251203_42 ERW_Stormwater Report

1.5 Downstream Stability The 25-year frequency discharge from the proposed infiltration basin is 5.27 cfs under failure conditions. This corresponds to a velocity of 3.52 fps discharging from the 18” diameter outflow pipe. The existing soils on site are Yalesville loam, 0 to 8 percent slopes, extremely stony, which have an allowable velocity of 5.0 fps. A 7.5 ft by 9 ft scour hole with a d50 of 4 in is proposed at the outflow of the proposed basin to provide additional conduit outlet protection and point of discharge stability. Scour hole sizing calculations are included in the appendix of this report. In accordance with the Standard for Off-Site Stability, the proposed peak flows are reduced to 50% and 75% of the predeveloped peak rates for 2 and 10 year storms, providing downstream stability. Stormwater Quality As per NJDEP Regulations, stormwater management measures shall only be required for water quality control if an additional ¼ acre of regulated motor vehicle surface is being proposed on a development site. For this project, the existing motor vehicle surface area totals approximately 0.13 acres and the proposed motor vehicle surface area totals approximately 0.35 acres, which results in an increase of 0.22 acres. Therefore, as the proposed improvements result in less than ¼ acre of new regulated motor vehicle surfaces, water quality treatment is not required. Groundwater Recharge (N.J.A.C. 7:8-5.4) To meet Township and NJDEP requirements for groundwater recharge, the proposed development is designed to maintain 100 percent of the average annual pre-construction groundwater recharge volume for the site. As the entire subject property falls within one (1) HUC-14 sub-watershed, compliance with the groundwater recharge standard is met by analyzing the site as a whole. The site is within the Passaic River Lower (Nwk Bay to Saddle) HUC-14 drainage area. The New Jersey Groundwater Recharge Spreadsheet, Version 2.0, November 2003 is utilized to calculate the post-development annual recharge deficit for the project site and annual recharge volume for each proposed BMP. The following table provides a summary of the post-development deficit and proposed recharge volume. Full calculation spreadsheets and a summary of the proposed recharge volume for each BMP are provided in the Appendix. Groundwater Recharge Summary HUC-14

opment deficit and proposed recharge volume. Full calculation spreadsheets and a summary of the proposed recharge volume for each BMP are provided in the Appendix. Groundwater Recharge Summary HUC-14 Drainage Area Pre-Development Total Annual Recharge (cubic feet) Proposed Annual Recharge Volume (cubic feet) Percentage of Average Annual Pre-Construction Groundwater Recharge Volume Passaic River Lower (Nwk Bay to Saddle) 150,467 200,794 133.45% Stormwater Management Report | October 18, 2024 – Last Revised December 3, 2025 Page 4