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HomeMy Public PortalAboutMillstone Watershed StudyMassachusetts Housing Partnership 2 Stormwater Study January 5, 2021 1.0 INTRODUCTION This report summarizes the stormwater characteristics of the site and surrounding area and makes recommendations for mitigating permanent and construction period impacts related to the proposed potential affordable housing development (Project) at the Site. This memorandum is based on a variety of available information, including a Site survey, GIS mapping, comments and photographs submitted by abutting property owners and a field visit of the site and adjacent properties. For the purpose of this memorandum the Project is based on the Concept 2 Plan, Bohler, dated March 10, 2020, but the stormwater design would be similar for any multi-building development on the property. The property survey, watershed mapping and Concept 2 are included in Appendix A – Exhibits. The reader is also directed to the July 9, 2019 Due Diligence Report prepared by Bohler for additional site information, some of which has been included with this memorandum. This memorandum specifically addresses the following scope:  Existing and Proposed Watershed Area Mapping- This identifies existing and proposed topographical and surface condition information as derived from the survey documents, master plan concepts and available GIS mapping.  Existing and Proposed Stormwater Modelling - Utilizing the topography and surface conditions, an analysis was performed in HydroCAD to quantify the rates of run-off, design points, potential flooding, etc. The analysis includes calculations for the 2-, 10-, 25-, 50- and 100-year storms. This modelling shows design points, related stormwater flow and proposed underground infiltration systems to manage increased run-off from development.  Review of Potential Stormwater Impacts to Abutting Properties – Based on the existing and proposed topography as well as the modelling, an assessment of impacts of abutting properties is provided. 2.0 EXISITNG SITE 2.1 Existing Surface Conditions The Site, located west of Millstone Road near the intersection with Fern Lane, includes the properties identified as “CAMA ID” 99-1 and 98-12 in the Town of Brewster, Massachusetts. One parcel (ID 99-1) has frontage along the western side of Millstone Road, between 598 and 560 Millstone Road, and is located along the northeastern edge of the second parcel. The second, rear parcel (ID 98-12) is located behind single family residential lots along both Millstone Road and Captains Village Lane, and extends to the west, along the Ocean Edge development and golf Massachusetts Housing Partnership 3 Stormwater Study January 5, 2021 course. The Site also abuts a portion of dedicated conservation land associated with the Captains Village Lane development. The subject properties encompass approximately 16.61 acres. The Site consists primarily of undeveloped wooded land and is bordered by single family residential lots to the east, south and west. Ocean Edge, a condominium development, borders the Site to the north with a portion of the associated golf course also located to the west. 2.2 Soil Conditions Based on the United States Department of Agriculture (USDA) Natural Resources Conservation Service’s (NRCS) Web Soil Survey, the soils identified across the Site consist of Barnstable- Plymouth-Nantucket complex (Hydrologic Soil Group A) and Plymouth loamy course sand (Hydrologic Soil Group A) which are expected to be very well drained soils. The NRCS Web Soil Survey report has been included in Appendix B for reference. In addition, based on the USDA NRCS Web Soil Survey, the depth to water table and any soil restrictive layer is greater than 6.5 feet. 2.3 Existing Topography The elevations across the Site range from approximately elevation 82 to 122. Slopes range from generally flat in the far southern extent of the property, up to 10% in the northwest portion of the property. The Existing Conditions Survey, included as in Appendix A, further details the location and existing topography at the Site. Flow arrows have been added to show the above description of the topography. Also included in Appendix A, is a Watershed Analysis Figure showing a larger area than the Site survey. This figure includes flow arrows, identified low points and design points. The design points correspond to locations where stormwater from the Site is measured in the existing and proposed condition. The Low Points (LP) are locations we evaluated near the site that collect stormwater. The northern portion of the Site primarily slopes from north to south, towards a low-lying area along the southern property line, near the inner corner of the “L-shaped” property. The southern portion of the Site is primarily sloped from south to north, towards the same low-lying area. This small depression appears to collect and infiltrate runoff in the vicinity and is considered Design Point 1 (DP1). As depicted on the Watershed Figure this design point collects approximately 52 acres. This low-lying area would be the likely location of a future stormwater detention and infiltration system in order to mimic the storage and infiltration that the area provides under existing conditions. There is a second design point at a low-lying area listed as DP2 on the Watershed Figure. This design point is on an adjacent property and collects stormwater from the project site, Millstone Road and property to the north. There would be a small stormwater detention and infiltration system in order to match existing flows from the Project site to this point to mimic existing conditions. Massachusetts Housing Partnership 4 Stormwater Study January 5, 2021 Adjacent to the site, on the Ocean Edge property to the north and west there are some identified low points. While some low-lying areas are on the golf course and wooded areas, one is on Fletcher Lane collecting runoff from a large part of developed area of Fletcher Lane and Howland Circle. Based on comments submitted by residents this area was reviewed and we agree that it appears to be prone to flooding. Unfortunately, it appears that a low point was built into the road design without a drainage overflow or drainage structures to help convey water from this depression. We are confident based on the topography along the northern property line as well as typical drainage protocol that the Project if designed properly will not have any negative stormwater impacts on the adjacent condominium development. 3.0 PROPOSED SITE 3.1 Proposed Topography and Stormwater System The proposed development will consist of a driveway, surface parking and likely multiple buildings across the property. The preferred design was depicted previously in Concept 2. The proposed topography should match the existing to the greatest extent practicable in order to help match stormwater characteristics, minimize grading and preserve as many existing trees as possible. Based on the existing topography and watershed there are two design points to be considered. Design Point 1 (DP1) is located in the northwest corner of the site and DP2 is at the northeast corner of the site. It is important to note that there is a natural ridge running along the northern property line, which will aid in minimizing stormwater conveyance from the Project travelling to the neighboring properties to the North (Ocean Edge property). This was reviewed in the field and can be seen in the topography depicted in the survey and watershed mapping in appendix A and photos in Appendix C. Assuming that there is a buffer along the property line, the high point of this ridge should remain intact and prevent stormwater from the development from travelling north to the adjacent condominium development. Near the western most edge of the property, where the ridge offers less relief, a berm, grading and a stormwater collection system of catch basins and manholes should be established to allow no increase in stormwater flow from the development to the northern abutter. 3.2 Proposed Stormwater System The proposed project will require a new storm drainage system consisting of a series of catch basins, water quality treatment units and an underground infiltration system or surface basin to collect, treat and manage stormwater runoff from the buildings and parking areas. Since the Site is undeveloped and wooded, the development will result in an increase in impervious area Massachusetts Housing Partnership 5 Stormwater Study January 5, 2021 and will require a stormwater management system to retain and infiltrate stormwater in order to match peak rates of runoff compared to that of existing conditions. We expect that two or more detention and infiltration systems will be designed as part of the project. We have defined two low points that are considered design points for the development. Each system is assumed to be an underground detention/infiltration system for the modelling but could also be open detention ponds. The proposed development would be a good candidate for the use of a low impact development drainage system, which is suggested under the Massachusetts Stormwater Management Standards. The storm drainage system will need to be further refined as the conceptual planning process moves forward. In addition, during soil testing for the proposed septic system design, Bohler recommends that soil testing be performed in areas identified for stormwater management in order to confirm site soil conditions and depth to groundwater. We understand there is great concern about adding to flooding issues to the north of the property in the Ocean Edge development. We have reviewed comment letters and provided responses in Appendix E. 4.0 STORMWATER MODELLING 4.1 Methodology The proposed stormwater management design will provide a decrease in peak stormwater runoff rates from the proposed facility for the 2-, 10-, 25-, 50- and 100-year design storm events utilizing the SCS TR-20 and TR-55 Urban Hydrology for Small Watersheds methods. The assessment of existing and proposed stormwater flows was performed using HydroCAD©. HydroCAD© input and output data is included in Appendix D. In the modelling DP1 is called POA1 (Point of Analysis 1) and DP2 is called POA2 (Point of Analysis 2). Times of concentration (Tc) utilized in the preparation of this report were generated utilizing the SCS TR-55 Urban Hydrology for Small Watersheds method. Runoff coefficients for the pre- and post-development conditions were calculated using widely accepted, and often utilized runoff coefficients and have been documented within the hydrology calculations in Appendix D of this report. Rainfall data for the storm events was obtained from the Northeast Regional Climate Center and the Natural Resources Conservation Service’s Extreme Precipitation in New York & New England. The following rainfall data was used in the calculations: Massachusetts Housing Partnership 6 Stormwater Study January 5, 2021 Table 1 – Rainfall Data 2-Yr 10-Yr 25-Yr 50-Yr 100-Yr Rainfall (inches) 3.23 4.69 5.60 6.29 7.0 Table 2 – Summary of Peak Runoff Rates to Design Point 1 (POA1 in HydroCAD) Storm Event (years) 2 10 25 50 100 Existing Flow, CFS 9.5 31.8 49.3 63.6 79.3 Proposed Flow, CFS 9.5 31.7 49.1 62.1 78.1 Change, CFS 0.0 -0.1 -0.2 -1.5 -1.2 Table 3 – Summary of Peak Runoff Rates to Design Point 2 (POA2 in HydroCAD) Storm Event (years) 2 10 25 50 100 Existing Flow, CFS 6.9 13.2 17.3 20.5 23.7 Proposed Flow, CFS 6.7 12.7 16.7 19.8 23.6 Change, CFS -0.2 -0.5 -0.6 -0.7 -0.1 The proposed stormwater management as designed will provide a decrease in peak rates of runoff from the proposed facility for the 2-, 10-, 25-, 50- and 100-year design storm events. Massachusetts Housing Partnership 7 Stormwater Study January 5, 2021 5.0 STORMWATER DESIGN RECOMMENDATIONS The Project should be required to meet the MA DEP Stormwater Standards. Below is a summary of the standards and recommendations for the project. Standard #1: No New Untreated Discharges The project should be designed so that proposed impervious paved areas at a minimum shall be collected and passed through the proposed drainage system for treatment prior to discharge. Roof areas, which are clean may be infiltrated directly without treatment. Standard #2: Peak Rate Attenuation The proposed stormwater management system should be designed so that post-development peak rates of runoff are below pre-development conditions for the 2-, 10-, 25- and 100-year storm events at all design points. Standard #3: Recharge The stormwater runoff from the project should be collected and diverted to infiltration systems to mimic existing drainage pattern at the site. Standard #4: Water Quality Water quality treatment should be provided to meet an 80% Total Suspended Solids (TSS) removal. Standard #5: Land Use with Higher Potential Pollutant Loads Not Applicable for this project. Standard #6: Critical Areas Not Applicable for this project. Standard #7: Redevelopment Not Applicable for this project. Massachusetts Housing Partnership 8 Stormwater Study January 5, 2021 Standard #8: Construction Period Pollution Prevention and Erosion and Sedimentation Control The proposed project should provide construction period erosion and sedimentation controls complying with MA DEP guidance and EPA NPDES guidance. This includes a proposed construction exit, protection for stormwater inlets, protection around temporary material stockpiles and various other techniques as outlined on the erosion and sediment control sheets. Additionally, the project is required to file a Notice of Intent with the US EPA and implement a Stormwater Pollution Prevention Plan (SWPPP) during the construction period. The SWPPP will be prepared prior to the start of construction and will be implemented by the site contractor under the guidance and responsibility of the project’s proponent. Standard #9: Operation and Maintenance Plan (O&M Plan) An Operation and Maintenance (O&M) Plan for this site should be prepared for the long term operation and maintenance of the proposed site stormwater management system, including initial inspections upon completion of construction, and periodic monitoring of the system components, in accordance with established practices and the manufacturer’s recommendations. The O&M Plan should include a list of responsible parties and an estimated budget for inspections and maintenance. The Town may want to require submission of an annual report summarizing the maintenance activities completed. Standard #10: Prohibition of Illicit Discharges The proposed stormwater system should only convey allowable non-stormwater discharges (firefighting waters, irrigation, air conditioning condensates, etc.) and should not contain any illicit discharges from prohibited sources. Massachusetts Housing Partnership 9 Stormwater Study January 5, 2021 6.0 SUMMARY The drainage analysis for the site and surrounding area indicate that with typcial stormwater controls and treatment, the proposed Project should not impact surrounding properties. The existing topography of the site aids in the ability to develop the site because there is a ridge or topographical break along the northern property line. We recommend that the project be required to complete infiltration testing during the design and permitting process, complete the stormwater design according to the DEP Stormwater Standards and submit a draft SWPPP to the town for review prior to finalizing the design under the EPA NPDES program.