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HomeMy WebLinkAbout5 Project NarrativeTighe&Bond Section 2 Project Description 2.1 Project Introduction On behalf of National Grid, Tighe & Bond has prepared the following Stormwater Management Report to support local permitting efforts for the South Yarmouth Liquified Natural Gas (LNG) Project located at 127 Whites Path in South Yarmouth, Massachusetts. The Town of Yarmouth maintains Stormwater Management Regulations that specify the required stormwater management procedures, and which require that proposed construction activities resulting in land disturbance of one acre or more to obtain a Stormwater Management Permit (SMP) from the Conservation Commission. This report has been prepared to document how the project complies with the Town's Regulations and the Massachusetts Stormwater Handbook. The project site is on the east side of the facility, and has an existing LNG tank and associated infrastructure, gravel vehicle paths, and an impoundment around the tank to contain LNG in the event of a leak. There is also an existing stormwater management system that collects runoff from the developed portion of the site and outlets it to a natural depression in the woods to the South. The proposed project includes the construction of a new LNG tank and associated infrastructure, and a Stormwater management system to mitigate runoff from the new impervious areas that will be generated by the project. The existing tank will remain online until the new tank is constructed and commissioned, at which point the existing tank will be demolished and removed. The drainage improvements associated with the new stormwater management system include trench drains, drainage swales, catch basins, drain manholes, and stormwater treatment structures to collect, pre- treat, and convey runoff away from the infrastructure and into the proposed stormwater infiltration basin. A United States Geological Survey (USGS) Site Location figure, Aerial Photograph, and Priority Resource figure of the Project site are provided in Appendix B as Figures 1-3, respectively. Project plans are provided separately. 2.2 Existing Conditions The Facility is located south of Rt. 6 in South Yarmouth within the Business 3 (B3) zoning district. The Project Site is on the east side of the facility, in a minimally developed area that currently consists of an LNG tank and associated infrastructure, gravel vehicle paths, and an impoundment. Under existing conditions, stormwater runoff from the project area flows to the West, where it is collected in a series of catch basins located in the paved access roads. The existing system does not include any pretreatment, and once collected, the runoff is discharged in a basin on the southern edge of the facility. The Natural Resources Conservation Service (NRCS) soil data was obtained through the Web Soil Survey portal on the United States Department of Agriculture (USDA) NRCS website. Soils within the project area, as published in the USDA Soil Survey for Barnstable County, Version 24, dated September 5, 2025, include Urban Land and Carver Coarse South Yarmouth LNG Project Stormwater Management Report 2-1 Tighe&Bond Sand. The NRCS Soils Mapping is provided in Appendix C. The hydrologic soil group (HSG) and further description for each soil association are presented in Table 2.1 below. Table 2.1 Soil Descriptions Soil Map Soil Name Hydrologic Soil Designation Group (HSG) 252A Carver coarse sand, 0 to 3 percent A slopes 252E Carver coarse sand, 3 to 8 percent A slopes 602 Urban Land N/A The hydrologic soil group designation (HSG) for this soil type is listed as A. The HSG rating for soil types is based on estimates of runoff potential. Soils are assigned to one of four groups according to the rate of water infiltration when the soils are not protected by vegetation, are thoroughly wet, and receive precipitation from long duration storms. Soils designated as HSG A are generally well draining and have a high capacity for water infiltration. Based on this, soils within the project area are consistent in their ability to infiltrate water and in the development of runoff. The NRCS Soils Mapping and USGS superficial geologic mapping is provided in Appendix C. Several borings were completed on March 20,2023 to confirm the soil characteristics and determine ground water levels. The results of the exploration program confirm USDA soils data and USGS surficial geologic information provided in Appendix C. The depth to groundwater on the day of borings ranged from 19.5 feet to 26.0 feet. For this analysis, the 19.5-foot reading will be used as the basis for calculations since it is most conservative. Several borings were completed on March 20,2023 that confirm soil characteristics and determined ground water levels. The depth to groundwater on the day of borings ranged from 19.5 feet to 26.0 feet below finish grade. For this analysis, the 19.5-foot reading will be used as the basis for calculations since it is most conservative. As per Section 2.4(1)(K) of the Town's Stormwater Management Regulations, the seasonal high groundwater elevation was determined using the Cape Cod Commission adjustment method specified in Technical Bulletin 92-001. The nearest Cape Cod Index Well is A1W- 247R, and the Cape Cod Commission's Index Well Readings as of March 22, 2023 shows a groundwater level of 19.67 feet below finish grade. The Table of Potential Water Level Rise for Index Well A1W-247R begins at 21.9 feet, which is closest to the 19.5-foot depth to groundwater on the day of explorations - indicating that an adjustment of 0 feet is appropriate. Therefore, a groundwater depth of 19.5 feet will be used in the design of the infiltration basin. The borings were completed at existing elevation 36, which puts the seasonal high groundwater at elevation 16.5. The hydraulic calculations included in this report have been completed using the groundwater information previously specified, and with the soil data from the USGS; and surficial geologic mapping from USGS, which will be confirmed with additional soil explorations that will be completed during construction. The Stormwater Infiltration Data Report from the borings is also provided in Appendix C. South Yarmouth LNG Project Stormwater Management Report 2-2 Tighe&Bond The runoff curve numbers (CN) used in the calculation of the composite CN for each drainage area is based on the values provided in TR-55, Urban Hydrology for Small Watersheds. CN values vary depending on the type of ground cover and soil HSG. Existing Conditions Drainage Areas were delineated based on topography and stormwater discharge location. A summary of each existing conditions drainage area, including sizing, CN and time of concentration calculations, are provided in the HydroCAD reports in Appendix D. An Existing Conditions Drainage Area Map is provided as Figure 4 in Appendix B. 2.3 Floodplain Management The Federal Emergency Management Agency's Flood Insurance Rate Map (FIRM) Community Panel Number 25001C0579J, effective July 16, 2014 shows the project site outside of any floodways or floodplains, as attached in Appendix B. Therefore, no floodplain is identified on this site. 2.4 Proposed Improvements The proposed project includes site clearing, regrading, and tree removal. It also includes the construction of a new LNG tank adjacent to the existing tank and associated infrastructure; as well as the construction of a new paved access road around the perimeter, new fencing in various locations, and a stormwater management system. The stormwater management system includes catch basins, trench drains, drain manholes, stormwater treatment units, sediment forebays and an infiltration basin. The design has been prepared in accordance with the Town's Stormwater Management Regulations, and the recommendations in the Massachusetts Department of Environmental Protection (MassDEP) Stormwater Handbook. Under proposed conditions, all of the stormwater runoff from the project area is collected and treated within the new stormwater management system. The new system is hydrologically disconnected from the existing system and no runoff from the redevelopment area will enter the existing stormwater system. Stormwater runoff from the new development generally flows to the south, where it is collected in catch basins, trench drains, or directly into a series of sediment forebays. Runoff entering the catch basins will flow through a series of drain manholes and a Water Quality Treatment Unit, otherwise it will flow into the sediment forebays for pretreatment. The site is located within the Aquifer Protection Overlay district, which will require that the proposed stormwater system achieve pretreatment of 44% Total Suspended Solids (TSS) removal prior to infiltration. All runoff from impervious vehicular areas will achieve the required 44% pretreatment before entering the basin for infiltration, and the basin has been sized to retain the volume of 1 inch per acre of impervious coverage. A summary of each proposed condition drainage area, including sizing, RCN and time of concentration calculations, are provided in the HydroCAD reports in Appendix D. A Proposed Conditions Drainage Area Map is provided as Figure 5 in Appendix B. The proposed stormwater management system treats both the quality and the quantity of stormwater discharge from the site. The system includes best management practices (BMP's) such as deep -sump, hooded catch basins, trench drains, drain manholes, a proprietary stormwater treatment unit, two sediment forebays, and an infiltration basin. South Yarmouth LNG Project Stormwater Management Report 2-3 Tighe&Bond A brief description of the proposed Best Management Practices incorporated into the stormwater management system are as follows: Deep -Sump, Hooded Catch Basins: Catch basins provided throughout the site collect stormwater runoff from the proposed vehicular areas and are connected to the project's stormwater collection system. The deep -sump and hooded outlet provide runoff an opportunity to separate from solids and floatable pollutants prior to discharge and are used as a pretreatment device throughout the project. Proprietary Treatment Devices: Structural stormwater treatment device, proposed as Stormceptor STC450i, is designed to mechanically separate pollutants from stormwater flows through centrifugal force and vortex separation. A unit is proposed at the end of the treatment train in the stormwater management system, prior to discharging into the infiltration basin. Each unit has been sized in accordance with guidance provided by MassDEP to ensure proper sediment removal efficiencies. Infiltration Basin: The proposed surface infiltration basin is the collection point for all runoff from the project and is located in the south area of the site. The infiltration basin has been designed in accordance with the Massachusetts Stormwater Handbook to provide the required groundwater recharge and water quality volume for the project. The basin is equipped with an emergency overflow spillway to minimize the potential for flooding during extreme storm events. Trench Drain: The proposed trench drains have been designed to accept untreated stormwater runoff from the site and convey it to either a series of sediment forebays or directly to the infiltration basin. 2.5 Method of Hydrologic and Hydraulic Analysis The following storm drainage design criteria were used in the hydrologic and hydraulic analyses: 1. Piped storm drainage system and the outlets are designed for a 25-year storm event. 2. Minimum time of concentration = 6 minutes. 3. For SCS peak flow calculations, Curve Number were as follows: a. >75% Grass Cover, Good (HSG A) = 39 b. Gravel Surface (HSG A) = 96 c. Paved Parking (HSG A) = 98 d. Woods, Good (HSG A) = 30 4. Minimum diameter pipes, excluding roof leaders, underdrains and foundation drains = 12 inches 5. Minimum pipe slope = 0.5 percent 6. Watershed areas delineated using polylines in AutoCAD Civil 3D 2025. South Yarmouth LNG Project Stormwater Management Report 2-4 Tighe&Bond 7. The storm water management plan for the site is designed to treat the water quality volume, remove total suspended solids and infiltrate the required recharge volume while reducing peak flow. 8. Comparative hydrology analyzed using HydroCAD Stormwater Modeling software Version 10.20-4c. A comparative hydrologic analysis of the pre -development and post -development site was performed to determine the impacts of the proposed project to peak discharge rates and stormwater runoff volumes. HydroCAD Release 10.20-4c is a hydrology and hydraulics software using Technical Release (TR) 20 and TR-55 methodologies for the determination of stormwater runoff quantities. The HydroCAD Report for both pre- and post - development conditions for each storm event is provided in Appendix D. Table 2.2 below presents the design rainfall depths for the storm events evaluated, as provided by the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service Atlas 14 PLUS. Table 2.2 Design Rainfall Depths Storm Event Rainfall Depth (inches) 1-Year 2.84 2-Year 3.35 5-Year 4.18 10-Year 4.87 25-Year 5.82 50-Year 6.54 100-Year 7.28 The proposed storm drain collection system was analyzed to ensure that the pipe capacities proposed can accommodate the 25-year storm event, as well as meeting minimum and maximum flow velocity. Results of that analysis are provided in Appendix D. South Yarmouth LNG Project Stormwater Management Report 2-5