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1.0 INTRODUCTION The proposed project consists of the construction of a 50 ft x 50 ft addition to an existing steel building located at 3254 Clinton Street in the Town of West Seneca. The site currently contains a 10,000 sq. ft steel building, with additional improvements including associated asphalt areas to support the proposed expansion. Under existing conditions, approximately 50,000 ± square feet of the site drains toward the western portion of the property and discharges to an existing swale. The remaining drainage area flows towards the southwest toward an existing end section along the property boundary. Under proposed conditions, stormwater runoff patterns will remain generally consistent with existing conditions. Runoff generated from the proposed building addition and associated impervious surfaces will be directed westward and discharged to the existing swale. The proposed development will not adversely impact off-site drainage patterns or downstream conveyance systems. 2.0 STORM WATER MANAGEMENT 2.1 Design Criteria Storm water drainage facilities for this development have been designed in accordance with the following: a. Storm Drainage Design Manual (ENCRPB) b. Town of West Seneca Storm Water Management Design Standards Under existing conditions, stormwater runoff from the site is collected by a swale located just west of the proposed action. The proposed site disturbance encompasses approximately 4,790 +1- sf (0.11 acres), which is below the 1.0-acre threshold requiring a Stormwater Discharge Permit (SWPPP) from the New York State Department of Environmental Conservation (NYSDEC). Post-development runoff rates will not exceed pre-development rates for the 10-year and 25-year design storm events. HydroCAD modeling calculations will be utilized to analyze and compare existing and proposed conditions, demonstrating that peak discharge rates to the existing swale are not increased. A detailed evaluation of the 10-year and 25-year design storm will be performed to confirm that runoff directed to the existing swale remains consistent with existing conditions and does not adversely impact downstream conveyance systems. Pre- and post-development hydrographs were developed using TR-55 and TR-20 methodologies within HydroCAD software. The hydrograph for the existing 10-year storm event is provided in Appendix A, while the hydrograph for the proposed 25-year storm event is included in Appendix B. Page 393 of 849