6.1. INTRODUCTION AND SUMMARY OF FINDINGS

Transcription

1 Chapter 6: Stormwater Management 6.1. INTRODUCTION AND SUMMARY OF FINDINGS A Stormwater Pollution Prevention Plan (SWPPP) has been prepared for the Proposed Project in accordance with the requirements of the City of White Plains and the New York State Department of Environmental Conservation (NYSDEC) State Pollutant Discharge Elimination System (SPDES) General Permit for Stormwater Discharges from Construction Activity (Permit No. GP ) (see Appendix C). The SWPPP calculates the existing rate and volume of stormwater runoff from the Project Site and designs a program for reducing the rate and volume of stormwater runoff from the Project Site with the construction of the Proposed Project, while also improving stormwater quality. Construction of the Proposed Project would result in an increase of 2.99 acres of buildings and other impervious areas on the Project Site (from 4.51 acres in the current condition to 7.50 acres in the proposed condition). As noted in Chapter 3, Land Use, Zoning, and Public Policy, this is less than the acres of impervious surface that is allowed under the Site s current zoning. To mitigate the potential for increased stormwater runoff from the increase in impervious surface with the Proposed Project, a system of stormwater management practices has been incorporated into the Proposed Project. Specifically, the Proposed Project would include more than 10,000 square feet (sf) of green roofs, a 30,000 gallon cistern that would collect rooftop runoff for use as Site irrigation, and a series of subsurface detention and infiltration systems. Together, these systems would meet the requirement of both the City of White Plains and the NYSDEC. Both the rate and volume of stormwater runoff from the Project Site would be reduced from the current condition in the Future with the Proposed Project in each of the modeled storm events. Therefore, stormwater discharge from the Proposed Project would not result in an adverse impact EXISTING CONDITIONS The Site is located within the Mamaroneck River drainage basin, which is within the Lower Long Island Sound drainage basin. The existing on-site drainage system consists of a network of underground stormwater pipes that discharge into three distinct points, each of which ultimately discharge into the City of White Plains drainage system. The three design points (DP) were identified for the purpose of comparing peak rates and volumes of runoff from the Project Site in the existing and proposed conditions. The area of the Project Site contributing stormwater runoff to each DP is identified in Figure 6-1 as the Site s existing drainage areas (EDA). DP-1 is located at the southwest corner of the Site. Drainage, either overland or through existing pipes, from EDA-1 flows through this point. EDA-1 is 4.92 acres, which is 31 percent of the Site. This area consists of pavement, grass, trees, the existing Mapleton building, and a portion of the Chapel. EDA-1 is bordered by North Broadway to the west, Elisabeth Haub School of Law at Pace University ( Pace Law School ) to the north, and the Convent to the east. DEIS 6-1 5/7/2018

2 52 North Broadway Redevelopment DEIS DP-2 is located at an existing catch basin on the north side of Stewart Place. Drainage, either overland or through existing pipes, from EDA-2 flows through this point. EDA-2 is approximately 4.91 acres, which is 31 percent of the Site. DP-3 is located at an existing catch basin on the north side of Ross Street. Drainage, either overland or through existing pipes, from EDA-3 flows through this point. EDA-3 is approximately 6.27 acres, which is 39 percent of the Site. The peak rates of runoff to each DP for each modeled storm event are shown in Table 6-1. Table 6-1 Existing Peak Runoff Rates Storm recurrence interval DP-1 DP-2 DP-3 1 year year year year year Note: Rates are given in cubic feet per second. The volume of runoff to each DP from each modeled storm event is shown in Table 6-2. Table 6-2 Existing Runoff Volume Storm recurrence interval DP-1 DP-2 DP-3 Total Site volume 1 year 11,966 15,858 18,316 46, year 39,283 45,857 54, , year 57,573 65,189 78, , year 75,445 83, , , year 97, , , ,714 Note: Volume is presented in cubic feet FUTURE WITHOUT THE PROPOSED PROJECT There are no known changes to drainage patterns, systems, or rates that are anticipated to occur in the Future without the Proposed Project POTENTIAL IMPACTS OF THE PROPOSED PROJECT AND MITIGATION MEASURES The preliminary SWPPP prepared for the Proposed Project (see Appendix C) includes an Erosion and Sediment Control Plan (ESCP), which would mitigate the potential adverse impact from stormwater runoff during construction of the Proposed Project. The ESCP is discussed in more detail in Chapter 17, Construction. During the Proposed Project s operation (i.e., after the Proposed Project is constructed), stormwater runoff from the impervious surfaces within the Proposed Project, including roadways, parking areas, driveways, and sidewalks, would be collected by catch basins and conveyed by a network of high density polyethylene (HDPE) drain pipe installed underground that would discharge to the proposed stormwater management systems. Catch basins and drain 5/7/ DEIS

3 Chapter 6: Stormwater Management inlets will be marked with a label that reads Dump No Waste-Drains to Local Waterway. Runoff from a portion of the buildings rooftops would be collected into roof drains that would discharge into stormwater practices, including green roofs, a rainwater cistern, or subsurface infiltration systems that treat stormwater. From these systems, stormwater would enter underground large diameter detention storm pipes or chambers before ultimately being discharged to the City of White Plains stormwater system. As with existing conditions, a portion of the stormwater runoff from the Site would continue to flow undetailed off-site. Though not required, the stormwater management program provides more than the minimum runoff reduction and incorporates green practices into the Site s design. To determine the rates of runoff generated by the Proposed Project, post-development drainage areas (DA) were determined, as described below and shown in Figure 6-2. DA-1 would include several detention and infiltration systems that would attenuate peak flows during storm events and improve the resultant quality of stormwater that is discharged to the City of White Plains stormwater system. As part of that system, a 30,000 gallon cistern would be constructed under the parking lot for the proposed assisted-living facility. The cistern would capture runoff from the roof of the assisted-living facility that would be used for irrigation of the Site s front lawn and other landscaping. Any flows exceeding the capacity of the cistern would flow into a detention system, followed by an infiltration system, followed by another detention system before being discharged at DP-1. DA-2, which would include the academic housing building, the relocated Mapleton building, the parking lot for the assisted-living facility, and the majority of the Main Site Driveway and porte cochere for the multi-family residential building, would continue to discharge to DP-2. Runoff from several areas within DA-2 would be conveyed to two connected infiltration systems, which would attenuate the volume and rate of stormwater flow to DP-2. DA-3, which would include the multi-family residential buildings and the area to the east of the parking area for the proposed assisted-living facility, would include several green infrastructure practices. Specifically, more than 10,000 sf of green roof would be installed on the proposed multi-family residential buildings. Additional stormwater attenuation would be provided by detention and infiltration systems. As a result of the stormwater design included in the Proposed Project, both the rate of stormwater exiting the Site and the volume of stormwater exiting the Site would be reduced from the existing condition for each modeled storm event. Table 6-3 and Table 6-4 summarize the reductions. The preliminary SWPPP prepared for the Proposed Project, which was summarized above, meets the requirements of the City of White Plains and the NYSDEC (see Appendix C). In addition to improving stormwater quality, the SWPPP demonstrates that with the Proposed Project, both the peak rate and total volume of stormwater runoff from the Project Site would decrease from the current condition. Therefore, the Proposed Project would not have an adverse impact as a result of stormwater runoff. DEIS 6-3 5/7/2018

4 52 North Broadway Redevelopment DEIS Table 6-3 Peak Runoff Rate Comparison DP Storm recurrence interval Existing peak runoff rate Proposed peak runoff rate Percent reduction 1 year % 10 year % 1 25 year % 50 year % 100 year % 1 year % 10 year % 2 25 year % 50 year % 100 year % 1 year % 10 year % 3 25 year % 50 year % 100 year % Note: Peak runoff rates are presented in cubic feet per second. Table 6-4 Runoff Volume Comparison Storm recurrence interval Total existing volume Total proposed volume Percent reduction 1 year 46,140 37, % 10 year 139, , % 25 year 201, , % 50 year 260, , % 100 Year 334, , % Note: Volume is presented in cubic feet. 5/7/ DEIS

5 Source: JMC Engineering 52 North Broadway Redevelopment DEIS Existing Drainage Areas Figure 6-1

6 Source: JMC Engineering 52 North Broadway Redevelopment DEIS Proposed Drainage Areas Figure 6-2