Loading...
HomeMy WebLinkAboutConsultant Review - 250 CLARK STREET 4/3/2013 Project No. NAND-0020 April 3,2013 WILLIAMS North Andover Planning Board Sp � s North Andover Conservation Commission ENGINEERS PLANNERS SURVEYORS 1600 Osgood Street C( North Andover,MA 01845 T Subject: Response to Stormwater Review by Eggleston Environmental 250 Clark Street(Flight Landata,Inc.) DEP File No. 242-1584 Dear Members of the Planning Board and Conservation Commission, The purpose of this letter is to respond to technical review comments that we received in a letter addressed to the Planning Board from Eggleston Environmental(EE) dated March 13,2013 for the proposed expansion of the Flight Landata facility located at 250 Clark Street. We have spoken with Lisa Eggleston regarding the comments and have communicated on how we intend to address them. The comments provided by Eggleston Environmental are shown in italics below, and, our responses are in the bold text that follows. L It appears from the plans and available aerial photography that runoff from the project area currently drains to three different watershed 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. The first topic was discussed at our presentation to the Conservation Commission on March 27,2013. The two wetland resource areas to the north and west of the existing building are joined at approximately 150'to the northwest of the existing edge of pavement and are tributary to an unnamed stream that ultimately reaches the Merrimack River. The wetland resource area to the southeast of the existing building is tributary to a different unnamed stream which passes under Holt Road and also reaches the Merrimack River. We have split the flows into three directions in our revised calculations to compare the flows to the wetland resource areas to the north,west and to the southeast as requested. A Volume Comparison Table was included in the initial Comparative Drainage Analysis on page 3 and will be updated to reflect the results of the revised calculations. We have included an analysis of the 1-year storm in the revised calculations. As discussed with Lisa Eggleston,we have revised the title of the structures known as Rain Garden#1 and Rain Garden#2 to Biofilter#1 and Biofilter#2 respectively. 191 South Main Street, Suite 103 • Middleton, MA 01949 • Tel: (978) 539-8088 • Fax: (978) 767-8579 Response to Stormwater Review 250 Clark Street North Andover, MA 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 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 ESHGWelevation is less than afoot below the surface, within the soil media layer. Per the DEP Stormwater Handbook, rain gardens designed to exfiltrate mush 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). As discussed with EE,the presence of shallow groundwater is an existing condition that we must work around. For example,the surface elevation of the edge of pavement where we are proposing additional parking spaces ranges between 134.54 to 134.06 (average=134.3). The estimated seasonal high groundwater table (ESHGWT)was observed at 133.2,only 1.1 feet below the average pavement grade in this area. We agree with EE's comment,but our position is that we have to make our best attempt to improve the water quality of the stormwater runoff even if it does not exactly fit into the criteria of the DEP Stormwater Handbook. If this were a new project,we could design a system that met each criteria,but as an expansion of an existing facility this option is not available to us. We agree with EE's comment regarding the requirement of 2-foot of separation from the high groundwater elevation,however,DEP does recognize that for sites comprised of hydrologic soil groups "C"and"D"and bedrock at the land surface,proponents are required to infiltrate the required recharge volume only to the maximum extent practicable (see Volume 1,Chapter 1 pages 6 &7 of the Stormwater Handbook). Our site is comprised solely of"C" soils and it is our belief that based on the existing conditions that we have to work with...we are infiltrating to the maximum extent practicable. We have raised the level of the bottom of Biofilter#1 in the revised calculations to elevation 133.2 in order to provide additional storage during periods of high groundwater. 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. As mentioned in our previous response,we have raised the bottom of Biofilter#1 to match the ESHGWT and therefore,will be able to count the volume below the spillway towards dead storage. The ESHGWT observed at Biofilter#2 was at elevation 129.1 and is approximately 0.9 feet below the proposed bottom of the biofilter at elevation 133.0 and no change to the provided dead storage will be required. Response to Stormwater Review 250 Clark Street North Andover, MA 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 four 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 that the hydrologic analysis be run without any exfiltration from the basins in order to fully account for the impacts to the rate and volume of runoff. We agree with EE's comment regarding the requirement of a mounding analysis and came to the same conclusion. We also removed the rate of exfiltration through the bottom of the biofilters as suggested by EE in the revised calculations to show the difference between providing for no exfiltration and providing some limited exfiltration. There will be many times during the year where some infiltration will occur. 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 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. We have revised the detail to specify that the bottoms of the Biofilters will be seeded with "New England Wetmix" or a suitable alternative. 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. See the revised calculations. i 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 down gradient of the proposed overflow weir and a paved road immediately to the north. The overflow from Biofilter#2 will discharge via stone spillway at elevation 133.25 and then flow overland across the existing lawn towards the utility pole and the lower grades at the existing limit of disturbance. Runoff from this lawn area then flows east,southeast away from the paved roadway towards the existing wetland system tributary to the unnamed stream flowing under Holt Road mentioned previously. 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. We have revised the bottom of the Biofilters to be treated as impervious surfaces in the revised calculations. We have discussed this with EE and agree that there will be some exfiltration occurring through the bottoms at certain times of the year. Response to Stormwater Review 250 Clark Street North Andover, MA 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 handles. 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. We have added a 157'long,4'wide and 2'deep stone-filled trench to distribute the southern half of the proposed roof runoff evenly over the length of the building,which will provide storage volume for the 2 year storm event and then overflow across the lawn area towards the southeast. We felt it prudent not to add a similar trench on the other side of the proposed building as there is not a lot of room between it and the existing building. On this side of the building,roof runoff will continue to be directed to the surface and the land between the buildings will be graded to allow surface water to flow out to the west. 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 a wetland resource area. According to the project architect,Mr. David Barsky,the roof shall be constructed of a patented sheet steel product having an acrylic coating of corrosion-resistant material applied to the steel sheets. 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 of the site, the storage and use of potentially hazardous materials as well as any vehicle (or airplane) washing,deicing, etc. should also be addressed. Please see the Erosion and Sediment Control Plan dated February 7,2013 included in the Notice of Intent(NOI) submittal package as well as an Operation&Maintenance Plan with the same date which will address the comment made by EE. A copy of the NOI package has been provided to EE. Response to Stormwater Review 250 Clark Street North Andover, MA We trust that you will find the responses above adequately address the comments made by the technical review agent and will allow you to approve the proposed expansion of the Flight Landata facility. If you have any questions,please do not hesitate to contact me directly. Very truly yours, v Peter M. Blaisdell,Jr.,P.E.,P.L.S. Project Engineer PMB:CPS Enclosures cc: Attorney Jill Mann Scott Stetson/KEYW Corporation Lisa Eggleston,P.E.