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HomeMy WebLinkAbout8807.Stormwater Management Report(5.2.23) STORMWATER MANAGEMENT REPORT Miracle Way & Faith Road Cluster Subdivision Assessors’ Map 125, Parcel 39, Route 6A Yarmouth, MA JUNE 2, 2023 PREPARED FOR: THE DAVENPORT COMPANIES 20 NORTH MAIN STREET SOUTH YARMOUTH, MA 02664 PREPARED BY: J.M. O’REILLY & ASSOCIATES, INC. 1573 MAIN STREET P.O. BOX 1773 BREWSTER, MA 02631 508-896-6601 Page 2 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 TABLE OF CONTENTS 1. Property Description and Information 2. Stormwater System Description 3. Stormwater System Specifications 4. Stormwater Management by Contributory Area 5. Erosion Control Plan 6. Massachusetts Stormwater Management Design Standards 7. Town of Yarmouth Stormwater Management Regulations Performance Standards 8. APPENDICES APPENDIX A – Checklist for Stormwater Report APPENDIX B – TSS Removal Calculation Sheets APPENDIX C - HydroCAD Modeling Reports Page 3 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Property Description: Lot Area: 15.30 acres (666,507 sf+/-) Parcel Improvements: The site is to be developed with a cluster subdivision which will contain (14) residential lots and an Open Space lot. Each residential lot will be developed with a (4) bedroom dwelling, for a total of (56) bedrooms. Wetlands: The parcel is abutted by a vegetated wetland at its Southwest corner. However, the wetland is greater than 250’ from the closest area of the parcel that is proposed for development. The wetland is on the far side of the 150’ electrical easement and is not within the same watershed as any of the developed portion of the parcel. Soils: The NRCS Soil Survey places the parcel within the soil unit 252C Soil, Carver Coarse Sand, 8 to 15 percent. Medium to coarse sands were found in all preliminary test holes conducted on the site. Groundwater: The USGS Cape Cod Groundwater Map places groundwater at approximately elevation 8. With the parcel’s topography ranging from EL=72 at the hilltop to EL=28 at the low point near the West property line, the depth to groundwater to range from 20 to 64 feet below the existing grade. Perched groundwater was found at the bottom of test holes conducted within low areas. Maximum elevation of perched water found at EL=23. Zone II: The parcel is not located within a Zone II Groundwater Recharge mapped area. Topography: The topography is hilly with a high point toward the back of the lot and the front portion of the lot sloping down toward Route 6A. Site Conditions: The existing lot is vacant woodlands directly adjacent to Route 6A. Page 4 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Stormwater System Description: The proposed post-development stormwater management plan consists of: Two main stormwater systems (M1 and M2) that address the roadway, driveway, road shoulder and swale areas; Five secondary stormwater systems (S1-S5) that address driveway and front yard areas, and a system of drywells to handle roof runoff from the 14 proposed dwellings (M3). The roadway will use Cape Cod berms to control and pitch stormwater toward a network of offline catch basins (9 total). The catch basins then flow into centralized conveyance systems of pipe and manholes that connect to a proprietary pre-treatment device (CDS 2015-4 by Contech) before outletting into water quality swales. Any overflow from the swales is then routed into subsurface leaching galleys. Landscaped areas and driveways are graded to direct stormwater runoff into a series of smaller dry water quality swales. Each of the swales is surrounded by vegetated strips that provide pre-treatment for some driveway runoff. Roof runoff will be directed into subsurface leaching facilities. Please see the Existing and Proposed Stormwater Contributory Area Plans for delineations of the site’s contributory areas. As shown in the HydroCAD Modeling report, the proposed stormwater controls will reduce the site-wide peak discharge rate for the 2, 10, 25 and 100 year storms. The Post-Development 100-year Storm Peak Discharge Rate relative to Pre-Development conditions has decreased from 4.41 CFS to 1.52 CFS. Table 1 provides a comparison of the site-wide Pre- to Post-Development peak discharge rate for each storm event. Table 1: Peak Discharge Comparison Storm Event Pre-Dev. Peak Discharge Rate (ft3/sec) Post-Dev. Discharge Rate: M1-M3 (ft3/sec) Post-Dev. Discharge Rate: S1-S6* (ft3/sec) Total Post-Dev. Peak Discharge Rate (worst case) (ft3/sec) 2 year 0 0 0 0 10 year 0.23 0 0.06 0.06 25 year 1.16 0 0.29 0.29 100 year 4.41 0.30 1.22 1.52 *S1-S5 are secondary stormwater systems. S6 is all non-recharge areas such as Open Space and backyards. For HydroCAD modeling analysis of the stormwater systems, the following methods and assumptions were used: • Simple Dynamic • Rawls Rate of 8.27 in/hr for sands within the subsoil layers for subsurface leaching galleys. • Rawls Rate of 2.41 in/hr for the loamy sands in the upper soil layers for dry water quality swales. • Exfiltration has been assumed to occur along horizontal surface area of bottoms only, sidewalls/slope have been excluded per MA Stormwater Handbook requirements. Page 5 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Stormwater System Specifications: Main Stormwater System, M1 (Adjacent to Miracle Way - Route 6A Intersection): • Roadway and shoulder runoff from the North portion of Miracle Way and surrounding area directed via grading and cape cod berms to (2) catch basins at the roadway low point. • Catch basins flow into a central manhole, which then flows into a CDS 2015-4 Pre-Treatment unit (manuf. by Contech, specifications attached) with built-in bypass system. • Outflow from CDS unit flows via (1) outfall pipe into a dry water quality swale. • Landscaped and natural areas in the vicinity of the swale will flow directly into the swale. • Swale sized so that it will fill to a maximum 18” water depth during storm events before overflowing into (1) catch basin installed within the swale with rim elevation 18” above bottom of swale. • Overflow enters into (1) 28’ x 14’ subsurface leaching facility consisting of (2) 6’ diameter, 6’ deep leach pits spaced 14’ on center, with stone around. Main Stormwater System, M2 (Adjacent to Faith Road Cul-De-Sac): • Roadway and shoulder runoff from the South portion of Miracle Way and all of Faith Road directed via grading and cape cod berms to a series of catch basins (7 total). • Catch basins flow into a centralized conveyance system of pipe and manholes, which then flows into a CDS 2015-4 Pre-Treatment unit (manuf. by Contech, specifications attached) with built-in bypass system. • Outflow from CDS unit flows via (2) outfall pipes into a dry water quality swale. • Landscaped and natural areas in the vicinity of the swale will flow directly into the swale. • Swale sized so that it will fill to a maximum 18” water depth during storm events before overflowing into (1) catch basin installed within the swale with rim elevation 18” above bottom of swale. • Overflow enters into (1) 70’ x 28’ subsurface leaching facility consisting of (10) 6’ diameter, 6’ deep leach pits spaced 14’ on center, with stone around. Roof Runoff, M3 (per dwelling): • Roof runoff for each dwelling will be directed via gutters and downspouts into two subsurface leaching facilities, one on each side of the dwelling. • Each leaching facility is 10.5’ x 6.33’ x 3.5’, consisting of (1) Cultec Recharger 330XLHD unit with stone around and underneath. Secondary Stormwater Systems (Total of 5): • Driveway and front/side yard runoff flows directly into several dry water quality swales sited adjacent to driveway parking and backyard areas on residential lots. • Swales have a maximum depth of 2’. Swales have all been sized to retain 100 year storm runoff for their contributory areas, with no overflow. Page 6 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Stormwater Management by Contributory Area: There were six existing subcatchment areas identified and labeled on the ‘Existing Stormwater Contributory Area Plan’. Each of those subcatchments is described below: Subcatchment Area #1 (100,376 SF) is in the northwest portion of the lot, with a discharge point close to the western property line. The abutting properties to the west generally slope upward from the western property line, so some stormwater from adjacent properties flows to the discharge point as well. • The subcatchment includes proposed Open Space area, including a portion of the M1 drainage easement. • The subcatchment also includes portions of Lots 7- 9, 11 and 12. • Proposed swales S1 and S2 will attenuate and treat driveway runoff from Lots 8, 9 and 12 within the subcatchment, along with overland flow from landscaped and natural areas. Vegetated strip areas in between the driveways and swale will provide pre-treatment prior to the swale, except in the driveway areas that are less than 25 feet from the swales. • The M1 drainage easement area will be graded to direct runoff into the M1 swale instead of the discharge point. Excess stormwater from the M1 swale will discharge into the M1 subsurface leaching facility. • All roof runoff within the subcatchment will be discharged into subsurface leaching facilities. • The existing discharge point and surrounding low area will remain undisturbed Open Space, allowing it’s continued use as a discharge point for surrounding backyard lawn, landscaped and natural areas. Subcatchment Area #2 (30,064 SF) is in the interior of the northern portion of the lot, with the discharge point centered in the low area. • The subcatchment includes a portion of Miracle Way’s road layout. • The subcatchment also includes portions of Lots 7-10. • The proposed regrading of the lot will eliminate the interior discharge point. Driveway runoff will be pitched toward swales S1 and S3, along with all other overland flow. Vegetated strip areas in between the driveways and swale will provide pre-treatment prior to the swale, except in the driveway areas that are less than 25 feet from the swales. • All roof runoff within the subcatchment will be discharged into subsurface leaching facilities. • The portion of the subcatchment within the roadway itself will be directed into the M1 stormwater system. Subcatchment Area #3 (230,874 SF) is the land north of the power line easement in the southwest portion of the lot, with a discharge point within a low area close to the western property line. The abutting properties to the west generally slope upward from the western property line, so some stormwater runoff from adjacent properties flows to the discharge point as well. • The subcatchment includes proposed Open Space area, including the M2 drainage easement. • The subcatchment also includes portions of Faith Road and Miracle Way. • The subcatchment also includes portions of Lots 5-6 and 10-14. Page 7 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 • Proposed swale S3 will attenuate and treat driveway runoff from Lots 9-13 within the subcatchment, along with overland flow from landscaped and natural areas. Vegetated strip areas in between the driveways and swale will provide pre-treatment prior to the swale, except in the driveway areas that are less than 25 feet from the swales. • The M2 drainage easement area will be graded to contain the M2 swale, creating a low point that will attenuate and treat driveway runoff from Lots 5-6 within the subcatchment, along with overland flow from landscaped and natural areas. Vegetated strip areas in between the driveways and swale will provide pre-treatment prior to the swale, except in the driveway areas that are less than 25 feet from the swales. Excess stormwater from the M2 swale will discharge into the M2 subsurface leaching facility. • All roof runoff within the subcatchment will be discharged into subsurface leaching facilities. • The existing discharge point will remain a low point, with proposed swale S3 constructed in the current low point location. Due to the grading and stormwater controls proposed within the subcatchment, runoff into the S3 swale will be reduced. The area will continue to be used as a discharge point for surrounding backyard lawn, landscaped and natural areas. Subcatchment Area #4 (115,350 SF) is the land north of the power line easement in the southeast portion of the lot, with a discharge point at the eastern property line. The abutting properties to the east generally slope downward from the eastern property line, so there is no additional stormwater flow from adjacent properties to the discharge point. • The subcatchment includes proposed Open Space area. • The subcatchment also includes portions of Faith Road and Miracle Way. • The subcatchment also includes portions of Lots 2-6 and 10. • Proposed swales S4 and S5 will attenuate and treat driveway runoff from Lots 3-4 within the subcatchment, along with overland flow from landscaped and natural areas. Vegetated strip areas in between the driveways and swale will provide pre-treatment prior to the swale, except in the driveway areas that are less than 25 feet from the swales. • Driveway areas and surrounding landscaped areas on Lots 2, 5 and 6 have been graded to be part of the M2 stormwater system’s watershed area. • All roof runoff within the subcatchment will be discharged into subsurface leaching facilities. • Due to the grading and stormwater controls proposed within the subcatchment, runoff to the discharge point will be reduced. The area will continue to be used as a discharge point for surrounding backyard lawn, landscaped and natural areas. Subcatchment Area #5 (19,999 SF) is land along the eastern portion of the lot, with a discharge point at the eastern property line. The abutting properties to the east generally slope downward from the eastern property line, so there is minimal additional stormwater flow from adjacent properties to the discharge point. • The subcatchment includes proposed Open Space area. • The subcatchment also includes portions of Lots 1 and 2. • Most of the driveway area and surrounding landscaped areas on Lots 1 and 2 have been graded to be part of the M2 stormwater system’s watershed area. A small portion of Lot 2’s driveway Page 8 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 pitches toward the rear of the property, eventually reaching the discharge point after flowing across vegetated landscaped and natural areas. • All roof runoff within the subcatchment will be discharged into subsurface leaching facilities. • Due to the grading proposed within the subcatchment, runoff to the discharge point will be reduced. The area will continue to be used as a discharge point for surrounding backyard lawn, landscaped and natural areas. Subcatchment Area #6 (24,480 SF) is in the northeast portion of the lot, with a discharge point at the Route 6A catch basin at the base of the subcatchment. The abutting properties to the east generally slope upward from the eastern property line, so some stormwater from adjacent properties flows to the discharge point as well. • The subcatchment includes proposed Open Space area, including a portion of the M1 drainage easement. • The subcatchment also includes portions of Miracle Way. • The subcatchment also includes portions of Lots 1, 2 and 7. • The subcatchment area has been graded to be part of the M1 stormwater system’s watershed area, instead of Route 6A’s stormwater systems. • The M1 drainage easement area will be graded to direct surrounding runoff into the M1 swale instead of the discharge point. Excess stormwater from the M1 swale will discharge into the M1 subsurface leaching facility. • Existing stormwater runoff from the project site into the Route 6A catch basin will be prevented by the proposed grading and stormwater controls. Page 9 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Erosion Control Plan - Temporary Siltation Barrier & Silt Socks: Prior to start of construction, a Stormwater Pollution Prevention Plan (SWPPP) will be prepared and a Notice of Intent will be filed with the US EPA under the EPA NPDES Construction Stormwater program. • The SWPPP & NOI shall address the erosion controls necessary prior to and during construction. • The erosion controls shall include a row of staked 9-inch straw wattles or an erosion (silt) fence surrounding the entire area to be disturbed. Erosion controls shall be installed around all areas of disturbance. • Once the driveway stormwater systems are installed, deep sump catch basins shall be fitted with filter fabric (silt socks) underneath the grates, to prevent silt and debris from clogging and/or damaging the dry water quality swales and subsurface leaching facility. • The erosion controls shall be monitored and corrected during the entire construction phase and until the site has been stabilized with ground cover and the proposed landscape. • Contractor shall be required to provide extra siltation controls in case a repair is needed to the straw wattles and/or catch basin filter fabric. The Stormwater Operations and Maintenance Plan is included as a separate document in order to address the long-term maintenance of the stormwater systems. Page 10 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Massachusetts Stormwater Management Design Standards: The following is a description of how the proposed project meets the Massachusetts Stormwater Handbook design standards. Standard 1: No new untreated discharges: This standard is met since there are no new untreated stormwater discharges proposed. See Standards 4- 6 calculations. Standard 2: Maintain Pre-development peak discharge rate: This standard has been met. As shown in the HydroCAD Modeling report, the proposed stormwater controls will reduce the site-wide peak discharge rate for the 2, 10, 25 and 100 year storms. The Post- Development 100-year Storm Peak Discharge Rate relative to Pre-Development conditions has decreased from 4.41 CFS to 1.52 CFS. Standard 3: Groundwater Recharge: This standard is met. The proposed stormwater management systems are sized so that the total recharge volume provided exceeds the minimum groundwater recharge volume specified in the handbook and the proposed stormwater recharge galleys will drawdown within 72 hours of a storm event. In accordance with the MA Stormwater Manual, the required recharge volume factor (F) required across the impervious area (A) is 0.6 inches per hour for hydraulic soil group A soils. Coarse Sand (Rawls Rate: 8.27 inches per hour) has been used in the sizing of the stormwater recharge galleys. Refer to the HydroCAD Stormwater Modeling Report in Appendix. The required recharge volume is calculated based on the total pavement and roof areas on site. • Required Recharge Volume Rv = F x A = (0.6 in)(1 ft/12 in)(73,698 sf) = 3,685 cf (pavement and roofs) • Recharge Storage Provided (Subsurface leaching facilities) = 12,415 cf > 3,685 cf • The drawdown for the subsurface leaching facilities for the driveway/parking system and roof runoff is 24 hours < 72 hour maximum allowance. Standard 4: Water Quality: This standard has been met. The roof runoff stormwater system will remove 80% of the annual load of Total Suspended Solids (TSS) via subsurface leaching facilities. Site-wide, stormwater controls for all non- roof runoff will provide 74.8% cumulative Pre-Treatment TSS Removal, and 90.7% cumulative Treatment TSS Removal. See Tables 2 and 3 below. TSS removal calculation tables for roadway and roof runoff are included in the Appendix. In accordance with the MA Stormwater Manual, the required water quality depth (Dwq) across the impervious area (A) is 1.0 inches per hour in areas containing soils with rapid infiltration rate greater than 2.4 in/hr. The required water quality volume is based on the total pavement area on site. • Required Water Quality Volume Vwq = Dwq x A = (1.0 in)(1 ft/12 in)(55,666 sf) = 4,639 cf (pavement) • Water Quality Storage Provided (Dry water quality swales M1 & M2) = 5,542 cf > 4,639 cf Page 11 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Table 2: TSS Removal – Pre-Treatment Stormwater System Type of Runoff Impervious Area (SF) Total TSS Removal (%) Incremental Removal (%) M1 To Catch Basins 6,286 96.5 10.90 M1 Overland 0 0 0 M2 To Catch Basins 33,633 89.8 54.26 M2 Overland 4089 45 3.31 S1 Overland, <25’ from Swale 1988 0 0 S1 Overland, 25’-50’ from Swale 584 10 0.10 S1 Overland, >50’ from Swale 3,425 45 2.77 S2 Overland 0 0 0 S3 Overland, >50’ from Swale 2900 45 2.34 S4 Overland, <25’ from Swale 791 0 0 S5 Overland, >50’ from Swale 1359 45 1.10 S6 Non-Recharge Areas 611 0 0 Cumulative Pre-Treatment Removal Efficiency = 74.8% Table 3: TSS Removal –Treatment Stormwater System Type of Runoff Impervious Area (SF) Total TSS Removal (%) Incremental Removal (%) M1 To Catch Basins 6,286 99.7 11.26 M1 Overland 0 0 0 M2 To Catch Basins 33,633 99.3 60.00 M2 Overland 4089 96.7 7.10 S1 Overland, <25’ from Swale 1988 0 0 S1 Overland, 25’-50’ from Swale 584 73 0.77 S1 Overland, >50’ from Swale 3,425 83.5 5.14 S2 Overland 0 0 0 S3 Overland, >50’ from Swale 2900 83.5 4.35 S4 Overland, <25’ from Swale 791 0 0 S5 Overland, >50’ from Swale 1359 83.5 2.04 S6 Non-Recharge Areas 611 0 0 Cumulative Treatment Removal Efficiency = 90.7% Standard 5: Land uses with higher potential pollutant loads: This standard has been met. The proposed condominium units are residential buildings. • Prior to construction a NPDES Construction General Permit for stormwater discharges associated with construction activities will be completed and submitted. • The Operation and Management Plan of the facility, as previously provided identifies the operational procedures for the reduction and elimination of potential exposure to hazardous materials. Page 12 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Standard 6: Stormwater discharges within Zone II or Interim Wellhead protection area of a public water supply and stormwater discharges near or to any critical area. This standard has been met. The 1 inch Required Water Quality Volume for discharges within a Groundwater Protection area has been met, see Standard 4 calculations. Standard 7: Redevelopment: This standard is not applicable given the new development within the site meets the performance standards of the Massachusetts Stormwater Design Standards. Standard 8: Construction Erosion Control Plan: The project is subject to the NPDES Construction General Permit. All applicable SWPPP and NOI submittals shall be completed and approvals issued prior to any work being started for the proposed development areas. Standard 9: Long Term Operation and Maintenance Plan: A long-term O&M plan has been submitted with this report, refer to Appendix. The Davenport Companies will operate and maintain the stormwater systems. Standard 10: Illicit Discharges: This standard is met since there are no illicit discharges at this site and no illicit discharges proposed. Page 13 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 Town of Yarmouth Stormwater Management Regulations Performance Standards: The following is a description of how the proposed project meets the Performance Standards of the Town of Yarmouth Conservation Commission’s Stormwater Management Regulations (Section 2.05). (1) Low Impact Development (LID) site planning and design strategies must be implemented unless infeasible in order to reduce the discharge of stormwater from development sites. Reduce the amount of runoff over paved surfaces through the implementation of LID techniques, such as infiltrating roof runoff at the source, planting large canopy trees over impervious areas to intercept rainfall, use of porous paving materials, etc. where feasible. The applicant must document in writing why LID strategies are not appropriate when not used to manage stormwater. Project proposes no disturbance to any Wetland Resource Areas. Project is a Clustered Subdivision per Yarmouth Zoning bylaws Section 402, providing significant Open Space and limiting impervious surface area required for the developed portion of the site. Project proposes dry water quality swales which will be vegetated with Conservation approved seed mix and native shrub plantings, see ‘C9 Landscape Plan’. (2) Good housekeeping procedures shall be used to reduce sources of sediment, phosphorus, nitrogen and other contaminants in stormwater runoff. These shall be documented in the Operation and Maintenance Plan. See the following sections of this report: • ‘Erosion Control Plan’ • ‘Schedule of Inspection and Maintenance of Stormwater Management Systems’ • ‘Schedule of Vegetation Control for Roadway, Driveway and Sight Easement Areas’ • ‘Long-term Lawn Care and Pollution Prevention Plan’ • ‘Emergency Spill Cleanup Plan’ (3) Stormwater management systems design shall be consistent with, or more stringent than, the requirements of the 2008 Massachusetts Stormwater Handbook (as amended) with the following additional requirements: (a) Post-development peak discharge rates do not exceed pre-development peak discharge rates for the 2, 10, 25, 50 and 100-year 24-hour storms. The 50-year post-development stormwater volume shall be retained onsite through design of the stormwater management system to the maximum extent practicable. The 100-year post-development stormwater volume shall be controlled onsite with no offsite discharge to the maximum extent practicable. This Standard may be waived for discharges to land subject to coastal storm Page 14 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 flowage as defined in the Massachusetts Wetlands Protection Regulations at 310 CMR 10.04. Project meets this standard. See Standard 2: Peak Discharge and Standard 3: Recharge calculations provided in the ‘Massachusetts Stormwater Management Design Standards’ section of this report. Pre- vs. Post-Construction Discharge rates table is shown in the ‘Stormwater Management System Description’ section of this report. (b) Structural pretreatment is required for all proposed infiltration devices to remove 44% TSS from runoff before it enters the infiltration device. Runoff from non-metal roofs may be infiltrated without pretreatment. Runoff from metal roofs may be infiltrated without pretreatment only if the roof is located outside the Zone II or Interim Wellhead Protection Area of a public water supply and outside an industrial site, otherwise, pretreatment is required as specified in the Stormwater Management Handbook. Project meets this standard. See TSS Calculations provided in Appendix B of this report. (c) The calculations of runoff volumes and peak rates required under Massachusetts Stormwater Management Standard 2 shall be based on precipitation data provided in National Oceanic and Atmospheric Administration (NOAA) – National Weather Service “NOAA Atlas 14” unless otherwise authorized by the Commission. Project meets this standard. (4) Stormwater management systems for new developments shall be designed to meet an average annual pollutant removal equivalent to 90% of the average annual load of TSS AND 60% of the average annual load of Total Phosphorus (TP) AND 30% of average annual load of Total Nitrogen (TN) related to the total post-construction impervious area on the site as achieved through one of the following methods: (b) Retaining the volume of runoff equivalent to, or greater than one (1.0) inch multiplied by the total post-construction impervious surface area on the new development site, with pretreatment provided in accordance with Section 2.05(3)(b) Project provides (2) water quality swales as part of the treatment train for both of the main roadway stormwater systems. Deep sump catch basins provide pre-treatment. Proprietary CDS-2015-4 pre-treatment units provide additional pre-treatment. See Standard 4: Water Quality calculations provided in the ‘Massachusetts Stormwater Management Design Standards’ section of this report. Page 15 of 15 DAVENPORT CLUSTER SUBDIVISION - MIRACLE WAY & FAITH ROAD STORMWATER MANAGEMENT REPORT ASSESSORS’ MAP 125, PARCEL 39, ROUTE 6A, YARMOUTH, MA JUNE 6, 2023 (5) Stormwater management systems for redevelopments shall be designed to meet an average annual pollutant removal equivalent to 80% of the average annual post-construction load of TSS AND 50% of the average annual load of TP AND 30% of the average annual load of TN related to the total post-construction impervious area on the site as achieved through one of the following methods: N/A. Project is a new development. (6) In complying with (4) and (5) above, the required removal percentage is not required for each storm; it is the average removal over a year that is required. Pollutant removal shall be calculated consistent with EPA Region 1’s BMP Accounting and Tracking Tool (2016) (https://www.epa.gov/npdes-permits/stormwater-tools-new-england) or other BMP performance evaluation tool provided by EPA Region 1 where available. If EPA Region 1 tools do not address the planned or installed BMP performance any federally or State approved BMP design guidance or performance standards (e.g. State stormwater handbooks and design guidance manuals) may be used to calculate BMP performance. TSS Calculation Sheet provided per MA Stormwater handbook standards in Appendix B of this report. (7) Discharges to water bodies or their tributaries subject to one or more approved Total Maximum Daily Load (TMDL) or impaired waterbodies and their tributaries, listed as Category 4b or 5 in the current Massachusetts Integrated List of Waters listed pursuant to the Federal Clean Water Act Sections 303(d) and 305(b) without an EPA approved TMDL shall adhere to the following: N/A. No discharges to water bodies or their tributaries are proposed.