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WAVES AE Engineering Memorandum TO: Town of West Seneca Planning Board FROM: Jason Havens, P.E. — Waves AE DATE: July 28, 2026 PROJECT: 4350 Seneca Street — Site Plan Application RE: Stormwater Management Summary (Pre- vs. Post-Construction Analysis) Purpose This memorandum summarizes the stormwater management analysis prepared in support of the Town of West Seneca Site Plan Review application for the referenced project. Total soil disturbance associated with this project is less than one (1) acre. Per the Town of West Seneca Site Plan Application Checklist (Appendix A, Section IV – Drainage/Grading), sites with less than one acre of disturbance are required to detain the difference between the 10- year pre-developed storm and the 25-year post-developed storm, with supporting calculations provided. Stormwater conveyance for this project has additionally been designed for the 25-year storm event, exceeding the Town's 10-year minimum conveyance design standard. Project Description The project consists of pavilion, walkway, and paver improvements at 4350 Seneca Street. Site soils are classified as Hydrologic Soil Group D. Post-development runoff is managed through a series of on-site infiltration practices — three pea gravel infiltration beds (3.0-ft, 2.5-ft, and 5.7-ft effective depths) and one sand-over-engineered-fill infiltration bed (4.0-ft) — prior to discharge. Overflow from these practices is conveyed to one of two site discharge points: Seneca Street and an on-site detention basin. Methodology Hydrologic modeling was performed using HydroCAD 10.20-7a, applying the SCS TR-20 unit hydrograph method with weighted Curve Numbers (CN) for each subcatchment. Rainfall depths were derived from NOAA Atlas 14 point precipitation data (Type II, 24-hour distribution). Pre-construction and post-construction conditions were each modeled for the 10-year and 25-year design storms, with peak flow and volume compared at two discharge points: Link AP1 (Seneca Street) and Link AP2 (Basin). Results Summary Table 1 summarizes peak discharge and runoff volume at each site discharge point for the governing storm events, comparing the 10-year pre-developed condition (the Town's allowable release standard) against the 25-year post- developed condition. Table 1: Pre- vs. Post-Development Peak Flow & Volume Comparison 10-yr Pre-Dev 25-yr Post-Dev 10-yr Pre-Dev 25-yr Post-Dev Discharge Point Peak (cfs) Peak (cfs) Volume (cf) Volume (cf) Seneca Street (AP1) 2.20 2.12 4,600 4,750 Basin (AP2) 1.39 0.52 2,309 526 Table 2: On-Site Infiltration Practice Performance — 25-Year Storm Page 49 of 84