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Easthampton Road stormwater excerpt.pdfPlatinum Self Storage Stormwater Drainage Report Easthampton Road Northampton, Massachusetts R Levesque Associates, Inc. Page 1-1 1. INTRODUCTION The applicant, Platinum Self Storage, is proposing to construct a self-storage facility located on a currently vacant parcel identified as lot 15-32-E in East Longmeadow, Massachusetts. Currently, there are no stormwater best management practices located on the project site. Generally, stormwater runoff from the property is tributary toward existing wetland areas north of the site and low points located off the property to the east. The proposed project involves the following: 1. Construction of 2 buildings to house storage units; 2. Construction of associated site improvements including an office building and access drives; 3. Installation of underground utilities to the office building including, water service, sanitary sewer, electric and communication services; 4. Construction of a stormwater management system including deep-sump hooded catch basins, underground infrastructure, proprietary sedimentation devices, bioretention areas and extended dry detention basins with sediment forebays. This report has been prepared to document the compliance of the proposed stormwater management system. The stormwater system has been designed to meet the standards set forth in the City of Northampton Stormwater Management Ordinance as well as the Massachusetts Department of Environmental Protection Stormwater Management Handbook. Platinum Self Storage Stormwater Drainage Report Easthampton Road Northampton, Massachusetts R Levesque Associates, Inc. Page 3-1 3. STORMWATER MANAGEMENT SYSTEM R. Levesque Associates, Inc. has prepared the following drainage system calculations for the proposed project site. These calculations were performed to document compliance with the guidelines set forth by Chapter 281 – Stormwater Management Ordinance of the Code of Ordinances of the City of Northampton, Massachusetts as well as the Massachusetts Department of Environmental Protection Stormwater Management Handbook (MassDEP Handbook). A detailed hydrologic analysis of the system was completed in order to evaluate the performance of the stormwater management system components, see Appendix C – Pre- and Post-Development Hydrologic Analysis. The proposed stormwater management system will collect runoff from on-site impervious areas and utilize stormwater best management practices to provide water quality treatment, groundwater recharge, and peak discharge rate attenuation. 3.1 Drainage Calculations R. Levesque Associates, Inc. utilized the HydroCAD software program, Version 10.0, developed by HydroCAD Software Solutions LLC, in order to create and analyze the site hydrology. The HydroCAD software is based upon the Soil Conservation Service (SCS) “Technical Release 20 – Urban Hydrology for Small Watersheds” and “Technical Release 55 – Urban Hydrology for small Watersheds” which are generally accepted industry standard methodologies. The analysis was conducted in order to establish the peak discharge rates and estimated run-off volume from the project site. This was accomplished to properly evaluate pre- and post-development conditions during various storm events. Contributing drainage areas were identified and soils, surface cover, watershed slope, and flow paths were evaluated to develop the necessary HydroCAD model input parameters. Drainage calculations were performed for the Pre- and Post-Development conditions for the 24- hour, 2, 10, and 100-year Type III storm events. The total rainfall for each of the storm events was based upon data provided by the United States Department of Commerce Technical Paper No. 40 – Rainfall Frequency Atlas of the United States. The total rainfall values used in the hydrologic modeling for each event are shown in the following table: Table 3.1: Design Rainfall Data 2-year, 24-hour storm 10-year, 24-hour storm 100-year, 24-hour storm 3.00 inches 4.50 inches 6.40 inches Platinum Self Storage Stormwater Drainage Report Easthampton Road Northampton, Massachusetts R Levesque Associates, Inc. Page 3-2 3.1.1 Design Points In order to compare the difference between pre- and post-development peak flows, existing and proposed watersheds were delineated. Three subcatchments draining to two Design Points (DP) were established with flow paths representing the longest time of concentration of run-off in each tributary watershed. For this analysis, the design points were determined as follows:  DP-1: This design point represents runoff from the property which is tributary to the existing drainage infrastructure in Easthampton Road;  DP-2: This design point represents the runoff from the property which is directed off-site towards the westerly property line. 3.1.2 Pre-Development Hydrology The project area under existing conditions was broken down into three (3) sub-catchments discharging to the two design points as described above. The sub-catchments were delineated based on the existing topography of the parcel and surrounding areas. The existing watershed areas are shown on the attached Figure 4 entitled “Pre-Development Watershed Plan”. Peak discharge rates for the design point are depicted in Table 3.1.4 below. 3.1.3 Post-Development Hydrology The project area under proposed conditions was broken down into seven (7) sub-catchments discharging to the same design points as the existing conditions. The proposed watershed areas are shown on the attached Figure 5 entitled “Post-Development Watershed Plan”. Peak discharge rates for the design point are depicted in Table 3.1.4 below. Platinum Self Storage Stormwater Drainage Report Easthampton Road Northampton, Massachusetts R Levesque Associates, Inc. Page 3-3 3.1.4 Peak Discharge Rates The table below summarizes the Pre and Post-Development peak discharge rates for each Design Point: Table 3.1.4 Pre- and Post-Development Peak Discharge Rates 2-year storm (cfs) 10-year storm (cfs) 100-year storm (cfs) Pre- Post- Pre- Post- Pre- Post- Design Point 1 1.64 1.63 5.95 5.55 13.16 12.89 Design Point 2 1.06 0.52 2.78 2.22 5.34 4.72 As depicted in the table, the post-development peak discharge rates do not increase over pre- development peak discharge rates for each of the storm events presented. This is accomplished by providing onsite attenuation with the proposed bioretention areas and detention basins. 3.2 Hydraulic Analysis R. Levesque Associates, Inc. utilized the Hydraflow Storm Sewer Extension for AutoCAD Civil 3D 2012 software program, Version 9, developed by Autodesk, Inc., in order to analyze the hydraulic capacity of the proposed underground infrastructure. The analysis was conducted in order to verify that the proposed conveyance piping has sufficient capacity to convey up to and including the 24-hour, 100-year storm event. The data from the analysis was used to properly size the conveyance piping such that there is no or minimal surcharge of stormwater above the rim elevations within the paved areas. As part of the input parameters required for the hydraulic analysis, the tributary inlet areas were delineated based on topography, any additional connected discharges, characteristic land use coverages and flow paths, see Figure 6 – Inlet Area Plan. A minimum Time of Concentration (Tc) of (6) minutes was used in the calculations. Please refer to Appendix D for the hydraulic analysis of the proposed stormwater management system. 3.3 MassDEP Stormwater Management Standards R. Levesque Associates, Inc. has designed the proposed stormwater management system to be in compliance with the MassDEP Stormwater Management Standards. Chapter 1, Volume 3 of the MassDEP Handbook outlines specific calculations, and other information, that must be submitted with each report to document compliance. The following summary highlights elements of the proposed project and how they apply to each standard.