is generated-frequently required as a condition for subdivision plat approval. Master Planning - A "systems" approach to the planning of facilities, programs and management organizations for comprehensive control and use of stormwater within a defined geographical area. Minor Drainage System - The conveyance drainage system con- sisting of street gutters, storm sewers, small open channels, and swales, etc. Off-stream Detention - Temprary storage accomplished off-line; i.e., not within a principal drainage system. Outfall - The conduit through which water is discharged to a watercourse. Percolation - The downward movement of water thrdugh soil. Pollutant Loading - The arithmetic product of the pollutant concentration and the runoff rate. Pollutograph - A graph of pollutant loading versus time (commonly referred to as a "mass emission pollutograph"). Units are kg/day or lbs/day, etc. D-2 Ponding - The occurrence of excessive depths of stormwater after a rainfall or snowmelt event. Precipitation - A basic part of the hydrologic cycle, pre- cipitation is the falling of water in the form of rain, snow, sleet, or hail. Receiving Waters - Streams, lakes, bays, etc., into which storm- waters are discharged. Recurrence Interval - The average interval of time within which the magnitude of a given event is likely to be equalled or exceeded. Regional Detention - Temporary storage of runoff for a large drainage area. Retention Facility - Any type of detention facility not provided with a positive outlet. Runoff - Water flowing overland or in a stream channel past any given section. Sedimentation - The process of depositing particles of waterborne soil, rock, or other materials. Storm Sewers - Usually, enclosed conduits that transport excess stormwater runoff toward points of dishcarge (sometimes called "storm drains"). Stormwater Management - Encompasses both "control" and "develop- mental" activities in which there is physical interaction with stormwater (a broader interpretation includes activities of an institutional nature - financing, staffing, etc.). Stormwater Storage - Temporary storage of excess runoff on, below, or above the surface of the earth for the purpose of attenuating excess runoff. Subbasin - A portion of a complete drainage area delineated by con- centration of flow at a certain point, which contributes in turn to flows in the overall drainage area. Subcatchment - A portion of a subbasin delineated by a concen-. tration of flow at a certain point. Time of Concentration - The time period necessary for surface runoff to reach the outlet of a subbasin from the hydrau- lically most remote point in the tributary drainage area. Transpiration - The process whereby moisture circulates through the structure of plants and is returned to the atmosphere. D-3 Travel Time - The sum of the time intervals for overland flow, sewer or gutter flow, and pipe and channel flow from the hydraulically most remote point in the tributary to the discharge point of interest. Trunk System - Major conveyance network which has the capacity to handle large areas. D-4 I @Aff- I I -. I 4' -1 I I I I I i I I i I 0- " I I I I REFERENCES I 'A REFERENCES Comprehensive Land Use Plan, Municipal Planning Department, September, 1981, rev-1-sed March 1982. Anchorage Wetlands Management Plan, Municipal Planning Department, April 1982. Hillside Wastwater Management Plan, Arctic Environmental Engineers and Municipal Physical Planning Division, May 1982. Interim Snow Dispoal Study, Municipal Planning Department, January 1981. Title 21 of the Anchorage Municipal Code - Land Use Regulation, January 1, 1982. 208 Areawide Water Quality Management Plan, Anchorage, Alaska, August 1979. Metropolitan Anchorage Urban Study, Volume 7, Anchorage Area Soil Survey, Army Corps of Engineers, 1979. Hydrology for Land Use Planning: The Hillside Area, Anchorage, Alaska, Open File Report 75-105, Department of the Interior, 1975. 1995 Employment Population and Land Use Forecasts, Municipal Planning Department, March, 1977. Campbell Creek Drainage Basin, Task Memorandum Number 7, Methodology Manual, CH 2 M-Hill, January 1979. Storage Treatment Overflow Runoff Model (STORM), User Manual, U.S. Army Corps of Engineers, 1976. Weather Tape: 1963-1979, National Oceanic and Atmospheric Administration. Drainage Management Plan for Homer, Alaska, CH 2 M-Hill, August 1979. Juanita Creek Drainage Plan, URS Engineers, 1977. Stormwater Drainage Study for the City of Soldotna, Ted Forsi & Associates, December 1979. Sand Lake Drainage and Water Quality Management Study., Quadra Engineering, August 1981. Urban Stormwater Management Special Report No. 49, American Public Works Association, 1981. "Solving Problems of Ice-Blocked Drainage Facilities", Special Report 77-25, K.L. Carey, Cold Regions Research and Engineering Laboratory, August 1977. Soil Erosion and Sediment Control for Anchorage, Municipal Department of Public Works, December 1978. "Insulated Roadway Subdrains in the Subarctic for the Prevention of Spring Icings", H. Livingston and E. Johnson. "Storm Drainage Design Considerations in Cold Regions", ASCE Conference Proceedings on Applied Techniques for Cold Environments, May 1978. "Icings Developed from Surface Water and Ground Water", CRREL Monograph 111-D3, Kevin Carey, May 1973. "Hydrologic Effects on Frozen Groundif, CRREL Special Report 218, S. L. Dingman, March 1975. 3 6668 14101 7790