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Stormwater Report_2016-05-23 - SUMMARYProposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts I. INTRODUCTION The applicant, Winterberry, LLC, is proposing to redevelop two adjacent parcels located at the corner of Bridge Street (Route 9) and Pomeroy Terrace in Northampton, Massachusetts. The parcels have been historically developed and are the locations of a dilapidated motel building with frontage on Bridge Street and two multi -unit apartment buildings with frontage on Pomeroy Terrace. Both parcels have been developed with the existing buildings and associated site improvements including a shared paved parking area, lawn areas, and landscaped areas. As part of the proposed project, the applicant will demolish the existing motel building as well as one of the existing multi -unit buildings. The applicant will renovate the existing multi -unit (6 -unit) building located at the corner of Bridge Street and Pomeroy Terrace. The applicant is proposing to construct three (3) new residential duplex buildings. Additionally, the project will demolish and reconfigure the existing access drives, driveways, and parking areas with associated site improvements. The site is located in close proximity to downtown and due to its urban setting has been mostly developed and cleared of natural vegetation. The existing stormwater best management practices on-site are limited to a single catch basin within the paved parking area. The following is a summary of the proposed site improvements: 1. Construction of three new residential duplex buildings: two buildings with frontage on Bridge Street and one building at approximately the location of the razed building with frontage on Pomeroy Terrace; 2. Associated site improvements including a parking lot, access drives, and common landscaped areas; 3. A stormwater management system to provide water quality treatment and attenuate peak discharge rates from the proposed development; The proposed project will qualify as a redevelopment project due to the overall reduction of impervious area located on the property. Furthermore, the inclusion of Low -Impact -Development Best Management Practices such as bio -retention areas, pervious pavers, and a subsurface infiltration basin will help mitigate the impacts of the proposed development. 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. Due to the eligibility of the project to qualify as a redevelopment, the proposed stormwater management system is subject to the MassDEP Stormwater Handbook "to the maximum extent practicable" R Levesque Associates, Inc. Page 1-1 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 2. SITE DESCRIPTION 2.1 Predevelopment Conditions The properties to be developed are located at 87 Bridge Street and 5 Pomeroy Terrace in an Urban Residence C (URC) zone and are listed as parcels 32A-185-001 and 32A-186-001 in the Northampton Assessor's Office. Figure 1 — USGS Map illustrates the location of the project. The properties are proposed to be combined and for the purposes of this report, the property will be referenced as a single parcel unless otherwise stated. The properties are bounded to the north by Bridge Street, to the south by a residential property, to the east by Pomeroy Terrace, and to the west by residential properties. The combined parcel area is approximately 0.71 acres in size. The properties have been historically developed and are mostly cleared of natural vegetation. There are four existing structures between the two parcels: • A dilapidated motel building with a detached garage accessory building, both to be razed, located at 87 Bridge Street; • Two multi -unit residential buildings, both located at 5 Pomeroy Terrace. The larger of the two buildings, the one with 6 -units located at the corner of Bridge Street and Pomeroy Terrace is to be rehabilitated while the other building is to be razed. There is an existing paved area located behind the buildings that served as a parking lot for both the motel building at 87 Bridge Street and the residential units located at 5 Pomeroy Terrace. The general parcel topography is relatively flat with approximately elevation 139 along the frontages with Bridge Street, Pomeroy Terrace, and along the southern property line. The grade rises slightly to approximately elevation 140 along the western property line. There is an existing catch basin on the property which is located at a low point within the paved parking area. The structure is to be demolished and removed as part of the proposed site improvements. 2.2 Resource Areas A majority of the parcel where the site improvements are proposed has been historically developed and cleared. There were no bordering or isolated vegetation wetland areas delineated on or adjacent to the parcel. 2.2.1 Floodplain R. Levesque & Associates, Inc. performed due diligence research on the property in regards to FEMA flood zone mapping. As demonstrated by the most recent FEMA Flood Insurance Rate Maps, the property is not located within any flood hazard areas, see Figure 2 — FEMA FIRM Map. R Levesque Associates, Inc. Page 2-1 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 2.2.2 Natural Heritage and Endangered Species Program R. Levesque Associates, Inc. performed due diligence research on the property in regards to Natural Heritage and Endangered Species Program (NHESP) areas. The property is not located within any areas delineated by NHESP as estimated or priority habitats of endangered species, see Figure 3 — NHESP Map. 2.3 Soils R. Levesque & Associates, Inc. researched the soils located on site with information readily available by the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS). Based on a review of the USDA Soil Survey of Hampden County, Massachusetts, Central Part, the site is comprised of the following soil types: Table 2.4: Hydrologic Soil Group Classification Soil Description Map Unit Symbol Hydrologic Soil Group Amostown-Windsor Silty Substratum — Urban Land 741A B Complex A series of test pits were conducted by Gary Weiner, P.E. SE# 2567 to evaluate the ability of the site to support the proposed stormwater drainage system components. A total of five test holes were dug in that vicinity with none showing evidence of estimate seasonal high groundwater, see Appendix B for additional soils information. In general, the soil evaluations corroborated the NRCS mapping of the area. 2.4 Post Development Conditions The applicant is proposing to demolish the existing dilapidated motel building located at 87 Bridge Street, the duplex apartment building of 5 Pomeroy Terrace, and all accessory buildings and associated site improvements. The applicant is proposing to construct three new residential duplex buildings as well as rehabilitate the existing multi -unit (6 -unit) residential apartment building located at 5 Pomeroy Terrace at the corner with Bridge Street. As part of the project, all of the existing paved areas will be demolished and a new parking and driveway configuration will be constructed. The proposed site improvements include the construction of a new stormwater management system on site which will incorporate low -impact development best management practices such as bio -retention areas, pervious paver parking areas, and a subsurface infiltration system. Water, gas, electric, and sanitary sewer services for the new duplex buildings will be R Levesque Associates, Inc. Page 2-2 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts brought on site via Pomeroy Terrace and the services for the existing multi -unit building will be maintained. The proposed stormwater management system will collect and convey runoff from the proposed site improvements via an underdrain and overflow system within the bio -retention areas to the proposed subsurface infiltration system. Subsurface manifolds will be installed to capture the roof runoff from the buildings and convey the runoff to the subsurface infiltration system. The subsurface infiltration system will provide groundwater recharge, peak rate attenuation, and water quality treatment for storm events up to and including the 100 -year storm event. There are no proposed stormwater infrastructure connections to the city drainage system. Should the existing catch basin with the parking area be connected to the drainage infrastructure on Pomeroy Terrace, the connection shall be capped and abandoned at the property line. Additionally, the site grading has been designed such that should an extreme storm event occur beyond the 100 - year storm event, water will surcharge into the parking lot and be conveyed off site via the proposed entrance on Pomeroy Terrace. Overall, the proposed site improvements will maintain the general drainage patterns of the site. R Levesque Associates, Inc. Page 2-3 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 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 the City of Northampton Stormwater Management Permit requirements and 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. A minimum Time of Concentration (Tc) of (6) minutes was used in the calculations. 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 R Levesque Associates, Inc. Page 3-1 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 3.1.1 Design Points In order to compare the difference between pre and post -development peak flows, existing and proposed watersheds were delineated. A single Design Points (DP) was established with flow paths representing the longest time of concentration of run-off in each tributary watershed. For this analysis, the design point was determined as follows: • DP -1 : City Drainage Infrastructure; This design point represents the runoff from the project area which flows offsite towards the city drainage infrastructure; 3.1.2 Pre -Development Hydrology The project area under existing conditions was analyzed as a single (1) sub -catchment discharging to the design points as described above. It has been assumed that the existing catch basin within the parking area behind the building connects to the drainage infrastructure on Pomeroy Terrace. The catch basin had high levels of accumulated sediment which may have been blocking the outlet invert pipe. Furthermore, in support of this assumption, there is a pipe in a drain manhole in Pomeroy Terrace that generally points towards the existing catch basin within the paved area behind the buildings. In any case, whether runoff from the site is conveyed overland by sheet runoff or through underground infrastructure, all runoff makes it to the city drainage infrastructure. The sub -catchment was delineated based on the existing developed areas and 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 five (5) sub -catchments discharging to the same design point as the existing conditions. The proposed stormwater management system will collect most of the runoff produced on site, however, due to challenges with grading areas such as the front yards of the residential buildings, a small portion of the property will still be conveyed to the city drainage infrastructure however the peak discharge rate has been drastically reduced. 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. The table below summarizes the Pre and Post -Development peak discharge rates for each Design Point: R Levesque Associates, Inc. Page 3-2 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 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 1.35 0.26 2.48 0.56 3.84 0.95 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 provided onsite attenuation through the proposed bio -retention areas and subsurface infiltration basin. 3.2 Hydraulic Analysis As part of the stormwater management system design, a hydraulic analysis was performed on the proposed underground infrastructure and is included in the HydroCAD model. The stormwater management system was analyzed to evaluate the performance of the proposed stormwater piping. The proposed stormwater infrastructure has been sized to convey storm events up to and including the 24-hour, 100 -year storm event. Please refer to Appendix C for the HydroCAD 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. • Standard #7 - No new stormwater conveyances e.g., outfalls) may discharge untreated stormwater directly to or cause erosion in wetlands or waters of the Commonwealth. The proposed project does not introduce any new outfalls and provides best management practices designed to the guidelines of the MassDEP Handbook. Therefore, no new untreated stormwater is discharged. • Standard #2 — Stormwater management systems shall ,be designed so that post - development peak discharge rates do not exceed pre -development peak discharge rates. This Standard may ,be waived for discharges to land subject to coastal storm flowage as defined in 370 CMR 70.04. R Levesque Associates, Inc. Page 3-3 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts The proposed stormwater management system has been designed such that the post - development peak discharge rates are less than the pre -development discharge rates for the 2 -year, 10 -year, and 100 -year 24-hour storms. See Appendix C for the Hydrologic Analysis. • Standard #3 - Loss of annual recharge to groundwater shall ,be eliminated or minimized through the use of environmentally sensitive site design, low impact development techniques, stormwater BMPs, and good operation and maintenance. At a minimum, the annual recharge from the post -development site shall approximate the annual recharge from pre -development conditions ,based on soil type. This Standard is met when the stormwater management system is designed to infiltrate the required volume as determined in accordance with the Massachusetts Stormwater Handbook. The proposed subsurface infiltration basin has been designed with the capacity to infiltrate the required recharge volume for the tributary impervious areas. See Appendix ® for the Required Recharge Volume Calculations. • Standard #4 - Stormwater management systems shall ,be designed to remove 80% of the average annual post -construction load of TSS. It is presumed that this standard is met when: a. Suitable practices for source control and pollution prevention are identified in a long term pollution prevention plan, and thereafter are implemented and maintained; ,b. Structural stormwater BMPs practices are sized to capture the required water quality volume determined in accordance with the Massachusetts Stormwater Handbook; and c. Pretreatment is provided in accordance with the Massachusetts Stormwater Handbook The stormwater management system has been designed to provide the required total suspended solids pre-treatment prior to discharge to the subsurface infiltration basin. The following describes the treatment train for the system: • Subsurface Infiltration basin #1: The treatment train for the subsurface infiltration basin includes filtration through the bio -retention areas and a proprietary sedimentation device to meet the water quality standards. The proposed proprietary treatment device has been sized to treat the water quality flow rate from the respective tributary areas. See Appendix ® for the Water Quality Volume Calculations. • Standard #5 - For land uses with higher potential pollutant loads, source control and pollution prevention shall ,be implemented in accordance with the Massachusetts Stormwater Handbook to eliminate or reduce the discharge of stormwater runoff from such land uses to the maximum extent practicable. If through source control and/or pollution prevention all R Levesque Associates, Inc. Page 3-4 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts land uses with higher potential pollutant loads cannot ,be completely protected from exposure to rain, snow, snow melt, and stormwater runoff, the proponent shall use the specific structural stormwater BMPs determined ,by MassDEP to ,be suitable for such uses as provided in the Massachusetts Stormwater Handbook. Stormwater discharges from land uses with higher potential pollutant loads shall also comply with the requirements of the Massachusetts Clean Waters Act, M. G.L. c. 27, §§ 26-53 and the regulations promulgated thereunder at 374 CMR 3.00, 374 CMR 4.00 and 374 CMR 5.00. • Standard #6 - Stormwater discharges within the Zone II or Interim Wellhead Protection Area of a public water supply, and stormwater discharges near or to any other critical area, require the use of the specific source control and pollution prevention measures and the specific structural stormwater BMPs determined ,by MassDEP to ,be suitable for managing discharges to such areas, as provided in the Massachusetts Stormwater Handbook. A discharge is near a critical area if there is a strong likelihood of a significant impact occurring to said area, taking into account site-specific factors. Stormwater discharges to Outstanding Resource Waters and Special Resource Waters shall ,be removed and set ,back from the receiving water or wetland and receive the highest and ,best practical method of treatment. A "storm water discharge" as defined in 374 CMR 3.04(2)(a) 7 or (b) to an Outstanding Resource Water or Special Resource Water shall comply with 374 CMR 3.00 and 374 CMR 4.00. Stormwater discharges to a Zone l or Zone A are prohibited unless essential to the operation of a public water supply. This standard is not applicable. • Standard #7 - A redevelopment project is required to meet the following Stormwater Management Standards only to the maximum extent practicable: Standard 2, Standard 3, and the pretreatment and structural BMPs requirements of Standards 4, 5, and 6. Existing stormwater discharges shall comply with Standard 7 only to the maximum extent practicable. A redevelopment project shall also comply with all other requirements of the Stormwater Management Standards and improve existing conditions. The project is considered a redevelopment due to the site being historically developed and the proposed site improvements reducing the overall impervious area. Under existing conditions, runoff is captured via a single catch basin in the paved parking area behind the buildings and is assumed to be conveyed to the city drainage infrastructure in Pomeroy Terrace. The proposed stormwater management system will disconnect from the city drainage infrastructure, provide water quality treatment, groundwater recharge, and peak rate attenuation which currently do not exist. Overall, the proposed stormwater management system will improve the existing conditions. R Levesque Associates, Inc. Page 3-5 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts • Standard #8 — A plan to control construction -related impacts including erosion, sedimentation and other pollutant sources during construction and land disturbance activities (construction period erosion, sedimentation, and pollution prevention) shall ,be developed and implemented. A Construction Period Erosion Control Plan has been provided in Appendix E. • Standard #9 — A long-term operation and maintenance plan shall ,be developed and implemented to ensure that stormwater management systems function as designed. A Long-term Operation & Maintenance Plan has been provided in Appendix F. • Standard #70 -A# illicit discharges to the stormwater management system are prohibited. An illicit discharge statement will be provided prior to discharge of stormwater to post - construction BMPs. See Appendix G for a copy of the Illicit Discharge Statement. 3.4 Stormwater Best Management Practices The proposed stormwater management system was designed utilizing stormwater best management practices (BMP) as set forth by the MassDEP Handbook. The BMPs utilized as part of the stormwater management system include deep -sump hooded catch basins, underground infrastructure, proprietary sedimentation devices, and subsurface infiltration basins. All of the BMPs were designed to meet the requirements of the MassDEP Handbook and will provide water quality treatment, groundwater recharge, and peak rate attenuation in order to mitigate the impacts of the proposed site improvements. See Appendix D — MassDEP Calculations for the calculations required to document compliance. The following section provides a description of the best management practices (BMPs) being utilized on site. 3.4.1 Bio -Retention Areas Bio -retention areas are a best management practice which is being utilized on site as part of the stormwater management system treatment train process to enhance water runoff quality. Bio - retention areas utilize soils, plants, and microbes to treat stormwater before it is discharged to downstream drainage infrastructure or infiltrated to provide groundwater recharge via an underdrain. The proposed bio -retention areas associated with this project will provide aesthetically pleasing vegetation within the paved parking area. Additionally, the bio -retention areas will provide stormwater pre-treatment prior to discharge to the subsurface infiltration system. 3.4.2 Proprietary Sedimentation Devices Proprietary sedimentation devices are being utilized on site for the pretreatment of stormwater runoff, in addition to the catch basins, prior to conveyance to the subsurface infiltration basins. R Levesque Associates, Inc. Page 3-6 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts Due to the stormwater management system utilizing underground infiltration systems, treatment of the stormwater runoff is essential to providing higher water quality than existing conditions. As much sediment should be removed from the stormwater runoff as possible to avoid clogging of the infiltration media. Therefore, maintenance of the proprietary device is crucial to the long- term effectiveness of the subsurface infiltration system. The stormwater management system is utilizing proprietary treatment devices in order to ensure that the amount of sediment reaching the subsurface infiltration basin is minimal. 3.4.3 Subsurface Infiltration Basin Subsurface Infiltration Basins are well suited to provide groundwater recharge an peak rate attenuation from watershed areas such as those associated with this project. The subsurface infiltration basin provides groundwater recharge by collecting and storing of runoff such that the underlying soils can absorb the water. The subsurface infiltration basins consist of underground stormwater chambers embedded in stone. The runoff is conveyed to the subsurface infiltration system via up -gradient underground drainage infrastructure. 3.5 Protection of Stormwater Best Management Practices during Construction Protection of the stormwater best management practices during construction will ensure the proper functioning of the stormwater management system and provide protection to the undisturbed areas until the site has been stabilized. Certain specific erosion and sedimentation controls and good practices to be performed by the site contractor have been documented in a Construction Period Erosion Control Plan. See Appendix E — Construction Period Erosion Control Plan. 3.6 Inspection and Maintenance of Stormwater Best Management Practices Frequent maintenance of the stormwater best management practices is essential to ensuring that the stormwater management system will function properly long-term. The MassDEP provides guidelines for the regular inspection and maintenance of the proposed stormwater best management practices. A Long -Term Stormwater Operation and Maintenance Plan has been prepared which dictates the inspection frequency and maintenance operations for each BMP. See Appendix F — Long -Term Operation and Maintenance Plan. 3.7 Illicit Discharge Compliance Statement RLA has prepared an Illicit Discharge Compliance Statement to document compliance with the Massachusetts Department of Environmental Protection Stormwater Management Handbook, see Appendix G. R Levesque Associates, Inc. Page 3-7 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 3.3 Low -Impact Development Alternatives Analysis Narrative RLA has prepared a Low -Impact Development Alternatives Analysis Narrative as part of the Stormwater Drainage Report, see Appendix H. 3.9 Northampton DPW Inspection Schedule Checklist RLA has prepared a checklist for the proposed project identify specific inspection milestones where the Department of Public Works shall make an inspection, see Appendix I. R Levesque Associates, Inc. Page 3-8 Proposed Residential Development Stormwater Drainage Report 87 Bridge Street & 5 Pomeroy Terrace Northampton, Massachusetts 4. CONCLUSION The proposed stormwater management system has been designed to mitigate the impacts of the proposed site improvements by providing a control for runoff water quality and water quantity. Implementation of Low -Impact -Development stormwater best management practices such as bio - retention areas, pervious paving, proprietary sedimentation devices, and a subsurface infiltration basin allows for a stormwater drainage design that is in conformance with the criteria set forth in the City of Northampton Stormwater Management Ordinance requirements and the Massachusetts Department of Environmental Protection Stormwater Management Handbook. As the project is considered a redevelopment, the proposed stormwater management system is subject to the MassDEP Stormwater Standards only to the "maximum extent practicable" however, every effort has been made such that the proposed stormwater management system is in full compliance with the standards. Additionally, the proposed site improvement and stormwater management system provides an improvement over existing conditions. R Levesque Associates, Inc. Page 4-1