Chapter 10: Stormwater Management A. INTRODUCTION AND SUMMARY OF FINDINGS This chapter describes existing and proposed stormwater management on the Site. Potential impacts to stormwater infrastructure are based on record utility data from City of White Plains. Runoff quality and quantity from Hamilton Green would meet or improve existing conditions. Onsite improvements for Hamilton Green would include green roofs and landscape features, and would have a net reduction in impervious area. Because of the anticipated decrease in impervious area resulting from the Proposed Action, no onsite detention would be required. B. EXISTING CONDITIONS The City of White Plans Department of Public Works (DPW) maintains the storm drains around the subject parcel. According to records provided by the City DPW summarized in Table 10-1 below, storm drains exist in all of the adjacent streets: Table 10-1 Adjacent Storm Drains Street Storm Drain Size Flow Direction Barker Avenue 54-inch x 83-inch West Cottage Place 15-inch to 18-inch North Hamilton Avenue 15-inch to 18-inch West Dr. Martin Luther King Jr. Boulevard (n/f Grove Street) 18-inch to 30-inch North The Site is connected to the surrounding storm drains at four locations. A French drain connects to the 54-inch x 83-inch storm drain in Barker Avenue. A 15-inch lateral connects to the 18-inch storm drain in Cottage Place. Two 15-inch laterals connect to the 18-inch and 30- inch storm drains in Dr. Martin Luther King Jr. Boulevard (MLK Blvd). The storm drains in Barker Avenue, Cottage Place and MLK Blvd converge at the intersection of Barker Avenue and MLK Blvd. The storm drain continues west along Barker Avenue in an 8.5-foot x 4.5-foot box culvert. It appears the drain continues westward to a discharge point directly along the Bronx River. The Site is nearly entirely impervious surfaces. Some landscape features and trees exist on the Site. Additional landscape features and tree pits exist between the site and the street curb but are considered off-site pervious areas. Due to curbs and sidewalks typically graded towards the roadways, no off-site area contributes to the Site. The entire site is collected by the municipal storm system. Hamilton Green Stormwater Management 10-1
The storm drains and laterals are depicted on the Grading, Drainage and Utility Plan in Appendix F. C. FUTURE WITHOUT THE PROPOSED ACTION The Site currently has no stormwater quality treatment or stormwater quantity control practices. Stormwater runoff from the Site enters the municipal storm drainage system undetained and untreated. In the future without the Proposed Action stormwater runoff would continue to enter the municipal drainage system un-detained and untreated. D. POTENTIAL IMPACTS OF THE PROPOSED ACTION A preliminary stormwater management report (SWMR) has been completed and is included in Appendix F. The SWMR is the basis from which a Stormwater Pollution Prevention Plan (SWPPP) would be developed. In accordance with the New York State Department of Environmental Conservation (NYSDEC) State Pollutant Discharge Elimination System (SPDES) General Permit for Stormwater Discharges from Construction Activity (GP-0-15-002), a SWPPP would be prepared and submitted to the City for review and approval as the Regulated, Traditional Land Use Control MS4 as part of the Site Plan Approval process. Following approval of the SWPPP by the City, a Notice of Intent would be submitted to the NYSDEC to gain permit coverage for Hamilton Green under the SPDES general permit. The SWMR includes calculations to demonstrate adherence to NYSDEC requirements for stormwater quality treatment and stormwater quantity control. The final SWPPP would include plans detailing the erosion control measures to be used during construction to avoid impacts from soil erosion/sedimentation. Through implementation and inspection of erosion control measures throughout construction, significant adverse impacts to onsite soils would be avoided. It is anticipated that Hamilton Green would not interfere with the functionality of the existing storm drain structures, such as catch basins and manholes. However, if it is necessary to relocate or alter drainage existing drainage structures, then the design team would work with the City to find a mutually agreeable solution. Existing French drain and storm drain laterals would be capped and abandoned in accordance with City requirements. New storm drain connections would be constructed. E. MITIGATION MEASURES Through the implementation of a section of green roof, onsite improvements associated with Hamilton Green would produce no increase in impervious area. Open space area, which would include green roofs and landscape features, is an integral part of the design of Hamilton Green. However, the exact limits, type and location of the green roof and landscape areas have not been fully designed and would be completed as part of the Site Plan Approval process. The proposed green roof would exceed 10,600 square feet and would be designed in accordance with the specifications of Chapter 5 of the New York State Stormwater Management Design Manual (SMDM). The provision of 10,600 square feet of intensive green roof and no net conversion of pervious area to impervious area enables Hamilton Green to be considered a full redevelopment project in accordance with Chapter 9 of the SMDM. Table 10-2 compares impervious site coverage under existing conditions and With Action conditions. 10-2 Hamilton Green Stormwater Management
Coverage Existing Conditions With Action Conditions Table 10-2 Site Coverage Change in Coverage Impervious 152,698 152,650-48 Pervious 10,552 10,600 +48 Total 163,250 163,250 0 By nature of being entirely a redevelopment project, there is no runoff reduction volume (RRv) requirement. Alternative stormwater management practices (SMPs), such as hydrodynamic separators, are proposed to treat the required water quality volume (WQv). Alternative SMPs would be installed on each of the roof drain connections to provide stormwater treatment prior to discharge into the municipal storm drain system. Because Hamilton Green is a redevelopment project and only alternative SMPs would be included as proposed, the required WQv is equal to 75 percent of the calculated WQv. Table 10-3 summarizes the required WQv and RRv for Hamilton Green. It should be noted the green roof of Hamilton Green would be used as an impervious area reduction practice and no credit would be taken for it as a volume reduction practice. Practice Water Quality Volume (WQv) Required Water Quality Volume (WQv) Provided 1 Table 10-3 Water Quality Summary Volume in Cubic Feet (CF) 13,644 CF 13,644 CF Runoff Reduction Volume (RRv) Required 0 CF Notes: 1 Minimum WQv provided by alternative SMPs. Actual WQv provided to be calculated once building roof drainage and plumbing designs are advanced. The inclusion of the green roof that would exceed 10,600 SF would enable Hamilton Green to claim no change in hydrologic conditions when compared to existing conditions. Considering this and the fact that Hamilton Green would be considered a redevelopment project, the tenyear storm, one hundred-year storm and channel protection volume requirements do not apply. Table10-4 below demonstrates no increase in peak flow rates for the 1, 10, and 100 year 24-hour storm events. No onsite detention would be required. Notes: Storm Event Rainfall Depth (inches) Existing Conditions Peak Flow Rate (CFS) Table 10-4 Water Quantity Summary With Action Conditions Peak Flow Rate (CFS) 1-Year 2.82 10.62 10.62 10-Year 5.10 18.38 18.38 100-Year 9.02 30.46 30.46 Cubic feet per second (CFS) Hamilton Green Stormwater Management 10-3
Construction activities have the potential to increase soil erosion and introduce sediment into stormwater runoff because of the necessary earth disturbance associated with the activities. Temporary erosion and sediment controls (ESCs) would be implemented during construction to mitigate and control stormwater pollution. Because of the urban nature of the Site, only limited ESCs are required. The anticipated ESCs are listed below. They would be designed and installed in accordance with the New York State Standards and Specifications for Erosion and Sediment Control (SSESC) (latest edition). Stabilized Construction Entrance/Exit (SCE): A stabilized construction entrance/exit would have a stabilized aggregate pad underlain with filter cloth to prevent construction vehicles from tracking sediment off-site. The stabilized construction entrance would be located at specific transition areas between concrete/asphalt to exposed earth. The contractor would not be permitted to track sediment into the public roadways. Silt Fence: Silt fence would be installed on the down gradient edge of disturbed areas parallel to existing or proposed contours or along the property line as perimeter control. Silt fence would be used where stakes can be properly driven into the ground as per the Silt Fence detail in the SSESC. Silt fence controls sediment runoff where the soil has been disturbed by slowing the flow of water and encouraging the deposition of sediment before the water passes through the straw bale or silt fence. Built-up sediment would be removed from silt fences when it has reached one-third the height of the bale/fence and properly disposed. Inlet Protection: Inlet protection would be installed at all drainage structure inlets subject to possible construction runoff during construction. Inlet protection would be constructed to pass stormwater through, but prevent silt and sediment from entering the drainage system. Generally, silt sacks are not an appropriate inlet protection. Spoil Stockpile: Stockpiled soil would be protected and stabilized. Soil stockpiles and exposed soil would be stabilized by seed, mulch, or other appropriate measures, when activities temporarily cease during construction for seven days or more in accordance with NYSDEC requirements. Sediment Trap: A sediment trap intercepts sediment-laden runoff and filters the sediment laden stormwater runoff leaving the disturbed area in order to protect downstream drainage ways, properties, and rights-of-way. The sediment traps may be installed down gradient of construction operations which expose critical areas to soil erosion. The basin would be maintained until the disturbed area is protected against erosion by permanent stabilization. The sediment traps would not be connected to the municipal storm sewer if the site is designated a brownfield/hotspot. After storm events, the collected water would be tested and deemed acceptable prior to discharge. Sediment traps would be drawn down by acceptable measures after storm events to restore volume for future storm events. 10-4 Hamilton Green Stormwater Management
Dust Control: During the demolition and construction process, debris and any disturbed earth would be wetted down with water, if necessary, to control dust. Other NYSDOT approved dust palliatives may be also be used; refer to their website. 1 After demolition and construction activities, all disturbed areas would be covered and/or vegetated mitigate the creation of dust. Soil Stabilization: In areas where construction activities have ceased, temporary seeding or permanent landscaping would be performed to control sediment-laden runoff and provide stabilization to control erosion during storm events. The contractor would apply soil stabilizer in accordance with NYSDEC specifications and manufacturer s requirements. This temporary seeding/stabilization or permanent landscaping would be in place no later than seven days after demolition and construction activities have ceased. Dewatering: If required, GP-0-15-002 authorizes limited non-stormwater discharges including uncontaminated discharges from construction site dewatering operations. Any contaminated discharges must be properly permitted by the applicable regulatory authority and are not covered under the SWPPP. If required, a Remedial Action Plan would be developed to detail the proper handling of contaminated discharges. Material Handling: The contractor would be required to store construction and waste materials as far as practical from any environmentally sensitive areas, including stormwater inlets. Where possible, materials would be stored in a covered area to minimize any potential runoff. The contractor would be required to implement spill prevention and response where practicable. Prior to commencing any construction activities, the contractor would be required to obtain all necessary permits and verify permits have been obtained. An Erosion and Sediment Control Plan (ESC Plan) would be developed to identify the location, type, and sizing of ESCs and additional detailed regarding the ESCs would be provided in the final SWPPP. Both the SWPPP and ESC Plan would be completed as part of the Site Plan Approval process. 1 https://www.dot.ny.gov/divisions/engineering/environmental-analysis/water-ecology/stormwatermanagement Hamilton Green Stormwater Management 10-5