Drainage Report

Introduction f : The West Essex Highlands property has been designated as the location for muléfam help meet the affordable housing requirements of the Township of West Orange’ Nib" AS HarEuf thes court approved development plan for this property the limit of disturbance on the 120-acre parcel is limited to 30 acres leaving the remainder as open space to be utilized as passive recreation in its natural condition. The project design includes a stormwater management design that meets state stormwater standards for water quality, groundwater recharge and discharge rate. The drainage design has small scale infiltration basins with pre-treatment devices to provide green technology solutions for quantity , quality and recharge. One large scale infiltration basin with NJDEP approved green technology device for water quality throughout the drainage area and two porous pavement systems to provide green technology solutions for quality, quantity and recharge. The stormwater control systems have been routed with exfiltration for the quantity calculation to demonstrate compliance without reliance on exfiltration. The groundwater recharge calculations use exfiltration based on field soil testing with the appropriate factor of safety to demonstrate compliance with the recharge requirement and a calculation is included to demonstrate the basins completely evacuate within 72 hours. The calculations utilize Hydrocad, version 9.10, NJDEP approved intensity curves, and the SCS TR- 20 methodology to calculate the pre-and post-development runoff volumes. Methodology The drainage for the development area of the property is calculated for the existing and proposed conditions for the areas going to each of the 6 detention systems. Overland flow areas that will not drain to the detention system are calculated individually in each of the areas. The existing conditions for the site are considered wooded in good conditions per the ordinance standards. Discharge rates for the existing conditions under the 2, 10 and 100-year storm events are calculated with The Hydrocad drainage program and the SCS TR-20 method. Hydrographs for the proposed conditions for areas detained and un-detained are developed. Allowable discharge rates for the detention system are calculated and the proposed hydrograph is routed through a proposed detention system until the desired discharge rate is

-detained are developed. Allowable discharge rates for the detention system are calculated and the proposed hydrograph is routed through a proposed detention system until the desired discharge rate is achieved for each storm. Discussion Soil Conditions & Groundwater Recharge An extensive geotechnical investigation has been performed on the subject property to establish existing conditions. 15 borings and 27 test pits were performed in the development area in 2021 during the January to April testing window and testing was previously performed with a past development application. The results indicated a fairly uniform soil conditions. The percolation rates onsite indicate primarily a type D soil however due to the published maps indicating C soils onsite the stormwater calculations utilize C soils when selecting curve numbers. 2 Anderson Consulting Services LLC 40 Miller Drive, Boonton, NJ 07005 973-975-0703 (O) 973-886-7336 (C) PAndersonpe@gmail.com