J_2024 05 09_West Orange Stormwater Management Plan

Stormwater Management Plan Revision 0 Hydrology and Hydraulics The post-developed peak runoff rates for watershed PO-DA N, which drains to an existing inlet manhole similar to watershed PO-DA 1, thus providing no adverse impact to Outfall O-2. The HydroCAD calculations show the post-construction runoff hydrographs for the current and projected 2-, 10-, and 100year storm events, do not exceed, at any point in time, the pre-construction runoff hydrographs through hydrologic and hydraulic analysis for stormwater leaving the site, for the same storm events. This is summarized in Table 2.7 below. The bioretention basins with underdrains will provide runoff quantity control in accordance with N.J.A.C. 7:8-5.6. This regulation requires the peak post developed runoff rates to not exceed the peak preconstruction runoff rates for 50% of the 2-year, 75% of the 10-year, and 80% of the 100-year storm events. Table 2.7 below summarizes the stormwater flow conditions for the project. Both the Current and Future Precipitation Change Factors for Essex County have been considered as per the July 2023 N.J.A.C. 7:8 updates and section . Table 2.6: Site Flow Results Pre Q(2-YR) Current (cfs) 4.09 3.34<4.09 OK 1.59 Q(2-YR) Future (cfs) 5.27 4.32<5.27 OK 2.25 Q(10-YR) Current (cfs) 7.37 6.09<7.37 OK 3.53 Q(10-YR) Future (cfs) 9.33 7.73<9.33 OK 4.74 Q(100-YR) Current (cfs) 14.11 11.79<14.11 OK 7.72 Q(100-YR) Future (cfs) 18.67 15.60<18.67 OK 10.46 Post 1.59 2.25 3.53 4.74 7.72 10.46 Pre 2.66*50% = 1.33 3.49*50% = 1.75 5.20*75% = 3.90 6.61*75% = 4.96 10.52*80% = 8.42 13.89*80% = 11.11 Post 0.87<1.33 OK 1.04<1.75 OK 1.31<3.90 OK 1.81<4.96 OK 3.61<8.42 OK 5.00<11.11 OK Outfall Discharge Point O-AquaShield (PO-DA 6 / A, DA 8 / M) O-1A (PO-DA K, noncontributory areas (L, H, F, D, B)) O-1Bb (bioretention areas totals (PODA 6 / C, 7 / E, G, J, I)) Pre Post 2.45 3.19 4.81 6.07 9.80 12.86 2.06<2.45 2.75<3.19 4.02<4.81 5.21<6.07 8.18<9.80 11.03<12.86 Post OK OK OK OK OK OK b N.J.A.C. 7:8-5.6 requires post-construction peak runoff rates for the 2-, 10-, and 100-year storm events to be 50%, 75%, and 80%, respectively, of the pre-construction peak runoff rates. O-2 (PO-DA 1 / N) Pre Public Service Electric & Gas Company 2-7 Burns & McDonnell