241220_42 ERW_Stormwater Report FULL

Stormwater Management Report | October 18, 2024 – Revised December 20, 2024 Page 3 Stormwater Quantity 1.1 Stormwater Management As mentioned above, a subsurface infiltration basin has been designed to provide water quantity mitigation for the runoff discharging to the existing drainage channel. As the site is located in HSG ‘C’ soils, infiltration is anticipated to be the minimum allowable 1 in/hr. Soil testing, including groundwater elevation determination, will be performed prior to construction. Full HydroCAD output calculations are provided in the appendix. The results are summarized in the table below. 1.2 Summary Table Description 2 Year Storm 10 Year Storm 100 Year Storm A. Existing Conditions Design Point 1 2.85 cfs 5.99 cfs 12.64 cfs Required Reductions (50%, 75%, 80%) 1.43 cfs 4.49 cfs 10.11 cfs Design Point 2 0.55 cfs 1.19 cfs 2.54 cfs Required Reductions (50%, 75%, 80%) 0.28 cfs 0.89 cfs 2.03 cfs B. Proposed Conditions Design Point 1 1.40 cfs 4.42 cfs 9.91 cfs Design Point 2 0.26 cfs 0.55 cfs 1.14 cfs According to §25-29.4.r of the Township of West Orange, stormwater management measures shall be designed so that the post-construction peak runoff rates for the 2-, 10-, and 100-year storm events are 50%, 75%, and 80%, respectively, of the preconstruction peak runoff rates. As can be seen in the Summary Table above, the post-construction peak runoff rates for the 2-, 10-, and 100-year storm events have been reduced below the required peak runoff rates. 1.3 Stormwater Management Facility Operations and Maintenance The long-term operations and maintenance of the proposed stormwater management facilities shall be performed in accordance with the Stormwater Management Facility Operations & Maintenance Manual prepared by Colliers Engineering & Design (under separate cover). This Manual explains how often maintenance should be performed and how long it should take for the proposed on-site stormwater management systems (porous pavement) to drain to ensure proper operation. Tables showing the data to support the listed drain times are included in the appendix of this report. 1.4 Conveyance System Pipe sizing calculations will be provided prior to final approval. Pipe sizing calculations will be performed using the Rational Method and 25-year design storm for capacity verification.