2025 12 08_West Orange SMP Memorandum

Stormwater Management Plan Memorandum 2.3 Revision A Hydrology and Hydraulics Stormwater Management Facilities This project is considered a major development but is only required to address stormwater quantity and quality. A modular underground detention basin is proposed to mitigate the effects of higher runoff rates from the proposed switching station expansion. Stormwater will be routed to the underground detention basin via a 4 foot wide flat bottom grass swale running along the east edge of the switching station, discharging into a catch basin and through a stormwater pipe network. Stormwater quality will be addressed via the use of the grass swale BMP. To prevent the non-contributory area drainage from reaching the switching station, and impacting the stormwater detention basin quantity control, an additional trench drain is proposed to the east of the grass swale. This trench drain will be connected to separate stormwater pipe network and shall connect to the existing stormwater infrastructure located within Mount Pleasant Avenue. The underground detention basin is designed to attenuate the post-construction peak runoff rates for the current and future 2-, 10- and 100-year storm events to rates that are less than 50%, 75% and 80%, respectively, of the pre-construction peak runoff rates as shown in Table 2.3 below. Table 2.3: Underground Detention System Discharge Return Frequency (yr) Inflow (cfs) Discharge (cfs) Stage Elevation (ft) WQ 1.59 0.17 591.17 2 (Current) 3.14 0.64 592.14 2 (Future) 4.07 1.48 592.29 10 (Current) 5.61 3.66 592.59 10 (Future) 7.14 4.92 592.85 100 (Current) 10.65 7.15 593.55 100 (Future) 14.18 8.71 594.19 Max Available Storage (cf) Bottom of Underground Detention Elevation (ft) 32,340 590.00 Table 2.4: Underground Detention Control Devices Outlet Control Structure Name CS-01 Public Service Electric & Gas Company Control Device Outlets (1) – 18” Concrete Pipe @ 590.00’ (1) – 2.5” Circular Orifice @ 590.00’ (1) – 30”W x 6”H Rectangular Orifice @ 592.00’ TOC-2-2 Burns & McDonnell