J_2024 05 09_West Orange Stormwater Management Plan

Stormwater Management Plan 2.3 Revision 0 Hydrology and Hydraulics Stormwater Management Facilities As previously stated in Section 1.3, this project is considered a major development, but is only required to treat quantity. A series of bioretention basins with underdrains is proposed to mitigate the effects of higher runoff rates from the proposed development. The bioretention basins consist of a topsoil layer of at least 18 inches in depth, a middle sand layer of at least 6 inches and a bottom gravel layer of at least 12 inches. Filter fabric will also be installed along the sides and bottom of each basin. Perforated underdrains, 4 inches in diameter, will be located throughout the basin. In accordance with the BMP manual, inverts will not be required to be at least 3 inches from the bottom because of the use of a concrete/non-earthen bottom. Pipe crowns will be required to be at least 3 inches from the top of the gravel layer. The bioretention basin systems are located to the east and south of the proposed station upgrades and are designed according to the New Jersey Stormwater Best Practices Manual, Chapter 9.7, Small-Scale Bioretention Systems. The bioretention basins are designed so that the post-construction peak runoff rates for the current and future 2-, 10- and 100-year storm events are less than 50%, 75% and 80%, respectively, of the preconstruction peak runoff rates as shown in Table 2.4. During storm events, the bioretention basins will capture stormwater runoff, which will filter through the basin’s media layers and drain out through a network of perforated underdrains at the bottom of each of the basins. Water will be released from the underdrain system into a stormwater conveyance system, ultimately connecting and discharging into the Township’s existing stormwater network, located within Mount Pleasant Avenue. Table 2.4: Bioretention Basins Discharge Return Frequency (yr) Inflow (cfs) Discharge (cfs) Stage Elevation (ft) Surface Area (sf) Max Available Storage (cf) Approx. SHWT Elevati on (ft) Bottom of Bioretention Basin Elevation (ft) 6,639 595.00 596.00 BR-1 WQ 0.44 0.12 596.68 2 (Current) 0.93 0.24 598.20 2 (Future) 1.21 0.30 599.34 10 (Current) 1.67 0.33 600.08 10 (Future) 2.13 0.42 600.58 100 (Current) 3.18 1.24 601.35 100 (Future) 4.23 1.65 602.16 Public Service Electric & Gas

0 2 (Future) 1.21 0.30 599.34 10 (Current) 1.67 0.33 600.08 10 (Future) 2.13 0.42 600.58 100 (Current) 3.18 1.24 601.35 100 (Future) 4.23 1.65 602.16 Public Service Electric & Gas Company 1,759 2-3 Burns & McDonnell