APPENDIX APPENDIX 6 STORMWATER MANAGEMENT REPORT
ECHO BAY NEW ROCHELLE, NEW YORK STORMWATER MANAGEMENT REPORT Prepared for: Forest City Residential Group 1 Metro Tech Center North Brooklyn, NY 11201 Prepared by: Divney Tung Schwalbe, LLP One North Broadway, Suite 1407 White Plains, NY 10601 August 31, 2012 Revised January 4, 2013
ECHO BAY NEW ROCHELLE, NEW YORK STORMWATER MANAGEMENT REPORT TABLE OF CONTENTS PAGE NO. EXECUTIVE SUMMARY I. SITE PLANNING...I-1 A. Site Location...I-1 B. Existing Site Conditions...I-1 C. Redevelopment Planning Strategies...I-4 D. Soil Types...I-4 II. WATER QUALITY TREATMENT VOLUME (WQv)...II-1 A. Redevelopment WQv Methodology...II-1 B. Redevelopment WQv Calculations...II-2 III. RUNOFF REDUCTION (RRv)...III-1 A. Redevelopment Requirements for RRv...III-1 B. RRv Strategy...III-1 IV. WATER QUANTITY CONTROL...IV-1 V. EROSION & SEDIMENT CONTROL...V-1 A. Temporary Erosion & Sediment Control Measures...V-1 B. Permanent Measures...V-2
ECHO BAY NEW ROCHELLE, NEW YORK STORMWATER MANAGEMENT REPORT LIST OF TABLES TABLE No. PAGE NO. 1 Existing Drainage Conditions... II-3 2 Redeveloped Drainage Conditions... II-4 3 Redevelopment Stormwater Requirements... II-5 4 Stormwater Quality Management Measures... II-6 5 Stormwater Quantity Design Flow Summary... IV-2 LIST OF FIGURES FIGURE No. PAGE NO. SWM-1 Site Location Aerial Photograph... I-2 SWM-2 Existing Drainage Conditions Map... I-3 SWM-3 Proposed Drainage Conditions Map... I-6
Executive Summary This Stormwater Management Report has been prepared to address changes to the stormwater characteristics within the City of New Rochelle that would result from the redevelopment of two parcels of publically owned property located along the Long Island Sound. Stormwater management requirements of the proposed redevelopment for the 9.44-acre site will be evaluated and quantified in terms of stormwater runoff quantity and quality associated with on-site stormwater discharge. The stormwater management design of the redeveloped facility adheres to a five step process outlined by the New York State Department of Environmental Conservation s (NYSDEC) New York State Stormwater Management Design Manual (NYSSMDM), dated August 2010. The following report is a summary of the strategies employed to redevelop the existing site while meeting the stormwater regulations and requirements included in the New York State Department of Environmental Conservation (NYSDEC) SPDES General Permit GP-0-10-001. Through the use of proposed site planning methods, reduction in impervious surfaces, and alternative stormwater management practices (SMPs) at the redeveloped site, the proposed stormwater peak rates of stormwater runoff are estimated to be below existing conditions for each 24-hour storm event up to and including the 100- storm event. In addition, the total runoff volume from the redeveloped site will be reduced due to a reduction in impervious area and the addition of new planting areas to promote infiltration into the ground. All stormwater runoff will eventually discharge to the Long Island Sound flowing either overland, through three (3) new proposed outfalls, or via the existing municipal storm drain system within the Main Street/US Rt. 1 right-of-way.
I. Site Planning A. Site Location The Echo Bay Center site is located on the south side of the intersection of Main Street and Huguenot Street in the City of New Rochelle, Westchester County, New York. The Project Site is bounded to the north by US Rt. 1 (Main Street), to the east by commercial structures and the Westchester County Wastewater Treatment Plant, to the west by commercial development, to the southwest across Echo Bay (Sutton Manor) a single family residential neighborhood, and to the south by Echo Bay and Long Island Sound. The majority of the site currently drains overland directly to the Long Island Sound, while the northwestern portion of the site drains to the storm sewer along US Rt. 1, before discharging to the Long Island Sound. Refer to the attached Site Location Aerial Photograph, Figure SWM-1 for an aerial view of the site and its surrounding areas. B. Existing Site Conditions: The Project Site consists of two (2) parcels totaling 9.44 acres and includes a number of buildings associated with the New Rochelle Armory and the City of New Rochelle Department of Public Works. The majority of the site is covered by buildings, asphalt pavement, or gravel. There are limited lawn areas and trees within the site. An Existing Drainage Conditions Map, Figure SWM-2, has been prepared to identify the existing watershed areas, drainage patterns, and discharge points of the Project Site. The project site is divided into two separate sub-watersheds: sub-watershed A and sub-watershed B. The project site drains to the southeast to the Long Island Sound (Design Point DP-1), and to the northwest to US Rt. 1 (Design Point DP-2). Design Point DP-1 discharges directly to the Long Island Sound, and Design Point DP-2 discharges to the municipal stormwater sewer along US Rt. 1 before discharging into the Long Island Sound. The entire existing study watershed, approximately 9.44 acres in size, includes approximately 6.69 acres of impervious surfaces which accounts for more than 70% of the study area. I-1
C. Redevelopment Planning Strategies: One of the redevelopment goals is to improve stormwater infiltration through new pervious areas consisting of planting areas, lawns, and low gradient slopes. A new mixed-use building, approximately 350,000 square feet, will be constructed on the site of the existing City of New Rochelle Department of Public Works maintenance yard. The new building will include residential apartments and a small amount of retail space facing Main Street. New publically accessible uses and waterfront open space improvements, including a waterfront walk with future links to adjoining parcels, planting and lawn areas, bulkhead repairs, a kayak dock, a walk-in ramp, sitting and activity areas along the Echo Bay shoreline, and public parking will be constructed along the shoreline. The existing Armory Building is planned to be maintained and renovated into other uses by the city and others. Several outbuildings and paved/gravel yard areas will be removed as part of the overall demolition and site preparation. In addition to careful site planning strategies to improve infiltration, stormwater management measures have been proposed to improve stormwater runoff conditions and maintain the pre-construction hydrology of the site. As shown on the Proposed Drainage Conditions Map, Figure SWM-3, the reduction in impervious cover on the site and installation of the hydrodynamic separator on the former Armory parcel to remove sediment and pollutants from the stormwater will significantly improve the overall water quality of the stormwater runoff from the site by providing infiltration and an increased travel time allowing for sediment removal and nutrient uptake into the planted regime. There are now three proposed watersheds: AA-1 (includes 3 new outfalls), AA-2, and BB. Sub-watersheds AA-1 and AA-2 both drain to DP-1, while sub-watershed BB drains to DP-2. D. Soil Types: The soil classifications and their limits were provided from mapping compiled by the U.S. Department of Agriculture Soil Service Soil Survey of Westchester County, New York which has classified the soil types as follows: I-4
Soil Symbol Hydrologic Soil Group Description Ub n/a Urban Land Hydrologic Group C was used for curve number calculations. I-5
II. Water Quality Treatment Volume (WQv) As a redevelopment project, the required water quality volume (WQv) is estimated as a percentage of WQv from the total impervious area equal to the total existing impervious area plus 100% of the WQv of the increased impervious area. Table No.1, Existing Drainage Conditions, illustrates a breakdown of the impervious and pervious areas within each subwatershed discharging to the two DPs. Table No. 2, Redeveloped Drainage Conditions, tabulates the proposed conditions of the redeveloped site for the Echo Bay Center. A. Redevelopment WQv Methodology Per Chapter 9, Redevelopment Projects, of the New York State Department of Environmental Conservation (NYSDEC) New York State Stormwater Management Design Manual (NYSSMDM), August 2010, required WQv is calculated using one or more of four separate options. Option 1 The reduction of impervious cover from existing conditions to redeveloped conditions by 25% or more. The Echo Bay Center project fulfills this requirement by reducing the impervious cover of the site by approximately 26%. Option 2 Redevelopment WQv calculation includes a minimum of 25% of the WQv from the existing disturbed impervious area to be captured and treated by the implementation of a NYSDEC standard practice or green infrastructure technique. Due to the proximity to the Long Island Sound and lack of available elevation, standard stormwater management practices are not proposed. Option 3 The use of alternative practices, sized to meet a minimum of 75% of the water quality volume from the disturbed, impervious area. The NYSDEC allows for the use of accepted verified manufactured technologies. It is proposed that one of these measures, a hydrodynamic separator, be used in sub-watershed AA-2 (DP 1). A hydrodynamic separator which removes up to 80% total suspended solids (TSS) from stormwater runoff is II-1
recommended due to the lack of space for a standard SMP or green infrastructure technique. The proposed alternative practice (hydrodynamic separator) will treat the runoff for approximately 12% of the WQv from the disturbed impervious surfaces. This will be done through treating the runoff from the new parking lot located west of the Echo Bay Center mixed use building. Option 4 A combination of the first three options, allowing for the use of standard and alternative practices. The proposed project therefore will include the following combined measures. 1. Reduction in impervious cover approximately 26% 2. Hydrodynamic separator to treat approximately 12% of water quality volume from disturbed impervious surfaces. B. Redevelopment WQv Calculations Table No. 3, Redevelopment Stormwater Requirements, provides a summary of how the proposed project complies with Chapter 9 of the NYSSMDM. Refer to Table No. 4, Stormwater Quality Management Measures, for a detailed breakdown of the estimated WQv for the site. II-2
TABLE NO. 1 ECHO BAY CENTER NEW ROCHELLE, NEW YORK EXISTING DRAINAGE CONDITIONS WATERSHED/ SUBBASIN ID AREA (AC) (1) (2) (3) (4) (4) IMPERVIOUS TOTAL I R v CN Ia Tc Tt PERVIOUS Roof Pavement IMP. TOTAL AREA (%) (HRS) (HRS) DESIGN POINT # A 1.47 4.79 6.26 2.17 8.43 74 0.72 91 0.2 0.083 --- 1 B 0.23 0.19 0.43 0.58 1.01 42 0.43 84 0.4 0.083 --- 2 TOTAL AREA 6.69 2.75 9.44 1. I=Percent Impervious, (Impervious Area/Total Area)*100% 2. R v = 0.05+0.009(I), Minimum Rv=0.2 3. CN=Curve Number 4. Tc=Time of Concentration, Tt=Travel Time 674 Stormwater Analysis GP-0-10-001 Revised Jan 2013 1/4/2013 Divney Tung Schwalbe, LLP
TABLE NO. 2 ECHO BAY CENTER NEW ROCHELLE, NEW YORK REDEVELOPED DRAINAGE CONDITIONS WATERSHED/ SUBBASIN ID AREA (AC) (1) (2) (3) (4) (4) IMPERVIOUS TOTAL I R v CN Ia Tc Tt PERVIOUS Roof Pavement IMP. TOTAL AREA (%) (HRS) (HRS) DESIGN POINT # AA-1 3.05 0.89 3.93 3.69 7.62 52 0.51 86 0.3 0.083 --- 1 AA-2 0.00 0.60 0.60 0.38 0.98 61 0.60 89 0.3 0.083 --- 1 BB 0.23 0.20 0.43 0.41 0.84 51 0.51 86 0.3 0.083 --- 2 TOTAL AREA 4.96 4.48 9.44 1. I=Percent Impervious, (Impervious Area/Total Area)*100% 2. R v = 0.05+0.009(I), Minimum Rv=0.2 3. CN=Curve Number 4. Tc=Time of Concentration, Tt=Travel Time 674 Stormwater Analysis GP-0-10-001 Revised Jan 2013 1/4/2013 Divney Tung Schwalbe, LLP
TABLE NO. 3 ECHO BAY CENTER NEW ROCHELLE, NEW YORK REDEVELOPMENT STORMWATER REQUIREMENTS A. Impervious Cover (IC) Reduction Site Area 9.44 ac Disturbed Existing Impervious 1 6.69 ac Total Proposed Impervious 4.96 ac Impervious Reduction 1.73 ac Impervious Reduction 26% B. Standard/RRV Stormwater Management Practices (SMP) Minimum Impervious Area Required to be Treated by SMP 0% - Not Required 2 C. Alternative Stormwater Management Practices Minimum Impervious Area Required to be treated by Alternative Measures 0% - Design Exceeds Min. Impervious treated by Alternative Measure (Hydrodynamic Separator) 0.60 ac % Impervious Area Treated by Alternative Measure 12% Notes: 1. Approximately 0.45 acres of existing impervious area include gravel roads and lots, which were treated as impervious area for this analysis per Chapter 4 of the New York State Stormwater Management Design Manual, August 2010. 2. Per NYSSMDM Chapter 9, if a redevelopment project reduces existing impervious cover by at least 25%, water quality and quantity criteria have been met, and no standard or alternative practices are required. 674 Stormwater Analysis GP-0-10-001 Revised Jan 2013 1/4/2013 Divney Tung Schwalbe, LLP
TABLE NO. 4 ECHO BAY CENTER NEW ROCHELLE, NEW YORK STORMWATER QUALITY MANAGEMENT MEASURES SUMMARY DISCHARGE POINT, DP 1 2 EX PROPOSED Total Exist Devel I 1 2 R v P 3,4 S 5 Alternative AREA AREA WQv 6 WQv WQv WQv 6 SMP Measure IMP IMP TOTAL Provided Proposed (ac) (ac) (ac) (%) (in) (cft) (cft) (cft) (cft) Hydrodyanamic 6.26 4.53 8.60 53 0.52 1.3 0.30 28,593 21,269-7,325 2,690 Separator 0.43 0.43 0.84 51 0.51 1.3 0.30 2,043 2,024-19 0 Total 6.69 4.96 9.44 30,636 23,293-7,343 2,690 NOTES 1. I=Impervious Cover (%) 2. Rv = 0.05+0.009(I), Minimum Rv=0.2 3. P=90% Rainfall Event Number 4. P=1.3 in. Westchester County (See Figure 4.1, NYSSMDM, August 2010) 5. S=Hydrologic Soil Group (HSG) Specific Reduction Factor 6. WQv=[(P)(Rv)(A)]/12 674 Stormwater Analysis GP-0-10-001 Revised Jan 2013 1/4/2013 Divney Tung Schwalbe, LLP
III. Runoff Reduction (RRv) A. Redevelopment Requirements for RRv Chapter 9, Redevelopment Projects, of the New York State Department of Environmental Conservation (NYSDEC) New York State Stormwater Management Design Manual (NYSSMDM), August 2010, explains that though encouraged, meeting the Runoff Reduction Volume (RRv) sizing criteria is not required for redevelopment projects such as the Echo Bay Center. 1 Runoff reduction is used to replicate pre-development hydrology by maintaining and/or improving preconstruction infiltration, peak runoff flow, and discharge volume. B. RRv Strategy There is a reduction in impervious cover under the proposed action such that the runoff volume from all storm events will be less than under existing conditions. Additionally, with the site located along the coast of the Long Island Sound, there is no need or requirement for peak flow reduction. 1 New York State Department of Environmental Conservation New York State Stormwater Management Design Manual, August 2010, page 9-4. III-1
IV. Water Quantity Control Through the use of the proposed site planning methods and the overall reduction in impervious cover the proposed peak rates of runoff from the site will be reduced. Stormwater discharge rates to both the municipal storm sewer in Main Street and directly to the Long Island Sound are equal to or less than existing conditions. Table No. 5, Stormwater Quantity Design Flow Summary, outlines the existing flow rates at each discharge point and the combined flow for the 1- through the 100-24-hour storm events. Also included are the anticipated design flow rates for the improved redeveloped site, and a delta indicating for each storm event the proposed peak rate of runoff is at or below existing conditions. IV-1
TABLE NO. 5 ECHO BAY CENTER NEW ROCHELLE, NEW YORK STORMWATER QUANTITY DESIGN FLOW SUMMARY DESIGN POINT NO. 1-YEAR 2-YEAR 10-YEAR 25-YEAR 100-YEAR SW Flow Runoff Volume SW Flow Runoff Volume SW Flow Runoff Volume SW Flow Runoff Volume SW Flow Runoff Volume (CFS) (AC-FT) (CFS) (AC-FT) (CFS) (AC-FT) (CFS) (AC-FT) (CFS) (AC-FT) Westchester County Rainfall 2.8 3.5 5.0 6.0 (IN) (1) 1 Existing 16.1 1.3 21.4 1.8 32.6 2.8 40.0 3.5 51.1 4.5 Developed 13.6 1.1 18.8 1.5 30.3 2.5 38.0 3.2 49.4 4.2 Delta -2.6-0.2-2.6-0.3-2.3-0.3-2.0-0.3-1.6-0.3 2 Existing 1.4 0.1 2.0 0.2 3.4 0.3 4.3 0.4 5.6 0.5 Developed 1.3 0.1 1.8 0.1 2.9 0.2 3.7 0.3 4.8 0.4 Delta -0.1 0.0-0.2 0.0-0.4 0.0-0.6 0.0-0.8-0.1 (1) Northeast Regional Climate Center, Data Ending 2003 674 Stormwater Analysis GP-0-10-001 Revised Jan 2013 1/4/2013 Divney Tung Schwalbe, LLP
V. Erosion & Sediment Control As described in the NYSDEC NYSSMDM, in order to protect the waters of the State of New York from adverse impacts of urban stormwater runoff, the typical objectives of stormwater management in redeveloped and newly developed areas include reducing the potential of flooding, flood damage, erosion, and water quality degradation. The following are proposed temporary and permanent stormwater management measures that are NYSDEC accepted methods and are designed to address these objectives. A. Temporary Erosion & Sediment Control Measures During construction, the objective of stormwater management is to minimize soil and sediment leaving the site. The following methods are proposed to achieve this goal: Limit and control the exposure of soil to the extent that can be managed. Install silt fencing, hay bales, inlet protection, sediment traps, swales, berms, energy dissipaters, anti-tracking pads, mulching and seeding. These methods will be regularly maintained and periodically inspected. Divert stormwater runoff away from disturbance areas. Meet NYSDEC New York State Standards and Specifications for Erosion and Sediment Control requirements. The NYSDEC General Permit GP-0-10-001 has limitations on the extent of soil disturbance during construction to no more than five acres maximum at any one time, without prior written approval from the municipality, the operators of a NYSDEC regulated Municipal Separate Storm Sewer Systems (MS4s). A disturbance of greater than five acres is not proposed. V-1
B. Permanent Measures After completion of the project there will be several permanent stormwater measures that will require regular inspections and maintenance. These stormwater measures include drain inlets, catch basins, manholes, pipes, and a hydrodynamic separator. V-2
ECHO BAY NEW ROCHELLE, NEW YORK STORMWATER MANAGEMENT REPORT - APPENDIX Prepared for: Forest City Residential Group 1 Metro Tech Center North Brooklyn, NY 11201 Prepared by: Divney Tung Schwalbe, LLP One North Broadway, Suite 1407 White Plains, NY 10601 August 20, 2012 Revised January 4, 2013
ECHO BAY NEW ROCHELLE, NEW YORK STORMWATER MANAGEMENT REPORT TECHNICAL APPENDIX A. METHODOLOGY 1. Zero Increase in Watershed Peak Runoff 2. Storm Frequencies 3. Technical Approach 4. Rainfall Intensity B. POND PACK MODELING
A. METHODOLOGY 1. Zero Increase in Watershed Peak Runoff In accordance with standard development practices, the peak rate of stormwater discharge from the site after the completion of development shall not exceed the estimated predevelopment peak discharge to offsite areas with conveyance systems. Discharge directly to Echo Bay which is tidal influenced is not subject to peak rate control. 2. Storm Frequencies The storm frequencies to be used as a basis for computing peak rate of discharge shall be storms expected once every 1, 2, 10, 25 and 100 s with a duration of 24 hours as defined by the U.S. Department of Agriculture Soil Conservation Service. 3. Technical Approach The method used for estimating peak discharge shall be as per the document released by the Engineering Division of the U.S. Department of Agriculture Soil Conservation Service titled Urban Hydrology for Small Watersheds, Technical Release No. 55, dated June 1986, Type III Storm Distribution. This criterion governs the data that is input into the software, namely the Haestead Methods Quick TR-55 computer program. A summary of the flows under existing and proposed conditions is provided. The complete input and output data is available upon request. A-1
4. Rainfall Intensity Frequency and intensities, which have been used in this report, are as follows: WESTCHESTER COUNTY RAINFALL INTENSITY BY STORM FREQUENCY 1 Storm Frequency Year Rainfall Intensity (24-Hour Period) (Inches) 100 7.5 25 6.0 10 5.0 2 3.5 1 2.8 1 Northeast Regional Climate Center, Data Ending 2003 A-2
B. POND PACK MODELING B-1
Subsection: Master Network Summary Existing Conditions Catchments Summary A A A A A B B B B B Label Scenario Pre-Development 1 Pre-Development 2 Pre-Development 10 Pre-Development 25 Pre-Development 100 Pre-Development 1 Pre-Development 2 Pre-Development 10 Pre-Development 25 Pre-Development 100 Return Event (s) 1 2 10 25 100 1 2 10 25 100 Hydrograph Volume (ac-ft) 1.324 1.784 2.795 3.480 4.515 0.114 0.163 0.275 0.353 0.472 Time to Peak (hours) Peak Flow (ft³/s) 16.13 21.40 32.61 40.02 51.05 1.43 2.03 3.37 4.27 5.62 Node Summary DP-1 DP-1 DP-1 DP-1 DP-1 DP-2 DP-2 DP-2 DP-2 DP-2 Label Scenario Pre-Development 1 Pre-Development 2 Pre-Development 10 Pre-Development 25 Pre-Development 100 Pre-Development 1 Pre-Development 2 Pre-Development 10 Pre-Development 25 Pre-Development 100 Return Event (s) 1 2 10 25 100 1 2 10 25 100 Hydrograph Volume (ac-ft) 1.324 1.784 2.795 3.480 4.515 0.114 0.163 0.275 0.353 0.472 Time to Peak (hours) Peak Flow (ft³/s) 16.13 21.40 32.61 40.02 51.05 1.43 2.03 3.37 4.27 5.62 Echo Bay - PondPack Summary 1/4/2013 Bentley Systems, Inc. Haestad Methods Solution Center 27 Siemon Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Bentley PondPack V8i [08.11.01.56]
Subsection: Master Network Summary Redeveloped Conditions Catchments Summary AA-1 AA-1 AA-1 AA-1 AA-1 AA-2 AA-2 AA-2 AA-2 AA-2 BB BB BB BB BB Label Scenario Post-Development 1- Post-Development 2- Post-Development 10 - Post-Development 25 - Post-Development 100- Post-Development 1- Post-Development 2- Post-Development 10 - Post-Development 25 - Post-Development 100- Post-Development 1- Post-Development 2- Post-Development 10 - Post-Development 25 - Post-Development 100- Return Event (s) 1 2 10 25 100 1 2 10 25 100 1 2 10 25 100 Hydrograph Volume (ac-ft) 0.947 1.331 2.200 2.798 3.710 0.140 0.192 0.308 0.386 0.506 0.104 0.147 0.242 0.308 0.409 Time to Peak (hours) Peak Flow (ft³/s) 11.83 16.49 26.66 33.46 43.61 1.73 2.34 3.65 4.52 5.82 1.30 1.82 2.94 3.69 4.81 Node Summary DP-1 DP-1 DP-1 DP-1 DP-1 Label Scenario Post-Development 1- Post-Development 2- Post-Development 10 - Post-Development 25 - Post-Development 100- Return Event (s) 1 2 10 25 100 Hydrograph Volume (ac-ft) 1.087 1.524 2.508 3.184 4.216 Time to Peak (hours) Peak Flow (ft³/s) 13.57 18.84 30.32 37.98 49.43 Echo Bay - PondPack Summary 1/4/2013 Bentley Systems, Inc. Haestad Methods Solution Center 27 Siemon Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Bentley PondPack V8i [08.11.01.56]
Subsection: Master Network Summary Redeveloped Conditions Node Summary DP-2 DP-2 DP-2 DP-2 DP-2 Label Scenario Post-Development 1- Post-Development 2- Post-Development 10 - Post-Development 25 - Post-Development 100- Return Event (s) 1 2 10 25 100 Hydrograph Volume (ac-ft) 0.104 0.147 0.242 0.308 0.409 Time to Peak (hours) Peak Flow (ft³/s) 1.30 1.82 2.94 3.69 4.81 Echo Bay - PondPack Summary 1/4/2013 Bentley Systems, Inc. Haestad Methods Solution Center 27 Siemon Company Drive Suite 200 W Watertown, CT 06795 USA +1-203-755-1666 Bentley PondPack V8i [08.11.01.56]