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HomeMy WebLinkAbout897 Route 28 Abstract Stormwater Management Report for Site Plan Review on August 12, 2025Abstract of Stormwater Management Report Project Overview This Stormwater Management Report, prepared by Merrill Engineers and Land Surveyors on July 24, 2025, evaluates stormwater impacts for the proposed redevelopment of a 1.088-acre site at 897 Main Street (Route 28), South Yarmouth, Massachusetts, by Autonomous CRE +. The project involves demolishing existing structures and constructing a 27-unit residential building with associated parking, driveways, and utilities. The site is located in a FEMA Flood Zone X (areas of minimal flood hazard) and is not within NHESP Priority Habitat or Estimated Habitat areas. Hydrologic Soil Group (HSG) A soils predominate, characterized by high infiltration rates. Methodology Stormwater modeling was conducted using HydroCAD® software (version 10.20-3h) with the SCS TR-20 method and Type III 24-hour rainfall distributions. Rainfall depths were sourced from NOAA Atlas 14 for South Yarmouth: 3.35 inches (2-year), 4.88 inches (10- year), 5.83 inches (25-year), and 7.30 inches (100-year). Existing and proposed conditions were analyzed for two design points (DP1: off-site to the north; DP2: off-site to the south). Existing Conditions The site comprises two watersheds totaling 1.088 acres, with 29.31% impervious cover (0.319 acres). Runoff primarily sheet-flows to adjacent properties or infiltrates on-site. Peak runoff rates and volumes are as follows: Storm Event DP1 Peak Rate (cfs) DP1 Volume (af) DP2 Peak Rate (cfs) DP2 Volume (af) 2-year 0.00 0.000 0.38 0.068 10-year 0.01 0.002 0.73 0.114 25-year 0.03 0.005 0.96 0.144 100-year 0.11 0.014 1.38 0.194 Existing features include a depressed area and assumed leaching pits for infiltration. Proposed Conditions The redevelopment increases impervious cover to 71% (0.773 acres). Stormwater management incorporates two subsurface infiltration systems (StormTech SC-740 chambers with Isolator Row Plus), proprietary treatment units (First Defense HC-3), and a preserved depressed area for additional storage. Proposed peak rates and volumes are reduced compared to existing conditions: Storm Event DP1 Peak Rate (cfs) DP1 Volume (af) DP2 Peak Rate (cfs) DP2 Volume (af) 2-year 0.00 0.000 0.00 0.000 10-year 0.00 0.001 0.01 0.003 897 Route 28 Stormwater Management Abstract for Site Plan Review on August 12, 2025 Storm Event DP1 Peak Rate (cfs) DP1 Volume (af) DP2 Peak Rate (cfs) DP2 Volume (af) 25-year 0.00 0.002 0.03 0.006 100-year 0.00 0.005 0.10 0.014 Recharge volume required is 2,228 cubic feet (cf), with 6,797 cf provided. Water quality volume (1 inch over impervious areas) is treated via proprietary units achieving 80% TSS removal, supplemented by infiltration for full compliance. Compliance with Massachusetts DEP Stormwater Standards  Standard 1 (No New Untreated Discharges): All discharges are treated and infiltrated on-site.  Standard 2 (Peak Rate Attenuation): Proposed peak rates are attenuated below existing levels for all storms.  Standard 3 (Recharge): Full recharge achieved via infiltration systems (drawdown within 72 hours).  Standard 4 (Water Quality): 80% TSS removal via pretreatment and infiltration; water quality volume provided.  Standard 5 (LUHP): Not applicable (residential use).  Standard 6 (Critical Areas): Not applicable (no discharges to critical areas).  Standard 7 (Redevelopment): Not applicable (full development).  Standard 8 (Erosion/Sediment Control): Construction-phase plan includes silt socks, stabilized entrances, and temporary basins.  Standard 9 (O&M Plan): Long-term plan includes annual inspections, vacuum sweeping, and pollution prevention measures.  Standard 10 (Illicit Discharges): Compliance statement prohibits illicit connections. Conclusions The proposed stormwater management system mitigates runoff, promotes recharge, and treats pollutants in full compliance with Massachusetts DEP standards. Appendices include HydroCAD reports, soil maps (Carver coarse sand, HSG A), recharge/drawdown calculations, TSS removal worksheets, erosion control plans, and manufacturer details for StormTech and First Defense systems. The design ensures no adverse impacts to adjacent properties or the environment.