The URL can be used to link to this page

Home My WebLink About2013-04-02 Planning Board Supplemental Materials (126) Eggleston Environmental 32 Old Framingham Rd Unit 29 Sudbury MA 01776 tel 508.259.1137 fax 866.820.7840 March 13, 2013 North Andover Planning Board 1600 Osgood Street North Andover, MA 01845 Attn: Judy Tymon, Town Planner RE: Stormwater Review 250 Clark Street Dear Ms. Tymon and Board Members: Per your request, I have conducted a technical review of the February 15, 2013 Special Permit Application packet submitted by Mann & Mann, P.C. for the construction of a 9,280 sf addition and associated alterations to the property at 250 Clark Street in North Andover. Included in the materials I received and reviewed were the following: § Special Permit – Site Plan Review Application, Flight Landata, Inc. 250 Clark Street North Andover, § Site Plan in North Andover, MA, Sheets 1-4 of 4, prepared by Williams & Sparages, dated February 7, 2013 and revised February 15, 2013. § Stormwater Report, 250 Clark Street, North Andover, prepared for Flight Landata Inc. by Williams & Sparages and dated February 7, 2013. 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 Town of North Andover’s zoning requirements for Site Plan Review, 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 filed concurrently with the Conservation Commission. My review is aimed at assisting both the Planning Board and the Conservation Commission in their respective reviews of the project. The project site is a 27,027 sf (proposed) lease parcel on the Lawrence Municipal Airport property located in North Andover. It is currently occupied by a 9,525 commercial building and associated parking and driveways. There are no stormwater management structures on the existing site; runoff from the property drains overland toward wetlands located to the west, north and east of the lease parcel. The proposed project includes construction of a new 9,680 sf building, 19 new parking spaces, relocation of the septic system serving the facility, and two rain gardens (bioretention areas) for stormwater treatment of runoff from the new paved areas and portion of the roof areas. 250 Clark Street, Stormwater Review 2 March 13, 2013 My comments on the proposed plan are outlined below: 1. It appears from the plans and available aerial photography that runoff from the project area currently drains to three different wetland areas; however it is not clear to what degree the three are hydrologically connected. In order to evaluate impacts on the hydrologic regime of the three wetlands as well as any potential downstream flooding the drainage analysis needs to be broken out on a subwatershed basis, with control points at each resource area. In accordance with the North Andover Wetlands Bylaw Regulations, the analysis should evaluate the volume of runoff discharged to each control point as well as the peak rate of flow. The local wetlands regulations also require analysis of the 1-year storm. 2. The proposed rain gardens are intended to exfiltrate captured runoff flow in order to provide the required groundwater recharge and water quality treatment (via filtration and pollutant uptake) of runoff flow. However, due to the shallow groundwater table on the site, in conjunction with the shallow depth (0.25 ft) of the proposed rain gardens, their capacity to provide either effective recharge or filtration of the recharge will be minimal to non-existent during periods of high groundwater, and may also be diminished during periods of lower groundwater. Based on the soil tests conducted, the surface elevation of Rain Garden #1 will intercept the groundwater table during high groundwater conditions, and in Rain Garden #2 the ESHGW elevation is less than a foot below the surface, within the soil media layer. Per the DEP Stormwater Handbook, rain gardens designed to exfiltrate must ensure two feet of vertical separation from the seasonal high groundwater table to the bottom of the bioretention cell (e.g. the bottom of the filtration media). 3. The dead storage volume in the rain gardens, e.g. for calculating water quality or recharge volume should only include that which is above the ESHGW elevation. During high groundwater conditions, there may be as little as 0.05 ft (0.6 in) of storage between the pool/groundwater elevation and the overflow weir of Rain Garden #1. 4. Since the proposed project calls for the infiltration BMPs to attenuate large storm flows (10-yr and larger) and the separation to seasonal high groundwater beneath the systems is less than 4 feet, the DEP Stormwater Standards require that a mounding analysis be performed to demonstrate that the systems will be fully dewatered within 72 hours. Per my comments above, the proposed design provides little to no separation to groundwater, hence a mounding analysis is unlikely to provide any benefit. I would suggest instead that the hydrologic analysis be run without any exfiltration from the basins in order to fully account for impacts to the rate and volume of runoff. 5. As they are currently designed, the proposed rain gardens are to be mulched areas with clustered grasses and shrubs. Frequent inundation and overflow of the 250 Clark Street, Stormwater Review 3 March 13, 2013 basins, particularly Rain Garden #1, has the potential to wash the mulch out of the basin and into the adjacent wetlands. Consideration should be given to vegetating the entire bottom of the basins and letting them function more as vegetated bioswales. This would both eliminate the potential for mulch washout and provide filtration/water quality enhancement of the runoff that is not infiltrated. 6. The design detail for the proposed rain gardens on Sheet 4 of 4 calls for a 6-inch ponding depth however, based on the spillway elevations, the effective ponding depth is only 3 inches. 7. Based on the plans, overflows from Rain Garden #1 will drain approximately 50 feet overland to the adjacent wetland. It is unclear, however, where the discharge from Rain Garden #2 will go, particularly as there is a stone wall immediately downgradient of the proposed overflow weir and a paved road immediately to the north. 8. The surface area of the proposed rain gardens should be treated as impervious areas in the HydroCAD analysis in order to avoid double-counting of the infiltration capacity. 9. Based on the plans the existing building has roof downspouts that appear to discharge to the ground around the building, with the runoff flow then draining across the pavement. It is not clear how the roof drainage from the proposed building will be handled. Given that the proposed rain gardens will be limited in the amount of groundwater recharge they can provide, it may be possible to direct some of the roof runoff to drip trenches or a shallow infiltration trench (e.g. in the vegetated area to the southeast of the new building) to provide the needed recharge. 10. Additional information is needed on the proposed roofing material for the new building. Unless they are coated or sealed, galvanized metal roofs can leach significant quantities of zinc into the runoff stream. The discharge from such roofs should not be infiltrated or discharged to wetland a resource area. 11. While it is appropriate to combine the Long-term Pollution Prevention Plan with the O&M plan for the permanent BMPs on the site, short-term construction- related pollution prevention measures should be incorporated in a separate Erosion & Sediment Control Plan for use by the site contractor. The Long-term Pollution Prevention/O&M Plan should be aimed at the facility manager and should address site specific good housekeeping measures such as pavement sweeping, pesticide and fertilizer use, deicing and snow storage locations for the project site, as well as the locations and maintenance of the permanent BMPs. Given the nature of the operations on the site, the storage and use of potentially hazardous materials as well as any vehicle (or airplane) washing, deicing, etc. should also be addressed. 250 Clark Street, Stormwater Review 4 March 13, 2013 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