J_2024 05 09_West Orange Stormwater Management Plan

Stormwater Management Plan Revision 0 Hydrology and Hydraulics A series of trench drains are also included, adjacent to each bioretention basin. These trench drains are intended to prevent off-site drainage from flowing onto the station pad or into the bioretention basins. These areas have been labeled as Non-Contributory and were not taken into consideration when evaluating the bioretention basins. The trench drains adjacent to the basins will connect into a separate stormwater conveyance system, ultimately connecting and discharging into the Township’s existing stormwater network, located within Mount Pleasant Avenue, at outfall 0-1A. As shown in Table 2.7, these areas are considered non-contributory because they are to remain in their existing condition and are only captured at the watershed lowpoint to directly route runoff to the stormwater network in Mount Pleasant Avenue to prevent any impact to the station. Each Discharge Point was analyzed to regard only watersheds that contributed to that outfall and were analyzed differently as not all areas are being disturbed/impacted by these improvements. In Table 2.7, the pre-development outfalls comparison versus the post-development outfalls show less flow leaving the site in the post-development condition for each of the current and future projected design storm outfalls per the criteria described in N.J.A.C 7:8. Stormwater flow from three of the post developed watersheds, PO-DA-A, PO-DA-M and portions of PODA C, will mimic the pre-developed drainage patterns of PO-DA 6 and PO-DA 8, utilizing the existing stormwater conveyance system, routing to the existing Aquashield treatment device, ultimately discharging into the Township’s existing stormwater network located within Mount Pleasant Avenue. As shown in Table 2.7, post-development flows to O-Aquashield were significantly reduced from predevelopment flows, this was accomplished by routing runoff to other BMPs. By reducing the footprint of the watersheds routing to O-Aquashield, we were able to reduce the load to this device and add new capture and treatment via bioretention. Soil tests were conducted by GZA GeoEnvironmental, Inc. between May 31st, 2022 and January 6th, 2023 (See Appendix E). During the soil investigation, GZA excavated five test pits in the bioretention areas, TP-203, TP-204, TP-205, TP-103 and TP-102, from north to south. The test

and January 6th, 2023 (See Appendix E). During the soil investigation, GZA excavated five test pits in the bioretention areas, TP-203, TP-204, TP-205, TP-103 and TP-102, from north to south. The test pits extended to depths of approximately 6 to 10 feet below grade. Groundwater seepage was encountered at varying depths. Table 2.5 provides estimated Seasonal High Water Table (SHWT) elevations used for each basin. Based off the acquired groundwater locations, each bioretention basin was initially designed to allow for a minimum of 1’ of separation between the SHWT and the bottom of the bioretention basin (See Table 2.5). Design was modified to utilize a sealed bioretention basin with concrete on all sides and so therefore in accordance with the New Jersey Stormwater Best Practices Manual, Chapter 9.7, Small-Scale Bioretention Systems, the basins are not required to maintain a separation from the Seasonal High Water Table. Public Service Electric & Gas Company 2-5 Burns & McDonnell