2025 12 08_West Orange SMP Memorandum

Stormwater Management Plan Memorandum 2.0 2.1 Revision A Hydrology and Hydraulics HYDROLOGY AND HYDRAULICS Rainfall Data Per the updated N.J.A.C. 7:8 regulations effective July 17, 2023, the Essex County adjustment factor is applied for both the current and future precipitation depth values for the 2-, 10-, and 100-year return frequencies used for stormwater design. Table 2.1: Design Storm Frequency with Depth Precipitation Adjustment Factors (AF) Return Frequency (yr) Water Quality (WQ) 2 10 100 NOAA Atlas 14 (in) 1.25a 3.42 5.20 8.71 24 Hour Depth Current Current Future Adjusted AF AF (in) Future Adjusted (in) 1.01 1.03 1.06 4.07 6.36 11.58 3.45 5.37 9.23 1.19 1.22 1.33 a N.J.A.C. 7:8-5.5 requires the WQv storm to be 1.25 inches of rainfall in two hours. 2.2 Runoff Data HydroCAD-10 software was utilized to model the stormwater runoff by the SCS TR-55 methodology to calculate the pre- and post-developed runoff rates. Table 2.2 provides detailed information regarding curve numbers and land coverages for pre- and post-developed watersheds, respectively. Table 2.2: Land Coverages Land Coverage CN HSG Pre-Development Impervious, Pavement 98 D >75% Grass cover, Good 80 D Yard Stone/Gravel Pad 85 D Brush, Good 73 D Non-Contributory* 73-85 D Post-Development Impervious, Pavement 98 D >75% Grass cover, Good 80 D Yard Stone/Gravel Pad 85 D Brush, Good 73 D Non-Contributory* 73-85 D Area (acres) 0.973 1.345 1.926 1.261 1.411 1.373 2.102 2.003 0.000 1.438 *Non-Contributory refers to areas that do not contribute to Underground Detention or the existing Aquashield system. Non-Contributory areas include the grassed slope areas and the transformer containment pits. Public Service Electric & Gas Company TOC-2-1 Burns & McDonnell