251203_42 ERW_Stormwater Report

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. Initial geotechnical investigations on the site found bedrock in the area of the proposed basin at an approximate elevation of 611.00, and infiltration rates of 2.2 in/hr and 30 in/hr. A design infiltration rate of 1.0 in/hr was used for the proposed infiltration basin sizing. The Geotechnical Data Report for 42 Eagle Ridge Way is included as an appendix to this report. Full HydroCAD output calculations are provided in the appendix. The results are summarized in the table below. 1.2 Summary Table Description A. Existing Conditions Design Point 1 Required Reductions (50%, 75%, 80%) Design Point 2 Required Reductions (50%, 75%, 80%) B. Proposed Conditions Design Point 1 Design Point 2 2 Year Storm 10 Year Storm 100 Year Storm 3.95 cfs 1.98 cfs 0.77 cfs 0.39 cfs 8.17 cfs 6.13 cfs 1.63 cfs 1.22 cfs 18.32 cfs 14.66 cfs 3.70 cfs 2.96 cfs 1.75 cfs 0.37 cfs 5.37 cfs 0.74 cfs 14.42 cfs 1.64 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, which will be prepared prior to construction. This Manual will explain how often maintenance should be performed and how long it should take for the proposed on-site stormwater management systems to drain to ensure proper operation. 1.4 Conveyance System Pipe sizing calculations are included in the appendix of this report. Pipe sizing calculations were be performed using the Rational Method and 25-year design storm for capacity verification. Stormwater Management Report | October 18, 2024 – Last Revised December 3, 2025 Page 3