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Home My WebLink About2011-09-06 Planning Board Supplemental Materials (62) Eggleston Environmental 32 Old Framingham Rd Unit 29 Sudbury MA 01776 tel 508.259.1137 fax 866.820.7840 August 29, 2011 North Andover Planning Board 1600 Osgood Street North Andover, MA 01845 Attn: Judy Tymon, Town Planner RE: Stormwater Management Review 2009 Salem Street Dear Ms. Tymon and Board Members: Per your request, I have conducted a stormwater review of the August 3, 2011 Application for Subdivision Approval for the proposed 3-lot subdivision (Salem Woods) at 2009 Salem Street. Specifically, the materials that I have received and reviewed to date include the following: August 3, 2011 submittal letter to the Planning Board from Lisa Mead and Adam Costa of Blatman, Bobrowski & Mead, LLC, Re: Application for Definitive Subdivision Approval, 2009 Salem Street, North Andover MA, w/ attachments. Drainage Analysis, Salem Woods, Salem Street, North Andover, prepared by Merrimack Engineering Services and dated August 1, 2011. Site Plans, Salem Woods, Sheets 1-6 of 6, prepared for 2009 Salem Street Realty Trust by Merrimack Engineering Services and dated July 13, 2011. My primary focus in this initial review is on the overall stormwater management approach and design concepts used in the project, as well as its compliance with the Rules and Regulations Governing the Subdivision of Land in Town of North Andover. The project site, located on the west side of Salem Street, is approximately 10 acres in size. There is presently one home on the site, with the remainder of the property being undeveloped and largely wooded. The site rises about 40 ft in elevation from Salem Street to a ridgeline that extends to the southern property boundary. This ridgeline forms a drainage divide down the undeveloped portion of the property; drainage to the west of the ridgeline flows to three onsite isolated wetlands/pockets (subareas B, C and D), two of which (B and C) are vernal pools. Drainage from the area east of the ridgeline, including the existing shed, garage, and a portion of the driveway, flows toward a large offsite bordering vegetated wetland to the east and onto an adjacent residential property ( Subareas F and G). The front, more developed portion of the site (Subarea A) drains in a northeasterly direction toward Salem Street and, ultimately, across the road to Boston Brook. In all likelihood, subsurface (groundwater) flow on the property is consistent with surface drainage patterns. 2009 Salem Street, Technical Review 2 August 29, 2011 The proposed project is a 3 lot residential subdivision, with the existing home occupying one lot. The existing driveway would be widened, and extended as a common driveway to serve the two back lots. As proposed, roof drainage from the two new houses would be directed to drywells on each lot, and runoff from the shared driveway and unpaved portions of Lots 2 and 3 would drain to infiltration trenches located between the double driveway and along the eastern edge of the shared driveway. The application seeks a waiver for a “modest” increase in runoff from the site, however based on a number of reasons outlined below, I believe that the hydrologic analysis understates the level of increase that would occur. My specific comments are outlined as follows: 1. The requirements of the North Andover Subdivision Regulations outlined in Section 6.14 (Storm Drainage) and Appendix V (Rules and Regulations Governing Storm Water Management) essentially mimic the Massachusetts Stormwater Standards with respect to their requirements for quantitative and qualitative control of stormwater runoff, with the exception that they do specify that the controls be designed to treat one inch of runoff from paved surfaces. In addition, however, the subdivision regulations call for several items that have not been addressed in this submittal: These include a nitrogen/phosphorus loading report, and predicted impacts on vegetation on the site. Due to the fact that this is a relatively small project, the Board may want to consider waiving these requirements. 2. The flow paths used to develop the times of concentration for each subarea should be shown on the drainage area maps. 3. The hydrologic analysis excludes the roof areas from the two proposed new houses as well as the new common driveway in the post development model, on the basis that the proposed infiltration systems will accommodate up to the 100-yr storm. However, the proposed roof drywells are sized to infiltrate only 0.35-inch (the required recharge volume); therefore even the smallest design storms would generate excess runoff flow. In addition, while the driveway infiltration trenches are sized for the 100-year storm, the design is based on an infiltration rate of 2 in/hr as opposed to the more conservative “Rawls” rate called for in the DEP Stormwater Handbook, which for the native soils would be 1.02 in/hr. The trench design calculations also assume that all of the drainage off of the driveway would instantaneously infiltrate into the stone trench and the soil beneath it, rather than backing up and/or flowing down the surface (or subsurface) of the trench. Given the steep (8%) slope of the trench and the fact that the design provides virtually no surface storage to capture and hold the runoff in the trench pending infiltration, I believe that only a small fraction of the drainage from and across the driveway would actually be effectively infiltrated, especially during large, intense storm events. 2009 Salem Street, Technical Review 3 August 29, 2011 4. Consideration should be given to adding an infiltration/drainage trench along the western edge of the common driveway, to prevent flow off of the cut slope from draining across the driveway. 5. As proposed, the infiltration trench would extend along the eastern edge of the existing/widened driveway to Salem Street. It is not clear whether the driveway will also be regraded to drain to the trench. 6. Confirmatory soil tests will be needed at locations proposed for stormwater infiltration. 7. The drywells and the infiltration trenches should be treated as separate treatment trains in the TSS removal calculations; the treatment provided by each is not additive. Most importantly, the treatment provided by the driveway infiltration trenches is only as effective as the infiltration itself. 8. I recommend against the use of filter fabric beneath the proposed infiltration systems (drywells and trenches), due to the potential for clogging. The UNH Stormwater Center recommends instead using a layer of bank-run gravel to prevent migration of material between layers. 9. The DEP Handbook calls for 100-ft setback from infiltration structures to private wells. The proposed drywells on the two new lots are approximately 50 ft from the proposed well locations. Given that they will discharge roof runoff only, this may be acceptable. 10. Responsibility for long-term implementation of the O&M plan needs to be clarified; the narrative suggests that the town may be asked to accept and maintain the roadway and associated drainage. 11. The design detail for the infiltration trench calls for crushed stone topped by peastone, yet the O&M plan references that the trench will need to be periodically mowed. Is it intended that the trench will be vegetated? 12. The roof infiltration systems are proposed as drywells filled with stone. It is not clear how they are to be cleaned out at least annually, as is called for in the O&M plan. 13. The reference to porous pavement driveways in the O&M plan narrative should be removed; no porous pavement is proposed for the site. 14. The frequency of maintenance activities in Table 3 of the O&M plan is not consistent with the frequencies called for in the plan narrative. 15. Table 3 also references storm drain inlets and grass lined channels; however there are none shown on the plan. 2009 Salem Street, Technical Review 4 August 29, 2011 16. The O&M Plan references designated snow stockpile locations located away from the infiltration trenches, however they are not shown on the plan. 17. Even without the roof runoff, the hydrologic calculations indicate that the volume of water discharged to and stored within the onsite vernal pool modeled as Pond 1 would be increased during storms. Due to the change in surface cover and the small decrease in drainage area, groundwater flow to the vernal pool would be correspondingly decreased. It is unclear whether this project is subject to Conservation Commission jurisdiction. I appreciate the opportunity to assist the North Andover Planning Board and the review of this project, and hope that this information is suitable for your needs. Please feel free to contact me if you or the applicants have any questions regarding the issues addressed herein. Sincerely, EGGLESTON ENVIRONMENTAL Lisa D. Eggleston, P.E. C: Jennifer Hughes, North Andover Conservation Commission