6.1. INTRODUCTION 6.2. EXISTING CONDITIONS

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1 Chapter 6: Stormwater Management 6.1. INTRODUCTION This Chapter describes the current drainage patterns on the Project Site, the stormwater management system that is proposed as part of the Proposed Project, and analyzes potential impacts stormwater flow as a result of the Proposed Project. As such, these analyses address the potential for the Proposed Action to have one or more significant adverse environmental impacts that were identified in the Lead Agency s Positive Declaration (see Appendix A-5). The Project Site generally slopes, and surface water runoff generally flows, from north to south. An existing detention basin (also identified as Wetland D) is located along the east side of the Project Site. This basin receives stormwater runoff from the existing building and parking lot, as well as runoff from adjacent Village properties to the east. Stormwater runoff exits the detention basin and is discharged to the municipal drainage system below Arbor Drive, through Harkness Park, and then along the Blind Brook High School driveway to King Street. There are two easements onto the Project Site containing storm drain lines that drain into the existing detention basin from the Rye Brook Village Hall, Rye Brook Police Department, and Rye Brook Fire Department (RBFD) property. The Proposed Project would result in a decrease in the amount of impervious area on the Project Site from the current condition. In order to manage stormwater runoff from the Proposed Project, the Applicant s engineer prepared a Stormwater Pollution Prevention Plan (SWPPP) in accordance with Chapter 9: Redevelopment Projects of the New York State Stormwater Management Design Manual and Chapter 217, Stormwater Management of the Village of Rye Brook (the Village ) Code (see Appendix C-1). The stormwater management system for the Proposed Project includes standard stormwater practices, including vegetated swales, subsurface infiltration systems, and improvements to the existing stormwater detention basin. To accommodate an increase in the Site s main drainage as a result of proposed Site grading and layout, the existing stormwater detention basin would be slightly expanded. The Project Site does not currently have any known stormwater practices with infiltration to provide water quality and runoff reduction. The SWPPP for the Proposed Project, however, includes practices that enhance water quality and provide runoff reduction volume through infiltration. Infiltration measures include the grasscrete paver emergency drive, vegetated swales, and disconnected impervious areas throughout the Site. As demonstrated in the SWPPP, the stormwater design of the Proposed Project would result in a reduction in both the rate and volume of stormwater exiting the Site for each modeled storm event when compared to the existing condition EXISTING CONDITIONS The Project Site, which is located in the Coastal Long Island Sound Drainage Basin, generally slopes, and surface water runoff generally flows, from north to south. An existing detention basin (also identified as Wetland D) is located along the east side of the Project Site. This basin receives stormwater runoff from the existing building and parking lot, as well as runoff from adjacent Village properties to the east. Stormwater runoff exits the detention basin and is discharged to the municipal drainage system within Arbor Drive, through Harkness Park, and then along the Blind Brook High School driveway to King Street. There are two easements onto the Project Site DEIS 6-1 9/12/2018

2 900 King Street Redevelopment containing storm drain lines that drain into the existing detention basin from the Village Hall, RBPD, and RBFD property. Along the northern property line, two storm drain lines convey runoff from the Hutchinson River Parkway (the Parkway ) onto the Project Site. All stormwater runoff from these outlets was included in all stormwater calculations. Two Design Points have been designated on the Site, DP-1 and DP-2. Each design point conveys the stormwater from one of the Site s Existing Drainage Areas (EDA s) (see Figure 6-1). DP-1 is located to the southeast of the Site and conveys stormwater runoff from EDA-1. EDA-1 includes the eastern portion of the Project Site, as well as portions of the Parkway and the Village-owned lots to the east of the Site. With the exception of a small area of the Site along Arbor Drive, stormwater from the Project Site is conveyed via an on-site pipe network into the existing detention basin. From the detention basin, stormwater flows into Stream S and under Arbor Drive through an existing 24- inch reinforced concrete pipe underneath Arbor Drive and through DP-1. The existing 24-inch pipe under Arbor Drive has a capacity to convey a maximum of cubic feet per second (cfs) (see Appendix C-2). Two off-site areas that include the Parkway and residential areas to the north of the Site, noted as EDA-1F and the bypass area on Figure 6-1, convey stormwater runoff through the Project Site in a separate piped network and discharge directly to Stream S prior to passing through the existing 24-inch pipe underneath Arbor Drive and through DP-1. DP-2 is located in the southwest corner of the Project Site and collects stormwater runoff from EDA- 2, which is made up primarily of the western portion of the Project Site, as well as off-site areas to the north and portions of the townhouse complex to the west. EDA-2 drains west into a small watercourse, Stream A, and then into a 15 inch diameter pipe at the southwestern corner of the site, which is designated as DP-2. The pipe is part of the Village s drainage conveyance system that runs beneath Arbor Drive and towards the adjacent school. The peak rates of runoff to each Design Point for each modeled storm event are shown in Table 6-1. The volume of runoff to each Design Point from each modeled storm event is shown in Table 6-2. Table 6-1 Existing Peak Runoff Rates Storm Recurrence Interval DP-1 (cfs) DP-2 (cfs) 1 year year year year year year Notes: cfs = cubic feet per second 9/12/ DEIS

3 Chapter 6: Stormwater Management Table 6-2 Existing Runoff Volume Storm Recurrence Interval DP-1 (cf) DP-2 (cf) 1 year 187,631 14,536 2 year 265,706 21, year 495,031 46, year 680,786 67, year 856,809 87, year 1,067, ,019 Notes: cf = cubic feet 6.3. POTENTIAL IMPACTS OF THE PROPOSED PROJECT The Proposed Project would redevelop the Project Site into an age-restricted residential community. The redevelopment would result in a decrease in the amount of impervious area on the Project Site. In order to manage stormwater runoff from the Proposed Project, the Applicant s engineer prepared a SWPPP in accordance with Chapter 9: Redevelopment Projects of the New York State Stormwater Management Design Manual and Chapter 217, Stormwater Management of the Village Code (see Appendix C-1). The stormwater management system for the Proposed Project includes standard stormwater practices, including vegetated swales, subsurface infiltration systems, and improvements to the existing stormwater detention basin. To accommodate an increase in the Site s main drainage as a result of proposed Site grading and layout, the existing stormwater detention basin would be slightly expanded. In addition to this slight expansion, the Proposed Project would enhance the functionality of this basin area by removing overgrown and dead vegetation, debris, etc. The Project Site does not currently have any known stormwater practices with infiltration to provide water quality and runoff reduction. The SWPPP for the Proposed Project, however, includes practices that enhance water quality and provide runoff reduction volume through infiltration. Infiltration measures include the grasscrete paver emergency drive, vegetated swales, and disconnected impervious areas throughout the Site. These practices will result in additional infiltration that was not considered in the SWPPP s hydrologic model, resulting in a conservative analysis presented in the SWPPP. Two Design Points, which are the same as the existing condition design points, were designated on- Site and convey water from the Site s two Proposed Drainage Areas (PDA s) (see Figure 6-2). In general, PDA-1 includes the developed portion of the Site, similar to the existing condition, and the same off-site areas and bypass. PDA-2 would continue to serve the western, undeveloped portion of the Site. As with the existing condition, stormwater from the developed portion of the Site (i.e., PDA-1) would flow through a piped network into the existing detention basin with the Proposed Project. However, several underground infiltration systems would be installed to promote infiltration and improve water quality 1. The stormwater generated from off-site locations that pass through the Project Site would be accommodated with the Proposed Project in the same manner as present. 1 Soil testing has been performed on-site. Soil testing for the infiltration practice located within the existing building will be performed when the site design is advanced and the results would be provided during the site plan approval process. DEIS 6-3 9/12/2018

4 900 King Street Redevelopment As with the existing condition, DP-2 would collect stormwater runoff from the western, undeveloped, portion of the Project Site, shown as PDA-2 in Figure 6-2. PDA-2 is approximately one acre smaller than EDA-2 due to slight changes in the grade in the adjacent area. As demonstrated in the SWPPP, the stormwater design of the Proposed Project would result in a reduction in both the rate and volume of stormwater exiting the Site for each modeled storm event when compared to the existing condition. Table 6-3 and Table 6-4 summarize the reductions. Table 6-3 Proposed Peak Runoff Rate Design Point Storm Recurrence Interval Existing Peak Runoff Rate (cfs) Proposed Peak Runoff Rate (cfs) Percent Reduction (%) 1 year year year year year year year year year year year year Notes: cfs = cubic feet per second Table 6-4 Proposed Runoff Volume Design Point Storm Recurrence Interval Existing Peak Runoff Rate (cf) Proposed Peak Runoff Rate (cf) Percent Reduction (%) 1 year 187, , year 265, , year 495, , year 680, , year 856, , year 1,067,613 1,067, year 14,536 12, year 21,963 18, year 46,072 39, year 67,072 56, year 87,696 73, year 113,019 94, Notes: cf = cubic feet The existing downstream drainage infrastructure accepting stormwater run-off from the site is comprised of two separate systems. The first system conveys stormwater run-off exiting the site from the southeastern area where Stream S flows beneath Arbor Drive via an existing 24-inch reinforced concrete pipe (RCP), (Design Point No.1). From this point the system continues south underground through Harkness Park. As noted above, the existing 24-inch pipe under Arbor Drive 9/12/ DEIS

5 Chapter 6: Stormwater Management has a maximum capacity of cfs. During periods of peak flows (e.g., the 50- and 100-year storms), the maximum flow rate through the 24-inch pipe would be cfs and cfs, respectively, which is a reduction from the current maximum flow rate (see Appendix C-2). Therefore, the existing pipe under Arbor Drive has the capacity to accommodate the projected stormwater flow from the Proposed Project MITIGATION MEASURES As summarized above, and presented in more detail in the SWPPP in Appendix C-1, the Proposed Project utilizes a variety of practices to enhance stormwater quality and reduce peak rates of runoff associated with the Proposed Project. With the implementation of the SWPPP, runoff volumes would be reduced in all the analyzed storms from the existing condition. As implementation of the SWPPP would provide water quantity and quality enhancements that exceed the regulatory requirements, stormwater runoff from the Proposed Project is not anticipated to have a significant adverse impact to the Project Site or downstream areas. DEIS 6-5 9/12/2018

6 COPYRIGHT 2017 by JMC All Rights Reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted in any form or by means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of JMC PLANNING, ENGINEERING, LANDSCAPE ARCHITECTURE & LAND SURVEYING, PLLC JMC SITE DEVELOPMENT CONSULTANTS, LLC JOHN MEYER CONSULTING, INC. (JMC). Any modifications or alterations to this document without the written permission of JMC shall render them invalid and unusable. Source: JMC Engineering No. Revision Date By KING STREET ANY ALTERATION OF PLANS, SPECIFICATIONS, PLATS AND REPORTS BEARING THE SEAL OF A LICENSED PROFESSIONAL ENGINEER OR LICENSED LAND SURVEYOR IS A VIOLATION OF SECTION 7209 OF THE NEW YORK STATE EDUCATION LAW, EXCEPT AS PROVIDED FOR BY SECTION 7209, SUBSECTION 2. Drawing: DRAINAGE Date: Time: 10:49 AM By: PROGRESS PLOTTING Scale: Drawn: Date: Project No: Approved: Drawing No: Existing Drainage Areas Figure 6-1

7 COPYRIGHT 2017 by JMC All Rights Reserved. No part of this document may be reproduced, stored in a retrieval system, or transmitted in any form or by means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of JMC PLANNING, ENGINEERING, LANDSCAPE ARCHITECTURE & LAND SURVEYING, PLLC JMC SITE DEVELOPMENT CONSULTANTS, LLC JOHN MEYER CONSULTING, INC. (JMC). Any modifications or alterations to this document without the written permission of JMC shall render them invalid and unusable. Source: JMC Engineering No. Revision Date By KING STREET ANY ALTERATION OF PLANS, SPECIFICATIONS, PLATS AND REPORTS BEARING THE SEAL OF A LICENSED PROFESSIONAL ENGINEER OR LICENSED LAND SURVEYOR IS A VIOLATION OF SECTION 7209 OF THE NEW YORK STATE EDUCATION LAW, EXCEPT AS PROVIDED FOR BY SECTION 7209, SUBSECTION 2. Drawing: DRAINAGE Date: Time: 10:46 AM By: PROGRESS PLOTTING Scale: Drawn: Date: Project No: Approved: Drawing No: Proposed Drainage Areas Figure 6-2