48 001 STORMWATER REPORT 20240823 Sealed

A. SCOPE OF REPORT: The purpose of this report is to present the analysis performed in support of the stormwater management design for the proposed building to be constructed on a 0.274 acres parcel of land located at Block 113, Lots 262, 263, 264 & 308 in West Orange, Essex County, New Jersey. This site has a frontage on Main Street. This report has been prepared in accordance with the requirements of the New Jersey Residential Site Improvements Standards (N.J.A.C. 5:21-7), New Jersey Department of Environmental Protection Stormwater Regulations (NJDEP 7:8) and Township of West Orange local ordinances. B. PROJECT INFORMATION: The applicant has applied to the Township of West Orange to construct a 3-story building with car parking at first floor & associated walkways. The project will disturb approximately 0.274 acres, of which approximately 0.233 acres will be used for the proposed impervious improvement including proposed building, sidewalk, and driveway. For the purpose of this report, approximately 0.274 acres considered as tributary will be considered for drainage calculations as shown in the attached “Drainage Area Map” (Exhibit 2). C. DESIGN CRITERIA: 1. Runoff Rate: The stormwater analysis described herein has been performed in accordance with Urban Hydrology for Small Watersheds (Technical Release 55), dated June 1986. The Pre and Post development runoff rates and volumes used in the design were calculated using computer software program Hydraflow Hydrographs Extensions of AutoCAD Civil 3D Version 2024. This program incorporates the methodology used in SCS TR-20 and TR-55 to compute and route flow hydrographs. 2. The proposed Subsurface Infiltration Basin is designed as per New Jersey Stormwater Best Management Practices Manual, Chapter 10.2. The basin is designed to temporarily accommodate the increased runoff volume from post development conditions and make sure the postdevelopment discharge does not exceed the pre-development discharge value for 2, 10, and 100year storm events. 3. The proposed Subsurface Infiltration Basin is designed as per Township of West Orange section §25-29.4.Q water quality standards. D. PRE-DEVELOPMENT CONDITION: The general topography of the site is characterized by a southerly slope. The highest elevation on the drainage area is approximately 258.01 feet and is in the location of the Northwest corner of the

The general topography of the site is characterized by a southerly slope. The highest elevation on the drainage area is approximately 258.01 feet and is in the location of the Northwest corner of the site. Therefore, runoff from the site generally drains to the south towards front of the site. The drainage area in the existing conditions was calculated using the Topographic Survey prepared by Allstate Mapping & Layout company dated December 5, 2023. The drainage area in the existing conditions was divided in two categories as follows: 1. Open Space (Grass good condition, hydrologic soil group C). CN value = 79, area = 0.19 Ac 2. Existing Impervious surface (driveway, walkways, ex. building). CN value = 98, area = 0.084 Ac Weighted CN value for whole drainage area = 84.82