Eggleston Environmental July 29, 2016 North Andover Planning Board 1600 Osgood Street North Andover, MA 01845 Attn: Rebecca Oldham, Staff Planner RE:
Stormwater Review Princeton at North Andover Apartment Homes
Dear Ms. Oldham and Board Members: Per your request, I have reviewed the following materials submitted in support of the Site Plan – Special Permit Applications for the above-referenced project: §
Special Permit – Site Plan Review Application for an approximately 40,000 sf commercial development on approximately 4.3 acres of land known and numbered as1210 Osgood Street, Princeton Development LLC – Petitioner, submitted by John T. Smolak, Esq.
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Special Permit – Site Plan Review Application for a market rate rental residential community on approximately 9.5 acres of land known and numbered as1210 Osgood Street, Princeton Development LLC – Petitioner, submitted by John T. Smolak, Esq.
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Stormwater Report in Support of: Princeton at North Andover Apartment Homes, prepared for Princeton Properties by Hancock Associates and stamped 6/21/16.
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Permit Site Plan (13 Sheets), Princeton at North Andover Apartment Homes, prepared for Princeton Properties by project team members in cooperation with Hancock Associates, dated 6/17/16.
The following additional materials were not part of the application package but were provided to me by Hancock Associates upon request: §
Electronic copy of the 6/17/16 Grading and Drainage Plan with test pit locations shown.
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Table of inverts, HSA – Princeton at North Andover Apartment Homes, dated 7/20/16
I also conducted a brief site visit to the property to observe existing drainage patterns. My primary focus in this technical review is on the overall stormwater management approach and design concepts used in the project, as well as its compliance with the 32 Old Framingham Rd Unit 29 Sudbury MA 01776 tel 508.259.1137
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Town of North Andover’s Site Plan Review regulations, the Massachusetts Department of Environmental Protection (DEP) Stormwater Management Standards and Regulations, and the North Andover Wetlands Bylaw. It is my understanding that a Notice of Intent (NOI) application for the project has been or will be filed concurrently with the Conservation Commission. My review is aimed at assisting both boards in their respective reviews of the project. The project site is an approximately 13.8-acre parcel on the west side of Osgood Street in North Andover. It is currently undeveloped and mostly vegetated. Drainage is overland to the north and west toward several wetland areas on and adjacent to the site. The proposed project calls for a mixed use development comprised of approximately 40,000 square feet (sf) of commercial development, four apartment buildings, a leasing office and pool facility, and associated landscaping, parking and driveway areas. As proposed, runoff from the impervious areas of the site would be drained through a closed drainage system to sixteen subsurface storage/infiltration systems, with overflow discharged toward the wetlands via stone aprons. My comments on the proposed design are outlined below: 1. Contrary to statements made in the Stormwater Report, the proposed project does not constitute Environmentally Sensitive Site Design or incorporate Low Impact Development (LID) techniques in the management of stormwater from the site, both of which entail minimizing impervious surfaces, disconnecting flow paths and using natural drainage systems to treat and recharge stormwater as an alternative to “curb and gutter” closed systems such as the one proposed. While the proposed plan does largely maintain existing drainage patterns it is not clear that any serious consideration was given to incorporating such measures in the design of the project, as is required by Stormwater Standard 1. 2. The test pit locations and drainage system inverts that were provided to me as supplemental materials are a fundamental component of the drainage design and need to be included on the permit plans. 3. The plans should clearly identify the pervious areas on the proposed site. The Landscape Plan identifies the proposed plantings and some areas to be loamed and seeded, but it is not clear how the areas immediately surrounding the buildings are to be treated and whether the plan is consistent with the runoff calculations. 4. The Stormwater Report includes sizing calculations for each of the proposed storm drain outlets in accordance with Stormwater Standard 1, however this information is not shown on the proposed plan and there is no design detail for the outlet aprons.
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5. Proposed Stormtech Systems #21, 33 and 34B are not labeled on the Grading and Drainage Plan and Stormtech System #38 is incorrectly identified as a second System #36. 6. In order to better quantify impacts on wetland resources I recommend setting the control points used in the hydrologic analysis at the wetlands themselves and including the two onsite wetland areas along the western boundary and the two offsite wetlands to the west and north of the site. It should also be clarified whether any of these wetland areas are hydrologically connected. 7. The drainage analysis assumes that all of the runoff from eighteen subcatchments on the post-development site is conveyed through the closed drainage system to the subsurface chambers for flow attenuation in all design storms up to and including the 100-year event. It therefore needs to be demonstrated that the catchbasin grates, roof drains and storm drains are designed to capture and convey the 100-yr flow. 8. The building roof areas are modeled as being unconnected (e.g. draining across pervious surfaces) in the post-development drainage analysis; this is inconsistent with the plan, which calls for roof drains directly connected to the subsurface structures. 9. According to the plan, the roof drainage from each of the proposed buildings on the site would be discharged at a single location on one side of the building. It should be confirmed (e.g. through architectural drawings) that this is consistent with design of the roofs and that all of the roof runoff from each building can be conveyed to that single location. 10. It is unclear from the plans how the runoff from the proposed carports would be handled. 11. The assumed flow path for post-development subcatchment area 300 is not consistent with the grading shown on the plan and artificially increases the Tc for the subcatchment. In general, the post-development Tc’s should not be greater than the pre-development ones. 12. The drainage analysis is not consistent with the outlet control structure detail for the subsurface Stormtech systems shown on Sheet 10 of the plan. The plan shows a weir with a 3-ft crest and four 4-inch orifices, while the drainage analysis assumes 4-ft weirs in all sixteen outlet structures, and a variety of configurations of 3-inch, 4-inch, 4.3-inch, 5-inch and 6-inch orifices. All such design assumptions need to be accurately reflected on the permit plans. 13. The drainage analysis assumes that eleven of the proposed subsurface systems are comprised of Stormtech MC-3500 chambers and that five of the systems are comprised of MC-4500 chambers. Only the MC-3500 chambers are shown on the
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detail sheet. The design detail also does not include the distribution manifolds or specify the amount of stone beneath the system. 14. As is identified in the Stormwater Report, the predominant soils on the site are mapped by NRCS as Paxton and Woodbridge fine sandy loams. These soils are relatively poorly drained; the Paxton series is assigned a Hydrologic Soil Group (HSG) classification of C, and the Woodbridge soils are classified HSG C/D, or only marginally suitable for infiltration. The soil test pits conducted on the site appear to confirm the mapped soils, and no additional grain size analyses or infiltration testing data were provided. I also note that the runoff calculations and the compensatory recharge calculations are based on HSG C soils. It is not clear, therefore, why both the drainage analysis and the drawdown calculations assume the Rawls infiltration rate for HSG B sandy loam (1.02 in/hr) for eleven of the sixteen proposed subsurface infiltration systems, and the Rawls rate for HSG loamy sand (2.41 in/hr) for infiltration system #21. These assumed infiltration rates are an order of magnitude greater than the Rawls rates for the mapped soils (0.17 to 0.27 in/hr) and significantly overestimate the amount of runoff that will be infiltrated during the design storm events. 15. I also note that based on the test pit data provided, four of the subsurface systems where exfiltration is assumed (Stormtech Systems 31, 32, 33 and 34) would not meet the 2-ft separation requirement between the bottom of the system and estimated seasonal high groundwater (ESHGW). No soil tests were conducted in another six of the proposed infiltration areas (Stormtech Systems 21, 31B, 34B, 35B, 38B, 39) where exfiltration is assumed. Two of those systems (38B and 39) are several feet below existing grade and would likely not have adequate separation to ESHGW. 16. The drainage analysis should not assume any storage volume in that portion of any of the systems that is below ESHGW. 17. The infiltration system elevations listed on pages 7 and 8 of the Stormwater Report are not consistent with the table of inverts that I was provided. In addition, the separation to ESHGW should be measured from the bottom of the system (e.g. the stone layer) and not the invert of the lowest structure, thus several of the recharge systems listed would not provide the required separation. The model assumes a foot of stone beneath each of the proposed infiltration systems. 18. In accordance with the requirements of Stormwater Standard #3, the recharge calculations should include a capture area adjustment to account for impervious areas that are not tributary to the proposed recharge structures. 19. A number of the proposed infiltration systems are to be constructed in fill. The plans should specify any fill material to be used beneath the systems.
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20. A portion of Stormtech System 10 is located within the 50-ft buffer to the onsite wetland. The MA Stormwater Handbook requires infiltration structures to have a minimum setback of 50-ft from wetlands. 21. Based on the relative inverts there appears to be potential for breakout of infiltrated flows downgradient of Stormtech Systems 10 and 21. 22. As proposed, stormwater treatment for the impervious areas on the site would be provided by the subsurface Stormtech Isolator Rows, for which 80 percent TSS removal credit is being claimed in the water quality calculations. I note that the Isolator Row is considered a proprietary treatment/pretreatment device and that no TSS removal rate has been assigned by MADEP, therefore third party verification of the stated removal rate must be provided. Although it was checked off in the Stormwater Checklist, no such third party verification was included in the materials I reviewed. The verified removal rate should also reflect the fact that some of the Isolator Rows would not be receiving raw runoff, but rather flow which has already undergone some sediment removal in upgradient deep sump catchbasins. (Most third party testing is based on raw runoff). I further note that the treatment provided in an Isolator Row system does rely on exfiltration through the woven geotextile that surrounds the chambers; hence it will not function effectively in those locations where adequate separation to groundwater is not provided. This would include proposed Stormtech Systems 11, 36, 37 and 38 as well as those identified in my previous comment # 13. 23. Calculations are needed to demonstrate that the entire water quality flow from all impervious areas on the site, including roof areas, would be directed through the proposed treatment BMPs and not bypassed. 24. The Stormwater Checklist indicates (under Standard 5) that the proposed land use is covered by the NPDES Multi-Sector General Permit (MSGP). I do not believe this to be the case since it is not an industrial use, however the proposed project may constitute a Land Use with Higher Potential Pollutant Loadings (LUHPPL) with moderate to higher concentrations of oil and grease if the projected traffic generation is greater than 1,000 vehicle trips per day. If that is the case, the treatment train for the pavement runoff on the site would need to include an oil grit separator, a filtering bioretention area, a sand filter, or equivalent. The treatment train currently proposed would not meet this requirement. 25. It should be confirmed that the wetlands on and adjacent to the site do not drain to Lake Cochechewick and that the requirements of Stormwater Standard 6 (Critical Area) do not apply. 26. The proposed plan calls for upwards of ten feet of fill across much of the project site. The Board may want to consider requesting cut and fill calculations for the proposed development, as well as specifications for any fill to be brought onto the site.
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27. I recommend against the placement of geotextile fabric on the bottom of the proposed infiltration systems, as it can be prone to clogging. A drainage net per Stormtech’s Tech Sheet #5 or a filter layer of graded stone are acceptable substitutes for the fabric. 28. The Conceptual Landscape Plan shows shade trees and shrubs over several of the proposed subsurface infiltration systems. Deeply rooted plantings should be avoided. 29. As is indicated in the Stormwater Checklist, the proposed project would entail the disturbance of more than one acre of land and be subject to EPA’s NPDES Construction General Permit (CGP). Prior to the initiation of work the selected contractor will need to file a Notice of Intent for coverage under the CGP, and prepare a Stormwater Pollution Prevention Plan (SWPPP) to be implemented during construction. This requirement should be clearly noted on the plans. The Planning Board and/or Conservation Commission may also want to require proof of filing of the NOI and the opportunity to review the SWPPP prior to the start of work. 30. The Erosion & Sediment Plan should include a detailed construction sequence and should address stabilization of the 2:1 slope between Stormtech System 10 and the adjacent wetland and protection of proposed infiltration areas during construction. 31. The minimum stone size on the proposed construction entrance should be 1-1/2 to 2 inches. 32. The Layout and Materials Plan shows several proposed snow storage locations between the developed areas of the site and the wetlands. Snow stockpiles should be located outside of the buffer zone and upgradient of stormwater BMPs, so that the meltwater undergoes treatment prior to discharge. 33. The plan should show inspection ports brought to grade on each row of subsurface chambers. 34. I have the following comments on the Operation and Maintenance (O&M) Plan included in the Stormwater Report: § Short-term, construction related measures should be removed from the O&M Plan and included in a separate construction SWPPP. The long-term plan should be structured as a standalone document to be maintained on site. § The plan should identify the owners and parties responsible for the ongoing maintenance of the stormwater system. If the commercial and residential portions of the site are to have different owners separate plans may be required.
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The O&M Plan should clearly identify the inspections and maintenance tasks that are to be performed for each pretreatment and treatment BMP, as well as the outlet aprons. Simply attaching design guidelines from the MA Stormwater Handbook and the manufacturer’s cut sheets is not adequate. Subsurface storage/infiltration systems should be inspected at least once a year following a storm of one inch or more to ensure that they are fully drained within 72 hours. The Long Term Pollution Prevention Plan should be expanded to include pavement sweeping, landscape maintenance, snow storage and any other source controls associated with the proposed land uses. The plan should also include a maintenance checklist and a simple figure showing the locations of all stormwater BMPs to be maintained as well as designated snow storage locations.
I appreciate the opportunity to assist the North Andover Planning Board and Conservation Commission with 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, Conservation Coordinator
