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HomeMy WebLinkAboutSheldrake River Watershed Hyrologic Study Appendix 5-3 of DGEIS for Conservation-Recreation (CR) Zone Volume I Revised 5/1/1991 Engineering Report • SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY TOWN OF MAMARONECK NEW YORK APPENDIX 5-3 OF DGEIS FOR CONSERVATION - RECREATION (CR) ZONE TOWN OF MAMARONECK, NEW YORK VOLUME I March 1991 (Revised May 1991) Project 0849-15-1 vorENVIRONMENTAL ENGINEERS,SCIENTISTS&PLANNERS Engineering Report SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY TOWN OF MAMARONECK NEW YORK APPENDIX 5-3 OF DGEIS FOR CONSERVATION - RECREATION (CR) ZONE TOWN OF MAMARONECK, NEW YORK VOLUME PROPS TOWN OF MAMARgi March 1991 (Revised May 1991) N��S a M ��� DEP Project 0849-15-1 � �N . vole ENVIRONMENTAL ENGINEERS,SCIENTISTS& PLANNERS IRNIMALCOLM PIRNIE, INC. ENVIRONMENTAL ENGINEERS, SCIENTISTS & PLANNERS May 10, 1991 Mr. Stephen Altieri Town Administrator Town of Mamaroneck 740 W. Boston Post Road Mamaroneck, NY 19543-3319 Re: Hydrologic Study of Sheldrake River Watershed Dear Steve: In accordance with the provisions of our contract dated August 10, 1990, we are pleased to submit the results of our Hydrologic Study of the Sheldrake River Watershed. This study establishes a baseline hydrologic relationship for the Sheldrake River and its main tributary, the East Branch within the Town of Mamaroneck. Hydrologic modeling of the River was performed to develop flood elevations and the extent of flooding for existing conditions is shown on a 1" = 200' scale map. The map was developed from aerial photography obtained in April, 1986. As such, the recent Fenbrook subdivision is not completely shown on the map. However, the study considers its effects both in the hydrologic and water surface profile models. This report also incorporates revisions resulting from the comments received from the Town Board at their April 15, 1991 work session. The study also evaluated potential impacts on flooding due to possible development of the Bonnie Briar Country Club and the Winged Foot Golf Club. Four potential development schemes developed by Ferrandino and Associates in their concurrent Draft Generic Environmental Impact Statement (DGEIS) was considered. These include: No Zoning Changes. 1. R30 Detached - Single family detached houses 2. R30 Townhouses - Attached townhouses. Re-zoning Required. 3. CR Detached - Single family detached houses 4. CR Townhouses - Attached townhouses. Conceptual measures for mitigating the estimated hydrologic impacts of the possible development scenarios are also presented. 2 CORPORATE PARK DR. BOX 751 WHITE PLAINS,NY 10602 914-694-2100 FAX 914-694-9286 • IRNI Mr. Stephen Altieri May 10, 1991 Town of Mamaroneck Page 2 The attached draft report describes the data utilized in the analysis, the methodology applied, models developed and the results and conclusions of the study. We appreciate the opportunity to assist the Town in developing a basis for sound land use planning in the Sheldrake River Watershed and look forward to discussing the contents of the draft report with Town officials and staff in conjunction with review of the DGEIS. If you have any questions regarding this report, please contact me. Very truly yours, MALCOLM PIRNIE, INC. nneth Hen ersd n, P.E. Hagop L. Shahabian, PhD., P.E. Vice President Associate • HLS.017/mjh c: G.B. Trachtman - MP I I 0 I TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY TABLE OF CONTENTS Page 1 . INTRODUCTION 1-1 1 . 1 Background 1-1 1 .2 Study Objectives 1-2 1 .3 Project Approach 1-2 2. METHODOLOGY 2-1 2. 1 Model Selection 2-1 2.2 Existing Information 2-2 3. ANALYSIS OF EXISTING CONDITIONS 3-1 3. 1 Description of the Sheldrake River System 3-1 3.2 Description of the Watershed 3-2 3.3 Development of HEC-1 Model (Peak Runoff Flows) 3-4 3.4 Calibration of HEC-1 Model 3-13 3.5 Development of HEC-2 Model (Water Surface Profiles) 3-18 3.6 Calibration of HEC-2 Model 3-19 3.7 Model Simulation of Different Storms for Existing Conditions 3-20 3.8 Flood Plain Areas for Existing Conditions 3-22 4. ANALYSIS OF POTENTIAL DEVELOPMENT SCHEMES 4-1 4. 1 Introduction 4-1 4.2 R30 Detached Development 4-4 • 4.3 R30 Townhouse Development 4-11 4.4 CR Detached Development 4-18 4.5 CR Townhouse Development 4-25 5. CONCLUSIONS AND RECOMMENDATIONS 5-1 • 5. 1 Conclusions 5-1 5.2 Recommendations 5-6 LIST OF REFERENCES APPENDICES VOLUME II • 0849-15-1095 i • TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY LIST OF TABLES Table Following No. Description Page 3-1 Summary of Hydrologic Features 3-7 of the Sheldrake River Watershed for Existing Conditions 3-2 Westchester 24-Hour Rainfall 3-8 at Various Storm Frequencies 3-3 Summary of Estimated Peak Runoff 3-14 Flows for Selected Storm Events at the Sheldrake River Watershed for Existing Conditions 4-1 Increase in Impervious Area for 4-5 R30 Detached Development 4-2 Summary of CN Values for R30 4-5 Detached Development 4-3 Summary of Peak Runoff Flows 4-5 for R30 Detached Development 4-4 Summary of Runoff Volumes for R30 4-6 Detached Development 4-5 Maximum Increase in Flood Plain 4-7 Elevation for R30 Detached Development 4-6 Increase in Impervious Area for 4-12 R30 Townhouse Development 4-7 Summary of CN Values for R30 4-12 Townhouse Development 4-8 Summary of Peak Runoff Flows 4-12 for R30 Townhouse Development 4-9 Summary of Runoff Volumes for R30 4-13 Townhouse Development 4-10 Maximum Increase in Flood Plain 4-14 Elevation for R30 Townhouse Development 0849-15-1095 ii TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY LIST OF TABLES (Continued) Table Following No. Description Page 4-11 Increase in Impervious Area for 4-19 CR Detached Development 4-12 Summary of CN Values for CR 4-19 Detached Development • 4-13 Summary of Peak Runoff Flows for 4-19 CR Detached Development 4-14 Summary of Runoff Volumes for R30 4-20 Detached Development • 4-15 Maximum Increase in Flood Plain 4-21 Elevation for CR Detached Development 4-16 Increase in Impervious Area for 4-26 CR Townhouse Development • 4-17 Summary of CN Values for CR 4-26 Townhouse Development 4-18 Summary of Peak Flows for CR Townhouse 4-26 • 4-19 Summary of Runoff Volumes for R30 4-27 Townhouse Development 4-20 Maximum Increase in Flood Plain 4-28 Evaluation for CR Townhouse Development • • • 0849-15-1095 iii • TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY LIST OF FIGURES Figure Following No. Description Page 3-1 Location of Sub-Basins Within 3-5 Sheldrake River Watershed 3-2 Schematic of Sheldrake River 3-8 Watershed 3-3 Calibration - May 1989 Storm 3-10 • Larchmont Reservoir 3-4 Calibration - May 1990 Storm 3-10 Larchmont Reservoir 3-5 Calibration - May 1990 Storm 3-11 • West Branch 4-1 R30 Detached Development 4-3 (Bonnie Briar Property) 4-2 R30 Detached Development Refer • (Winged Foot Property) to 'Note: Figure Not Provided Note 4-3 R30 Townhouse Development 4-11 (Bonnie Briar Property) • 4-4 R30 Townhouse Development 4-11 (Winged Foot Property) 4-5 CR Detached Development 4-18 (Bonnie Briar Property) • 4-6 CR Detached Development 4-18 (Winged Foot Property) 4-7 CR Townhouse Development 4-31 (Bonnie Briar Property) • 4-8 CR Townhouse Development 4-31 (Winged Foot Property) • 0849-15-1095 iv TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY LIST OF APPENDICES Appendix Description • A Summary of Runoff Flows for Existing Conditions g Summary of Runoff Flows for Developed Conditions (CR Detached, CR Townhouse, and R30 Townhouse) C Summary of Runoff Flows for Developed Conditions • (R30 Detached) p Summary of Water Surface Elevations for Existing Conditions E Summary of Water Surface Elevations for Developed • Conditions (CR Detached, CR Townhouse, and R30 Townhouse) F Summary of Water Surface Elevations for Developed Conditions (R30 Detached) • • • • 0849-15-1095 v I I a P Cardinal 1. INTRODUCTION 1.1 BACKGROUND The Town of Mamaroneck currently experiences flooding along the Sheldrake River. The flooding problems appear to have worsened as a result of urbanization within the Watershed. At present, the majority of the Watershed is developed. However, two large tracts of land within the Town, the Bonnie Briar Country Club and the Winged Foot Golf Club, remain undeveloped. Their potential development is of concern to the Town residents, especially those along the Sheldrake River who fear flooding problems may become worse. Accordingly, a Generic Environmental Impact Statement (GEIS) is currently being prepared by Ferrandino & Associates to assess potential impacts from future development. In order to evaluate flooding conditions within the Sheldrake River Watershed, the Town retained Malcolm Pirnie, Inc. (Pirnie) , to perform a hydrologic study to be prepared concurrently and incorporated into the GEIS. The hydrologic study consists of the development of a hydrologic model to simulate the flows in the Sheldrake River, and additional analyses required to mitigate potential increases in flooding due to development. The model incorporates the Sheldrake River Watershed upstream of the New England Thruway. A second model used in tandem with the hydrologic model estimates flood elevations along the Sheldrake River. The second model is limited to the Sheldrake River and its tributaries within the Town of Mamaroneck. 0849-15-1095 1-1 1.2 STUDY OBJECTIVES The purpose of the hydrologic study is three-fold. It will enable the Town to (1) define the existing flooding conditions within the Town of Mamaroneck, (2) evaluate potential flooding conditions associated with • future development of Bonnie Briar Country Club and Winged Foot Golf Club, and (3) provide the tools to evaluate benefits of potential larger scale flood control projects within the Town. • 1.3 PROJECT APPROACH • In order to accomplish the study objectives, our approach is as follows: 1. Review existing information on hydrologic conditions within the Sheldrake River Watershed. • 2. Develop and calibrate a hydrologic model to determine peak runoff flow rates and the resulting water surface profiles along the Sheldrake River and its associated tributaries. 3. Run the hydrologic model under existing conditions to verify • that the model results simulate actual observed flooding conditions. 4. Run the hydrologic model for different development scenarios to determine the extent of any additional flooding which may result from the proposed land use. 5. Evaluate potential mitigation measures to minimize flooding impacts as a result of potential developments, if necessary. r 0849-15-1095 1-2 1 • • • • 2 • • • • r Cardinal' • 2. METHODOLOGY 2.1 MODEL SELECTION Two models were selected for the analysis; HEC-1 which develops streamflow hydrographs and HEC-2 which develops water surface profiles. The models were developed by the Hydrologic Engineering Center (HEC) , U.S. Corps of Engineers, Department of the Army. The models are commonly used • for flood plain management. The most recent versions of the models were used in the Sheldrake Hydrologic Study. • DESCRIPTION OF HEC-1 The HEC-1 model (Flood Hydrograph Package, March 1987 edition) is designed • to simulate the surface runoff response of a river basin to precipitation. The model represents the basin as an interconnected system of hydrologic and hydraulic components. Each component models an aspect of the • rainfall -runoff process within a portion of the Watershed. A component may represent the surface runoff within a sub-basin, a stream channel , or a reservoir. Representation of a component requires a set of parameters RIO which characterize the component and mathematical relations which describe the physical processes. The result of the modeling process is the computation of streamflow hydrographs at desired locations in the river y . basin. A hydrograph is a representation of the flow rate within a stream with respect to time. The model results are expressed in terms of discharge (flow) but not stage (elevation) . The HEC-2 model is generally used in conjunction with HEC-1 to obtain stages. 0849-15-1095 2-1 DESCRIPTION OF HEC-2 The HEC-2 model (Water Surface Profiles, September 1982 edition) is designed to calculate water levels in a stream for a given flow rate. The geometry of the stream, either natural or man-made, is used to character- ize and define the capacity of the stream. The geometry of the stream is provided through cross-sectional information along the stream. Both subcritical and supercritical flow profiles can be calculated. The • effects of various obstructions such as bridges, culverts, weirs, and structures in the flood plain may also be considered in the computations. The computational procedure is based on the one-dimensional energy • equation with energy loss due to friction evaluated using Manning's equation. The program is designed for application in flood plain delineation and flood insurance studies to evaluate floodway encroachments • and to designate flood hazard zones. Capabilities are also available for assessing the effects of channel improvements and structural controls on water surface profiles. • 2.2 EXISTING INFORMATION • The development of the hydrologic model requires specific information about the Sheldrake River Watershed to characterize the basin. The required data includes topographic features of the area, soil classifica- 11. tion maps, land use information for existing and future conditions, zoning maps, stream cross-section information, storage information for Larchmont Reservoir, and rainfall data for the watershed area. The relevant information was collected from various sources including government 0849-15-1095 2-2 offices, municipalities, and various individuals. A list of the sources and the supplied data from each is given below: U. S. Army Corps of Engineers (COE) : The COE data, developed in April 1989, includes stream cross-section information for several locations along the West Branch of the Sheldrake River from the Bonnie Briar Lane bridge downstream to the New England Thruway (I- 95) , including cross-sections of culverts and bridges found along the Sheldrake River. This information does not include the East Branch of the Sheldrake River. Westchester Country Soil and Water Conservation District: The WCSWCD has developed photographic maps of soil characteristics for • the entire county. We obtained copies comprising the study area. We also obtained copies of the TR-20 computer program input data used by the WCSWCD in their 1984 analyses for the West Branch of the Sheldrake River upstream of the Larchmont Reservoir. Town of Mamaroneck: A 1" = 200' topographic map of the Town of • Mamaroneck within the study area was obtained. The map shows 2-foot contour intervals in this area. In addition, the following was obtained: land use information based on assessment maps, previous FEMA reports for this area, maps of critical environmental areas, maps of drainage facilities for the study area, and other miscella- neous information. • Village of Scarsdale: We obtained prints of the Village's "House Number Map", which can be used to determine land use information, and drainage maps for the Village. City of White Plains: A print of the "Land Use Plan" for the City • was obtained. In addition, we obtained drainage maps for those areas of the City located within the Watershed. City of New Rochelle: We obtained prints of drainage maps and zoning maps for those areas of the City that are within the Watershed boundaries. Village of Larchmont: After visiting the Village Engineer's office, we obtained miscellaneous information regarding the Larchmont Reservoir and its dam. Larchmont Reservoir Information: Information regarding water surface elevations at the Larchmont Reservoir and a location downstream of the Reservoir (on the West Branch) , as well as data on operating parameters for the discharge control valve for recent storm events were obtained from Dr. Alan Mason. Dr. Mason, who is a member of the Town of Mamaroneck's Conservation Advisory Commis- sion, operates two gaging stations along the Sheldrake River. • • 0849-15-1095 2-3 Precipitation Information: Rainfall data for the Watershed area were collected from two independent sources: the National Oceanic and Atmospheric Administration (NOAA) and Mr. Thomas Guinan, a respected collector of rainfall data for the study area. The NOAA data comprised two different gage stations in the area (Maple Moor in White Plains and Scarsdale) . Mr. Guinan's data are collected at his gage station in Larchmont. The rainfall data obtained corre- spond to the storm events recorded by Dr. Mason. • • • • • • • 0849-15-1095 2-4 . . . . . . 3 . . . Cardinal 3. ANALYSIS OF EXISTING CONDITIONS 3.1 DESCRIPTION OF THE SHELDRAKE RIVER SYSTEM The Sheldrake River is a tributary of the Mamaroneck River. The Sheldrake River drainage system comprises the upper Sheldrake River (above and including Larchmont Reservoir) and lower Sheldrake River (below Larchmont Reservoir) , the East Branch, the main tributary to the East Branch (East Tributary) , and various smaller tributary streams. The upper portion of the Sheldrake River, which is longer than the lower portion, rises in a residential area of White Plains (near Cushman Road) and flows south across Scarsdale, New Rochelle, and the Town of Mamaroneck before entering the Sheldrake Lake (Larchmont Reservoir) . From the Reservoir, the Sheldrake River flows south across the Town of Mamaroneck (flowing almost • parallel to Weaver Street) downstream to Valley Stream Road. At this point, the Sheldrake River is joined by the East Branch. The Sheldrake River then flows through a series of small waterfalls to Gardens Lake. 41 Downstream from Gardens Lake, the river turns northeast and flows under the New England Thruway (I-95) , to join the Mamaroneck River which empties into Mamaroneck Harbor. • The East Branch of the Sheldrake River rises in Scarsdale and flows south across the Bonnie Briar Country Club to meet the East Tributary at • Fenimore Road. Below Fenimore Road, the East Branch flows south across Rockland Avenue to meet the Sheldrake River below Valley Stream Road. The Larchmont Reservoir, formerly a source of water supply, began 4 operating as a means of storing runoff for the Town of Mamaroneck in the 0849-15-1095 3-1 mid 1970's. The Reservoir's dam was constructed in 1924 and raised in 1935 by the construction of a concrete cap on the existing masonry structure. The 50-foot long spillway is located approximately 650 feet from the south abutment. The drain and blow-off lines from the dam consist of 20-inch cast iron pipes controlled by gate valves. The 100- foot long masonry receiving channel downstream from the spillway discharges into a smaller pond located near the Reservoir. The pond has been used as a conservation area by the community. In 1981, the Reservoir was equipped with an automated butterfly valve to control the Reservoir's water level to improve the utilization of the Lake for flood control . In 1984, the Village of Larchmont dedicated the Reservoir to specific public uses, among which is flood control . The Sheldrake River, Larchmont Reservoir, East Branch and East Tributary have all been classified by the New York Department of Environmental Conservation as class "C" waters. According to New York State Water Quality Regulations, class "C" waters are suitable for fishing and fish • propagation. Class "C" waters are also suitable for primary and secondary contact recreation even though other factors may limit the use for that purpose. Water quality standards for Class "C" waters include limitations • on fecal coliform, pH, total dissolved solids, and dissolved oxygen. 3.2 DESCRIPTION OF THE WATERSHED • The upper portion of the Sheldrake River Watershed is located in White Plains, starting just below and west of Prescott Avenue. It continues • southward across Scarsdale, parallel to Soundview Avenue on its eastern side and Mamaroneck Road on its western side. Inside New Rochelle, the 0849-15-1095 3-2 • western Watershed boundary is parallel to and approximately 3,500 feet west of Weaver Street to the Larchmont Reservoir. On the east, the Watershed boundary lies within Scarsdale and continues southward parallel to Mamaroneck Road. Within Mamaroneck, the eastern boundary continues southward across the Winged Foot Golf Club down to Rockland Avenue and then eastward toward the New England Thruway near Gardens Lake. On the west, the Watershed boundary runs southward from the Larchmont Reservoir to Wood Hollow Lane and then east toward Gardens Lake. The majority of the Watershed contributing to the Sheldrake River above the New York Thruway is essentially developed except for the Bonnie Briar Country Club and the Winged Foot Golf Club. The southern portion of the Bonnie Briar property, along Weaver Street, is frequently flooded during moderate to heavy rainfall events. As such, it acts as a retention basin to the Sheldrake River. These low lying areas should be considered as the Sheldrake River flood plain. Removal of these areas would have an effect both upstream and downstream along the Sheldrake. Constricting the flows • in the upstream section of the River, below the Sheldrake Lake, could result in increased flooding levels. By removing the storage volume provided lying bythese low 1 in areas, an increase in flood flow rates could • be experienced in the downstream reaches of the River. Similarly, a low lying area exists within the Winged Foot Golf Club • property. It is situated in the northern central section of the property. However, unlike the low lying areas of Bonnie Briar, this area is not in the direct flow path of the stream but in the backwater areas. Its • removal would not affect the upstream flood stages but would remove a 0849-15-1095 3-3 natural retention site from the River system and thus, potentially increase flood flow rates in downstream reaches. Several wetland areas identified by the Town Wetlands Commission are located within the Bonnie Briar Country Club property. These areas are shown on the Plates 1A and 1B. These wetlands constitute a depression area and would tend to decrease the runoff volumes in the Watershed. These wetlands and the adjacent area contributing to them could be considered as non-contributing watershed areas within the sub-basin except during rare storm events. Their elimination should be regarded as a decrease in the natural flood retention capacity of the Watershed and could result in increased flooding conditions. 3.3 DEVELOPMENT OF THE HEC-1 MODEL (PEAK RUNOFF FLOWS) Development of the hydrologic model for the Sheldrake River Watershed included delineation of sub-basin boundaries within the Watershed and development of hydrological features for each sub-basin including composite runoff curve numbers, travel times and times of concentration. Development of the model was performed in the following steps: • Delineation of Subbasin Boundaries • The Sheldrake River Watershed boundary was developed and delineated on a U. S. Geological Survey (USGS) map at a scale of 1" = 2,000' . In order to limit the scope of the hydrologic modeling effort, the watershed boundary • was defined as the drainage area which contributes runoff to the Sheldrake River within the Town of Mamaroneck. In addition, the Watershed was 0849-15-1095 3-4 • divided into sub-basins 1-7. The entire Bonnie Briar country Club property and half of the Winged Foot Golf Club properly fall inside the Watershed. Two additional sub-basins (identified as sub-basins 8 and 9) were delineated in order to study the other half of the Winged Foot Golf Club property which lies outside the Watershed and contributes runoff to the Sheldrake River and the Mamaroneck River below the Town of Mamaroneck. Sub-basins 8 and 9 were not included in the hydrologic modeling since this would have required a substantial increase in the scope of work. However, the sub-basins were evaluated using the Soil Conservation Service (SCS) runoff curve number method. Figure 3-1 presents the delineation of the Sheldrake River Watershed into nine sub-basins for the purposes of this study. This delineation allowed the Sheldrake River to be studied in sections as follows: ▪ Upper Reaches of the Sheldrake River (from White Plains downstream to the Larchmont Reservoir, inclusive) : Sub-basins No. 1 and 2. • Lower Reach of the Sheldrake River (from the Larchmont Reservoir downstream to the confluence with the East Branch) : Sub-basin No. 4. ▪ Upper Reach of the East Branch Above Fenimore Road: Sub-basin No. 3. • Upper Reach of the East Tributary (above Fenimore Road) : Sub- basin No. 6. Lower Reach of the East Branch (below Fenimore Road downstream to confluence with the Sheldrake River) : Sub-basin No. 7. • Lower Reach of the Sheldrake River (downstream from the confluence with the East Branch) : Sub-basin No. 5. This delineation intends to separate the different reaches of the • Sheldrake River System. 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' R),:-.Quaker 1—1,, I%wpb F.., o ��. l�,b z :!.� '` � _ O ilia Rldge - , .• I� � ` rt-arprrel��•�.�ll. • ' f jl �ese/zr . •. � / ., ^ -;p Quaker FZ e • • r I( t_Jj ' White , o' SchgQuakeS.Ri.. '• �y �!I� r N ,1 / i • 10� " Bircheso - , .- .", ',1::9n.o,.-g�h- • ' ' 11 lirr o Golf I hCi/ •uNICI f'. .rt` )- naSch } ,.p_+ /. �2-:, C r .I ��J,tuel $Gq hAount �, e _ BRAC °W • �: -� �• a.' Vim, / • 20(i--;"-- ,.. 1 ' ® , , Map 3 .�`� I IPyITERCHANG �a �R .MU/,,ck Wo ds , 1a .o (;rdl,eil 'y,; NI1 �1 Si' ,'';11 °" 1•ineorook i ,' -2% �♦,� P(�/fri. /l I uT°HIN 1 ti w water.. � f J • II H °T: Cn • Tank a°r _ - ° I / t d 1. " -- \ `r.•e No !1 1./ • - a. I 225 �� �� Wing.. Foot I 1 Sta' T3 r 11 t t ��� 6,•t. �Eti, U If Cl P 7$4 V j Itis ( rest :�' .Ris^f/ n'opF r - 4- - INIERCHANGE0 ��/'I / W t //` P� u \ 1 Sch G 'a , l S'-'11 µ°°' - Bonnie R i F I. ire A •• �7 ra-I • i ,;�" -. 1 � 1 I 1 Country Club li' • „ �, � ' Parke •I� r"`� / , « v ,, �La s , tsta ng ,� ,� )� �,i� • / till(4L"hL G:1{. ... H u9tinn ( .... I. (' �jl � untr, , MAIVIARONECI I1 / 611 � � ��� �. �' J l/)' Q01 I;t -. 04- `170 _.i 1 \��° - )5/,;.7 I t , I. a I � ✓ " u � � oIi) • ,� I , o . I . /lIlage ,ri1,•t ,� � I ,., y' • Hall ''-' � Leonard 'fir �_ �\ 0 b.. 5t TI o,r�1. / 1 154 • i • Jr High Scher ��'� { r .� I_AS f 23 K /� r `\r tt-' ret rV \ �wd, ss ,,ryt \, \astir ., • S•ewa , �/ e y 1 f'.., 1 - Dispo al �1 �� t Wyk I / R i+ I. ,r �I^I �Ha o Isla d .��t� r J� 4.,,' S C-0try Club I'O \, ° Ma,ray ,1 , - \,�-) • We3t �)1 ' �, �,_J ii le t ,,^;.,, �� )� Y�l' �jo° h ;! ::''.•� Pam?? '''' o TER -. /.r- aVL �. ps, I ,,. ��� � �• ''..\. \a /�194�;r R° � � v� Fti,1nhn awl Pali( °•� :,,Lr . v Hihh l • ��.:''`�� o � Ro� 1 „1 eti't o\ ,e 1 =.N/ �� m �� t�!,.� Snr)11 Est �. t-- �� • A k l E'' I, atior 0 �J / r \ e--I 'I i�ee c.hn1 on�t " p10 1� t/� \ 3E 15 �� ovLE a ���I % �) aV<iod5 ,.r��+'� tiq>, �� � � ��0yM p use / __ gi FII / ��� C y�� Hampshire ' ! ! % rt,...' � T: Country Club% 4 and 7) from the Winged Foot Golf Club (located within Sub-basin No. 6) so as to facilitate the study of hydrologic impact due to potential development in these areas. Surface areas were computed for each sub- basin in the Watershed, and are presented in Table 3-1. The total Watershed of the Sheldrake River to the New England Thruway is estimated to be 5.6 square miles. Two additional sub-basins (identified on Figure 3-1 as sub-basins 8 and 9) are located on the Winged Foot Golf Club property but do not contribute runoff to the Sheldrake River within the Town of Mamaroneck. The combined areas of these two sub-basins represent approximately fifty percent of the Winged Foot property. Sub-basin 8, with an area of 74 acres drains into two ponds located on the property and used for irrigation. Overflow from these ponds drain to a series of smaller ponds towards the New England Thruway approximately one mile north of where the Sheldrake River crosses the Thruway. These waters ultimately reach the Mamaroneck River in the Village of Mamaroneck above the USGS gaging station. Sub-basin 9, with an area of 66 acres drains through a network of open channels and storm sewers along Country and Fenimore Roads towards the New England Thruway and into the Sheldrake River in the Village of Mamaroneck. Flooding has been experienced along Country and Fenimore Roads from a combination of limiting storm drainage network capacity and high stages in rr the Sheldrake River. These sub-basins have not been incorporated into the Sheldrake River • watershed model , however, the impact of their potential development have been considered separately and are presented in Chapter 4. 0849-15-1095 3-6 • Development of Composite Runoff Curve Numbers (CN) As rain falls on an area, a portion of it is intercepted by the leaves and vegetation. Another portion is used to replenish the soil moisture in pervious areas and could be used to replenish the groundwater. That portion of the rainfall which is not intercepted nor percolates through the soil is available as direct runoff. The rainfall -runoff process is a complicated one and is a function of the type of soils present (a pervious soil strata will allow more infiltration and less runoff) , the type of land use (a commercial area with parking lots would interfere with infiltration and allow more runoff) , and the type of vegetation present (a forested area would intercept more rainfall than a well maintained grassed area) . The U.S. Department of Agriculture's Soil Conservation Service has developed an index known as composite Curve Number (CN) method that incorporates the above parameters into a numerical value. The development of the CN value takes into consideration the hydrologic properties of the native soil types, the amount of imperviousness associated with various land uses and the type of vegetation cover in open areas. Information obtained from the WCSWCD regarding soil characteristics and land use information obtained from appropriate municipalities, was utilized to develop composite runoff Curve Numbers (CN) for each sub-basin following the Soil Conservation Service's (SCS) methodology. The average CN for this Watershed is approximately 75. The CN value for each sub- basin is presented in Table 3-1. 0849-15-1095 3-7 TABLE 3-1 SUMMARY OF HYDROLOGIC FEATURES OF THE SHELDRAKE RIVER WATERSHED EXISTING CONDITIONS DRAINAGE TIME OF SUB-BASIN AREA SCS CONCENTRATION No. (ACRES) CURVE NUMBER (hours) 1 1110 80 1 .33 2 818 79 0.55 3 726 74 2.00 4 252 72 1 . 17 5 95 78 0.53 6 312 70 1 .42 7 244 71 0.68 • 8 74 62 -- 9 66 63 -- • • • • Development of Times of Concentration (Tc) Another parameter in the development of the rainfall -runoff relationship is the time it takes for a sub-basin to contribute fully to the runoff measured at a particular point. In general , the shorter this time, the faster and more intense is the response of the runoff. This time is known as the time of concentration (Tc) . The Tc, physically represents the time it takes for runoff to travel from the most remote point within the watershed to the downstream point of interest. The Tc is developed from topographic information as well as the characteristics of the storm water drainage network, natural or manmade. Utilizing topographic information, times of travel (Tt) and times of concentration (Tc) were developed for each sub-basin within the Watershed. The SCS TR-55 methods were used (i .e. , Manning's kinematic solution to compute sheet flow, SCS equation for shallow concentrated flow, and Manning's equation for open channel flow) . Development of the Sheldrake River Watershed Hydrologic Model The COE's Hydrologic Engineering Center's HEC-1 Flood Hydrograph Package computer program was utilized to model the surface runoff response of the Sheldrake River Watershed to precipitation. A schematic of the Sheldrake River Watershed Hydrologic Model built for this study is presented in Figure 3-2. The figure shows the routing sequence for the seven sub- basins. Table 3-2 presents a summary of the most important hydrological features for each of the seven sub-basins for existing land use condi- tions. 0849-15-1095 3-8 TABLE 3-2 WESTCHESTER 24-HOUR RAINFALL AT VARIOUS STORM FREQUENCIES Frequency in years Rainfall in inches 1 2.6 2 3.3 5 4.3 10 5.0 25 5.7 50 6.3 100 7.2 Note: Values are interpolated values taken directly from Weather Bureau Technical Paper No. 40, "Rainfall Frequency Atlas of the United States. " FIGURE 3-2 1111118111111111111 Larchmo t ................ ................. Reservoir . ........................... ............................ .......................... ............................ ........................... ............................ ........................... .............. ........... ........................... .........................: ........................... IP 40 SHELDRAKE RIVER HYDROLOGIC STUDY SCHEMATIC OF ItiP3M1144 SHELDRAKE RIVER WATERSHED The procedure followed in the development of the runoff model consisted of two major steps: (1) preliminary development of the hydrologic model using collected data for existing conditions in the watershed; and (2) calibration of the model using actual rainfall data and observed runoff flows. • 3.4 CALIBRATION OF THE HEC-1 MODEL • Before a computer program is accepted as a model , a calibration process needs to be performed. The calibration consists of comparing the computed results of a known precipitation event and actual measured conditions in • the field during the same event. If necessary, model parameters need to be adjusted to minimize the variance between the observed and the modeled values. When this comparison is satisfactory, the computer program is • accepted as a model . After developing the hydrologic model for the watershed, it was then • calibrated to the extent that flow data information was available. Calibration was performed for the upper reaches of the Sheldrake River (upstream portion of sub-basin 1 down to Larchmont Reservoir) and for a • lower section of the Sheldrake River (from Larchmont Reservoir down to the crossing at Rockland Avenue) . • The data utilized in the calibration process corresponded to two independent storm events, one of which caused water to go over the Reservoir's spillway. The flow data for the two storm events was • collected by Dr. Alan Mason of the Town of Mamaroneck's Conservation Advisory Commission. The provided information was collected during the 0849-15-1095 3-9 to 41 storms of May 16-17, 1989 and May 16-17, 1990 and included water surface elevations at the Reservoir, degree of opening in the controlling • butterfly valve, and stream water surface elevations for a point downstream from the Larchmont Reservoir (for the May 1990 storm only) . Rainfall information for these storms was obtained from two different • sources: the National Oceanic and Atmospheric Administration (NOAA) and from Mr. Thomas Guinan. • The watershed model was initially calibrated with data from the May 1989 storm. A comparison between the observed and computed outflow hydrographs at the Larchmont Reservoir is presented in Figure 3-3. No data was • available for observed water surface elevations in the Sheldrake River downstream of the Reservoir for this storm event. Thus, no comparison could be made for calibration purposes. • The initial calibration process involved comparing volumes of runoff generated in the watershed with the observed volumes at the Larchmont Reservoir. The computed volumes adequately represented the observed volumes at the Reservoir and no adjustment in CN values was required. The channel routing process used in the development of the model was the Muskingum routing procedure. The lag time between watersheds used in this Imethod was derived from the observed data. The results of the initial calibration indicate that the model adequately represents the runoff rates IP observed at the Larchmont Reservoir. The second calibration was performed utilizing the data from the May 1990 storm event. Figure 3-4 presents a comparison between observed and computed stage hydrographs at the Larchmont Reservoir. For this second 0849-15-1095 3-10 • 0 Flow (cfs) 45 - 40 - Computed 35 - ---- Observed 30 25 - 20 - 5 -20 / 15 - 10 - 5 1 5 17191111131151171191211231 1 13 1 517191 4 6 8 10 12 14 16 18 20 22 24 2 4 6 8 10 Hour of Day (from May 16 , 1989 ) -n0 SHELDRAKE RIVER HYDROLOGIC STUDY CC CALIBRATION - MAY 1989 STORM W IRNI LARCHMONT RESERVOIR 145 - 140 - o 135 Observed > 130 - Computed 125 - a� U (ti 4-1$4 120 - U) N 115 - a� ' 110 - 105 - 100 1 „ Il III III III II 11 III III IIII II 6 10 14 18 22 2 6 10 14 18 22 Hour of Day (from May 16 , 1990) T SHELDRAKE RIVER HYDROLOGIC STUDY C Valle CALIBRATION - MAY 1990 STORM m LARCHMONT RESERVOIR calibration, the comparison between observed and computed runoff volumes showed a higher observed flow volume. Further examination of the rainfall records showed moderate to large rainfall volumes in the 6 days prior to the observed rainfall event (a total of 2.12 inches of rain) . These antecedent rainfalls would have saturated the soils, thus increasing the actual runoff volumes. Accordingly, the original computed CN values were increased to reflect these conditions based on the relationship provided in the SCS National Engineers Handbook - Section 4 (NEH-4) . The adjusted CN values produced the observed runoff volumes at the Reservoir. It should be noticed, in Figure 3-4, that the computed hydrograph is very close to the observed one in timing as well as reservoir levels for the period of observed values. Figure 3-5 presents a comparison between observed and computed outflow hydrographs for a location on the lower portion of the Sheldrake River downstream of the Reservoir and upstream of the confluence point with the East Branch. This is a monitoring station located within Dr. Alan Mason's property. The releases from the Reservoir are observed further downstream at that monitoring location. The comparison between the observed and computed hydrographs presented in Figure 3-5 show that the hydrographs appear very close to each other. A discrepancy appears at the beginning of the storm whereby the observed values are higher than the computed ones. This could be explained by the fact that the low level outlet at the Reservoir was left open in anticipation of the forthcoming storm. Thus, the observed flows at the downstream location reflect the emptying of the Reservoir. Since no data prior to hour 22 on May 16, 1990 was available at either location, this emptying procedure could not be 0849-15-1095 3-11 280 - 260 - 240 - 220 - 200 - /---\ N 180 - ; N 160 - 140 - M 40 - 0 120 - �N_ Observed 100 - 80 - \ so - 40 - Computed ______________ 20 - 22 23 24 1 2 3 4 5 6 7 8 9 10 11 1213 1415 1617 18 19 20 21 22 Hour of Day (from May 16, 1990) T SHELDRAKE RIVER HYDROLOGIC STUDY C CALIBRATION - MAY 1990 STORM w RNI WEST BRANCH duplicated in the model , which considered the levels in the Reservoir to be the same as the observed levels as of the first reading. It should again be noted that shapes, timing, and magnitude of the hydrographs at the downstream location are within less than 10 percent. Based on the above results, the model was considered calibrated to reflect the existing conditions. The printed results of the two computer calibra- tion runs are presented in the Appendix. 3.5 DEVELOPMENT OF HEC-2 MODEL (WATER SURFACE PROFILES) The output data from the HEC-1 modeling, in the form of peak runoff flows, was used as input data for the HEC-2 model to obtain flood level elevations along the Sheldrake River and its tributaries. In order to obtain stage information, a rating curve must be developed for the drainage system. The rating curve for the watershed model is developed by entering stream cross-section data for all reaches of concern. For the Sheldrake Watershed Model , cross-section data was obtained for the Sheldrake River, the East Branch, and the tributary to the East Branch. Cross-sectional data for the upper reach of the Sheldrake River was mostly obtained from the Corps of Engineers. The data was supplemented by field surveying upstream of the Rockland Avenue bridge. The work was performed on November 26, 1990 and provided data for the development of stream cross-sections for three additional stations located on the crossings at Bonnie Briar Lane and Weaver Street. 0849-15-1095 3-12 To the best of our knowledge, there was no stream cross-section informa- tion available on the East Branch of the Sheldrake River prior to this study. Therefore, a field survey of the East Branch was performed on November 26, 1990. This resulted in the development of approximately 54 stream cross-sections which characterize the East Branch and its tributary. All significant structures (i .e. , bridges or culverts) were also surveyed. Field surveys supplemented by existing topographic maps with 2-ft contour intervals were used to develop the rating curves for the East Branch and the East Tributary. The HEC-2 model was utilized to calculate water surface elevations corresponding to the different flows generated from the HEC-1 output for the Sheldrake River, East Branch, and the East Tributary. The effects of all major obstructions such as bridges and culverts were considered in the computations. Estimates of the water surface profiles were attained by concurrently modeling the three reaches. 3.6 CALIBRATION OF HEC-2 MODEL A rigorous calibration of the water surface profiles was not performed because no known high water marks have been identified in the watershed. However, the results were reviewed for their representation of known flooding along Weaver Street. The computer-generated water surface profile shows close agreement with actual floodings experienced along Weaver Street. 0849-15-1095 3-13 3.7 MODEL SIMULATION OF DIFFERENT STORMS FOR EXISTING CONDITIONS Once development and calibration of the computer model was completed, a series of simulations was performed utilizing data for the 5, 10, 25, 50, and 100-year return period storms for the Westchester Country area. The return period is defined as the average number of years within which a storm event will be equaled or exceeded once. The rainfall depth for Westchester Country corresponding to various return periods are presented in Table 3-2. Data on the rainfall distribution for a type III storm, applicable to the study area, was obtained from the SCS office in Somerset Country, NJ. The Type III synthetic 24-hour rainfall distribution represents Gulf of Mexico and Atlantic coastal areas where tropical storms bring large 24-hour rainfall amounts. Surface areas, runoff curve numbers, travel times, and times of concentrations used for developing the model were used for simulating the five storm events since the characteristics used for model development represent existing conditions. Table 3-3 presents a summary of the estimated runoff flows for the five return period storms. Results are given for each of the seven sub-basins identified within the Sheldrake River Watershed Model for existing land use conditions. It should be noted that the peak values presented in Table 3-3 are for each individual sub-basin. They do not reflect the significant attenuation in peak flow that occurs through the reaches of the river and the routing through the Reservoir. In addition, these peak flow rates occur at different times and cannot be directly added together. 0849-15-1095 3-14 TABLE 3-3 SUMMARY OF ESTIMATED PEAK RUNOFF FLOWS FOR SELECTED STORM EVENTS AT THE SHELDRAKE RIVER WATERSHED EXISTING CONDITIONS 5-YEAR 10-YEAR 25-YEAR 50-YEAR 100-YEAR DRAINAGE STORM STORM STORM STORM STORM SUB-BASIN FLOW FLOW FLOW FLOW FLOW No. (cfs) (cfs) (cfs) (cfs) (cfs) 1 1008 1275 1547 1782 2138 2 1125 1489 1738 2007 2413 3 395 519 649 763 939 4 174 232 293 347 430 5 128 164 200 232 280 6 176 238 304 362 453 7 214 288 364 432 536 Note: cfs signifies cubic feet per second. The computer printed results of these simulations are presented in the Appendix. 3.8 FLOOD PLAIN AREAS FOR EXISTING CONDITIONS The United States Army Corps of Engineers (COE) Hydrologic Engineering Center's HEC-2 Water Surface Profiles computer program was utilized to calculate water surface elevations corresponding to the above mentioned flows in the Sheldrake River, the East Branch, and the East Tributary. The effects of all major obstructions such as bridges and culverts were considered in the computations. Based on the results of this modeling, flood hazard areas were delineated for the peak runoff flows obtained by analyzing the five return period storms. The flood plain areas delineated in this study include only those reaches of the Sheldrake River Watershed that are within the Town of Mamaroneck. A summary of the flood plain and affected dwellings for the 100-year, 25-year, and 5-year storm events are presented below. Flood plain elevations for the 10 and 50-year storms fall within the range given for the 5 and 100-year storms. The flood plain elevations for the five storm frequencies are shown graphically on Plates 1A and 1B. SHELDRAKE RIVER FLOOD PLAIN The Sheldrake River flows south and east through the Town of Mamaroneck. The Sheldrake River is the major drainage source for the entire watershed. The section of the River which was modeled in this study includes the portion from the lower pond of the Larchmont Reservoir to the crossing with the New England Thruway (I-95) . This area includes sub-basins 3 and 0849-15-1095 3-15 4. The flood plain for this section of the Sheldrake River is relatively narrow with two wide flood plain areas; one at the upstream side of the Rockland Avenue crossing and one at the upstream side of the I-95 crossing. Although the flood plain along this portion of Sheldrake River is relatively narrow, the area is heavily populated and residents have experienced flooding problems in the past. Upper Reach Between the lower pond of the Larchmont Reservoir and the bridge at Weaver Street, the flood plain covers an area below Elevation 84.7, approximately. The bridge at Weaver Street is at a high elevation and acts as a dam in this scenario allowing the water to flow only through the opening underneath the bridge. Five dwellings with appurtenant structures located along Bonnie Way are within the 100- year flood plain. Four are within the 25-year flood plain and none are within the 5-year flood plain. The area located between the Weaver Street bridge and the bridge at Bonnie Briar Lane is within the boundary of the Bonnie Briar Country Club and is sparsely populated. In this area, the flood plain varies between Elevation 84.6 and Elevation 81.5, approximately. Six dwellings with appurtenant structures are within the 100-year flood plain, and four are within the 25-year and 5-year flood plains. The bridge crossing the Sheldrake River on Rockland Avenue is high enough to act as a dam, allowing the flows to pass only through the opening below it. This tends to back up the water upstream of the 0849-15-1095 3-16 bridge. Thus, from Rockland Avenue upstream to Bonnie Briar Lane the flood plain varies between Elevation 80 and Elevation 81 .5, approximately. Eleven dwellings or appurtenant structures are within the 100-year and 25-year flood plains, while 7 are within the 5-year flood plain. In the area between Forest Avenue and Rockland Avenue, the flood plain varies between Elevation 74 and Elevation 78. Approximately 34 dwellings or appurtenant structures located on either bank of the Sheldrake River are within the 100-year flood plain. For the 25- year and 5-year flood plains, 28 and 27 dwellings are affected respectively. From the junction of Valley Stream Road and Brookside Drive upstream to Forest Avenue, the flood plain varies between Elevation 62 and Elevation 74, approximately. Twenty dwellings or appurtenant structures are within the 100-year flood plain in this area. Sixteen are within the 25-year flood plain and 13 are within the 5- year flood plain. Lower Reach The flood plain between the junction of Valley Stream Road and Brookside Drive down to Fernwood Road varies between Elevation 62 and Elevation 60, approximately. In this area, 18 dwellings or appurtenant structures are within the 100-year flood plain, while 11 are within the 25-year flood plain and none are within the 5-year flood plain) . 0849-15-1095 3-17 I 41 From Fernwood Road downstream to Hickory Grove Drive, the flood plain varies between Elevation 60 and Elevation 59.8. Twenty-nine dwellings or appurtenant structures are within the 100-year flood plain area. Twenty are within the 25-year flood plain while none are within the 5-year flood plain. • In the area between Hickory Grove Drive and the New England Thruway (I-95) , the flood plain varies between Elevation 59.7 and Elevation 30.2. Ten dwellings or appurtenant structures are affected by the 100-year flood plain in this area surrounding the Gardens Lake. Four dwellings are within the 25-year flood plain and none within the 5-year flood plain. Finally, downstream of the New England Thruway, the 100-year flood plain is below Elevation 30 and a maximum of 6 dwellings or appurtenant structures may be affected. None are affected by the 25-year or the 5-year flood plains. EAST BRANCH FLOOD PLAIN • The East Branch flows south across Murdock Woods in sub-basin 3 and joins with the East Tributary at Fenimore Road. On this portion of the East Branch, the 100-year flood plain includes areas below Elevation 102.2 (USGS datum) . The upper portion of the East Branch along Fenimore Road has a wide flood plain, however, there are no dwellings or appurtenant structures within the 100-year flood plain. The only structures flooded are bridges across the river, including those located on Fenimore Road. 0849-15-1095 3-18 40 South of Fenimore Road, the East Tributary combines with the East Branch. The East Branch flows south, collecting runoff from sub-basin 7 and 40 eventually joins the Sheldrake River. The flood plain area along the East Branch between Fenimore Road and the confluence with the Sheldrake River is relatively narrow with one wide area between Fenimore Road and York • Road. The majority of the area along the East Branch is sparsely populated except for a portion downstream of Rockland Avenue to the confluence with the Sheldrake River. 40 The flood plain for the East Branch below Fenimore Road downstream to York Road covers an area below approximately Elevation 98.7. Eight dwellings • or appurtenant structures are affected by the 100-year flood plain, 5 dwellings are affected by the 25-year flood plain, and 4 dwellings are 4 affected by the 5-year flood plain. Between York Road and Hilltop Road, the flood plain includes an area below Elevation 93 (upper reach) to Elevation 79.4 (lower reach) . One dwelling is affected in this area by the 100-year flood plain. That same dwelling is within the 25-year and 5- year flood plains. The flood plain between Hilltop Road and Rockland Avenue covers an area y • below Elevation 79.4 at Hilltop Road to Elevation 72.6 at Rockland Avenue. This is a sparsely populated area and no dwellings or appurtenant structures are within the flood plains. However, below Rockland Avenue 44. and downstream to the point of confluence with the Sheldrake River, the 100-year flood plain includes an area below Elevation 72.7, which affects approximately six dwellings or appurtenant structures. Three dwellings are within the 25-year flood plain and two are within the 5-year flood plain. 0849-15-1095 3-19 moo EAST TRIBUTARY FLOOD PLAIN The East Tributary flows south across the Winged Foot Golf Club and then across the Murdock Woods neighborhood community. The Tributary collects storm water runoff from sub-basin 6. The confluence of the East Tributary and the East Branch occurs at Fenimore Road. The area along the East Tributary and the surrounding grounds are sparsely populated at present. A wide flood plain signifies that an area is more susceptible to flooding than a narrow flood plain. The East Tributary between Griffin Avenue and Fenimore Road has a relatively narrow flood plain with the widest portions occurring at Adrian Circle and at the crossing with Griffin Avenue. On the East Tributary, the flood plain includes areas below Elevation 177.9 on the uppermost section of the study area, near Griffin Avenue. Three dwellings in this area are within the 100-year and 25-year flood plains, while 2 dwellings are within the 5-year flood plain. Below Griffin Avenue, the flood plain for this tributary includes Adrian Circle and Kolbert Drive. In this area, approximately 11 dwellings or appurtenant structures are within the 100-year flood plain, 6 dwellings are within the 25-year flood plain, and 5 are within the 5-year flood plain. 0849-15-1095 3-20 4 Cardinal' 4. ANALYSIS OF POTENTIAL DEVELOPMENT SCHEMES 4.1 INTRODUCTION A hydrologic model was developed for the Sheldrake River Watershed. The model was developed based on existing land use conditions. Next, the developed model was calibrated using rainfall and stream gage data collected during actual storm events. The calibrated model was then used to simulate flooding impacts under existing conditions for five storm frequencies (5, 10, 25, 50, and 100-year) . The final part of the modeling effort was to analyze potential flooding impacts which may result if future development is undertaken. For the two sub-basins on the Winged Food Golf Club property draining to the Village of Mamaroneck and previously identified in Section 3.2 as sub- basins 8 and 9, hydrologic characteristics for present (undeveloped) and the four alternative development schemes, discussed below, were estimated. Based on these estimates, mitigation measures to minimize and/or eliminate potential impacts were developed separately for these two sub-basins. The following section is a summary of the work performed for the four development schemes. The proposed developments are to be located in the areas of the existing Bonnie Briar Country Club and Winged Foot Golf Club. The proposed development schemes fall into two different types of land use; R30 zoning and CR zoning. The R-30 zoning represents the current land use zoning 0849-15-1095 4-1 which is classified as residential at a density of one unit per 30,000 square feet. The CR zoning represents the proposed Conservation- Recreation zone which is based on preserving environmentally sensitive areas and maintaining the maximum usable open space. The four development schemes include: No Zoning Changes 1. R30 Detached - Single family detached houses 2. R30 Townhouses - Attached townhouses Re-zoning Required 3. CR Detached - Single family detached houses 4. CR Townhouses - Attached townhouses Detailed information on land use, zoning, and development programs were obtained from Ferrandino and Associates. Schematic designs for CR Detached and CR Townhouses for both Bonnie Briar Country Club and Winged Foot Golf Club were supplied by Ferrandino and Associates. They also supplied us with schematic designs for R30 Detached and R30 Townhouses for the Winged Foot Golf Club. The schematic designs for R30 Detached and R30 Townhouses for the Bonnie Briar Country Club previously developed by Shuster and Associates were used in this study. In general , two major parameters are affected within a watershed as a result of potential development; the increase in imperviousness and decrease in time of concentration. Both of these would tend to increase the peak flow rates contributed from a watershed. In addition, the 0849-15-1095 4-2 increase in imperviousness would also increase the volume of runoff from a given storm because of reduced infiltration. The increases in peak flow rate are usually accommodated by the incorporation of storm water detention basins. However, the increase in runoff volumes would remain. In this study, it is implicitly assumed that any potential development would incorporate storm water detention basins which would eliminate the peak runoff rate increases from the individual tract. The increases in runoff volumes are considered because they impact directly on the overall runoff from their respective sub-basins. 0849-15-1095 4-3 FIGURE 4-1 • I1 r ' a ` ' \ • • l S MO1 R ' , %o , \ � ` .. �' '^ Q 1 1 r ,. ; 6 .• � z\/ jt- � !, / . .t 1 , _ _��� / ' / p) f /�\ Iniii t 1\tom, toof— . �• / ce'\� /i ; \ ;°"--- \ \ \\ '• I \ t \ 1.5 • `` _ 1 4C \ iT ,F r / ., 2T i I ,. `‘ . q 69 vtv `r` �V Al ' cAIR \ 1 I IN t l t\ ` "tf "10 1 , 53 5�\� 't f 1 i . i . 4q ,�, ct 111, \ i / , .t1• ";.ir �', _ "/ t 1 \\ �` J-7,4 I 1 110 V/ t. 1 J'f .*��✓✓''��af'�t J / \, 1 e4 /it t 1t ) r t ,; 1 ot4opt • '''%4.* ',) ! 7 A \pak t_. -1 I I 1 , r-, i \ l v 2 oc �i� !) • •,9 ' '151 Yo , 1 /�g�_ 1 1 . \ �� ��i r\ tr- .,e 1 \ _ t - \ • / \‘,. • 1C'°-'i ' . 1 I • � �1� ., .� 1 0 100 200 400 • °0 r \O-\ \,:`1, /• r_ 1,1\ I \",t _ ` \� i 1 �; fi° FEET • r► G r t !_� .' , ��, 1 . I \\. ... ., t\ r --sj'�1 , 1.13 - ti 1 \ �1 \ �? > >� t i q0\\\ "24' g' ‘‘)Z/ `/ 118\ T N s —� tel' . \ \ r • \":4 -, \ e/ , N, r ,' •:....7:- g':',, if7:2,:',.;:-.4",-!''',.'•,'"i''':i -7z,'":' . 4,44, • \--•: \ 43 •\ ..r.,-.4--'', ..'-' -- ,..../4-111kk ''• cam•#7• er• ,. -• x/��� ' .')A t '• V. - ` River Z y0. :42.'74c)::r - I. QQ. lork122 /� .y'b 123 124 125 a il h- 4:6$> \"11 , SN 6 1 SOURCE: SHUSTER ASSOCIATES SHELDRAKE RIVER HYDROLOGIC STUDY BONNIE BRIAR PROPERTY 0 IRNI R30 DETACHED DEVELOPMENT r 4.2 R30 DETACHED DEVELOPMENT • INTRODUCTION The R30 Detached development is also called the "no action" alternative • because it represents residential development of the country club properties under existing zoning. Under this development scheme, 125 single family homes can be developed on the Bonnie Briar property and 285 • single family homes can be developed on the Winged Foot property. The size of the unit or "footprint" was assumed to be 3,000 square feet at a density of one unit per 30,000 square feet. Under this scenario, the golf course would be lost to development, but the existing recreational facilities would be preserved. Environmentally sensitive areas would be developed. A schematic design for the R30 Detached development of Bonnie Briar property is illustrated in Figure 4-1. HYDROLOGIC ANALYSIS Watershed boundaries and land use plans for the development scheme were used to determine that six sub-basins would be affected by the develop- - • ments. The Bonnie Briar development will affect sub-basins 3, 4, and 7, and the Winged Foot development will affect sub-basins 6, 8 and 9. r For each sub-basin, the increase in impervious surface area was calculat- ed. This was accomplished by using land use maps and site area calcula- tions for roofs, roads, parking areas, etc. , as supplied by Ferrandino and Associates. The increase in impervious area represents between 1 percent 0849-15-1095 4-4 • • and 14 percent of the sub-basin area. A summary of the change in imperviousness due to the development is presented in Table 4-1 . The increase in imperviousness for each sub-basin resulted in an increase in the composite runoff Curve Number (CN) . Information obtained from the WCSWCD regarding soil characteristics and land use information was utilized to develop the CN values for the affected sub-basins. For the calculated impervious areas, a CN value of 98 was used. Increases in CN values of less than 0.3 were considered to be negligible. Comparison of CN values for undeveloped (existing) and developed conditions show increases in five of the six affected sub-basins. (Refer to Table 4-2) . Using the watershed hydrologic model which was calibrated for existing conditions, a series of peak runoff flows were simulated for the R30 Detached development condition under 5, 10, 25, 50, and 100-year return period storms. The model predicted increases in peak storm water runoff in the sub-basins as a result of the development. The model results are summarized in Table 4-3. The increases range from 3 to 11 percent. It can also be seen that the impact is higher for the more frequent storms (5-year) than for the less frequent storms (100-year) . This is due to the fact that the larger storms tend to saturate the soils to a degree whereby their unpaved areas contribute to runoff almost to the same degree as the paved areas. The model results indicate no increase in peak runoff flow for sub-basin 3 for the developed condition. (Note that the CN values did not change for this sub-basin.) For sub-basin 4, a 5 to 10 percent increase in peak runoff flow resulted. A 3 to 5 percent increase is predicted for sub-basin 7, and for sub-basin 6 the increase ranges from 6 to 11 percent . Because the individual sub-basin peak flows occur at 0849-15-1095 4-5 • TABLE 4-1 • INCREASE IN IMPERVIOUS AREA FOR R30 DETACHED DEVELOPMENT • INCREASE IN IMPERVIOUS AREA LOCATION AFFECTED SUB-BASIN ACRES % OF PROPERTY % OF BASIN • BONNIE BRIAR 3 7.1 14 1 BONNIE BRIAR 4 14.2 18 6 BONNIE BRIAR 7 4.7 26 2 • WINGED FOOT 6 26.7 18 9 WINGED FOOT 8 6.7 9 9 WINGED FOOT 9 9.2 14 14 • I • 4 • 4 • 4 y ,. 4 TABLE 4-2 SUMMARY OF CN VALUES FOR R30 DETACHED DEVELOPMENT COMPARISON OF CN VALUES LOCATION AFFECTED SUB-BASIN UNDEVELOPED DEVELOPED BONNIE BRIAR 3 74 74 BONNIE BRIAR 4 72 74 BONNIE BRIAR 7 71 72 WINGED FOOT 6 70 72 WINGED FOOT 8 62 66 WINGED FOOT 9 63 67 • TABLE 4-3 • SUMMARY OF PEAK RUNOFF FLOWS FOR R30 DETACHED DEVELOPMENT • PEAK RUNOFF FLOWS (CFS) LOCATION AFFECTED SUB BASIN 5-YR 10-YR 25-YR 50-YR 100-YR • BONNIE BRIAR 3 395 519 649 763 939 (0%) (0%) (0%) (0%) (0%) BONNIE BRIAR 4 192 252 314 369 453 (10%) (9%) (7%) (6%) (5%) BONNIE BRIAR 7 225 300 377 446 551 (5%) (4%) (4%) (3%) (3%) WINGED FOOT 6 195 260 327 387 479 (11%) (9%) (8%) (7%) (6%) Note: (%) signifies percent increase in flow over existing undeveloped conditions. I 4 • mi 41 different times, the overall flow increase in the lower reaches of the Watershed along the Sheldrake River is less. The computer printed results • for the development simulations are presented in the Appendix. For sub-basins 8 and 9, the hydrologic parameters developed were used to 111 compute the volumetric increases of runoff during a 10 and 100-year return period storms. The results indicate that under present undeveloped conditions, during the 10-year storm, approximately 8 and 9 acre-feet of * runoff is generated in sub-basins 8 and 9 and during the 100-year storm, the runoff volumes are increased to 16 and 19 acre-feet respectively. It should be noted that an acre-foot is the volume of water one foot deep w • covering a one acre area. Under the R-30 detached development alterna- tive, during the 10-year storm, these volumes would increase to 10 and 11 acre-feet for sub-basins 8 and 9 and during the 100-year storm the runoff volumes would increase to 19 and 21 acre-feet respectively. (Refer to Table 4-4) . Since downstream flooding exists under present conditions, it is proposed that future development on these portions of Winged Foot incorporate a retention basin to contain all runoff during storms of up to 100-year N . frequency. These retention basins would be emptied when the waters in the Sheldrake and Mamaroneck Rivers recede. As indicated Section 3.3, subbasin 8 drains into an existing pond on the Winged Foot property. If I . the levels in this pond are maintained at their present level , the pond could provide a retention volume of 11.3 acre-feet. Therefore, additional volume of storage needs to be provided within Winged Foot for both sub- basins 8 and 9. 0849-15-1095 4-6 S TABLE 4-4 SUMMARY OF RUNOFF VOLUMES FOR R30 DETACHED DEVELOPMENT RUNOFF VOLUME (ACRE-FEET) EXISTING ENTIRE DEVELOPMENT CONDITIONS SUB-BASIN ONLY AFFECTED SUBBASIN 10-YR 100-YR 10-YR 100-YR 10-YR 100-YR 8 7.91 16.22 9.50 18.49 9.50 18.49 (20%) (14%) 9 9.30 18.82 11 . 12 21 .38 11. 12 21.38 (20%) (14%) Note: - "Existing conditions" represent runoff from undeveloped sub-basin area. - "Entire sub-basin" represents runoff from the entire sub-basin due to develoment. - "Development only" represents runoff from the developed portion of the property only. - (%) signifies percent increase over existing undeveloped conditions. t • IMPACTS • Peak storm water runoff flows generated by the HEC-1 model were used as input for the HEC-2 model to determine flood plain elevations. The water surface elevations corresponding to the peak runoff flows from the • developed sub-basins were determined for all five storm frequencies. The HEC-2 computer program results showed an increase in water surface elevations of less than 0.3 feet in the Sheldrake River Watershed for all 40 return storm frequencies. A summary of the increase in flood plain elevations is given in Table 4-5. Results of the computer program runs are presented in the Appendix. • The water surface profile results show much variation in flood elevation increases. Higher storm water runoff flows do not necessarily show r correspondingly higher increases in flood elevations. The varying 4 geometry of the stream cross sections results in varying flood levels for different storm events. For sub-basin 3, the maximum increase in flood plain elevation for all storm events occurs along the East Branch where the stream parallels Fenimore Avenue between Cornell Street and Winding Brook Drive. For sub-basin 4, the maximum increase for all storms except RI the 10-year, occurs at one of two different stations near the confluence of the East Branch with the Sheldrake River. For the 10-year storm frequency, the maximum increase occurs just upstream of Valley Stream 4 Road. The maximum flood plain elevation increase for sub-basin 7 varies with the I different storm frequencies. For the 5-year storm, the maximum increase occurs along the East Branch where the stream parallels Ridgeway Road. 0849-15-1095 4-7 y , TABLE 4-5 MAXIMUM INCREASE IN FLOOD PLAIN ELEVATION FOR R30 DETACHED DEVELOPMENT MAX. INCREASE IN FLOOD PLAIN ELEV. (FT) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR BONNIE BRIAR 3 0.01 0.01 0.02 0.03 0.04 BONNIE BRIAR 4 0.10 0.01 0.04 0.05 0.04 BONNIE BRIAR 7 0.21 0.12 0. 12 0. 13 0.20 WINGED FOOT 6 0.16 0. 11 0.24 0. 10 0.19 Note: Maximum increase occurs at different locations for varying storm events. The maximum increase for the 10, 25, and 50-year storms occurs along Hilltop Road and at the Rockland Avenue Crossing. For the 100-year storm, the maximum increase occurs along the East Branch between Hilltop Road and York Road. The maximum increase in flood plain elevation for sub-basin 6 varies with different storm frequencies. For the 5 and 50 year storms, the maximum increase occurs at two different stations along the East Tributary upstream of the confluence with the East Branch at Fenimore Avenue. For the 10, 25, and 100-year storms, the maximum increase occurs along the East Tributary at a section of the stream which approaches Salem Drive. The increase in water surface elevations (less than 0.3 feet) due to the R30 Detached development had no observable affect on the existing flood plain area. The increase along the river sections within each sub-basin was such that no additional dwellings were affected. Therefore the number of dwellings within the flood plain areas for the developed conditions is the same as the number of dwellings which are being flooded under existing conditions. However, localized flooding within the developed area itself or in areas immediately adjacent to the property may be a concern during and after construction. This can happen for instance when existing culvert and storm drains are not adequate for increased runoff flow. The properties should be developed with storm water controls in mind to reduce peak runoff flows to the drainage system. This could be accomplished with detention ponds. Under this development scheme, the low lying areas along the Sheldrake River in the Bonnie Briar property are somewhat preserved. However, four 0849-15-1095 4-8 41 lots are envisioned along Weaver Street which might encroach on the existing flood plain. In addition, the existing wetlands are not specifically preserved. This approach would tend to exacerbate the existing flooding conditions along Weaver Street. The proposed control method would effectively eliminate the runoff contribution of sub-basins 8 and 9 from a portion of the existing watershed during a storm condition. This, in turn, would reduce the flows in the Fenimore and Country Roads drainage systems and help alleviate existing flooding. I . Currently the Sheldrake River and its tributaries are class "C" waters and must meet standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality 0 . aspects were performed in this study, we expect the highest degree of water quality degradation with the R30 Detached development since this alternative results in the greatest increase in road surface area. Approximately 10 percent of the Bonnie Briar and Winged Foot properties are intended to be paved for roads. Increases in road surface area typically result in an increased dissolved solids load to the drainage system as well as Polycyclic Aromatic Hydrocarbons (PAH) , and heavey metals such as lead and zinc. The proposed road surface area is three to four times greater under the R30 Detached scenario than under the other development schemes. Although we expect an increase in runoff suspended solids on the property itself, we believe that water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water 0849-15-1095 4-9 • runoff. The class "C" classification of the River is not expected to change. • s I . • • 40 0849-15-1095 4-10 4.3 R30 TOWNHOUSE DEVELOPMENT INTRODUCTION The R30 Townhouse development represents no change in the existing zoning of the property but assumes the site is developed for townhouses under the cluster provision. Under this development scheme, units would be arrayed in townhouse clusters in a way that preserves 18 holes of the golf courses and many natural features on the properties. For the Bonnie Briar property 125 townhouses are allowed and for the Winged Foot property 285 townhouses are allowed. The size of the unit or "footprint" is considered to be 1500 square feet. Schematic designs for R30 Townhouse development of Bonnie Briar and Winged Foot properties is illustrated in Figures 4-3 and 4-4. HYDROLOGIC ANALYSIS Watershed boundaries and land use plans for the development scheme indicate that five sub-basins would be affected by the developments. The Bonnie Briar development will affect sub-basins 3 and 4. Although a portion of the Bonnie Briar property falls into sub-basin 7, no develop- ment is planned in that sub-basin area for this scenario. The Winged Foot development will affect sub-basins 6, 8, and 9. For each sub-basin, the increase in impervious surface area was calculat- ed. 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'.•r ,/ 5 0 a �C I 41111111b. .) - * �- LSI- ? 3 r 'f Q1 ��� ° • `AO \ �_ a►L �, t N - 1101 - - t3 JCS __,• �x tl �- 1. �� - - s.� �~ I 'o \ 4T A000-- - - ,--t" -4 ....41011141101 '11*.i. a ,. ,,,3- ,.‘ -. ./- ‘./.5, - N,...„4. 4 7. 0_ ,„,_ •,_ , 0 , 4 • k0 4000"4114/ ��!�./��' A N• 4`.., Lam- b �40 Ow U ` O ~ ' ,� / AP �� •00-"-- ... �? y `tet f C l/' IOWA �/ ,_ .„, ce, 7 �J o" �/ •r�...-:, r �t a �� X _ S0 ,Q i U -- - ,. �� '�" ti,�; -h^+ '6611 .' / / `A \u•Y p 3 m i �` `'�� � � t•• r * s ;� - .v'. -} �"f - G/ oppe 44104SN, ir 4111. c,.- it 74r„.., ), :.), ..,iii- _ ,_ o° % •� I / 1. •� --4:11111144-`%:-. �!• :O „e. p,-1117.................„. • '._ -A + _ m �. ► - .7° • • :a" : ' 0 *° li i _ 0 O % °, ``o,- oilfilyNaisofe. "7111Prif Aegoti ."0. A 1 •;-,S. - . ' ` -MOW p -+fie 9,'I „ ' .0 , ` - t - _ '!`/, - •`/c ' . . a.1- 14111- 7-- 0 ° 40,1 O O c.--) O o J O p\ 'Op 'ti °-‘. ---.4 , .----. ) . - . ..... 0 POTENTIALcok '/� 9� _` o o - pofl ° zao v O - :go:• i•-:,,,- S::: t41,. ..„ . ° ,"*'O {"""~"�,, ° ‘, 0 oCg''''' ° - _ �vik. , ``,.�`\\ ._ o °O LEGEND *Az\ it. , .. O vfl ° s`' � ' ° SUB-BASIN BOUNDARY LINE 00 o y--- I Ij o ---7? '. eO„ f• PROPERTY LINE u� Y x- J 71.;I r,� t o « _ FLOOD PLAIN BOUNDARY ‘, POTENTIAL11(>`v SUB-eAS%N NUMBER o\'� _ a EXPANSION OF EXISTING POND r x SNELDRAKE RIVER HYDROLOGIC STUDY WINGED FOOT PROPERTY ��``'' R30 TOWNHOUSE DEVELOPMENT tlNI SCALE: 1° = 400' Associates. The increase in impervious area represents between 1 percent and 9 percent of the sub-basin area. A summary of the change in • imperviousness due to the development is presented in Table 4-6. The increase in imperviousness for each sub-basin resulted in an increase Uip in the composite runoff Curve Number (CN) . Information obtained from the WCSWCD regarding soil characteristics and land use information was utilized to develop the CN values for the affected sub-basins. For the U • calculated impervious areas, a CN value of 98 was used. Increases in CN values of less than 0.3 were considered to be negligible. Comparison of CN values for undeveloped (existing) and developed conditions shows increases in four of the five affected sub-basins. These increases are less significant than the R30 Detached development scheme. (Refer to Table 4-7) . Using the watershed hydrologic model which was calibrated for existing conditions, a series of peak runoff flows were simulated for the R30 • • Townhouse development condition under 5, 10, 25, 50, and 100-year return period storms. The model predicted increases in peak storm water runoff in the sub-basins as a result of the development. The model results are .000 summarized in Table 4-8. The increases range from 3 to 5 percent. It can also be seen that the impact is higher for the more frequent storms (5- year) than for the less frequent storms (100-year) . This is due to the fact that the larger storms tend to saturate the soils to a degree whereby their unpaved areas contribute to runoff almost to the same degree as the paved areas. The model results indicate no increase in peak runoff flow 0 for sub-basins 3 or 7 for the developed condition. For sub-basins 4 and 6, a 3 to 5 percent increase in peak runoff flow resulted. Because the 0849-15-1095 4-12 4 TABLE 4-6 INCREASE IN IMPERVIOUS AREA FOR R30 TOWNHOUSE DEVELOPMENT INCREASE IN IMPERVIOUS AREA LOCATION AFFECTED SUB BASIN ACRES % OF PROPERTY % OF BASIN BONNIE BRIAR 3 3.9 8 <1 BONNIE BRIAR 4 10.2 13 4 BONNIE BRIAR 7 0.0 -- -- WINGED FOOT 6 14.7 10 5 WINGED FOOT 8 5.7 8 8 WINGED FOOT 9 5.9 9 9 TABLE 4-7 SUMMARY OF CN VALUES FOR R30 TOWNHOUSE DEVELOPMENT COMPARISON OF CN VALUES LOCATION AFFECTED SUB-BASIN UNDEVELOPED DEVELOPED BONNIE BRIAR 3 74 74 BONNIE BRIAR 4 72 73 BONNIE BRIAR 7 71 71 WINGED FOOT 6 70 71 WINGED FOOT 8 62 65 WINGED FOOT 9 63 66 TABLE 4-8 SUMMARY OF PEAK RUNOFF FLOWS FOR R30 TOWNHOUSE DEVELOPMENT PEAK RUNOFF FLOWS (CFS) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR BONNIE BRIAR 3 395 519 649 763 939 (0%) (0%) (0%) (0%) (0%) BONNIE BRIAR 4 183 242 304 358 441 (5%) (4%) (4%) (3%) (3%) BONNIE BRIAR 7 214 288 364 432 536 (0%) (0%) (0%) (0%) (0%) WINGED FOOT 6 185 249 316 375 466 (5%) (5%) (3%) (3%) (3%) Note: (%) signifies percent increase in flow over existing undeveloped conditions. individual sub-basin peak flows occur at different times, the overall flow increase in the lower reaches of the Watershed along the Sheldrake River is even less. The computer printed results for the development simula- tions are presented in the Appendix. For sub-basins 8 and 9, the hydrologic parameters developed were used to compute the volumetric increases of runoff during a 10-and 100-year return period storms. The results indicate that under present conditions, during the 10-year storm, approximately 8 and 9 acre-feet of runoff is generated in sub-basins 8 and 9 and during the 100-year storm, the runoff volumes are increased to 16 to 19 acre-feet respectively. It should be noted that an acre-foot is the volume of water one foot deep covering a one acre area. Under the R-30 Townhouse development alternative, during the 10-year storm, these volumes would increase to 9 and 11 acre-feet for sub- basins 8 and 9 and during the 100-year storm the runoff volumes would increase to 18 and 21 acre-feet respectively. (Refer to Table 4-9) . Since downstream flooding exists under present conditions, it is proposed that future development on these portions of Winged Foot incorporate a retention basin to contain all runoff during storms of up to 100-year frequency. These retention basins would be emptied when the waters in the Sheldrake and Mamaroneck Rivers recede. As indicated Section 3.3, sub- basin 8 drains into an existing pond on the Winged Foot property. If the levels in this pond are maintained at their present level , the pond could provide a retention volume of 11.3 acre-feet. Therefore, additional volume of storage needs to be provided within Winged Foot for both sub- basins 8 and 9. 0849-15-1095 4-13 • TABLE 4-9 • SUMMARY OF RUNOFF VOLUMES FOR R30 TOWNHOUSE DEVELOPMENT • RUNOFF VOLUME (ACRE-FEET) EXISTING ENTIRE DEVELOPMENT • CONDITIONS SUB-BASIN ONLY AFFECTED SUBBASIN 10-YR 100-YR 10-YR 100-YR 10-YR 100-YR • 8 7.91 16.22 9.09 17.92 2.94 5.04 (15%) (10%) 9 9.30 18.82 10.65 20.73 3.06 5.30 (15%) (10%) • • Note: 4 - "Existing conditions" represent runoff from undeveloped sub-basin area. y • - "Entire sub-basin" represents runoff from the entire sub-basin due to develoment. - "Development only" represents runoff from the developed portion of .1 • the property only. - (%) signifies percent increase over existing undeveloped conditions. 0 Alternatively, only the storm runoff from the developed areas could be retained. Therefore, the retention volume requirements would decrease to • a more manageable level . For the R-30 Townhouse alternative, the developed portions of sub-basins 8 and 9 would generate 3 acre-feet of runoff each during the 10-year storm and 5 acre-feet of runoff during the • 100-year storm. Potential retention areas within sub-basins 8 and 9 are shown on Figure 4-4 for the limited control alternative. It should be noted that the existing pond would potentially be modified to provide a • dedicated retention volume for the developed portion as undeveloped areas naturally drain into the existing Pond. • IMPACTS 4 Peak storm water runoff flows generated by the HEC-1 model were used as input for the HEC-2 model to determine flood plain elevations. The water surface elevations corresponding to the peak runoff flows from the developed sub-basins were determined for all five storm frequencies. The HEC-2 computer program results showed an increase in water surface elevations of 0.1 feet or less in the Sheldrake River Watershed for all return storm frequencies. A summary of the increase in flood plain r elevations is given in Table 4-10. Results of the computer program runs are presented in the Appendix. The water surface profile results show variations in flood elevation increases. Higher storm water runoff flows do not necessarily show correspondingly higher increases in flood elevations. The varying geometry of the stream cross sections results in varying flood levels for different storm events. For sub-basin 3, the maximum increase in flood 0849-15-1095 4-14 r • TABLE 4-10 • MAXIMUM INCREASE IN FLOOD PLAIN ELEVATION FOR R30 TOWNHOUSE DEVELOPMENT • MAX. INCREASE IN FLOOD PLAIN ELEV. (FT) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR • BONNIE BRIAR 3 0.00 0.00 0.00 0.04 0.03 BONNIE BRIAR 4 0.05 0.01 0.01 0.02 0.02 BONNIE BRIAR 7 0. 10 0.08 0.06 0.07 0. 10 • WINGED FOOT 6 0.07 0.06 0.07 0.06 0.09 Note: Maximum increase occurs at different locations for varying • storm events. • • • • 41 plain elevation for all storm events occurs along the East Branch where the stream parallels Fenimore Avenue between Cornell Street and Winding rr Brook Drive. For sub-basin 4, the maximum increase for the 5, 50, and 100-year occurs at one of two different stations near the confluence of the East Branch with the Sheldrake River. For the 10-year storm 11 frequency, the maximum increase occurs just upstream of Valley Stream Road. The maximum increase in flood elevation for the 25-year storm occurs just upstream of Lansdowne Drive. 10 The maximum flood plain elevation increase for sub-basin 7 varies with the different storm frequencies. For the 5-year storm, the maximum increase • occurs along the East Branch where the stream parallels Ridgeway Road. The maximum increase for the 25 and 50-year storms occurs along Hilltop Road and at the Rockland Avenue Crossing. For the 10 and 100-year storms, • the maximum increase occurs along the East Branch between Hilltop Road and York Road. The maximum increase in flood plain elevation for sub-basin 6 varies with different storm frequencies. For the 5, 25, and 50 year storms, the maximum increase occurs at two different stations along the East Tributary upstream of the confluence with the East Branch at Fenimore Avenue. For the 10 and 100-year storms, the maximum increase occurs along the East Tributary at a section of the stream which approaches Salem Drive. The increase in water surface elevations (0.1 feet or less) due to the R30 Detached development had no observable affect on the existing flood plain area. The increase along the river sections within each sub-basin was such that no additional dwellings were affected. Therefore the number 0849-15-1095 4-15 of dwellings within the flood plain areas for the developed conditions is the same as the number of dwellings which are being flooded under existing conditions. However, localized flooding within the developed area itself or in areas immediately adjacent to the property may be a concern during and after construction. This can happen for instance when existing culvert and storm drains are not adequate for increased runoff flow. The properties should be developed with storm water controls in mind to reduce peak runoff flows to the existing drainage system downstream • Under this development alternative, the low lying area along the Sheldrake River in the Bonnie Briar property is completely preserved. The existing • wetlands are also preserved. Therefore no increases in flood flows due to encroachment of these areas are expected in this development alternative. • Each of the proposed control methods would eliminate runoff contribution from a portion of the present sub-basins 8 and 9 during a storm condition. Therefore, a decrease in downstream flows and flooding would be achieved • especially along the Fenimore and Country Road drainage system. The alternative limited control is expected to achieve smaller decreases. • Currently the Sheldrake River and its tributaries are class "C" waters and must meet standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality 4r aspects were performed for this study, we expect a minimal degree of water 4 quality degradation with the R30 Townhouse development since this alternative results in only a small increase in road surface area. Approximately 4 percent of the Bonnie Briar and Winged Foot properties are intended to be paved for roads. Increases in road surface area typically 0849-15-1095 4-16 result in an increased dissolved solids load to the drainage system as well as PAH and heavy metals such as lead and zinc. The proposed road surface area under the R30 Townhouse alternative is only one-third of the road area under the R30 Detached alternative. Although we expect an increase in runoff suspended solids on the property itself, we believe that water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water • runoff. The potential locations of the detention ponds are shown in Figures 4-2 and 4-3. The class "C" classification of the River is not expected to change. • • • • • • 0849-15-1095 4-17 • 4.4 CR DETACHED DEVELOPMENT INTRODUCTION The CR Detached development represents a change in the existing zoning of the property. Under this development scheme, the site's environmentally critical areas are preserved and maximum open space is maintained. The developments are concentrated so as to avoid wetlands, waterbodies, steep slopes, significant wooded areas, and critical drainage areas. By avoiding dispersion of the lots throughout the property, road construction and impervious surface areas are minimized. Under this development scheme, a maximum of 75 single family units are allowed for Bonnie Briar and 171 units for Winged Foot. The size of the unit or "footprint" was assumed to be 3,000 square feet. Schematic designs for CR Detached development of Bonnie Briar and Winged Foot properties is illustrated in Figures 4-5 and 4-6. HYDROLOGIC ANALYSIS Watershed boundaries and land use plans for the development scheme were used to determine that five sub-basins would be affected by the develop- ments. The Bonnie Briar development will affect sub-basins 3 and 4. Although a portion of the Bonnie Briar property falls into sub-basin 7, no development is planned in the sub-basin area for this scenario. The Winged Foot development will affect sub-basins 6, 8 and 9. For each sub-basin, the increase in impervious surface area was calculat- ed. This was accomplished by using land use maps and site area calcula- 0849-15-1095 4-18 • • • • • • • • I j FIGURE 4-5 • g w T f ! M of �soµlg Q .ow MAMAS 13 �� �`� s \� LEGEND T1 � ' _ l� \ SUB BASIN BOUNDARY LINE x 2 a-,4%- d��i �w ,�t 1' PROPERTY LINE ,) \\v --$�4 '�) b N, r°�'�, FLOOD PLAIN BOUNDARY t . _ •C� �'g ' s yy &-i "- r ' -sem /�� 44,01111 isoks b���J 6O SUB-BASIN NUNBER , IR ./ r fill „10140„, \/ �r, i ,.,` i ,Ala 111Ork = \ • -I qt \-1-%. ' ':"'?")- - 'W-(- ----- -' . NNW -.4- �$ c• •. 'SBO NIE,BRIARI , -- 4, ^ :_t_ __N :. ,.' 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PROPERTY LINE POTENTIAL to o o• o FLOOD PLAIN BOUNDARY RETENTION +, POTENTIAL POND S JFK d EXPANSION OF O SUB-BASIN NUMBER EXISTING POND i f• ` SHELDRAKE RIVER HYDROLOGIC STUDY WINGED FOOT PROPERTY •nKl • CR DETACHED DEVELOPMENT SCALE: 1'=900' tions for roofs, roads, parking areas, etc. , as supplied by Ferrandino and Associates. The increase in impervious area represents between 1 percent and 12 percent of the sub-basin area. A summary of the change in imperviousness due to the development is presented in Table 4-11. The increase in imperviousness for each sub-basin resulted in an increase in the composite runoff Curve Number (CN) . Information obtained from the WCSWCD regarding soil characteristics and land use information was utilized to develop the CN values for the affected sub-basins. For the calculated impervious areas, a CN value of 98 was used. Increases in CN values of less than 0.3 were considered to be negligible. Comparison of • CN values for undeveloped (existing) and developed conditions shows increases in four of the five affected sub-basins. (Refer to Table 4-12) . • Using the watershed hydrologic model which was calibrated for existing conditions, a series of peak runoff flows were simulated for the CR Detached development condition under 5, 10, 25, 50, and 100-year return • period storms. The model predicted increases in peak storm water runoff in the sub-basins as a result of the development. The model results are summarized in Table 4-13. The increases range from 3 to 5 percent. It • can also be seen that the impact is higher for the more frequent storms (5-year) than for the less frequent storm (100-year) . This is due to the fact that the larger storms tend to saturate the soils to a degree whereby their unpaved areas contribute to runoff almost to the same degree as the paved areas. The model results indicate no increase in peak runoff flow for sub-basins 3 or 7 for the developed condition. (Note that the CN values did not change for these sub-basins.) For sub-basins 4 and 6, a 3 to 5 percent increase in peak runoff flow resulted. Because the 0849-15-1095 4-19 • TABLE 4-11 • INCREASE IN IMPERVIOUS AREA FOR CR DETACHED DEVELOPMENT INCREASE IN IMPERVIOUS AREA • LOCATION AFFECTED SUB BASIN ACRES % OF PROPERTY % OF BASIN BONNIE BRIAR 3 5.8 11 1 • BONNIE BRIAR 4 9.0 12 4 BONNIE BRIAR 7 0.0 -- -- WINGED FOOT 6 10.9 7 3 • WINGED FOOT 8 8 10 10 WINTED FOOT 9 8 12 12 • • • • 4 4 rt TABLE 4-12 • SUMMARY OF CN VALUES FOR CR DETACHED DEVELOPMENT • COMPARISON OF CN VALUES LOCATION AFFECTED SUB-BASIN UNDEVELOPED DEVELOPED • BONNIE BRIAR 3 74 74 BONNIE BRIAR 4 72 73 BONNIE BRIAR 7 71 71 • WINGED FOOT 6 70 71 WINGED FOOT 8 62 66 WINGED FOOT 9 63 67 • • • • • 1 • TABLE 4-13 • SUMMARY OF PEAK RUNOFF FLOWS FOR CR DETACHED DEVELOPMENT • PEAK RUNOFF FLOWS (CFS) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR • BONNIE BRIAR 3 395 519 649 763 939 (0%) (0%) (0%) (0%) (0%) BONNIE BRIAR 4 183 242 304 358 441 • (5%) (4%) (4%) (3%) (3%) BONNIE BRIAR 7 214 288 364 432 536 (0%) (0%) (0%) (0%) (0%) WINGED FOOT 6 185 249 316 375 466 • (5%) (5%) (3%) (3%) (3%) Note: (%) signifies percent increase in flow over existing undeveloped conditions. • tir • I 10 11 individual sub-basin peak flows occur at different times, the overall flow increase in the lower reaches of the Watershed along the Sheldrake River • is less. The computer printed results for the development simulations are presented in the Appendix. • For sub-basins 8 and 9 the hydrologic parameters developed were used to compute the volumetric increases of runoff during a 10 and 100-year return period storms. The results indicate that under present conditions, during • the 10-year storm, approximately 8 and 9 acre-feet of runoff is generated in sub-basins 8 and 9 and during the 100-year storm the runoff volumes are increased to 16 to 19 acre-feet respectively. It should be noted that an acre-foot is the volume of water one foot deep covering a one acre area. Under the CR-30 Detached development alternative, during the 10-year storm, these volumes would increase to 10 and 11 acre-feet for sub-basins 8 and 9 and during the 100-year storm the runoff volumes would increase to 19 and 21 acre-feet respectively. (Refer to Table 4-14) . Since downstream flooding exists under present conditions, it is proposed that future development on these portions of Winged Foot incorporate a retention basin to contain all runoff during storms of up to 100-year • frequency. These retention basins would be emptied when the waters in the Sheldrake/Mamaroneck Rivers recede. As indicated above, subbasin 8 drains into an existing pond on the Winged Foot property. If the levels in this pond are maintained at their present level , the pond could provide a retention volume of 11.3 acre-feet. Therefore, additional volume of storage needs to be provided within the property for both sub-basins 8 and 9. 0849-15-1095 4-20 TABLE 4-14 SUMMARY OF RUNOFF VOLUMES FOR R30 DETACHED DEVELOPMENT RUNOFF VOLUME (ACRE-FEET) EXISTING ENTIRE DEVELOPMENT CONDITIONS SUB-BASIN ONLY AFFECTED SUBBASIN 10-YR 100-YR 10-YR 100-YR 10-YR 100-YR 8 7.91 16.22 9.50 18.49 5.31 9.67 (20%) (14%) 9 9.30 18.82 11 . 12 21 .38 6.04 11 .34 (20%) (14%) Note: - "Existing conditions" represent runoff from undeveloped sub-basin area. - "Entire sub-basin" represents runoff from the entire sub-basin due to develoment. - "Development only" represents runoff from the developed portion of the property only. - (%) signifies percent increase over existing undeveloped conditions. Alternatively, only the storm runoff from the developed areas could be retained. Therefore, the retention volume requirements would decrease to a more manageable level . For the CR Detached alternative, the developed portions of sub-basins 8 and 9 would generate 5 and 6 acre-feet of runoff respectively, during the 10-year storm and 10 and 11 acre-feet of runoff during the 100-year storm. Potential retention areas within sub-basins 8 and 9 are shown of Figure 4-6 for the limited control alternative. It should be noted that the existing pond would potentially be modified to provide a dedicated retention volume for the developed portion as undeveloped areas naturally drain into the existing Pond. IMPACTS Peak storm water runoff flows generated by the HEC-1 model were used as input for the HEC-2 model to determine flood plain elevations. The water surface elevations corresponding to the peak runoff flows from the developed sub-basins were determined for all five storm frequencies. The HEC-2 computer program results showed an increase in water surface elevations of 0.1 feet or less in the Sheldrake River Watershed for all return storm frequencies. A summary of the increase in flood plain elevations is given in Table 4-15. Results of the computer program runs are presented in the Appendix. • The water surface profile results show much variation in flood elevation increases. Higher storm water runoff flows do not necessarily show correspondingly higher increases in flood elevations. The varying • geometry of the stream cross sections results in varying flood levels for different storm events. For sub-basin 3, the maximum increase in flood 0849-15-1095 4-21 • TABLE 4-15 MAXIMUM INCREASE IN FLOOD PLAIN ELEVATION FOR CR DETACHED DEVELOPMENT MAX. INCREASE IN FLOOD PLAIN ELEV. (FT) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR BONNIE BRIAR 3 0.00 0.00 0.00 0.04 0.03 BONNIE BRIAR 4 0.05 0.01 0.01 0.02 0.02 BONNIE BRIAR 7 0.10 0.08 0.06 0.07 0. 10 WINGED FOOT 6 0.07 0.06 0.07 0.06 0.09 Note: Maximum increase occurs at different locations for varying storm events. plain elevation for all storm events occurs along the East Branch where the stream parallels Fenimore Avenue between Cornell Street and Winding Brook Drive. For sub-basin 4, the maximum increase for the 5, 50, and 100-year occurs at one of two different stations near the confluence of the East Branch with the Sheldrake River. For the 10-year storm frequency, the maximum increase occurs just upstream of Valley Stream Road. The maximum increase in flood elevation for the 25-year storm occurs just upstream of Lansdowne Drive. The maximum flood plain elevation increase for sub-basin 7 varies with the different storm frequencies. For the 5-year storm, the maximum increase occurs along the East Branch where the stream parallels Ridgeway Road. The maximum increase for the 25 and 50-year storms occurs along Hilltop Road and at the Rockland Avenue Crossing. For the 10 and 100-year storms, the maximum increase occurs along the East Branch between Hilltop Road and York Road. The maximum increase in flood plain elevation for sub-basin 6 varies with I different storm frequencies. For the 5, 25, and 50 year storms, the maximum increase occurs at two different stations along the East Tributary upstream of the confluence with the East Branch at Fenimore Avenue. For the 10 and 100-year storms, the maximum increase occurs along the East Tributary at a section of the stream which approaches Salem Drive. The increase in water surface elevations (0.1 feet or less) due to the CR Detached development had no observable affect on the existing flood plain area. The increase along the river sections within each sub-basin was such that no additional dwellings were affected. Therefore the number of 0849-15-1095 4-22 dwellings within the flood plain areas for the developed conditions is the same as the number of dwellings which are being flooded under existing conditions. However, localized flooding within the developed area itself or in areas immediately adjacent to the property may be a concern during and after construction. This can happen for instance when existing culvert and storm drains are not adequate for increased runoff flow. The properties should be developed with storm water controls in mind to reduce peak runoff flows to the existing downstream drainage system. Under this development alternative, the low lying area along the Sheldrake River in the Bonnie Briar property is completey preserved. The existing wetlands are also preserved. Therefore no increases in flood flows due to encroachment of these areas are expected in this development alternative. Each of the proposed control methods would eliminate runoff from a portion of the present sub-basins 8 and 9 during a storm condition. Therefore, a decrease in downstream flows and flooding would be achieved especially along the Fenimore and Country Road drainage system. The alternative involving limited control is expected to achieve smaller decreases. Currently the Sheldrake River and its tributaries are class "C" waters and must meet standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality aspects were performed for this study, we expect a minimal degree of water quality degradation under the CR Detached development since this alternative results in only a small increase in road surface area. Approximately 3 percent of the Bonnie Briar and Winged Foot properties are intended to be paved for roads. Increases in road surface area typically 0849-15-1095 4-23 result in an increased dissolved solids load to the drainage system as well as PAH and heavy metals. The proposed road surface area under the CR Detached alternative is only one-fourth of the road area under the R30 Detached alternative. Although we expect an increase in runoff suspended solids on the property itself, we believe that water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class "C" classification of the River is not expected to change. The potential locations of the detention ponds are shown in Figures 4-5 and 4-6. 0849-15-1095 4-24 4.5 CR TOWNHOUSE DEVELOPMENT INTRODUCTION The CR Townhouse development represents a change in the existing zoning of the property. Under this development scheme, townhouse development would be the principal land use with some of the site's environmentally critical areas preserved and maximum open space maintained. The development is designed to concentrate the construction on the most buildable area of the site. The design intends to preserve 18 holes of golf, maximize protection of the environment, minimize roadway through the site, and provide views and open space for each unit. By avoiding dispersion of the lots throughout the property, road construction and impervious surface areas are minimized. For the Bonnie Briar property, a maximum of 75 units are allowed and for the Winged Foot property, a maximum of 171 units are allowed. The size of the unit or "footprint" was assumed to be 1500 square feet. Schematic designs for CR Townhouse development of Bonnie Briar and Winged Foot properties is illustrated in Figures 4-7 and 4-8. HYDROLOGIC ANALYSES Watershed boundaries and land use plans for the development scheme were used to determine that five watershed sub-basins would be affected by the developments. Although a portion of the Bonnie Briar property falls into sub-basin 7, no development is planned in the sub-basin area for this scenario. The Bonnie Briar development will affect sub-basins 3 and 4. The Winged Foot development will affect sub-basins 6, 8 and 9. 0849-15-1095 4-25 For each sub-basin, the increase in impervious surface area was calculat- ed. This was accomplished by using land use maps and site area calcula- tions for roofs, roads, parking areas, etc. , as supplied by Ferrandino and Associates. The increase in impervious area represents between less than 1 percent and 8 percent of the sub-basin area. A summary of the change in imperviousness due to the development is presented in Table 4-16. The increase in imperviousness for each sub-basin resulted in an increase in the composite runoff Curve Number (CN) . Information obtained from the WCSWCD regarding soil characteristics and land use information was utilized to develop the CN values for the affected sub-basins. For the calculated impervious areas, a CN value of 98 was used. Increases in CN values of less than 0.3 were considered to be negligible. Comparison of CN values for undeveloped (existing) and developed conditions shows increases in four of the ffive sub-basins affected. These increases are the same as the R30 Townhouse alternative. (Refer to Table 4-17) . Using the watershed hydrologic model which was calibrated for existing conditions, a series of peak runoff flows were simulated for the CR Townhouse development condition under 5, 10, 25, 50, and 100-year return period storms. The model predicted increases in peak storm water runoff in the sub-basins as a result of the development. The model results are summarized in Table 4-18. The increases range from three to five percent. It can also be seen that the impact is higher for the more frequent storms (5-year) than the for the less frequent storm (100-year) . This is due to the fact that the larger storms tend to saturate the soils to a degree whereby their unpaved areas contribute to runoff almost to the same degree as the paved areas. 0849-15-1095 4-26 TABLE 4-16 INCREASE IN IMPERVIOUS AREA FOR CR TOWNHOUSE DEVELOPMENT INCREASE IN IMPERVIOUS AREA LOCATION AFFECTED SUB-BASIN ACRES % OF PROPERTY % OF BASIN BONNIE BRIAR 3 4.5 9 <1 BONNIE BRIAR 4 6.4 8 3 BONNIE BRIAR 7 0.0 -- -- WINGED FOOT 6 10.9 7 3 WINGED FOOT 8 5.4 7 7 WINGED FOOT 9 5.6 8 8 TABLE 4-17 SUMMARY OF CN VALUES FOR CR TOWNHOUSE DEVELOPMENT COMPARISON OF CN VALUES LOCATION AFFECTED SUB-BASIN UNDEVELOPED DEVELOPED BONNIE BRIAR 3 74 74 BONNIE BRIAR 4 72 73 BONNIE BRIAR 7 71 71 WINGED FOOT 6 70 71 WINGED FOOT 8 62 65 WINGED FOOT 9 63 66 TABLE 4-18 SUMMARY OF PEAK RUNOFF FLOWS FOR CR TOWNHOUSE DEVELOPMENT PEAK RUNOFF FLOWS (CFS) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR BONNIE BRIAR 3 395 519 649 763 939 (0%) (0%) (0%) (0%) (0%) BONNIE BRIAR 4 183 242 304 358 441 (5%) (4%) (4%) (3%) (3%) BONNIE BRIAR 7 214 288 364 432 536 (0%) (0%) (0%) (0%) (0%) WINGED FOOT 6 185 249 316 375 466 (5%) (5%) (3%) (3%) (3%) Note: (%) signifies percent increase in flow over existing undeveloped conditions. The model results indicate no increase in peak runoff flow for sub-basins 3 or 7 for the developed condition. (Note that the CN values did not change for these sub-basins.) For sub-basins 4 and 6, a 3 to 5 percent increase in peak runoff flow resulted. Because the individual sub-basin peak flows occur at different times, the overall flow increase in the lower reaches of the Watershed along the Sheldrake River is less. The computer printed results for the development simulations are presented in the Appendix. For sub-basins 8 and 9 the hydrologic parameters developed were used to compute the volumetric increases of runoff during a 10 and 100-year return period storms. The results indicate that under present conditions, during the 10-year storm, approximately 8 and 9 acre-feet of runoff is generated in sub-basins 8 and 9 and during the 100-year storm the runoff volumes are increased to 16 to 19 acre-feet respectively. It should be noted that an acre-foot is the volume of water one foot deep covering a one acre area. Under the CR Townhouse Detached development alternative, during the 10- year storm, these volumes would increase to 10 and 11 acre-feet for sub- basins 8 and 9 and during the 100-year storm the runoff volumes would increase to 19 and 21 acre-feet respectively. (Refer to Table 4-19) . Since downstream flooding exists under present conditions, it is proposed that future development on these portions of Winged Foot incorporate a retention basin to contain all runoff during storms of up to 100-year frequency. These retention basins would be emptied when the waters in the Sheldrake and Mamaroneck Rivers recede. As indicated above, sub-basin 8 drains into an existing pond on the Winged Foot property. If the levels in this pond are maintained at their present level , the pond could provide 0849-15-1095 4-27 TABLE 4-19 SUMMARY OF RUNOFF VOLUMES FOR R30 TOWNHOUSE DEVELOPMENT RUNOFF VOLUME (ACRE-FEET) EXISTING ENTIRE DEVELOPMENT CONDITIONS SUB-BASIN ONLY AFFECTED SUBBASIN 10-YR 100-YR 10-YR 100-YR 10-YR 100-YR 8 7.91 16.22 9.09 17.92 2.57 4.25 (15%) (10%) 9 9.30 18.82 10.65 20.73 2.84 4.92 (15%) (10%) Note: - "Existing conditions" represent runoff from undeveloped sub-basin area. - "Entire sub-basin" represents runoff from the entire sub-basin due to develoment. - "Development only" represents runoff from the developed portion of the property only. - (%) signifies percent increase over existing undeveloped conditions. a retention volume of 11 .3 acre-feet. Therefore, additional volume of storage needs to be provided within the property for both sub-basins 8 and 9. Alternatively, only the storm runoff from the developed areas could be retained. Therefore, the retention volume requirements would decrease to a more manageable level . For the CR-Townhouse alternative, the developed portions of sub-basins 8 and 9 would each generate 3-feet of runoff during the 10-year storm and 4 and 5 acre-feet of runoff respectively during the 100-year storm. Potential retention areas within sub-basins 8 and 9 are shown on Figure 4-6 for the limited control alternative. It should be noted that the existing pond would potentially be modified to provide a dedicated retention volume for the developed portion as undeveloped areas naturally drain into the existing Pond. IMPACTS Peak storm water runoff flows generated by the HEC-1 model were used as input for the HEC-2 model to determine flood plain elevations. The water surface elevations corresponding to the peak runoff flows from the developed sub-basins were determined for all five storm frequencies. The HEC-2 computer program results showed an increase in water surface elevations of 0.1 feet or less in the Sheldrake River Watershed for all return storm frequencies. A summary of the increase in flood plain elevations is given in Table 4-20. Results of the computer program runs are presented in the Appendix. 0849-15-1095 4-28 TABLE 4-20 MAXIMUM INCREASE IN FLOOD PLAIN ELEVATION FOR CR TOWNHOUSE DEVELOPMENT MAX. INCREASE IN FLOOD PLAIN ELEV. (FT) LOCATION AFFECTED SUB-BASIN 5-YR 10-YR 25-YR 50-YR 100-YR BONNIE BRIAR 3 0.00 0.00 0.00 0.04 0.03 BONNIE BRIAR 4 0.05 0.01 0.01 0.02 0.02 BONNIE BRIAR 7 0. 10 0.08 0.06 0.07 0. 10 WINGED FOOT 6 0.07 0.06 0.07 0.06 0.09 Note: Maximum increase occurs at different locations for varying storm events. The water surface profile results show much variation in flood elevation increases. Higher storm water runoff flows do not necessarily show correspondingly higher increases in flood elevations. The varying geometry of the stream cross sections results in varying flood levels for different storm events. For sub-basin 3, the maximum increase in flood plain elevation for all storm events occurs along the East Branch where the stream parallels Fenimore Avenue between Cornell Street and Winding Brook Drive. For sub-basin 4, the maximum increase for the 5, 50, and 100-year occurs at one of two different stations near the confluence of the East Branch with the Sheldrake River. For the 10-year storm frequency, the maximum increase occurs just upstream of Valley Stream Road. The maximum increase in flood elevation for the 25-year storm occurs just upstream of Lansdowne Drive. The maximum flood plain elevation increase for sub-basin 7 varies with the different storm frequencies. For the 5-year storm, the maximum increase occurs along the East Branch where the stream parallels Ridgeway Road. The maximum increase for the 25 and 50-year storms occurs along Hilltop Road and at the Rockland Avenue Crossing. For the 10 and 100-year storms, the maximum increase occurs along the East Branch between Hilltop Road and York Road. The maximum increase in flood plain elevation for sub-basin 6 varies with different storm frequencies. For the 5, 25, and 50 year storms, the maximum increase occurs at two different stations along the East Tributary upstream of the confluence with the East Branch at Fenimore Road. For the 10 and 100-year storms, the maximum increase occurs along the East Tributary at a section of the stream which approaches Salem Drive. 0849-15-1095 4-29 The increase in water surface elevations (0.1 feet or less) due to the CR Townhouse development had no observable affect on the existing flood plain area. The increase along the river sections within each sub-basin was such that no additional dwellings were affected. Therefore the number of dwellings within the flood plain areas for the developed conditions is the same as the number of dwellings which are being flooded under existing conditions. However, localized flooding within the developed area itself or in areas immediately adjacent to the property may be a concern during and after construction. This can happen for instance when existing culvert and storm drains are not adequate for increased runoff flow. The properties should be developed with storm water controls in mind to reduce peak runoff flows to the existing downstream drainage system. Under this development alternative the low lying area along the Sheldrake River in the Bonnie Briar property are completey preserved. The existing wetlands are also preserved. Therefore, no increases in flood flows due to encroachment of these areas are expected in this development alterna- tive. Each of the proposed control methods would eliminate runoff from a portion of the present sub-basins 8 and 9 during a storm condition. Therefore, a decrease in downstream flows and flooding would be achieved especially along the Fenimore Country Road drainage system. The alternative involving limited control is expected to achieve smaller decreases. Currently the Sheldrake River and its tributaries are class "C" waters and must meet standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality 0849-15-1095 4-30 aspects were performed for this study, we expect a minimal degree of water quality degradation under the CR Townhouse development since this alternative results in only a small increase in road surface area. Approximately 3 percent of the Bonnie Briar and Winged Foot properties are intended to be paved for roads. Increases in road surface area typically result in an increased dissolved solids load to the drainage system, as well as PAH and heavy metals such as lead and zinc. The proposed road surface area under the CR Townhouse alternative is only one-fourth of the road area under the R30 Detached alternative. Although we expect an increase in runoff suspended solids on the property itself, we believe that water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class "C" classification of the River is not expected to change. The potential locations of the detention ponds are shown in Figures 4-7 and 4-8. 0849-15-1095 4-31 • • • • • • • FIGURE 4.1 2F344---144-40.• 4V: • 4 • 4 . 1 .." / ,..? -.......*-841-."7 \i ‹,i., I ) c'.1 I. Cf.C234 . c\- j 3\ ? iLEGENDFLO " 1.. SUB-BASIN BOUNDARY LINE '' . 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FLOOD PLAIN BOUNDARY ... Nti, POTENTIAL 0 EXPANSION OF SUB-BASIN NUMB• Es� -Ar f w EXISTING POND SHELDRAKE RIVER HYDROLOGIC STUDY WINGED FOOT PROPERTY M CR TOWNHOUSE DEVELOPMENT r�"" SCALE: 1" = 400' n C � • • • • • • • • • • it' l- L e;j g / • `I' • All' C •may:1.. 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O' /y • °o C�''llofQ i V' J ❑ L� fr .Y/ 4 0 � t. 31 �_ 11 ',d o, 0°.° ,6,°:0e°°% o O 0 �� / O AQ �0 000 WOO I • 1 •/•>1,.• •8 o RETENTd N or- ....i. y o .� ��,—�� �! . wir o4. Doo• 6 ► 're- ( + 3 000 `VIs.,-,‘NS �� _^-`� LEGEND o O C �/ 0 I. �__ rt o/avois , CP - r �� SUB-BASIN BOUNDARY LINE " ' . PROPERTY LINE "..;.T... x .)/ ® FLOOD PLAIN BOUNDARY POTENTIAL `-.EXPANSION OF O SUB-BASIN NUMBER EXISTING POND o ! SHELDRAKE RIVER HYDROLOGIC STUDY WINGED FOOT PROPERTY weCR TOWNHOUSE DEVELOPMENT SCALE t'..400 • • • • • • • • • 0 5 Carder' • 5. CONCLUSIONS AND RECOMMENDATIONS 5.1 CONCLUSIONS A hydrologic study of the Sheldrake River Watershed has been performed. The main objectives of the study was to assess the existing flooding conditions in the Town of Mamaroneck along the Sheldrake River and analyze the potential impacts in flooding associated with failure development of Bonnie Briar Country Club and Winged Foot Golf Club. The study consisted in the development of a hydrologic model of the Sheldrake River within the Town limits. A second model was developed to determine flood elevations associated with various return frequency storms. In addition, for portions of the Winged Foot Golf Culb property draining to the Village of Mamaroneck a separate analysis was performed and mitigation measures proposed to eliminate potential impacts due to future developments. This chapter summarizes our findings, conclusions and recommendations. MODEL CALIBRATION RESULTS The HEC-1 runoff hydrograph model developed for the Sheldrake River Watershed was calibrated using actual rainfall informa- tion from two independent storm events. The model was calibrated at an upstream and downstream station along the Sheldrake River. Comparison of computer-generated runoff volumes using the model and actual observed runoff volumes showed close agreement. Therefore the HEC-1 model adequately represents runoff rates for the Sheldrake River Watershed. 0849-15-1095 5-1 The HEC-2 water surface profile model developed for the Sheldrake River Watershed shows close agreement with actual flooding experienced at certain residences along Weaver Street. Therefore, the HEC-2 model adequately represents flood elevations for the Watershed. EXISTING FLOODING CONDITIONS The HEC-1 and HEC-2 models were run to determine flood plain areas within the Watershed for existing conditions. Three drainage systems in the Watershed were analyzed; the Sheldrake River, the East Branch, and the East Tributary. For the upper reach of the Sheldrake River between Larchmont Reservoir and Valley Stream Road, 76 dwellings or appurtenant structures are within the 100-year flood plain while 51 dwellings are within the 5-year flood plain. For the lower reach of the Sheldrake River between Valley Stream Road and the New England Thruway, 63 dwellings or appurtenant structures are affected by the 100-year flood plain while none are affected by the 5-year flood plain. Along the East Branch, approximately 16 dwellings are within the 100-year flood plain while 7 are within the 5-year flood plain. Along the East Tributary, 14 dwellings are within the 100-year flood plain and 7 are within the 5-year flood plain. RUNOFF FLOW INCREASES DUE TO DEVELOPMENT The HEC-1 runoff hydrograph model developed for the Sheldrake River Watershed was used to determine increases in storm water 0849-15-1095 5-2 runoff resulting from development of Bonnie Briar Country Club and Winged Foot Golf Club. Generally, the increases are small within the sub-basins. For the R30 Detached Development, the model predicted an increase in storm water runoff ranging from 3 to 11 percent for five storm frequencies (5-year, 10-year, 25-year, 50-year, and 100-year) . For the R30 Townhouse, CR Townhouse, and CR Detached developments, the model predicted an increase in storm water runoff ranging from 3 to 5 percent. Increases in storm water runoff in the lower portions of the Sheldrake River are even less because the individual sub-basin peak flows occur at different times. The overall increase in peak runoff flows in the lower reaches of the Watershed along the Sheldrake River, is only 1 to 2 percent for the R30 Detached development and less than 1 percent for the R30 Townhouse, CR Townhouse, and CR Detached developments. FLOOD PLAIN ELEVATION INCREASES DUE TO DEVELOPMENT The HEC-2 water surface profile model developed for the Sheldrake River Watershed was used to determine increases in flood plain elevation resulting from development of Bonnie Briar Country Club and Winged Foot Golf Club. The increases in flood plain elevations along the drainage systems (Shel- drake River, East Branch, and East Tributary) are small . For the R30 Detached development, a maximum increase in flood elevation of 0.24 feet was predicted. This maximum increase occurred in the Winged Foot property (sub-basin 6) during the 25-year storm event. For the R30 Townhouse, CR Townhouse, and 0849-15-1095 5-3 CR Detached developments, a maximum increase in flood eleva- tion of 0.10 feet was predicted. This maximum increase occurred in the Bonnie Briar property (sub-basin 7) during the 5-year and 100-year storm events. The increase in water surface elevations along the river sections within each sub-basin due to development had no observable effect on the existing flood plain area. The increase in flood elevations due to each of the four develop- ment schemes was such that no additional dwellings were affected. However, localized flooding within the developed area itself may be a concern during and after construction. This will depend on the capacity of existing and future storm drains and culverts. IMPACTS ON LOW LYING AREAS DUE TO DEVELOPMENT Under the R30 Detached development alternative, the low lying areas along the Sheldrake River in the Bonnie Briar property are somewhat preserved. However, the development of four proposed lots along Weaver Street might encroach on the existing flood plain. In addition, the existing wetlands are not specifically preserved. This approach would tend to exacerbate the existing flooding conditions along Weaver Street. Under the R30 Townhouse, CR Townhouse, and CR Detached development schemes, the low lying areas along the Sheldrake River in the Bonnie Briar property are completely preserved. 0849-15-1095 5-4 The existing wetlands are also preserved. Therefore no increases in flood flows due to encroachment of these areas are expected for these development alternatives. WATER QUALITY IMPACTS DUE TO DEVELOPMENT No water quality analyses were performed during this study. However, we expect some degree of water quality degradation in the receiving streams as a result of the developments. Increases in road surface area typically result in more dissolved solids, heavy metals, and PAHs to drainage systems. The highest degree of degradation would occur with the R30 Detached development since this alternative results in the greatest increase in road surface area. The proposed road surface area is three to four times greater under the R30 Detached scheme than under the other development schemes. A minimal degree of water quality degradation is expected with the R30 Townhouse development since this alternative results • in only a small increase in road surface area. The proposed road surface area is only one-third of the road area under the R30 Detached alternative. • The lowest degree of degradation would occur with the CR Townhouse and CR Detached developments since these alterna- • tives result in the smallest increase in road surface area. The proposed road surface area is only one-fourth of the road area under the R30 Detached alternative. • 0849-15-1095 5-5 • 5.2 RECOMMENDATIONS The development of Bonnie Briar Country Club and Winged Foot Golf Club properties will result in impacts to the Sheldrake River Watershed. The two major concerns associated with the property developments are as follows: The increase in impervious area associated with the develop- ments will result in increased storm water runoff flows. The increased flows may cause localized flooding within the properties, especially in low lying areas. The increase in road surface area within the properties is expected to cause some degree of water quality degradation in the receiving streams which collect storm water runoff. • In order to address both of these concerns, we recommend that the properties be developed with storm water controls in mind. One such • control is the installation of storm water detention ponds within the developed areas. The detention ponds would eliminate the peak runoff flow increases from the individual tracts so that the overall runoff flows in the lower portions of the Watershed would not be affected. 41 For portions of the Winged Foot Golf Club property, retention basins are recommended to eliminate stormwater runoff from the developed portions of 11 the property during storm conditions. These basins would be emptied after the flood flows in the Mamaroneck and Sheldrake Rivers recede. The 0849-15-1095 5-6 proposed retention basins would help reduce downstream flooding that is presently being experienced along the Fenimore and Country Road drainage system. Existing culverts and storm drains may require upgrading to accommodate increased runoff flows within the properties. Storm water detention ponds within the developed areas can also minimize water quality degradation in the receiving streams. The potential locations of the detention ponds within the properties is shown in Figure 40 4-3 through 4-7. • • • • • 0849-15-1095 5-7 • C LIST OF REFERENCES • U.S. Army Corp. of Engineers, The Hydrologic Engineering Center, HEC-1 Flood Hydrograph Package - Computer Program, March 1987. U.S. Army Corp. of Engineers, The Hydrologic Engineering Center, HEC-2 • Water Surface Profiles - Computer Program, September 1982. Technical Release No. 55, "Urban Hydrology For Small Watersheds, " PB87- 101580, U.S. Dept. of Agriculture, Soil Conservation Service, June 1986. • National Engineering Handbook - Section 4 (NEH-4) , "Hydrology, " U.S. Dept. of Agriculture, Soil Conservation Service, August 1972. Zoning Code, Chapters 89 and 331 , Town of Mamamoneck. • House Number Maps, Village of Scarsdale, Westchester County. Drainage Map of Village of Scarsdale, Engineering Department, January • 1937. Drains Map, Department of Development, City of New Rochelle. • Map of Drainage Facilities, Town of Mamaroneck, Westchester County. Zoning Map, City of New Rochelle, Westchester County. • Map of Reservoir Showing Location to Proposed Parkway, Larchmont Water Works, Hazen and Whipple, 1928. Plans of Masonry Dam and Storage Reservoir On Sheldrake River, Larchmont Water Works, Hazen and Whipple, 1928. • Soil Classification Map of the Village of Scarsdale, Soil Conservation Service, September 1976. Map of Critical Environmental Areas, Draft Local Waterfront Revitalization Program, Town of Mamaroneck, May 1986. 4 Map of The Premium Salt March Complex, Draft Local Waterfront Revitaliza- tion Program, Town of Mamaroneck, May 1986. 41 Map of The Hommocks Salt March Complex, Draft Local Waterfront Revitaliza- tion Program, Town of Mamaroneck, May 1986. • Cross Section Survey Map, Study of Existing Conditions for Sheldrake River, Town of Mamaroneck, U.S. Army Corps. of Engineers, April 1989. (Used to obtain survey data for the Sheldrake River) • • • • • • • • •