Stormwater Report 1

METHODOLOGY The USDA Natural Resources Conservation Service (NRCS) methodology described in the Technical Release 55 – Urban Hydrology for Small Water Sheds (TR-55) is used for computing stormwater runoff rates, volumes, and hydrographs for both existing and proposed conditions. In computing stormwater runoff from all design storms, the relative stormwater runoff rates and volumes of pervious and impervious surfaces are analyzed separately. The rainfall-durationfrequency data was taken from the NOAA National Weather Service. (See attached.) The NRCS Type D Design Storm Rainfall Distribution was used to analyze both pre- and post-conditions for both current adjusted and future projected rainfall projections. Hydraflow Hydrographs Extension 2024 Modeling Software was used to compute the site's pre- and post-development runoff of the site. Table 1 – Rainfall Data Current Rainfall Current Adjusted Storm Event (in) Rainfall (in) 2-Yr 3.44 3.47 10-Yr 5.22 5.38 100-Yr 8.66 9.18 Future Rainfall (in) 4.09 6.37 11.52 According to the Township of West Orange Stormwater Management Rules and the NJDEP Stormwater Management regulations, the proposed development is considered a major development since there is more than one acre of disturbance. For major developments, stormwater management measures designed to provide groundwater recharge, stormwater runoff quantity control, and stormwater runoff quality treatment, utilizing green infrastructure. The development does not have to meet the groundwater recharge requirement since it is located within the Metropolitan Planning Area. Also, since the proposed development does not result in a net increase of ¼ of an acre or more of impervious surface, the water quality aspect of the Stormwater Management Rules is not required. Therefore, the stormwater management measures have been designed to meet only the stormwater runoff quantity requirements. According to Section 7:8-5.6(b)1. the design must “demonstrate through hydrologic and hydraulic analysis that for stormwater leaving the site, post-construction runoff hydrographs for the 2, 10 and 100-year storm events do not exceed, at any point in time, the pre-construction runoff hydrographs for the same storm events.” The proposed development calls for a decrease in the amount of impervious on the site, the amount of runoff will be reduced. Since the stormwater quantity is

ff hydrographs for the same storm events.” The proposed development calls for a decrease in the amount of impervious on the site, the amount of runoff will be reduced. Since the stormwater quantity is satisfied through the use of non-structural strategies, green infrastructure requirements are not required. 4