21 Stormwater Mgmt Report_082025

STORMWATER MANAGEMENT REPORT Prepared for: Proposed Retail Store Block 157, Lot 5 457 Mount Pleasant Avenue Township of West Orange Essex County, New Jersey Prepared by: N.J. Certificate of Authorization 24GA28161700 30 Independence Blvd., Suite 200 Warren, NJ 07059 908-668-8300 BENJ File No. NJA230139.00 ________________________________________________ Tung-To Lam, P.E. New Jersey Professional Engineer License No. 47863 August 2025

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Table of Contents Introduction ............................................................................................................................. 1 Stormwater Management Design Methodology ...................................................................... 1 2.1 Rainfall Data .................................................................................................................... 2 2.2 Site Soils .......................................................................................................................... 3 2.3 Curve Numbers................................................................................................................ 3 2.4 Time of Concentration ..................................................................................................... 3 Pre-Development Site Conditions ........................................................................................... 3 3.1 Point of Analysis #1 ......................................................................................................... 4 3.1.1 Existing Drainage Area #1 .................................................................................. 4 Post-Development Site Conditions ......................................................................................... 4 4.1 Point of Analysis #1 ......................................................................................................... 4 4.1.1 Proposed Drainage Area #1................................................................................ 5 4.2 Stormwater Quantity Controls.......................................................................................... 5 4.3 Water Quality Controls..................................................................................................... 6 4.3.1 Pervious Paving Systems ................................................................................... 6 5.1 Groundwater Recharge ................................................................................................... 7 5.2 Soil Erosion and Sediment Control .................................................................................. 7 5.3 Low-Impact Development and Non-Structural Stormwater Management Facilities ........

rosion and Sediment Control .................................................................................. 7 5.3 Low-Impact Development and Non-Structural Stormwater Management Facilities ........ 7 Conclusions............................................................................................................................. 9 ▪ Stormwater Management Report for Proposed Retail Store▪ i ▪ August 2025 ▪

Appendices A. Pre- vs. Post-Development Hydrographs - Current Adjusted Precipitation Depths ⬧ McCuen-Spiess Sheet Flow Length Calculations ⬧ 2-Year Storm Event ⬧ 10-Year Storm Event ⬧ 100-Year Storm Event ⬧ Pre- vs. Post Comparison B. Pre- vs. Post-Development Hydrographs - Future Adjusted Precipitation Depths ⬧ 2-Year Storm Event ⬧ 10-Year Storm Event ⬧ 25-Year Storm Event ⬧ 100-Year Storm Event ⬧ Pre- vs. Post Comparison C. Water Quality ⬧ Water Quality Storm Hydrograph D. Conveyance Calculations ⬧ Pipe Sizing ⬧ Pervious Pavement Underdrain Sizing E. Supporting documents ⬧ NOAA Atlas 14 Precipitation Estimate Report F. Maps ⬧ Tax Map ⬧ Zoning Map ⬧ Aerial Map ⬧ USGS Map ⬧ State Planning Area Map ⬧ NJDEP Potential Freshwater Wetlands Map ⬧ NJDEP Stream Map ⬧ HUC-14 Sub-Watershed Map ⬧ FEMA FIRM Map ⬧ NJDEP Potential Threatened / Endangered Species Habitat Map ⬧ NJDEP Known Contaminated Sites Map ⬧ Hydrologic Soils Group Map ⬧ Depth to Seasonal High Water Table Map ⬧ Drainage Area Maps o Existing Drainage Area Map o Proposed Drainage Area Map ▪ Stormwater Management Report for Proposed Retail Store▪ ii ▪ August 2025 ▪

Introduction The subject property is located at 457 Mount Pleasant Avenue in the Township of West Orange, Essex County, New Jersey. The property is identified as Block 157, Lot 5 on the Township of West Orange tax maps and is a total of 3.71 acres in size and will hereafter be referred to as “the site”. The site is bordered to the north by Mount Pleasant Place with residential properties and a Township Fire Department building beyond; to the east by residential properties and Mount Pleasant Place, to the west by Exxon Gas Station, the Rahway River, Pleasant Valley Way, and Trojan Self Storage with residential properties beyond; and to the south by Mount Pleasant Ave with Francis A. Byrne golf course beyond. A tax map and aerial map are included at the beginning of Appendix F for reference. The site is currently an existing shopping center within the B-2 Zone. The proposed development consists of converting the existing florist footprint within the shopping center to a proposed retail store with ancillary sidewalks and parking reconfiguration. While this project proposes a decrease in total impervious surface, an increase less than 0.25 acres of regulated motor vehicle surface area, and disturbs less than 1 acre, this project is a major development due to the Township of West Orange Ordinance 2918-25, adopted May 20, 2025. Based on the Township Ordinance, a “Major Development” means an individual “development,” including redevelopment, as well as multiple developments that individually or collectively result in: 1. The disturbance of 0.5 acres of land since February 2, 2004. 2. The creation of 5,000 SF or more of “regulated impervious surface” since February 2, 2004. 3. The creation of 5,000 SF or more of “regulated motor vehicle surface” since March 2, 2021, or 4. A combination of 2 and 3 above that totals an area of 5,000 SF or more. The same surface shall not be counted twice when determining if the combination area equals one-quarter acre or more. The proposed project will disturb more than 0.5 acres of land and create more than 5,000 SF of regulated motor vehicle surface. Therefore, the proposed project will be required to comply with the Township requirements for water quality and quantity treatment. The site is previously disturbed and located in Planning Area-1, therefore recharge requirements will not apply. This report summarizes the design

equirements for water quality and quantity treatment. The site is previously disturbed and located in Planning Area-1, therefore recharge requirements will not apply. This report summarizes the design objectives, methodology, and calculations for the conveyance, attenuation, and discharge of stormwater runoff leaving the site and is meant to accompany the Site Plan documents prepared by Bohler. Pre-development and post-development conditions are examined for analysis of the stormwater quantity, water quality, and groundwater recharge based on the NJDEP Stormwater Management Regulations (NJAC 7:8) and soil erosion and sediment control based on The Standards for Soil Erosion and Sediment Control in New Jersey. Stormwater Management Design Methodology In accordance with the NJDEP Stormwater Management Regulations, the proposed development must meet the requirements, if appropriate, for stormwater quantity reductions, water quality, groundwater recharge, etc. The assessment of stormwater runoff has been based upon the Soil Conservation Service Method (SCS) Unit Hydrograph as described in Chapters 7, 9, 10, 15 and 16, Part 630 Hydrology, National Engineering Handbook (NEH) (previously described in Technical ▪ Stormwater Management Report for Proposed Retail Store▪ 1 ▪ August 2025 ▪

Release Number 55 (TR55), “Urban Hydrology for Small Watersheds”). Theoretical storms are modeled with the 24-Hour SCS Unit Dimensionless Hydrograph using the NOAA Atlas 14 Type D rainfall distribution and recurrence intervals of 2, 10, and 100 years. Hydrograph creation and routings are accomplished using the HydroCAD stormwater modeling program by HydroCAD Software Solutions, LLC. 2.1 Rainfall Data Rainfall data used in the stormwater calculations of this report are obtained from different sources based on the latest NJ stormwater management regulations (NJAC 7:8). The Water Quality storm event is based on the NJ BMP Manual Chapter 5 definition of having a total rainfall depth of 1.25 inches and a total duration of two (2) hours. Twenty-four-hour rainfall frequency data in Essex County for all other storms has been obtained from the Engineering Field Handbook NJ Supplement last revised August 2012 (developed from data contained in NOAA Atlas 14 Volume 2). Per amended NJ stormwater management regulations dated July 17, 2023 the precipitation depths for the 2-, 10-, and 100-year storm events are determined using adjustment factors applied to the NOAA Atlas 14 rainfall estimates. The rainfall depth values used in this project are listed in Table 2.1 and 2.2. TABLE 2.1 – Current Adjusted, 24-Hour Rainfall Frequency Data Event (year) 2 10 100 NOAA Atlas 14 Rainfall (inches) 3.44 5.22 8.66 Adjustment Factor 1.01 1.03 1.06 Current Adjusted Rainfall (inches) 3.47 5.38 9.18 TABLE 2.2 – Future Adjusted, 24-Hour Rainfall Frequency Data Event (year) 2 10 25** 100 NOAA Atlas 14 Rainfall (inches) 3.44 5.22 6.44 8.66 Adjustment Factor 1.19 1.22 1.25 1.33 Future Adjusted Rainfall (inches) 4.09 6.37 8.02 11.52 ** NJAC 7:8 does not contain adjustment factors for the 25-year storm. The factors listed have been obtained from NJ Extreme Precipitation Projection Tool (https://njprojectedprecipitationchanges.com/) in order to estimate the increased rainfall. ▪ Stormwater Management Report for Proposed Retail Store▪ 2 ▪ August 2025 ▪

2.2 Site Soils Site soil information has been obtained from the USDA Natural Resources Conservation Service (NRCS) Web Soil Survey database. The soil types present within the studied watershed are shown in Table 2.2. The soils consist of hydrologic soil group (HSG) Type A, therefore the CN values used in calculations are those associated with Type A soils. TABLE 2.3 - Site Soil Properties 2.3 Soil Symbol Soil Name Characteristics Hydrologic Soil Group Rating DunC Dunellen Sandy Loam 8-15% slopes A DuuB Dunellen – Urban land 0-8% slopes A Curve Numbers For determining the runoff flows, the cover types in combination with the HSG rating must be determined for each drainage area. These are used to determine the runoff curve number (CN) values that are used in the hydraulic calculations. The CN values used in this project are listed in Table 2.3. TABLE 2.4 - CN Values 2.4 Land Cover Description HSG A Open space, Good Condition (grass cover >75%) 39 Impervious Surfaces 98 Time of Concentration The time of concentration (Tc) is calculated in accordance with Chapter 15 of the NRCS National Engineering Handbook and has been calculated separately for the pervious and impervious areas within each drainage area. Where there is a long stretch of sheet flow in the proposed condition, the McCuen-Spiess formula is used to determine the appropriate length, not to exceed 100 feet. McCuen-Spiess sheet flow length calculations are provided in the beginning of Appendix A, and full Tc calculations for each drainage area can be found in the HydroCAD summaries in Appendix A. Pre-Development Site Conditions The studied watershed area is a total of 1.36 acres in size and consists of one unique drainage areas: Existing Drainage Area EDA 1A, which is described in more detail below. In the predevelopment condition, the site is an existing florist shop within a shopping center with associated ▪ Stormwater Management Report for Proposed Retail Store▪ 3 ▪ August 2025 ▪

parking. Currently, the runoff generated on site outfalls to the Rahway River along the western property line via overland flow. The Existing Drainage Area Map in Appendix F illustrates the limits of the existing drainage area and how it relates to the existing site conditions. 3.1 Point of Analysis #1 The existing drainage area flows to Point of Analysis #1 located along the western property line. The Existing Drainage Area Map in Appendix F illustrates the identified point of analysis and how it relates to the existing topography on the site. 3.1.1 Existing Drainage Area #1 Existing Drainage Area #1 contains 1.36 acres of land, of which 1.34 acres are impervious surface, and includes the portion of the existing site and building on which improvements are proposed. The portion of the shopping center which will not be disturbed is excluded from this analysis. The topography of the area slopes from east to west from a maximum elevation of approximately 378 to a minimum elevation of approximately 360 with slopes ranging from approximately 1% to 66%. The runoff from Existing Drainage Area #1 flows from east to west and ultimately discharges to Point of Analysis #1. Post-Development Site Conditions The post-development condition for the site includes the repurposing of the existing florist space to a proposed retail store with parking reconfiguration, sidewalks, stormwater infrastructure, and other site improvements. The proposed site is designed in a manner that generally maintains the existing drainage patterns. The studied watershed area in the post-development condition contains the same 1.36-acre area that was studied in the pre-development condition and consists of 1 unique drainage area: Proposed Drainage Area PDA 1A, which is described in more detail below. A proposed stormwater conveyance system will collect the runoff from the proposed building and impervious areas via pervious pavement, an inlet, manholes, and stormwater piping and redirect it to the pervious pavement system and ultimately the existing inlet located along Mount Pleasant Place. The construction of the proposed improvements will disturb approximately 0.65 acres of land and will result in a reduction of approximately 0.07 acres of impervious coverage on the site in the post-development condition. The Proposed Drainage Area Map in Appendix F illustrates the limits of the proposed