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HomeMy WebLinkAboutSupplement to Draft GEIS for Town of Mamaroneck Sheldrake River Watershed Hyrologic Study 4/1/1992 Special Study Report SUPPLEMENT TO DRAFT GEIS FOR TOWN OF MAMARONECK SHELDRAKE RIVER WATERSHED HYDROLOGIC STUDY FOR TOWN OF MAMARONECK APRIL 1992 PROJECT 0849-197 IRNI ENVIRONMENTAL ENGINEERS,SCIENTISTS&PLANNERS 0 IRNIMALCOLM PIRNIE,INC. ENVIRONMENTAL ENGINEERS, SCIENTISTS& PLANNERS April 22, 1992 Mr. Stephen Alfieri Town Administrator Town of Mamaroneck 740 W. Boston Post Rd. Mamaroneck, NY 19543-3319 Re: Supplemental Hydrologic Study of Sheldrake River Watershed s Dear Steve: We have responded to your comments and are pleased to submit our revised supplemental hydrologic study of the Sheldrake River Watershed. Per our contract, the results of the study are presented in a format for direct incorporation in the body of the Draft SGEIS, concurrently being prepared by Ferrandino & Associates. 0 The study evaluates the potential increases in flood flows in the Village of Mamaroneck and increases in flood elevations in the Town resulting from four additional development _ scenarios for the Bonnie Briar Country Club; three development plans prepared by Parish & Weiner (Alternatives A, B and C) and one development plan (CR Modified) prepared by Ferrandino & Associates, concurrent development of the Winged Foot Golf Club is considered in our evaluation. The study follows the methodology of our original Hydrologic Study of the Sheldrake River Watershed dated March 1990 and draws on the results of the modeling performed in that Study. p- We appreciate the opportunity to assist the Town in its planning efforts and look forward to discussing the contents of the draft report with Town Officials and Staff in conjunction with review of the Draft SGEIS. If you have any questions regarding this report please contact me. Very truly yours, MALCOLM PIRNIE, INC. /Kenneth W. Henderson, P.E. _ agop L' hahabian,Ph.D.,P.E. Vice President Associate /lb • Enclosure cc: Ferrandino & Associates (w/enclosure) • 2 CORPORATE PARK DRIVE P. 0. BOX 751 WHITE PLAINS,NY 10602-0751 914-694-2100 FAX 914-694-9286 r 3.1 Description of Parish & Weiner Alternative B - Bonnie Briar Country Club a 3.1.3 Probable Significant Environmental Impacts Upon and Mitigation Measures for Natural Resources B. Water Resources The following hydrologic analysis conducted by Malcolm Pirnie Inc., is based on cumulative impact,assuming similar development scenarios for Bonnie Briar Country Club and Winged Foot Golf Club. Conceptual plans provided by Parish & Weiner, Inc., for development under Alternative B, provided the basis for the hydrologic analysis of the Bonnie Briar property. However,no plans were available for the Winged Foot property under Alternative • B. For Alternative B, no hydrologic modeling was performed. All hydrologic modeling was performed earlier in the DGEIS for CR Townhouse, CR Detached, R30 Townhouse, and R30 Detached alternatives. Therefore, to predict impacts in the Town and Village as a result of development under the Alternative B scenario, estimates of peak flow and flood • • elevation were determined by selecting the development scheme from the DGEIS which was most similar to Alternative B. The selection was based on the location and extent of imperviousness (from construction of buildings and roadways) resulting from the proposed Alternative B schematic. These two factors, location and extent of imperviousness, effect the runoff curve number (CN) parameter within each sub-basin. This parameter in turn • allows the selection of one of the alternatives previously analyzed in detail in the DGEIS. For Alternative B,the most similar development scheme in Bonnie Briar property which was modeled as part of the DGEIS,was the R30 Townhouse alternative. Since no similar plans were available for Winged Foot, the R30 Townhouse development scenario was also assumed for the Winged Foot site. (Refer to Appendix A of the SGEIS). Therefore impacts in the Town and Village under Alternative B will be similar to impacts under R30 0-- Townhouse. (See Appendix 5-5 of DGEIS for further detail). Again, it should be noted that the assumptions made regarding similar development scenarios(studied in the DGEIS), are predicated on the imperviousness and location of the proposed units, not the actual zoning description of R30 Townhouse. 1. Impacts on the Town of Mamaroneck (unincorporated area) • • Affected Sub-basins: Under the proposed development scheme,sub-basins 3,4,and 7 would be affected by the Bonnie Briar development. The majority of the development(approximately 70 percent)would occur in sub-basin 4. Approximately 15 percent of the development would occur in each of sub-basins 3 and 7. For the ••-= Winged Foot development, sub-basin 6 would be affected. Sub-basin 6 is within the Sheldrake Watershed upstream of the I-95 culvert. (Sub-basins 8 and 9 would also be affected by the Winged Foot development but they are outside of the Sheldrake Watershed. Hence, development in 8 and 9 would impact the Village but not the Town). • Peak Runoff Flow: For each sub-basin affected by the proposed development, a series of runoff flows was estimated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. • to Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. Bonnie Briar: Since the increase in impervious surfaces in sub-basins 3 and 7 is relatively small in comparison to the total sub-basin areas, the runoff curve numbers remain unaffected. Therefore,less than 1 percent increase in peak runoff flow over existing conditions is projected within sub-basins 3 and 7. However,within sub-basin 4, the increase in imperviousness results in an increase in the runoff curve number similar to the increase experienced under the R30 Townhouse development scheme. (See Table A-1 in Appendix A). Within sub-basin 4, a 5 percent increase in peak flow would be experienced during a 5-year storm and a 3 percent increase during a 100-year storm. (See Table A-2 in Appendix A). Winged Foot: For the Winged Foot development, an increase in the runoff curve number is projected for sub-basin 6 similar to the increase under the R30 Townhouse development scheme. Within sub-basin 6, a 5 percent increase in peak runoff is projected during a 5-year storm and a 3 percent increase during a 100-year storm. (See Table A-2 in Appendix A). ,r Overall: Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. Therefore, the overall peak flow increase from development (under Alternative B) of both Bonnie Briar and Winged Foot properties estimated for the lower Sheldrake River near the I-95 crossing is 1 percent for the 5-year storm and 0.6 percent for the 100-year storm. These overall peak flow estimates are based on hydrologic modeling results performed for the R30 Townhouse development scheme. (See Table A-3 in Appendix A). • Flood Plain Elevations: Water surface elevations corresponding to the peak runoff flows from the developed sub-basins are determined for the series of five storm frequencies. Water surface profile results based on hydrologic modeling of the Watershed, show variations in flood elevation increases along the Sheldrake River. Since flood elevations are also dependent on stream geometry which constantly varies, higher runoff flows do not necessarily correspond to higher flood elevations. Bonnie Briar: Within sub-basin 3, a maximum increase in flood plain elevation of 0.04 feet is projected during the 50-year storm along the East Branch where the stream parallels Fenimore Road between Cornell Street and Winding Brook Drive. Within sub-basin 4, a maximum increase in flood plain elevation of 0.05 feet is projected during the 5-year storm at two different locations near the confluence of the East Branch with the Sheldrake River. Within sub-basin 7, a maximum increase in flood plain elevation of 0.10 feet is projected during the 5-year and 100-year storms at two locations; along the East Branch where the stream parallels Ridgeway Road and along the East Branch between Hilltop Road and York Road. (See Table A-4 in Appendix A). •_ Winged Foot: Within sub-basin 6, a maximum increase in flood plain elevation of 0.09 feet is projected during the 100-year storm along the East Tributary near Salem Drive. • • Flooding: The increase in water surface elevation (0.1 feet or less) due to the proposed development scheme (Alternative B), has no observable effect on the existing flood plain area. The increase along the river sections within each sub-basin does not affect any additional dwellings. As a result of the proposed Alternative B development, we do not project any additional houses to be flooded in the Town of Mamaroneck along the Sheldrake River. However, those houses that are presently being flooded could experience increased flooding up to 0.1 feet during the 5-year and 100-year storms. (For the extent of flooded areas, refer to Appendix 5-5 in the DGEIS). 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. • Wetlands: Under the proposed development scheme, the low lying area along the Sheldrake River in the Bonnie Briar property would be completely preserved. The existing wetlands are also preserved. Therefore no increase in flood flows due to encroachment upon these areas is expected under this development alternative. • Water Quality: The Sheldrake River and its tributaries are classified "C"waters and must meet the corresponding standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality aspects was performed for the DGEIS, a minimal degree of water quality degradation is expected under the proposed development because this alternative results in a small increase in road surface area. Less than 5 percent of the Bonnie Briar property is intended to be paved for roads. Increased road surface area typically results in increased dissolved oxygen demands, pH and heavy metals in the drainage system. • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the _ development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is expected, the water quality degradation in the receiving streams can be minimized • by providing detention ponds to retain the storm water runoff. The class "C" designation for the Sheldrake River is expected to continue. r: 2. Impacts on the Village of Mamaroneck Impacts in the Village, downstream of the Sheldrake River Watershed above the I-95 culvert, were not determined by detailed hydrologic computer modeling. For the two sub- basins in the Winged Foot property which lie outside the Sheldrake Watershed, a similar but simplified "desktop" methodology was used to determine peak runoff flows. The • methodology developed by the SCS (TR-55 Method) uses runoff curve numbers (CN) and times of concentration to estimate peak runoff flow. Both CN values and times of concentration were determined based on the assumption that a corresponding Alternative B development for the Winged Foot property would be most similar to R30 Townhouse development for this area. Cummulative impacts in the Village were determined by adding impacts in the Town (determined from modeling) and impacts from sub-basins 8 and 9 • (determined from SCS TR-55 method). The individual impacts from sub-basins 8 and 9 added to the impacts from the Town, do not consider dampening of peak flows which normally occurs during a storm event as individual flows reach a river at different times. The estimated cummulative impacts in the Village are therefore somewhat conservative since they assume that all flows reach the river simultaneously. • Affected Sub-basins: Under the proposed development scheme, sub-basins 3, 4, and 7 draining to the Sheldrake River would be affected by the Bonnie Briar develop- - ment. Sub-basin 6, also draining to the Sheldrake River, would be affected by the Winged Foot development. Additionally, sub-basins 8 and 9 draining to the Mamaroneck River would be affected by the Winged Foot development. Because the Sheldrake River is tributary to the Mamaroneck River, development upstream in sub-basins 3, 4, 6 and 7 would not only potentially impact the Town but also the Village of Mamaroneck. Impacts in the Village will not only include the peak flow increases from sub-basins 3, 4, 6 and 7 (in the Town), but will also include peak flow increases from sub-basin 8 and 9 as discussed below. • Peak Runoff Flow: For sub-basins 8 and 9 the proposed Alternative B development for the Winged Foot property was assumed to be similar to the R30 Townhouse development scheme. A series of runoff flows was simulated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. The percentage of sub-basin area proposed for development in 8 and 9 is relatively larger than for other affected sub-basins because of the drainage area delineation. The increase in impervious surface area in sub-basins 8 and 9 due to development is approximately 10 percent compared to 3 percent or less for sub-basins 3, 4, 6 and 7. Therefore, the individual peak flow increases within sub-basins 8 and 9 is much higher than in sub-basins 3, 4, 6 and 7 which have a smaller percentage of the sub- basin area being developed. The runoff curve numbers for sub-basins 8 and 9 increase over existing conditions. vo— (See Table B-1 in Appendix B). Within sub-basin 8, a 33 percent increase in peak flow would be experienced during a 5-year storm and a 20 percent increase during a 100-year storm. Within sub-basin 9, a 23 percent increase in peak flow would be experienced during a 5-year storm and a 14 percent increase would be experienced during a 100-year storm. These projected increases do not take into consideration any mitigation measures. (See Table B-2 in Appendix B). Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. For sub-basins 8 and 9, hydrologic modeling was not performed and therefore dampening effects were not determined. The overall peak flow increase from development of both Bonnie Briar • and Winged Foot properties (under Alternative B) is determined for a location along the Mamaroneck River in the Village at a gaging station near the Conrail Line. The overall peak flow increase in the Village is estimated to be 1.2 percent during a 5-year storm and 0.9 percent during a 100-year storm. These peak flow 0 estimates are slightly conservative due to the fact that some dampening of sub-basin 8 and 9 flows will occur which was not accounted for. Peak flows for sub-basins 3, 4, 6, and 7 were determined by hydrologic modeling which includes dampening effects, while peak flows for sub-basins 8 and 9 are determined using SCS TR-55 methodology which does not include dampening effects. (See Table B-3 in Appendix B). • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class"C" classification for the River is not expected to change. 3.2 Description of Parish & Weiner Alternative A - Bonnie Briar Country Club 3.2.3 Probable Significant Environmental Impacts Upon and Mitigation Measures for Natural Resources B. Water Resources The following hydrologic analysis conducted by Malcolm Pirnie Inc., is based on cumulative impact,assuming similar development scenarios for Bonnie Briar Country Club and Winged Foot Golf Club. Conceptual plans provided by Parish & Weiner, Inc., for development under Alternative A, provided the basis for the hydrologic analysis of the Bonnie Briar property. However,no plans were available for the Winged Foot property under Alternative A. For Alternative A, no hydrologic modeling was performed. All hydrologic modeling was performed earlier in the DGEIS for CR Townhouse, CR Detached, R30 Townhouse, and R30 Detached alternatives. Therefore, to predict impacts in the Town and Village as a result of development under the Alternative A scenario, estimates of peak flow and flood elevation were determined by selecting the development scheme from the DGEIS which was most similar to Alternative A. The selection was based on the location and extent of imperviousness (from construction of buildings and roadways) resulting from the proposed Alternative A schematic. These two factors, location and extent of imperviousness, effect the runoff curve number (CN) parameter within each sub-basin. This parameter in turn allows the selection of one of the alternatives previously analyzed in detail in the DGEIS. For the Bonnie Briar property, the hydrologic parameters corresponding to Alternative A, are within the range of parameters associated with the CR Detached and the R30 Detached — alternatives which were modeled in detail as part of the DGEIS. Since no similar plans were available for Winged Foot property, it is assumed that a similar development on Winged Foot will also have impacts ranging between these two alternatives. Because the Alternative A scheme fell somewhere inbetween the two modeled scenarios, the impacts in the Town and Village from Alternative A development are expressed as a range; greater than impacts from CR Detached, but less than impacts from R30 Detached. (Refer to Appendix A of the SGEIS). Again, it should be noted that the assumptions made regarding similar development scenarios (studied in the DGEIS), are predicated on the imperviousness and location of the proposed units, not the actual zoning description of R30 Detached nor CR Detached. 1. Impacts on the Town of Mamaroneck (unincorporated area) • Affected Sub-basins: Under the proposed development scheme, sub-basins 3, 4, and 7 would be affected by the Bonnie Briar development. The majority of the development (approximately 50 percent)would occur in sub-basin 4. Approximately 25 percent of the development would occur in each of sub-basins 3 and 7. For the Winged Foot development, sub-basin 6 would be affected. Sub-basin 6 is within the Sheldrake Watershed upstream of the I-95 culvert. (Sub-basins 8 and 9 would also be affected by the Winged Foot development but they are outside of the Sheldrake Watershed. Hence, development in 8 and 9 would impact the Village but not the Town). • Peak Runoff Flow: For each sub-basin affected by the proposed development, a series of runoff flows was estimated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. Bonnie Briar: Since the increase in impervious surfaces in sub-basin 3 is relatively small in comparison to the total sub-basin area, the runoff curve number remains unaffected. Therefore,less than 1 percent increase in peak runoff flow over existing conditions is projected within sub-basin 3. However, within sub-basins 4 and 7, the increase in imperviousness results in an increase in the runoff curve numbers in the range of increases experienced under the CR Detached and R30 Detached development schemes. Under the R30 Detached scheme there is an increase in the sub-basin 3 runoff curve number which is not experienced in the Alternative A scheme. The impacts in the Town from development under the Alternative A scheme fall in the range between impacts from the CR Detached and R30 Detached schemes. (See Table A-1 in Appendix A). Within sub-basin 4, a 5 to 10 percent increase in peak flow would be experienced during a 5-year storm and a 3 to 6 percent increase during a 100-year storm. Within sub-basin 7, a 0 to 5 percent increase in peak flow would be experienced during a 5-year storm and a 0 to 3 percent increase during a 100-year storm. (See Table A-2 in Appendix A). Winged Foot: For the Winged Foot development, an increase in the runoff curve number is projected for sub-basin 6 which is in the range of increases estimated for the CR Detached and R30 Detached development schemes. Within sub-basin 6, a 5 to 11 percent increase in peak runoff flow is projected during a 5-year storm and •— a 3 to 6 percent increase during a 100-year storm. (See Table A-2 in Appendix A). Overall: Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. Therefore, the overall • peak flow increase from development (under Alternative A) of both Bonnie Briar and Winged Foot properties estimated for the lower Sheldrake River near the I-95 crossing is 1 to 2 percent for the 5-year storm and 0.6 to 1.1 percent for the 100-year storm. These overall peak flow estiamtes are based on hydrologic modeling results performed for the R30 Detached and CR Detached development schemes. (See Table A-3 in Appendix A). • - - • Flood Plain Elevations: Water surface elevations corresponding to the peak runoff flows from the developed sub-basins are determined for the series of five storm frequencies. Water surface profile results based on hydrologic modeling of the Watershed, show variations in flood elevation increases along the Sheldrake River. Since flood elevations are also dependent on stream geometry which constantly • varies, higher runoff flows do not necessarily correspond to higher flood elevations. Bonnie Briar: Within sub-basin 3, a maximum increase in flood plain elevation of _ 0.03 to 0.04 feet is projected during the 50-year and 100-year storms along the East Branch where the stream parallels Fenimore Road between Cornell Street and • Winding Brook Drive. Within sub-basin 4, a maximum increase in flood plain elevation of 0.05 to 0.10 feet is projected during the 5-year storm at two different locations near the confluence of the East Branch with the Sheldrake River. Within sub-basin 7, a maximum increase in flood plain elevation of 0.10 to 0.21 feet is projected during the 5-year storm at two locations; along the East Branch where the stream parallels Ridgeway Road and along the East Branch between Hilltop Road and York Road. (See Table A-4 in Appendix A). Winged Foot: Within sub-basin 6, maximum increase in flood plain elevation of 0.07 to 0.24 feet is projected during the 25-year storm along the East Tributary near Salem Drive. • Flooding: The increase in water surface elevation (0.24 feet or less) due to the proposed development scheme (Alternative A), has no observable effect on the existing flood plain area. The increase along the river sections within each sub-basin does not affect any additional dwellings. As a result of the proposed Alternative A development, we do not project any additional houses to be flooded in the Town of Mamaroneck along the Sheldrake River. However, those houses that are presently being flooded could experience increased flooding up to 0.24 feet during the 5-year and 25-year storms. (For the extent of flooded areas, refer to Appendix 5-5 in the DGEIS). 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. • Wetlands: Under the proposed development scheme, the low lying area along the Sheldrake River in the Bonnie Briar property would be completely preserved. The existing wetlands are also preserved. Therefore no increase in flood flows due to encroachment upon these areas is expected under this development alternative. • Water Quality: The Sheldrake River and its tributaries are classified"C"waters and must meet the standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality aspects was performed for the DGEIS, a minimal degree of water quality degradation os expected under • the proposed development because this alternative results in a small increase in road surface area. Less than 5 percent of the Bonnie Briar property is intended to be paved for roads. Increased road surface area typically results in increased dissolved oxygen demands, pH and heavy metals in the drainage system. • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is • expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class "C" designation for the Sheldrake River is expected to continue. • 2. Impacts on the Village of Mamaroneck Impacts in the Village, downstream of the Sheldrake River Watershed above the I-95 culvert, were not determined by detailed hydrologic computer modeling. For the two sub- basins in the Winged Foot property which lie outside the Sheldrake Watershed, a similar but simplified "desktop" methodology was used to determine peak runoff flows. The methodology developed by the SCS (TR-55 Method) uses runoff curve numbers (CN) and times of concentration to estimate peak runoff flow. Both CN values and times of concentration were determined based on the assumption that a corresponding Alternative A development for the Winged Foot property would have hydrologic features inbetween the range estimated for CR Detached and R30 Detached scenarios. Cummulative impacts in the Village were determined by adding impacts in the Town (determined from modeling) and impacts from sub-basins 8 and 9 (determined from SCS TR-55 method). The individual impacts from sub-basins 8 and 9 added to the impacts from the Town, do not consider dampening of peak flows which normally occurs during a storm event as individual flows reach a river at different times. The estimated cummulative impacts in the Village are therefore somewhat conservative since they assume that all flows reach the river simultaneously. • Affected Sub-basins: Under the proposed development scheme, sub-basins 3,4, and 7 draining to the Sheldrake River would be affected by the Bonnie Briar develop- ment. Sub-basin 6, also draining to the Sheldrake River, would be affected by the Winged Foot development. Additionally, sub-basins 8 and 9 draining to the Mamaroneck River would be affected by the Winged Foot development. Because the Sheldrake River is tributary to the Mamaroneck River, development upstream in sub-basins 3, 4, 6 and 7 would not only potentially impact the Town but also the Village of Mamaroneck. Impacts in the Village will not only include the peak flow increases from sub-basins 3, 4, 6 and 7 (in the Town), but will also include peak flow increases from sub-basin 8 and 9 as discussed below. •_ • Peak Runoff Flow: For sub-basins 8 and 9 the proposed Alternative A development for the Winged Foot property was assumed to be somewhere inbetween CR Detached and R30 Detached development schemes. A series of runoff flows was projected for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). -- Larger storms tend to saturate soils to a degree whereby unpaved areas contribute • to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. The percentage of sub-basin area proposed for development in 8 and 9 is relatively larger than for other affected sub-basins because of the drainage area delineation. • -- The increase in impervious surface area in sub-basins 8 and 9 due to development is approximately 10 percent compared to 3 percent or less for sub-basins 3, 4, 6 and 7. Therefore, the individual peak flow increases within sub-basins 8 and 9 is much higher than in sub-basins 3, 4, 6 and 7 which have a smaller percentage of the sub- basin area being developed. • The runoff curve numbers for sub-basins 8 and 9 increase over existing conditions. (See Table B-1 in Appendix B). Within sub-basin 8, a 39-71 percent increase in peak flow would be experienced during a 5-year storm and a 27 to 52 percent increase during a 100-year storm. Within sub-basin 9, a 27 to 68 percent increase in peak flow would be experienced during a 5-year storm and a 14 to 54 percent • increase would be experienced during a 100-year storm. These projected increases do not take into consideration any mitigation measures. (See Table B-2 in Appendix B). Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. For sub-basins 8 and 9, hydrologic modeling was not performed and therefore dampening effects were not determined. The overall peak flow increase from development of both Bonnie Briar and Winged Foot properties (under Alternative A) is determined for a location along the Mamaroneck River in the Village at a USGS gaging station near the Conrail Line. The overall peak flow increase in the Village is estimated to be 1.4 to 2.9 percent during 5-year storm and 1.0 to 2.5 percent during a 100-year storm. These peak flow estimates are slightly conservative due to the fact that some dampening of sub-basin 8 and 9 flows will occur which was not accounted for. Peak flows for sub-basins 3, 4, 6, and 7 were determined by hydrologic modeling which includes dampening effects, while peak flows for sub-basins 8 and 9 are determined using SCS TR-55 methodology which does not include dampening effects. (See Table B-3 in Appendix B). • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class"C' classification for the River is not expected to change. • •V • • 3.3 Description of Parish & Weiner Alternative C - Bonnie Briar Country Club 3.2.3 Probable Significant Environmental Impacts Upon and Mitigation Measures for Natural Resources B. Water Resources The following hydrologic analysis conducted by Malcolm Pirnie Inc., is based on cumulative impact, assuming similar development scenarios for Bonnie Briar Country Club and Winged Foot Golf Club. Conceptual plans provided by Parish & Weiner, Inc., for development under Alternative C, provided the basis for the hydrologic analysis of the Bonnie Briar property. However,no plans were available for the Winged Foot property under Alternative C. For Alternative C, no hydrologic modeling was performed. All hydrologic modeling was performed earlier in the DGEIS for CR Townhouse, CR Detached, R30 Townhouse, and R30 Detached alternatives. Therefore, to predict impacts in the Town and Village as a result of development under the Alternative C scenario, estimates of peak flow and flood elevation were determined by selecting the development scheme from the DGEIS which was most similar to Alternative C. The selection was based on the location and extent of imperviousness (from construction of buildings and roadways) resulting from the proposed Alternative C schematic. These two factors, location and extent of imperviousness, effect the runoff curve number (CN) parameter within each sub-basin. This parameter in turn allows the selection of one of the alternatives previously analyzed in detail in the DGEIS. For the Bonnie Briar property, the hydrologic parameters corresponding to Alternative C, are within the range of parameters associated with the CR Detached and the R30 Detached alternatives which were modeled in detail as part of the DGEIS. Since no similar plans were available for Winged Foot property, it is assumed that a similar development on Winged Foot will also have impacts ranging between these two alternatives. •.,. Because the Alternative C scheme fell somewhere inbetween the two modeled scenarios,the impacts in the Town and Village from Alternative C development are expressed as a range; greater than impacts from CR Detached, but less than impacts from R30 Detached. (Refer to Appendix A of the SGEIS). Again, it should be noted that the assumptions made regarding similar development scenarios (studied in the DGEIS), are predicated on the imperviousness and location of the proposed units, not the actual zoning description of R30 • Detached nor CR Detached. 1. Impacts on the Town of Mamaroneck (unincorporated area) •— • Affected Sub-basins: Under the proposed development scheme, sub-basins 3,4, and 7 would be affected by the Bonnie Briar development. The majority of the development (approximately 60 percent)would occur in sub-basin 4. Approximately 20 percent of the development would occur in each of sub-basins 3 and 7. For the Winged Foot development, sub-basin 6 would be affected. Sub-basin 6 is within the Sheldrake Watershed upstream of the I-95 culvert. (Sub-basins 8 and 9 would also • be affected by the Winged Foot development but they are outside of the Sheldrake Watershed. Hence, development in 8 and 9 would impact the Village but not the Town). • Peak Runoff Flow: For each sub-basin affected by the proposed development, a series of runoff flows was estimated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. Bonnie Briar: Since the increase in impervious surfaces in sub-basin 3 is relatively _ small in comparison to the total sub-basin area, the runoff curve number remains unaffected. Therefore,less than 1 percent increase in peak runoff flow over existing conditions is projected within sub-basin 3. However, within sub-basins 4 and 7, the increase in imperviousness results in an increase in the runoff curve numbers in the range of increases experienced under the CR Detached and R30 Detached development schemes. Under the R30 Detached scheme there is an increase in the sub-basin 3 runoff curve number which is not experienced in the Alternative C scheme. The impacts in the Town from development under the Alternative C scheme fall in the range between impacts from the CR Detached and R30 Detached schemes. (See Table A-1 in Appendix A). • Within sub-basin 4, a 5 to 10 percent increase in peak flow would be experienced during a 5-year storm and a 3 to 6 percent increase during a 100-year storm. Within sub-basin 7, a 0 to 5 percent increase in peak flow would be experienced during a 5-year storm and a 0 to 3 percent increase during a 100-year storm. (See Table A-2 �-- in Appendix A). Winged Foot: For the Winged Foot development, an increase in the runoff curve number is projected for sub-basin 6 which is in the range of increases estimated for the CR Detached and R30 Detached development schemes. Within sub-basin 6, a 5 to 11 percent increase in peak runoff flow is projected during a 5-year storm and a 3 to 6 percent increase during a 100-year storm. (See Table A-2 in Appendix A). Overall: Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. Therefore, the overall peak flow increase from development (under Alternative C) of both Bonnie Briar and Winged Foot properties estimated for the lower Sheldrake River near the 1-95 crossing is 1 to 2 percent for the 5-year storm and 0.6 to 1.1 percent for the 100-year storm. These overall peak flow estiamtes are based on hydrologic modeling results performed for the R30 Detached and CR Detached development schemes. (See Table A-3 in Appendix A). • Flood Plain Elevations: Water surface elevations corresponding to the peak runoff flows from the developed sub-basins are determined for the series of five storm frequencies. Water surface profile results based on hydrologic modeling of the Watershed, show variations in flood elevation increases along the Sheldrake River. Since flood elevations are also dependent on stream geometry which constantly varies, higher runoff flows do not necessarily correspond to higher flood elevations. Bonnie Briar: Within sub-basin 3, a maximum increase in flood plain elevation of 0.03 to 0.04 feet is projected during the 50-year and 100-year storms along the East Branch where the stream parallels Fenimore Road between Cornell Street and • Winding Brook Drive. Within sub-basin 4, a maximum increase in flood plain elevation of 0.05 to 0.10 feet is projected during the 5-year storm at two different locations near the confluence of the East Branch with the Sheldrake River. Within sub-basin 7, a maximum increase in flood plain elevation of 0.10 to 0.21 feet is projected during the 5-year storm at two locations; along the East Branch where the stream parallels Ridgeway Road and along the East Branch between Hilltop Road and York Road. (See Table A-4 in Appendix A). Winged Foot: Within sub-basin 6,maximum increase in flood plain elevation of 0.07 to 0.24 feet is projected during the 25-year storm along the East Tributary near Salem Drive. • Flooding: The increase in water surface elevation (0.24 feet or less) due to the proposed development scheme (Alternative C), has no observable effect on the existing flood plain area. The increase along the river sections within each sub-basin does not affect any additional dwellings. As a result of the proposed Alternative C development, we do not project any additional houses to be flooded in the Town of Mamaroneck along the Sheldrake River. However, those houses that are presently being flooded could experience increased flooding up to 0.24 feet during the 5-year and 25-year storms. (For the extent of flooded areas, refer to Appendix 5-5 in the DGEIS). 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. • Wetlands: Under the proposed development scheme, the low lying area along the Sheldrake River in the Bonnie Briar property would be completely preserved. The existing wetlands are also preserved. Therefore no increase in flood flows due to encroachment upon these areas is expected under this development alternative. • Water Quality: The Sheldrake River and its tributaries are classified "C"waters and must meet the standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality aspects was performed for the DGEIS, a minimal degree of water quality degradation os expected under r the proposed development because this alternative results in a small increase in road surface area. Less than 5 percent of the Bonnie Briar property is intended to be paved for roads. Increased road surface area typically results in increased dissolved oxygen demands, pH and heavy metals in the drainage system. • • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is • expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class "C" designation for the Sheldrake River is expected to continue. • 3.5 Description of Modified CR Single Family Detached With Eighteen Hole Golf Course- Bonnie Briar Country Club 3.5.4 Probable Significant Environmental Impacts Upon and Mitigation Measures for Natural Resources B. Water Resources The following hydrologic analysis conducted by Malcolm Pirnie Inc., is based on cumulative impact,assuming similar development scenarios for Bonnie Briar Country Club and Winged Foot Golf Club. Conceptual plans provided by Ferrandino and Associates, Inc., for development under Modified CR Alternative,provided the basis for the hydrologic analysis of the Bonnie Briar property. However, no plans were available for the Winged Foot property under Modified CR. For the Modified CR alternative, no hydrologic modeling was performed. All hydrologic modeling was performed earlier in the DGEIS for CR Townhouse, CR Detached, R30 Townhouse, and R30 Detached alternatives. Therefore, to predict impacts in the Town and Village as a result of development under the Modified CR scenario, estimates of peak flow and flood elevation were determined by selecting the development scheme from the DGEIS which was most similar to the Modified CR alternative. The selection was based on the location and extent of imperviousness (from construction of buildings and roadways) resulting from the proposed Modified CR schematic. These two factors,location and extent of imperviousness, effect the runoff curve number (CN) parameter within each sub-basin. This parameter in turn allows the selection of one of the alternatives previously analyzed in detail in the DGEIS. For the Modified CR alternative, the most similar development scheme in Bonnie Briar property which was modeled as part of the DGEIS, was the R30 Townhouse alternative. Since no similar plans were available for Winged Foot, the R30 Townhouse development was also assumed for the Winged Foot site. (Refer to Appendix A of the SGEIS). Therefore impacts in the Town and Village under Modified CR will be similar to impacts under R30 Townhouse. (See Appendix 5-5 of DGEIS for further detail). Again, it should be noted that the assumptions made regarding similar development scenarios (studied in the DGEIS),are predicated on the imperviousness and location of the proposed units, not the actual zoning description of R30 Townhouse. 1. Impacts on the Town of Mamaroneck (unincorporated area) • Affected Sub-basins: Under the proposed development scheme, sub-basins 3, 4 and 7 would be affected by the Bonnie Briar development. Approximately 40 percent of the development would occur in sub-basin 3 and approximately 60 percent would occur in sub-basin 4. Although no development would occur in sub-basin 7 under the CR Modified scheme, impacts could potentially occur in this sub-basin from -. increased flow upstream in sub-basin 3. For the Winged Foot development, sub- basin 6 would be affected. Sub-basin 6 is within the Sheldrake Watershed upstream of the I-95 culvert. (Sub-basins 8 and 9 would also be affected by the Winged Foot development but they are outside of the Sheldrake Watershed. Hence, development in 8 and 9 would impact the Village but not the Town). • Peak Runoff Flow: For each sub-basin affected by the proposed development, a series of runoff flows was estimated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less noticeable for the 100-year storms. Bonnie Briar: Since the increase in impervious surfaces in sub-basin 3 is relatively small in comparison to the total sub-basin area, the runoff curve number remains unaffected. No development is planned for sub-basin 7 under the CR Modifed scheme. Therefore, less than 1 percent increase in peak runoff flow over existing conditions is projected within sub-basins 3 and 7. Any small increase in peak flow in sub-basin 7 would be a direct result of peak flow increase in sub-basin 3 which is upstream. Within sub-basin 4, the increase in imperviousness results in an increase in the runoff curve number similar to the increase experienced under the CR Townhouse development scheme. (See Table A-1 in Appendix A). Within sub-basin 4, a 5 percent increase in peak flow would be experienced during a 5-year storm and a 3 percent increase during a 100-year storm. (See Table A-2 in Appendix A). Winged Foot: For the Winged Foot development, an increase in the runoff curve number is projected for sub-basin 6 similar to the increase under the CR Townhouse development scheme. Within sub-basin 6, a 5 percent increase in peak runoff is projected during a 5-year storm and a 3 percent increase during a 100-year storm. �-- (See Table A-2 in Appendix A). Overall: Individual sub-basin runoff flows peak at different times in the Watershed. -- This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. Therefore, the overall peak flow increase from development (under Modified CR) of both Bonnie Briar •.. and Winged Foot properties estimated for the lower Sheldrake River near the 1-95 crossing is 1 percent for the 5-year storm and 0.6 percent for the 100-year storm. These overall peak flow estimates are based on hydrologic modeling results performed for the CR Townhouse development scheme. (See Table A-3 in Appendix A). • • Flood Plain Elevations: Water surface elevations corresponding to the peak runoff flows from the developed sub-basins are determined for the series of five storm frequencies. Water surface profile results based on hydrologic modeling of the Watershed, show variations in flood elevation increases along the Sheldrake River. Since flood elevations are also dependent on stream geometry which constantly varies, higher runoff flows do not necessarily correspond to higher flood elevations. Bonnie Briar: Within sub-basin 3, a maximum increase in flood plain elevation of 0.04 feet is projected during the 50-year storm along the East Branch where the stream parallels Fenimore Road between Cornell Street and Winding Brook Drive. Within sub-basin 4, a maximum increase in flood plain elevation of 0.05 feet is projected during the 5-year storm at two different locations near the confluence of the East Branch with the Sheldrake River. Within sub-basin 7, a maximum increase in flood plain elevation of 0.10 feet is projected during the 5-year and 100-year storms at two locations; along the East Branch where the stream parallels Ridgeway Road and along the East Branch between Hilltop Road and York Road. (See Table A-4 in Appendix A). Winged Foot: Within sub-basin 6, a maximum increase in flood plain elevation of 0.09 feet is projected during the 100-year storm along the East Tributary near Salem Drive. • Flooding: The increase in water surface elevation (0.1 feet or less) due to the proposed development scheme (CR Modified), has no observable effect on the existing flood plain area. The increase along the river sections within each sub-basin does not affect any additional dwellings. As a result of the proposed Modified CR development, we do not project any additional houses to be flooded in the Town of Mamaroneck along the Sheldrake River. However, those houses that are presently being flooded could experience increased flooding up to 0.1 feet during the 5-year and 100-year storms. The number of dwellings within the flood plain areass for the developed conditions is the same as the number of dwellings which are being flooded under existing conditions. (For the extent of flooded areas, refer to Appendix 5-5 in the DGEIS). 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. • • Wetlands: Under the proposed development scheme, the low lying area along the Sheldrake River in the Bonnie Briar property would be completely preserved. The existing wetlands are also preserved. Therefore no increase in flood flows due to encroachment upon these areas is expected under this development alternative. • Water Quality: The Sheldrake River and its tributaries are classified"C"waters and �.. must meet the corresponding standards for fecal coliform, pH, dissolved solids, and dissolved oxygen. Although no detailed analysis of the water quality aspects was performed for the DGEIS, a minimal degree of water quality degradation is expected under the proposed development because this alternative results in a small increase in road surface area. Less than 5 percent of the Bonnie Briar property is intended to be paved for roads. Increased road surface area typically results in • increased dissolved oxygen demands, pH and heavy metals in the drainage system. • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to " reduce peak runoff flows to the drainage system. Increase in runoff is partially • - mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class "C" designation for the Sheldrake River is expected to continue. • 2. Impacts on the Village of Mamaroneck • Impacts in the Village, downstream of the Sheldrake River Watershed above the I-95 culvert, were not determined by detailed hydrologic computer modeling. For the two sub- - basins in the Winged Foot property which lie outside the Sheldrake Watershed, a similar but simplified "desktop" methodology was used to determine peak runoff flows. The methodology developed by the SCS (TR-55 Method) uses runoff curve numbers (CN) and times of concentration to estimate peak runoff flow. Both CN values and times of concentration were determined based on the assumption that a corresponding Modified CR development for the Winged Foot property would be most similar to R30 Townhouse development for this area. Cummulative impacts in the Village were determined by adding impacts in the Town (determined from modeling) and impacts from sub-basins 8 and 9 (determined from SCS TR-55 method). The individual impacts from sub-basins 8 and 9 added to the impacts from the Town, do not consider dampening of peak flows which normally occurs during a storm event as individual flows reach a river at different times. The estimated cummulative impacts in the Village are therefore somewhat conservative since they assume that all flows reach the river simultaneously. • Affected Sub-basins: Under the proposed development scheme, sub-basins 3, 4 and 7 draining to the Sheldrake River would be affected by the Bonnie Briar develop- ment. Sub-basin 6, also draining to the Sheldrake River, would be affected by the Winged Foot development. Additionally, sub-basins 8 and 9 draining to the Mamaroneck River would be affected by the Winged Foot development. Because the Sheldrake River is tributary to the Mamaroneck River, development upstream in sub-basins 3, 4, 6 and 7 would not only potentially impact the Town but also the Village of Mamaroneck. Impacts in the Village will not only include the peak flow • increases from sub-basins 3,4, 6 and 7 (in the Town),but will also include peak flow increases from sub-basin 8 and 9 as discussed below. • Peak Runoff Flow: For sub-basins 8 and 9 the proposed Alternative B development for the Winged Foot property was assumed to be similar to the CR Townhouse •_ development scheme. A series of runoff flows was simulated for the 5, 10, 25, 50, and 100-year storms. The impacts are highest in the more frequent storms (5-year) and lowest in the less frequent storms (100-year). Larger storms tend to saturate _ soils to a degree whereby unpaved areas contribute to runoff almost as much as paved areas. Therefore, the increase in runoff due to imperviousness is less • noticeable for the 100-year storms. • The percentage of sub-basin area proposed for development in 8 and 9 is relatively larger than for other affected sub-basins because of the drainage area delineation. The increase in impervious surface area in sub-basins 8 and 9 due to development is approximately 10 percent compared to 3 percent or less for sub-basins 3, 4, and • 6. Therefore, the individual peak flow increases within sub-basins 8 and 9 is much higher than in sub-basins 3, 4, and 6 which have a smaller percentage of the sub- basin area being developed. The runoff curve numbers for sub-basins 8 and 9 increase over existing conditions. (See Table B-1 in Appendix B). Within sub-basin 8, a 33 percent increase in peak flow would be experienced during a 5-year storm and a 20 percent increase during a 100-year storm. Within sub-basin 9, a 23 percent increase in peak flow would be experienced during a 5-year storm and a 14 percent increase would be experienced during a 100-year storm. These projected increases do not take into consideration any mitigation measures. (See Table B-2 in Appendix B). Individual sub-basin runoff flows peak at different times in the Watershed. This routing of sub-basin flows through the drainage area results in a dampening of peak runoff flows in the lower reaches of the Watershed. For sub-basins 8 and 9, hydrologic modeling was not performed and therefore dampening effects were not determined. The overall peak flow increase from development of both Bonnie Briar and Winged Foot properties(under CR Modified)is determined for a location along the Mamaroneck River in the Village at a gaging station near the Conrail Line. The overall peak flow increase in the Village is estimated to be 1.2 percent during a 5- year storm and 0.9 percent during a 100-year storm. These peak flow estimates are slightly conservative due to the fact that some dampening of sub-basin 8 and 9 flows will occur which was not accounted for. Peak flows for sub-basins 3, 4, 6, and 7 were determined by hydrologic modeling which includes dampening effects, while peak flows for sub-basins 8 and 9 are determined using SCS TR-55 methodology which does not include dampening effects. (See Table B-3 in Appendix B). • Mitigation Measures: The properties should be developed with storm water controls in mind (i.e., properly sized culverts, storm drains and retention ponds) to reduce peak runoff flows to the drainage system. Increase in runoff is partially mitigated by the protected large wooded areas interspersed throughout the development which can absorb runoff and minimize potential flooding both on and off site. Although an increase in runoff of suspended solids on the property is expected, the water quality degradation in the receiving streams can be minimized by providing detention ponds to retain the storm water runoff. The class"C"classification for the River is not expected to change. • wit • • • APPENDICES FOR HYDROLOGIC STUDY (Performed by Malcolm Pirnie, Inc.) SHELDRAKE RIVER HYDROLOGIC STUDY r SUMMARY OF IMPACTS DUE TO DEVELOPMENT ASSUMPTIONS: • • Impacts in subbasins 3, 4, and 7 are for Bonnie Briar only. Assumed that development in Winged Foot area(subbasins 6, 8, and 9) under Alternatives A, B, C, and Modifed CR would be most similar to one of the following: CR Townhouse,R30 Townhouse,CR Detached,or R30 Detached. Selection was based on similarities in subbasin impervious areas and resulting CN values. (See Table A-1). • Impacts in the Town of Mamaroneck due to development of Bonnie Briar and Winged Foot properties was determined by flow increase in the Sheldrake River at the I-95 crossing. Impacts in the Village of Mamaroneck due to the developments was determined by flow increase in the Mamarone- ck River at the gaging station near the Conrail Line in the Village. • Peak flow increases for sub-basins 3, 4, 6, and 7 (within Sheldrake Water- shed) were determined through hydrologic modeling. Peak flow increases for sub-basins 8 and 9 were determined by SCS TR-55 methodology. Impacts in the Town will tend to be slightly more conservative(greater) than 41/ impacts in the Village since routing of sub-basins 8 and 9 and dampening of flows at downstream locations in the watershed are not considered in the TR-55 method. • Impacts for CR Townhouse, R30 Townhouse, CR Detached, and R30 • Detached obtained through use of hydrologic models. • Impacts for Alternatives A, B, C, and Modified CR obtained by comparing runoff curve numbers to modeled development scenarios and interpolating model results. • - Alternative A results between CR Detached and R30 Detached - Alternative B results similar to R30 Townhouse . - Alternative C results between CR Detached and R30 Detached - Modified CR results similar to CR Townhouse •— • • APPENDIX A • IMPACTS IN TOWN OF MAMARONECK • • • • - • • TABLE A-1 TOWN OF MAMARONECK BONNIE BRIAR PROPERTY SUMMARY OF DEVELOPED CONDITIONS SUBBASIN PERCENT INCREASE RUNOFF CURVE NUM- IN BER NUMBER IMPERVIOUS AREA (CN) 3 4 6 7 3 4 6 7 EXISTING 1.13 0.39 0.49 0.38 74 72 70 71 CONDITION mit mit miZ mit CR TOWNHOUSE <1 3 3 <1 74 73 71 71 R30 TOWNHOUSE <1 4 5 <1 74 73 71 71 CR DETACHED <1 4 3 <1 74 73 71 71 R30 DETACHED <1 6 9 2 74 74 72 72 ALTA (P&W) <1 5 9 _ 2 74 73 72 72 ALT B (P&W) <1 5 5 1 74 73 71 71 ALT C (P&W) <1 5 9 2 74 73 72 72 MODIFIED CR <1 2 3 <1 74 73 71 71 (F&A) • �r • • TABLE A-2 TOWN OF MAMARONECK BONNIE BRIAR PROPERTY PEAK FLOW INCREASES WITHIN SUB-BASINS DUE TO DEVELOPMENT SUBBASIN/CONDITION PERCENT INCREASE IN PEAK FLOW 5-year 10-year 25-year 50-year 100-yr 3 EXISTING FLOWS 395 519 649 763 939 CR TOWNHOUSE <1 <1 <1 <1 < 1 R30 TOWNHOUSE <1 <1 <1 <1 < 1 CR DETACHED <1 <1 <1 <1 <1 R30 DETACHED <1 <1 <1 <1 < 1 ALTERNATIVE A <1 <1 <1 <1 < 1 ALTERNATIVE B <1 <1 <1 <1 < 1 ALTERNATIVE C <1 <1 <1 <1 < 1 MODIFIED CR <1 <1 <1 <1 <1 4 EXISTING FLOWS 174 232 293 347 430 CR TOWNHOUSE 5 4 4 3 3 R30 TOWNHOUSE 5 4 4 3 3 CR DETACHED 5 4 4 3 3 R30 DETACHED 10 9 7 6 5 ALTERNATIVE A 5-10 4-9 4-7 3-6 3-6 — ALTERNATIVE B 5 4 4 3 3 •— ALTERNATIVE C 5-10 4-9 4-7 3-6 3-6 • MODIFIED CR 5 4 4 3 3 6 EXISTING FLOWS 176 238 304 362 453 CR TOWNHOUSE 5 5 3 3 3 • R30 TOWNHOUSE 5 5 3 3 3 CR DETACHED 5 5 3 3 3 — R30 DETACHED 11 9 8 7 6 •` ALTERNATIVE A 5-11 5-9 3-8 3-7 3-6 ALTERNATIVE B 5 5 3 3 3 ALTERNATIVE C 5-11 5-9 3-8 3-7 3-6 MODIFIED CR 5 5 3 3 3 TABLE A-2 CONTINUED SUBBASIN/CONDITION PERCENT INCREASE IN PEAK FLOW — 5-year 10-year 25-year 50-year 100-year 7 EXISTING FLOWS 214 288 364 432 536 CR TOWNHOUSE <1 <1 <1 <1 <1 R30 TOWNHOUSE <1 <1 <1 <1 <1 CR DETACHED <1 <1 <1 <1 <1 R30 DETACHED 5 4 4 3 3 ALTERNATIVE A 0-5 0-4 0-4 0-3 0-3 ALTERNATIVE B <1 <1 <1 <1 <1 ALTERNATIVE C 0-5 0-4 0-4 0-3 0-3 MODIFIED CR <1 <1 <1 <1 <1 TABLE A-3 TOWN OF MAMARONECK TOTAL PERCENT FLOW INCREASE IN SHELDRAKE RIVER DUE TO DEVELOPMENT OF BONNIE BRIAR AND WINGED FOOT PROPERTIES DEVELOPMENT SCENARIO PERCENT FLOW INCREASE AT I-95 5-YR 10-YR 25-YR 50-YR 100-YR CR TOWNHOUSE 1.0 0.7 0.6 0.6 0.6 R30 TOWNHOUSE 1.0 0.7 0.6 0.6 0.6 CR DETACHED 1.0 0.7 0.6 0.6 0.6 R30 DETACHED 2.0 1.5 1.4 1.2 1.1 ALTERNATIVE A 1.0-2.0 0.7-1.5 0.6-1.4 0.6-1.2 0.6-1.1 ALTERNATIVE B 1.0 0.7 0.6 0.6 0.6 ALTERNATIVE C 1.0-2.0 0.7-1.5 0.6-1.4 0.6-1.2 0.6-1.1 MODIFIED CR 1.0 0.7 0.6 0.6 0.6 • •- • • • • TABLE A-4 TOWN OF MAMARONECK BONNIE BRIAR PROPERTY FLOOD PLAIN INCREASE WITHIN SUB-BASINS DUE TO DEVELOPMENT SUBBASIN/LOCATION INCREASE IN FLOOD ELEVATION (FT) 5-year 10-year 25-year 50-year 100-yr 3 CR TOWNHOUSE 0.00 0.00 0.00 0.04 0.03 R30 TOWNHOUSE 0.00 0.00 0.00 0.04 0.03 CR DETACHED 0.00 0.00 0.00 0.04 0.03 R30 DETACHED 0.01 0.01 0.02 0.03 0.04 ALTERNATIVE A 0-0.01 0-0.01 0-0.02 .03-.04 .03-.04 ALTERNATIVE B 0.00 0.00 0.00 0.04 0.03 ALTERNATIVE C 0-0.01 0-0.01 0-0.02 .03-.04 .03-.04 MODIFIED CR 0.00 0.00 0.00 0.04 0.03 4 CR TOWNHOUSE 0.05 0.01 0.01 0.02 0.02 R30 TOWNHOUSE 0.05 0.01 0.01 0.02 0.02 CR DETACHED 0.05 0.01 0.01 0.02 0.02 R30 DETACHED 0.10 0.01 0.04 0.05 0.04 ALTERNATIVE A .05-.10 0.01 .01-.04 .02-.05 .02-.04 ALTERNATIVE B 0.05 0.01 0.01 0.02 0.02 ALTERNATIVE C .05-.10 0.01 .01-.04 .02-.05 .02-.04 MODIFIED CR 0.05 0.01 0.01 0.02 0.02 6 CR TOWNHOUSE 0.07 0.06 0.07 0.06 0.09 R30 TOWNHOUSE 0.07 0.06 0.07 0.06 0.09 CR DETACHED 0.07 0.06 0.07 0.06 0.09 R30 DETACHED 0.16 0.11 0.24 0.10 0.19 ALTERNATIVE A .07-.16 .06-.11 .07-.24 .06-.10 .09-.19 ALTERNATIVE B 0.07 0.06 0.07 0.06 0.09 ALTERNATIVE C .07-.16 .06-.11 .07-.24 .06-.10 .09-.19 MODIFIED CR 0.07 0.06 0.07 0.06 0.09 • TABLE A-4 CONTINUED SUBBASIN/LOCATION INCREASE IN FLOOD ELEVATION (FT) 5-year 10-year 25-year 50-year 100-year 7 CR TOWNHOUSE 0.10 0.08 0.06 0.07 0.10 R30 TOWNHOUSE 0.10 0.08 0.06 0.07 0.10 CR DETACHED 0.10 0.08 0.06 0.07 0.10 R30 DETACHED 0.21 0.12 0.12 0.13 0.20 ALTERNATIVE A .10-.21 .08-.12 .06-.12 .07-.13 .10-.20 ALTERNATIVE B 0.10 0.08 0.06 0.07 0.10 ALTERNATIVE C .10-.21 .08-.12 .06-.12 .07-.13 .10-.20 MODIFIED CR 0.10 0.08 0.06 0.07 0.10 s APPENDIX B IMPACTS IN VILLAGE OF MAMARONECK • •- • • - •..._ • . TABLE B-1 VILLAGE OF MAMARONECK WINGED FOOT PROPERTY SUMMARY OF DEVELOPED CONDITIONS SUBBASIN PERCENT INCREASE IN RUNOFF CURVE NO. IMPERVIOUS AREA (CN) NUMBER 8 9 8 9 EXISTING 0.10 0.12 62 63 CONDITIONS (sq mi) (sq mi) CR TOWNHOUSE 8 8 65 66 R30 TOWNHOUSE 9 8 65 66 CR DETACHED 12 11 66 67 R30 DETACHED 10 13 66 67 ALT A (P&W) 10-11 11-13 66 67 ALT B (P&W) 9 8 65 66 ALT C (P&W) 10-11 11-13 66 67 MODIFIED CR 8 8 65 66 0_ • • • - TABLE B-2 VILLAGE OF MAMARONECK WINGED FOOT PROPERTY PEAK FLOW INCREASES WITHIN SUB-BASINS DUE TO DEVELOPMENT SUBBASIN/CONDITION PERCENT INCREASE IN PEAK FLOW 5-yr 10-yr 25-yr 50-yr 100-yr 8 EXISTING FLOWS (CFS) 33.37 47.47 64.19 80.44 106.16 CR TOWNHOUSE 33 31 28 24 20 R30 TOWNHOUSE 33 31 28 24 20 CR DETACHED 39 36 31 29 27 R30 DETACHED 71 67 62 57 52 ALTERNATIVE A 39-71 36-67 31-62 29-57 27-52 ALTERNATIVE B 33 31 28 24 20 ALTERNATIVE C 39-71 36-67 31-62 29-57 27-52 MODIFIED CR 33 31 28 24 20 - 9 EXISTING FLOWS (CFS) 40.38 58.85 77.99 96.02 124.49 CR TOWNHOUSE 23 19 17 15 14 R30 TOWNHOUSE 23 19 17 15 14 CR DETACHED 27 20 17 16 14 R30 DETACHED 68 62 60 57 54 ALTERNATIVE A 27-68 20-62 17-60 16-57 14-54 ALTERNATIVE B 23 19 17 15 14 ALTERNATIVE C 27-68 20-62 17-60 16-57 14-54 MODIFIED CR 23 19 17 15 14 TABLE B-3 VILLAGE OF MAMARONECK TOTAL PERCENT FLOW INCREASE IN MAMARONECK RIVER DUE TO DEVELOPMENT OF BONNIE BRIAR AND WINGED FOOT PROPERTIES DEVELOPMENT PERCENT FLOW INCREASE IN VILLAGE SCENARIO 5-YR 10-YR 25-YR 50-YR 100-YR CR TOWNHOUSE 1.2 1.2 1.0 0.9 0.9 R30 TOWNHOUSE 1.2 1.2 1.0 0.9 0.9 CR DETACHED 1.4 1.3 1.1 1.0 1.0 R30 DETACHED 2.9 3.0 2.7 2.5 2.5 ALTERNATIVE A 1.4-2.9 1.3-3.0 1.1-2.7 1.0-2.5 1.0-2.5 ALTERNATIVE B 1.2 1.2 1.0 0.9 0.9 ALTERNATIVE C 1.4-2.9 1.3-3.0 1.1-2.7 1.0-2.5 1.0-2.5 MODIFIED CR 1.2 1.2 1.0 0.9 0.9 TABLE B-4 VILLAGE OF MAMARONECK WINGED FOOT PROPERTY SUMMARY OF TIME OF CONCENTRATIONS SUBBASIN TIME OF CONCENTRATION (HOURS) CONDITION Sheet Shallow Open Channel Total Time Flow Concentated Flow For All Flow SUBBASIN 8 Existing Cond. 0.455 0.139 0 0.594 CR Townhouse 0.011 0.139 0 0.531 R30 Townhouse 0.011 0.139 0 0.531 CR Detached 0.017 0.139 0 0.475 R30 Detached 0.205 0 0.046 0.283 SUBBASIN 9 Existing Cond. 0.516 0.172 0 0.688 CR Townhouse 0.516 0.172 0 0.688 R30 Townhouse 0.516 0.172 0 0.688 CR Detached 0.385 0 0.058 0.493 R30 Detached 0.242 0 0.083 0.344 SCENARIO: ALTERNATIVE A ,-- Description: Bonnie Briar Development 164 detached single family units Affected subbasins = 3,4,7 • Area 3 = 1.13 sq mi Existing CN = 74 Developed CN = 74 • Area 4 = 0.39 sq mi Existing CN = 72 Developed CN = 73 • Area 7 = 0.38 sq mi Existing CN = 71 Developed CN = 72 SUMMARY OF IMPERVIOUS SURFACE AREAS DUE TO DEVELOPMENT AREA SUBBASIN 3 SUBBASIN 4 SUBBASIN 7 No. of Lots 44 86 34 Rooftops 132,000 258,000 102,000 Driveways 52,058 101,749 40,226 Roads 80,600 130,000 88,400 Rec Facilities 0 57,500 0 TOTAL (SQ FT) 264,658 547,249 230,626 ASSUMPTIONS: • Roads are 26 feet wide. • Driveways are proportional to no. of lots in each sub-basin. • Rooftops are 3000 sq ft footprint. i • Recreation facilities are the same for all Bonnie Briar development schemes. I ,r� • d SCENARIO: ALTERNATIVE B r_ Description: Bonnie Briar Development 114 detached single family units 18 hole golf course 40 Affected subbasins = 3,4,7 • Area 3 = 1.13 sq mi Existing CN = 74 Developed CN = 74 • Area 4 = 0.39 sq mi Existing CN = 72 Developed CN = 72 • Area 7 = 0.38 sq mi Existing CN = 71 Developed CN = 71 SUMMARY OF IMPERVIOUS SURFACE AREAS DUE TO DEVELOPMENT AREA SUBBASIN 3 SUBBASIN 4 SUBBASIN 7 No. of Lots 21 78 15 Rooftops 63,000 234,000 45,000 Driveways 27,426 101,869 19,591 Roads 88,400 132,600 40,300 Rec Facilities 0 57,500 0 TOTAL (SQ FT) 178,826 525,969 104,891 ASSUMPTIONS: • Roads are 26 feet wide. • Driveways are proportional to no. of lots in each sub-basin. • Rooftops are 3000 sq ft footprint. • Recreation facilities are the same for all Bonnie Briar development schemes. f _. • SCENARIO: ALTERNATIVE C • Description: Bonnie Briar Development 108 detached single family units 56 attached single family units Affected subbasins = 3,4,7 • Area 3 = 1.13 sq mi Existing CN = 74 Developed CN = 74 • Area 4 = 0.39 sq mi Existing CN = 72 Developed CN = 72 • Area 7 = 0.38 sq mi Existing CN = 71 Developed CN = 71 SUMMARY OF IMPERVIOUS SURFACE AREAS DUE TO DEVELOPMENT AREA SUBBASIN 3 SUBBASIN 4 SUBBASIN 7 No. of Lots 4 attached 52 attached 24 detached 37 detached 47 detached Rooftops 67,500 219,000 72,000 Driveways 44,309 143,759 47,263 Roads 80,600 124,800 88,400 Rec Facilities 0 57,500 0 TOTAL (SQ FT) 192,409 545,059 207,663 ASSUMPTIONS: • Roads are 26 feet wide. • Driveways are proportional to no. of lots in each sub-basin. • Rooftops are 3000 sq ft footprint. • Recreation facilities are the same for all Bonnie Briar development schemes. 4 SCENARIO: CR MODIFIED (FERRANDINO & ASSOS) 4- Description: Bonnie Briar Development 33 detached single family units 18 hole golf course O Affected subbasins = 3,4,7 • Area 3 = 1.13 sq mi Existing CN = 74 Developed CN = 74 • Area 4 = 0.39 sq mi Existing CN = 72 Developed CN = 73 • Area 7 = 0.38 sq mi Existing CN = 71 Developed CN = 71 • SUMMARY OF IMPERVIOUS SURFACE AREAS DUE TO DEVELOPMENT AREA SUBBASIN 3 SUBBASIN 4 SUBBASIN 7 No. of Lots 15 18 0 Rooftops 73,909 88,691 0 Driveways 43,318 51,982 0 Roads 24,936 29,924 0 0 Rec Facilities 0 57,500 0 TOTAL (SQ FT) 142,163 228,097 0 ASSUMPTIONS: • Roads are 26 feet wide. _ • Driveways are proportional to no. of lots in each sub-basin. • Rooftops are 3000 sq ft footprint. • Recreation facilities are the same for all Bonnie Briar development schemes. 4 • .. SHELDRAKE WATERSHED HYDROLOGIC STUDY SUMMARY OF TIME OF CONCENTRATIONS (HOURS) EXISTING CR R30 CR R30 SUBBASIN PREDEVEL TOWNHOUSE TOWNHOUSE DETACHED DETACHED 8 0 . 594 0. 531 0. 531 0 . 475 0 . 283 9 0. 688 0. 688 0. 688 0 . 493 0 . 344 li 41 411 41 • t ) ! i 1. fid. ;1 I I® I I i 41 ` t t i iI ! ! VILLAGE OF MAMARONECK CALCULATION OF RUNOFF IN INCHES SCENARIO: CR TOWNHOUSE/R30 TOWNHOUSE EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED SUBBASIN FREQUENCY CN VALUE CN VALUE S S Ia Ia P (in) RUNOFF (in) RUNOFF (in) 8 1-YR 62 66 6.13 5.15 1.23 1.03 2.6 0.25 0.37 2-YR 62 66 6.13 5.15 1.23 1.03 3.3 0.52 0.69 5-YR 62 66 6.13 5.15 1.23 1.03 4.3 1.03 1.27 10-YR 62 66 6.13 5.15 1.23 1.03 5.0 1.44 1.73 25-YR 62 66 6.13 5.15 1.23 1.03 5.7 1.89 2.22 50-YR 62 66 6.13 5.15 1.23 1.03 6.3 2.30 2.66 100-YR 62 66 6.13 5.15 1.23 1.03 7.2 2.95 3.36 9 1-YR 63 67 5.87 4.93 1.17 0.99 2.6 0.28 0.40 2-YR 63 67 5.87 4.93 1.17 0.99 3.3 0.56 0.74 5-YR 63 67 5.87 4.93 1.17 0.99 4.3 1.09 1.33 10-YR 63 67 5.87 4.93 1.17 0.99 5.0 1.51 1.80 25-YR 63 67 5.87 4.93 1.17 0.99 5.7 1.97 2.31 50-YR 63 67 5.87 4.93 1.17 0.99 6.3 2.39 2.76 100-YR 63 67 5.87 4.93 1.17 0.99 7.2 3.05 3.47 NOTES: 1. CN values from development schematics based on impervious surfaces, ground cover, and soil types. 2. S = (1000/CN)-10 3. Ia = 0.2S • 4. P is precipitation for type III rainfall distribution 5. Runoff = (P - Ia)"2/((P - Ia) + S) from SCS runoff curve method i I I ? L t . ) l I I ) ! 1 I I ' i VILLAGE OF MAMARONECK CALCULATION OF PEAK RUNOFF FLOWS SCENARIO: CR TOWNHOUSE/R30 TOWNHOUSE EXISTING DEVELOPED UNIT PEAK UNIT PEAK EXISTING DEVELOPED AREA EXISTING DEVELOPED EXISTING DEVELOPED DISCHARGE DISCHARGE PEAK RUNOFF PEAK RUNOFF INCREASE IN SUBBASIN FREQUENCY (SQ MI) Ia/P Ia/P Tc (hr) Tc (hr) (csm/in) (csm/in) (cfs) (cfs) PEAK RUNOFF 8 1-YR 0.10 0.47 0.40 0.594 0.531 200.00 265.00 5.03 9.71 4.68 2-YR 0.10 0.37 0.31 0.594 0.531 270.00 330.00 14.16 22.91 8.75 5-YR 0.10 0.29 0.24 0.594 0.531 325.00 350.00 33.37 44.43 11.06 10-YR 0.10 0.25 0.21 0.594 0.531 330.00 360.00 47.47 62.20 14.73 25-YR 0.10 0.22 0.18 0.594 0.531 340.00 370.00 64.19 82.15 17.96 50-YR 0.10 0.19 0.16 0.594 0.531 350.00 375.00 80.44 99.93 19.49 100-YR 0.10 0.17 0.14 0.594 0.531 360.00 380.00 106.16 127.77 21.61 9 1-YR 0.12 0.45 0.38 0.688 0.688 235.00 235.00 7.85 11.24 3.39 2-YR 0.12 0.36 0.30 0.688 0.688 265.00 265.00 17.96 23.54 5.58 5-YR 0.12 0.27 0.23 0.688 0.688 310.00 310.00 40.38 49.61 9.23 10-YR 0.12 0.23 0.20 0.688 0.688 325.00 325.00 58.85 70.32 11.47 25-YR 0.12 0.21 0.17 0.688 0.688 330.00 330.00 77.99 91.32 13.33 50-YR 0.12 0.19 0.16 0.688 0.688 335.00 335.00 96.02 110.89 14.88 100-YR 0.12 0.16 0.14 0.688 0.688 340.00 340.00 124.49 141.46 16.97 NOTES: 1. Time of concentration , T calculated by determining watercourse and using sheet flow, shallow concentrated flow, or open channel flow equations. 2. Unit peak discharge determined by using SCS type III rainfall distribution and inputting Tc and la/P. 3. Total discharge for existing and developed conditions calculated by using graphical peak discharge method. (Peak disharge = unit discharge x area x runoff) I 1 i i i - ) 1 1 i 1 1 1 I Ii VILLAGE OF MAMARONECK CALCULATION OF RUNOFF IN INCHES SCENARIO: CR DETACHED EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED SUBBASIN FREQUENCY CN VALUE CN VALUE S S Ia Ia P (in) RUNOFF (in) RUNOFF (in) 8 1-YR 62 66 6.13 5.15 1.23 1.03 2.6 0.25 0.37 2-YR 62 66 6.13 5.15 1.23 1.03 3.3 0.52 0.69 5-YR 62 66 6.13 5.15 1.23 1.03 4.3 1.03 1.27 10-YR 62 66 6.13 5.15 1.23 1.03 5.0 1.44 1.73 25-YR 62 66 6.13 5.15 1.23 1.03 5.7 1.89 2.22 50-YR 62 66 6.13 5.15 1.23 1.03 6.3 2.30 2.66 100-YR 62 66 6.13 5.15 1.23 1.03 7.2 2.95 3.36 9 1-YR 63 67 5.87 4.93 1.17 0.99 2.6 0.28 0.40 2-YR 63 67 5.87 4.93 1.17 0.99 3.3 0.56 0.74 5-YR 63 67 5.87 4.93 1.17 0.99 4.3 1.09 1.33 10-YR 63 67 5.87 4.93 1.17 0.99 5.0 1.51 1.80 25-YR 63 67 5.87 4.93 1.17 0.99 5.7 1.97 2.31 50-YR 63 67 5.87 4.93 1.17 0.99 6.3 2.39 2.76 100-YR 63 67 5.87 4.93 1.17 0.99 7.2 3.05 3.47 NOTES: 1. CN values from development schematics based on impervious surfaces, ground cover, and soil types. 2. S = (1000/CN)-10 3. Ia = 0.2S 4. P is precipitation for type III rainfall distribution 5. Runoff = (P - Ia)-2/((P - Ia) + S) from SCS runoff curve method i I. ) i I ) 1 i f i 1 VILLAGE OF MAMARONECK CALCULATION OF PEAK RUNOFF FLOWS SCENARIO: CR DETACHED EXISTING DEVELOPED UNIT PEAK UNIT PEAK EXISTING DEVELOPED AREA EXISTING DEVELOPED EXISTING DEVELOPED DISCHARGE DISCHARGE PEAK RUNOFF PEAK RUNOFF INCREASE IN SUBBASIN FREQUENCY (SQ MI) Ia/P Ia/P Tc (hr) Tc (hr) (csm/in) (csm/in) (cfs) (cfs) PEAK RUNOFF 8 1-YR 0.10 0.47 0.40 0.594 0.475 200.00 280.00 5.03 10.26 5.23 2-YR 0.10 0.37 0.31 0.594 0.475 270.00 340.00 14.16 23.60 9.44 5-YR 0.10 0.29 0.24 0.594 0.475 325.00 365.00 33.37 46.34 12.96 10-YR 0.10 0.25 0.21 0.594 0.475 330.00 375.00 47.47 64.79 17.32 25-YR 0.10 0.22 0.18 0.594 0.475 340.00 380.00 64.19 84.37 20.18 50-YR 0.10 0.19 0.16 0.594 0.475 350.00 390.00 80.44 103.92 23.49 100-YR 0.10 0.17 0.14 0.594 0.475 360.00 400.00 106.16 134.49 28.33 9 1-YR 0.12 0.45 0.38 0.688 0.493 235.00 245.00 7.85 11.72 3.87 2-YR 0.12 0.36 0.30 0.688 0.493 265.00 300.00 17.96 26.65 8.69 5-YR 0.12 0.27 0.23 0.688 0.493 310.00 320.00 40.38 51.21 10.83 10-YR 0.12 0.23 0.20 0.688 0.493 325.00 325.00 58.85 70.32 11.47 25-YR 0.12 0.21 0.17 0.688 0.493 330.00 330.00 77.99 91.32 13.33 50-YR 0.12 0.19 0.16 0.688 0.493 335.00 335.00 96.02 110.89 14.88 100-YR 0.12 0.16 0.14 0.688 0.493 340.00 340.00 124.49 141.46 16.97 NOTES: 1. Time of concentration , T calculated by determining watercourse and using sheet flow, shallow concentrated flow, or open channel flow equations. 2. Unit peak discharge determined by using SCS type III rainfall distribution and inputting Tc and Ia/P. 3. Total discharge for existing and developed conditions calculated by using graphical peak discharge method. (Peak disharge = unit discharge x area x runoff) ( ! r ( I t t 1 I I VILLAGE OF MAMARONECK CALCULATION OF RUNOFF IN INCHES SCENARIO: R30 DETACHED EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED EXISTING DEVELOPED SUBBASIN FREQUENCY CN VALUE CN VALUE S S Ia Ia P (in) RUNOFF (in) RUNOFF (in) 8 1-YR 62 66 6.13 5.15 1.23 1.03 2.6 0.25 0.37 2-YR 62 66 6.13 5.15 1.23 1.03 3.3 0.52 0.69 5-YR 62 66 6.13 5.15 1.23 1.03 4.3 1.03 1.27 10-YR 62 66 6.13 5.15 1.23 1.03 5.0 1.44 1.73 . 25-YR 62 66 6.13 5.15 1.23 1.03 5.7 1.89 2.22 50-YR 62 66 6.13 5.15 1.23 1.03 6.3 2.30 2.66 100-YR 62 66 6.13 5.15 1.23 1.03 7.2 2.95 3.36 9 1-YR 63 67 5.87 4.93 1.17 0.99 2.6 0.28 0.40 2-YR 63 67 5.87 4.93 1.17 0.99 3.3 0.56 0.74 5-YR 63 67 5.87 4.93 1.17 0.99 4.3 1.09 1.33 10-YR 63 67 5.87 4.93 1.17 0.99 5.0 1.51 1.80 25-YR 63 67 5.87 4.93 1.17 0.99 5.7 1.97 2.31 50-YR 63 67 5.87 4.93 1.17 0.99 6.3 2.39 2.76 100-YR 63 67 5.87 4.93 1.17 0.99 7.2 3.05 3.47 NOTES: 1. CN values from development schematics based on impervious surfaces, ground cover, and soil types. 2. S = (1000/CN)-10 3. Ia = 0.2S 4. P is precipitation for type III rainfall distribution 5. Runoff = (P - Ia)-2/((P - Ia) + S) from SCS runoff curve method 1 ) ; 1 i Ii 1 1 I ) L ) ) i VILLAGE OF MAMARONECK CALCULATION OF PEAK RUNOFF FLOWS SCENARIO: R30 DETACHED EXISTING DEVELOPED UNIT PEAK UNIT PEAK EXISTING DEVELOPED AREA EXISTING DEVELOPED EXISTING DEVELOPED DISCHARGE DISCHARGE PEAK RUNOFF PEAK RUNOFF INCREASE IN SUBBASIN FREQUENCY (SQ MI) Ia/P Ia/P Tc (hr) Tc (hr) (csm/in) (csm/in) (cfs) (cfs) PEAK RUNOFF 8 1-YR 0.10 0.47 0.40 0.594 0.531 200.00 340.00 5.03 12.46 7.43 2-YR 0.10 0.37 0.31 0.594 0.531 270.00 425.00 14.16 29.50 15.34 5-YR 0.10 0.29 0.24 0.594 0.531 325.00 450.00 33.37 57.13 23.75 10-YR 0.10 0.25 0.21 0.594 0.531 330.00 460.00 47.47 79.47 32.01 25-YR 0.10 0.22 0.18 0.594 0.531 340.00 470.00 64.19 104.35 40.16 50-YR 0.10 0.19 0.16 0.594 0.531 350.00 475.00 80.44 126.57 46.14 100-YR 0.10 0.17 0.14 0.594 0.531 360.00 480.00 106.16 161.39 55.23 9 1-YR 0.12 0.45 0.38 0.688 0.688 235.00 350.00 7.85 16.75 8.90 2-YR 0.12 0.36 0.30 0.688 0.688 265.00 400.00 17.96 35.53 17.57 5-YR 0.12 0.27 0.23 0.688 0.688 310.00 425.00 40.38 68.01 27.63 10-YR 0.12 0.23 0.20 0.688 0.688 325.00 440.00 58.85 95.20 36.35 25-YR 0.12 0.21 0.17 0.688 0.688 330.00 450.00 77.99 124.52 46.53 50-YR 0.12 0.19 0.16 0.688 0.688 335.00 455.00 96.02 150.62 54.60 100-YR 0.12 0.16 0.14 0.688 0.688 340.00 460.00 124.49 191.39 66.90 NOTES: 1. Time of concentration , T calculated by determining watercourse and using sheet flow, shallow concentrated flow, or open channel flow equations. 2. Unit peak discharge determined by using SCS type III rainfall distribution and inputting Tc and Ia/P. 3. Total discharge for existing and developed conditions calculated by using graphical peak discharge method. (Peak disharge = unit discharge x area x runoff) 1 ! ' I �. _ . ) / v ? , I i I ! I ! I VILLAGE OF MAMARONECK SUMMARY OF PEAK RUNOFF FLOWS SCENARIO: CR TOWNHOUSE AND R30 TOWNHOUSE (2) (1) STORM INCREASE IN SUBBASIN 8 SUBBASIN 9 EXISTING TOTAL FLOW INCREASE FREQUENCY SHELDRAKE AT (CFS) (CFS) MAM RIVER IN MAMARONECK RIVER (YEARS) 1-95 (CFS) EXISTING DEVELOP EXISTING DEVELOP FLOWS(CFS) CFS PERCENT 5 9 33 44 40 50 2400 29 1 10 9 47 62 59 70 3020 35 1 25 10 64 82 78 91 4000 41 1 50 11 80 100 96 111 5000 45 1 100 13 106 128 124 141 6000 52 1 NOTES: (1) See MPI Report, Appendix 5-5 of DGEIS (2) Reference: U.S. Army Engineer District, New York, "Westchester County Streams, Sheldrake River, Flood Control Project" (May 1991) * Flow increase in subbasins 8 and 9 developed using TR-55 Methodology. * Flow increases from alternative B and CR Modified developments are most similar to increases from CR Townhouse and R30 Townhouse. i i i ( t ! ! 1 t ) 4 i I i VILLAGE OF MAMARONECK SUMMARY OF PEAK RUNOFF FLOWS SCENARIO: CR DETACHED (1) (2) STORM INCREASE IN SUBBASIN 8 SUBBASIN 9 EXISTING TOTAL FLOW INCREASE FREQUENCY SHELDRAKE AT (CFS) (CFS) MAM RIVER IN MAMARONECK RIVER (YEARS) 1-95 (CFS) EXISTING DEVELOP EXISTING DEVELOP FLOWS(CFS) CFS PERCENT 5 9 33 46 40 51 2400 33 1.4 10 9 47 65 59 70 3020 38 1.3 25 10 64 84 78 91 4000 44 1.1 50 11 80 104 96 111 5000 49 1.0 100 13 106 134 124 141 6000 58 1.0 NOTES: (1) See MPI Report, Appendix 5-5 of DGEIS. (2) Reference: U.S. Army Engineer District, New York, "Westchester County Streams. Sheldrake River. Flood Control Project" (May 1991). * Flow increase in subbasins 8 and 9 developed using TR-55 Methodology. * Flow increases from alternative A and C developments are between increases from CR Detached and R30 Detached. VILLAGE OF MAMARONECK SUMMARY OF PEAK RUNOFF FLOWS SCENARIO: R30 DETACHED (1) (2) STORM INCREASE IN SUBBASIN 8 SUBBASIN 9 EXISTING TOTAL FLOW INCREASE FREQUENCY SHELDRAKE AT FLOWS (CFS) FLOWS (CFS) MAM RIVER IN MAMARONECK RIVER (YEARS) 1-95 (CFS) EXISTING DEVELOP EXISTING DEVELOP FLOWS(CFS) CFS PERCENT 5 18 33 57 40 68 2400 69 2.9 10 21 47 79 59 95 3020 89 3.0 25 22 64 104 78 125 4000 109 2.7 50 23 80 127 96 151 5000 124 2.5 100 26 106 161 124 191 6000 148 2.5 NOTES: (1) See MPI Report, Appendix 5-5 of DGEIS. (2) Reference: U.S. Army Engineer District, New York, "Westchester County Streams, Sheldrake River, Flood Control Project" (May 1991) * Flow increase in subbasins 8 and 9 developed using TR-55 Methodology. * Flow increases from alternative A and C developments are between increases from CR Detached and R30 Detached.