48 001 STORMWATER REPORT 20240823 Sealed

STORMWATER REPORT 410 Main Street West Orange Block 113, Lots 262, 263, 264 & 308 PREPARED BY: SMBT CONSULTANTS LLC August 23, 2024 ____________________________ Ali Rangwala, P.E. NJ License No. GE53255

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

The peak discharge and runoff volume calculated from the existing drainage area is as indicated in Table No. 1. Drainage Area Combined, CN=84.8 Table No. 1 Summary of Pre-Development Condition (See Exhibit 1 for Stormwater Modeling Report) Peak Discharge (cfs) Runoff Volume (cu ft) Area 1010010100(ac) 2-Year 2-Year Year Year Year Year 0.274 0.637 1.156 2.183 1,963 3,615 7,023

E. POST-DEVELOPMENT CONDITION: The drainage area in the proposed conditions was divided in three catch basins: Basin 1 has categories as follows: 1. Proposed impervious surface (building & walkways). CN value = 98, area = 0.2217 Ac 2. Open Space (Grass good condition, hydrologic soil group C). CN value = 79, area = 0.19 Ac Weighted CN value for whole drainage area = 96 Basin 2 has categories as follows: 1. Open Space (grass good condition, hydrologic soil group C). CN value = 79, area = 0.0226 Ac 2. Proposed impervious surface (walkway). CN value = 98, area = 0.0099 Ac Weighted CN value for whole drainage area = 84.8 Basin 3 has categories as follows: 1. Open Space (Grass good condition, hydrologic soil group C). CN value = 79, area = 0.0027 Ac Total runoff from the proposed improvements will increase due to the increase of proposed impervious surfaces which will replace the existing grass area. The runoff from walkway and lawn areas will be intercepted by catch basins, which is connected to the proposed underground detention system with HDPE storm drains. Runoff from roof drain system will also be directed to the proposed underground detention system. Runoff from Basin 2 & Basin 3 will overflow outside the site. The peak discharge calculated from the Post-Development drainage area is as indicated in Table No. 2 Drainage Area Basin 1, CN = 96 Table No. 2 Summary of Post-Development Condition (See Exhibit 1 for Stormwater Modeling Report) Peak Discharge (cfs) Runoff Volume (cu ft) Area 1010010100(ac) 2-Year 2-Year Year Year Year Year 0.2479 0.843 1.317 2.231 2,477 3,974 6,907 Basin 2, CN = 84.8 0.0325 0.065 0.111 0.200 179 313 583 Basin 3, CN = 79 0.0027 0.005 0.010 0.020 13.6 27.1 56 This increased runoff volume from proposed conditions will be temporarily stored in an underground detention SYSTEM which has an outlet drainage pipe draining into the existing catch basin which is located on Park Drive N. F. WATER QUALITY VOLUME According to 25-29.4.Q, water quality volume should be calculated for proposed net increase of motor vehicle surface. Since the proposed parking area will be located on the first floor of the proposed building, which is a covered driveway, and new driveway is much smaller than existing (in pre-development conditions), no increase in motor vehicle surface will be provided with new development.

G. UNDERGROUND DETENSION SYSTEM DESIGN: The underground detention system is designed to temporarily store the increased runoff from proposed conditions of Basin 1 and making sure that total post-development peak discharge is equal to or less than the pre-development peak discharge. Detention system will be constructed using Stormtech SC-310 chambers. Below are the design metrics of the detention system: Chamber Model Units Number of Chambers Number of End Caps Voids in the stone (porosity) Base of Stone Elevation Amount of Stone Above Chambers Amount of Stone Below Chambers Area of system - • SC-310 Imperial 24 12 40 240.70 12 18 541 % ft in in sf Outlet Structure: • • • 2x9 INCH.ORIFICE HOLE @ I.E.:243.36 2x9 INCH.ORIFICE HOLE @ I.E.:242.95 5 DIA.ORIFICE HOLE @ I.E.:242.20 Refer to the attached Exhibit 1 – Stormwater Modeling Report and Sheet 7 – Stormwater management plan for design details of the proposed detention system. Table No. 3 Comparison of Peak Discharge (See Exhibit 1 for Stormwater Modeling Report) Peak Discharge (cfs) 2-Year 10-Year 100-Year Existing Condition 0.637 1.156 2.183 Proposed Condition (Discharge from Detention System + Basin 2 & 3) 0.546 1.063 2.081 H. CONCLUSIONS The proposed underground detention system as described herein will meet the design objectives set forth in Section C above. As indicated in Table No. 3 above, the discharge from the site in proposed conditions is less than discharge in existing condition for each of the 2, 10, and 100-year rainfall events. It is my professional opinion that the proposed BMP improvements to the site can be constructed without negatively impacting the offsite drainage.

EXHIBIT 1 Stormwater Modeling Report prepared using Hydraflow Hydrograph Extension for Autodesk Civil 3D Ver 2021

1 Watershed Model Schematic Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2024 1 2 3 4 5 6 7 Legend Hyd. Origin Description 1 2 3 4 5 6 7 EX-1 BASIN 1 BASIN 2 BASIN 3 BASIN 2 & 3 CHAMBERS TOTAL POST SCS Runoff SCS Runoff SCS Runoff SCS Runoff Combine Reservoir Combine Project: PRE & POST FINALLY.gpw Wednesday, 06 / 5 / 2024

2 Hydrograph Summary Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2024 Hyd. Hydrograph Peak Time Hyd. Inflow Maximum Total Hydrograph No. type flow interval Peak volume hyd(s) elevation strge used Description (origin) (cfs) (min) (cuft) (ft) (cuft) 1 SCS Runoff 0.637 1 725 1,963 ------ ------ ------ EX-1 2 SCS Runoff 0.843 1 722 2,477 ------ ------ ------ BASIN 1 3 SCS Runoff 0.065 1 722 179 ------ ------ ------ BASIN 2 4 SCS Runoff 0.005 1 722 14 ------ ------ ------ BASIN 3 5 Combine 0.070 1 722 193 3, 4 ------ ------ BASIN 2 & 3 6 Reservoir 0.500 1 726 2,349 2 100.77 7 Combine 0.546 1 726 2,542 5, 6 ------ PRE & POST FINALLY.gpw Time to (min) Return Period: 2 Year 455 ------ CHAMBERS TOTAL POST Wednesday, 06 / 5 / 2024

3 Hydrograph Report Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2024 Wednesday, 06 / 5 / 2024 Hyd. No. 1 EX-1 Hydrograph type Storm frequency Time interval Drainage area Basin Slope Tc method Total precip. Storm duration = SCS Runoff = 2 yrs = 1 min = 0.274 ac = 0.0 % = TR55 = 3.40 in = 24 hrs Peak discharge Time to peak Hyd. volume Curve number Hydraulic length Time of conc. (Tc) Distribution Shape factor = 0.637 cfs = 725 min = 1,963 cuft = 84.8 = 0 ft = 6.50 min = Type III = 484 EX-1 Q (cfs) Q (cfs) Hyd. No. 1 -- 2 Year 1.00 1.00 0.90 0.90 0.80 0.80 0.70 0.70 0.60 0.60 0.50 0.50 0.40 0.40 0.30 0.30 0.20 0.20 0.10 0.10 0.00 0 120 240 Hyd No. 1 360 480 600 720 840 960 1080 1200 1320 1440 1560 0.00 Time (min)