240916_40 ERW Stormwater Report FULL

Stormwater Management Report | September 16, 2024 Page 3 Under proposed conditions, the site will generally mimic the existing drainage pattern; however, the majority of the proposed development will be captured and routed through a subsurface infiltration basin to attenuate peak flows. New inlets and pipes will be installed to convey the stormwater runoff from certain areas to the infiltration basin. Tributary areas to the infiltration basin are outlined on the Proposed Drainage Area Map and include the house, the pool house, and surrounding lawn area. The previously installed drywell system is also included within this model and has a tributary area including the surrounding impervious areas around the Pool house and pool area. The area, curve number, and time of concentration for each proposed watershed can be found on the Proposed Drainage Area Map and HydroCAD output included in the appendix. 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 1.41 cfs 2.64 cfs 5.22 cfs Required Reductions (50%, 75%, 80%) 0.70 cfs 2.23 cfs 4.10 cfs Design Point 2 2.08 cfs 4.38 cfs 9.04 cfs Required Reductions (50%, 75%, 80%) 1.04 cfs 3.28 cfs 7.23 cfs Design Point 3 2.47 cfs 5.16 cfs 10.82 cfs Required Reductions (50%, 75%, 80%) 1.23 cfs 3.87 cfs 8.65 cfs B. Proposed Conditions Design Point 1 0.65 cfs 1.37 cfs 2.88 cfs Design Point 2 0.99 cfs 1.97 cfs 4.04 cfs Design Point 3 1.23 cfs 2.53 cfs 5.44 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

ts 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.