12 2026 02 12 Crestmont CC Stormwater Management Report

channel design standards require utilizing the 10-year design storm and providing a minimum of 1 foot of freeboard, while the Soil Erosion standards require utilizing the 25-year design storm and providing a minimum of 0.3 feet of freeboard. Both storm events and freeboard requirements were calculated in order to determine which standard governs. The proposed off-site subsurface collection and conveyance system within the County Route 611 right-of-way is designed to convey the 25-year design storm event. Inlet spread calculations must be performed for all inlets which may have an effect on the County Road right-of-way. The inlet spread calculations analyze the spread of flow across roadways and driveways for the 10- year design storm event utilizing the minimum 10-minute duration rainfall intensity. The inlet spread calculations are performed utilizing the criteria set forth within the NJDOT Roadway Design Manual. 3.5.2 Design Methodology The conveyance system was analyzed using the rational method for estimating runoff for the 25- year design storm event. The project site was divided into subareas based upon topography to determine the contributing runoff to each individual inlet or roof drain. Weighted runoff coefficients were calculated based upon the land cover type within each delineated sub-area. A runoff coefficient (C) was selected in accordance with Table 10-4: Recommended Coefficient of Runoff Values for Various Selected Land Uses from Section 10 of the 2015 New Jersey Department of Transportation (NJDOT) Roadway Design Manual. Values of time of concentration were chosen based on land cover and slope of the flow path from the hydraulically most distant point in the subarea to the appropriate inlet. Unless otherwise specified, the minimum time of concentration used for each inlet is 10 minutes per Section 10.3.5.C.1.e of the NJDOT Roadway Design Manual. Rainfall intensities were taken from the National Oceanic and Atmospheric Administration (NOAA) Precipitation Frequency Data Server for the project site; refer to Appendix D. The proposed conveyance systems have been analyzed utilizing a starting tailwater elevation corresponding to the maximum design water elevation for the corresponding design storm event for the respective BMP that each conveyance network discharges into. The outflow from the proposed detention BMPs is inputted as a known

esign water elevation for the corresponding design storm event for the respective BMP that each conveyance network discharges into. The outflow from the proposed detention BMPs is inputted as a known discharge rate from the outlet control structure for the purpose of analyzing the capacity of the proposed conveyance system. In order to account for the time difference between the peak flow discharged from the detention BMPs and the peak flow discharged from the undetained areas of the site, the quantity control analysis performed within the Hydrology Studio software was analyzed to determine the time difference. The outflow from the detention BMP that corresponded to the time of the peak flow from the undetained watersheds was utilized as the known discharge rate for the outlet 20