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HomeMy WebLinkAbout2012-04-03 Planning Board Supplemental Materials (47)26 Main Street page 1 of 4 March 19, 2012 North Andover Planning Board Town Offices 1600 Osgood Street North Andover, MA 01845 North Andover Conservation Commission Town Offices 1600 Osgood Street North Andover, MA 01845 RE: Site Plan Special Permit/ Notice of Intent 26 Main Street Applicant: Jeffco, Inc. Dear Board and Commission Members: We are in receipt of a peer review made by your consultant, Eggleston Environmental, in a letter to the Board and Commission dated March 8, 2012, for the project captioned above. We have summarized our responses to these comments below: Comment 2. The proposed drywell is sized to infiltrate only the recharge volume (0.35 in) from half of the building roof, or 20 percent of the post-development impervious area. There is nothing in the Stormwater Report documenting why the project cannot comply fully with the recharge requirement of Standard 3, e.g. by infiltrating the entire recharge volume for the post- development site, as is warranted under the "maximum extent practicable" requirement for a redevelopment project. Response: The project is a five - nine unit or less condominium project and as such is subject to the stormwater management standards to the maximum extent practicable. The project is also a redevelopment project as it is proposed on a parcel previously developed with buildings and pavement. Importantly, as a redevelopment project, the post developed impervious area is less than the pre developed condition and there is no loss of recharge, or more importantly, the post developed recharge rate is comparable to the pre-developed recharge rate. Since the drywell is not required, the drywell has been removed minimizing excavation in the buffer zone. The rear of the roof will discharge to the ground surface, thus disconnecting the roof in compliance with low impact development practice. 3. The TSS removal calculations are based on 77 percent TSS removal by the proposed Stormceptor unit. As is indicated in the supporting documentation submitted, the Stormceptor units are currently certified by the NJCAT (New Jersey Corporation for Advanced Technology) 26 Main Street page 2 of 4 through the TARP (Technology Acceptance and Reciprocity Partnership) Tier 1 program for a TSS removal rate of 50 percent, not 77 percent. The proposed treatment train, consisting of a deep sump catchbasin followed by the Stormceptor, therefore does not meet the 80 percent TSS removal requirement of Standard 4. Response: The project is not in New Jersey. Note the following excerpted from the Introduction of the Technology Acceptance Reciprocity Partnership (TARP) Protocol for Stormwater Best Management Practice Demonstrations, Section 1.2 Purpose: “The requirements for a stormwater BMP demonstration are minimized in the Protocol to a common set of uniform criteria, acceptable to all participating states. However, specific state requirements must be considered when a technology proponent is pursuing certification or verification of a stormwater BMP in that state; specific requirements for the endorsing states are described in, but are not limited by, Appendix D. In addition, the Protocol does not completely eliminate all state review or approval of projects proposing to use the stormwater technology, nor does it require any state to “rubber stamp” the approval or permit of another state or regulator.” This site was designed using the Massachusetts STEP fact sheet (fact sheet #4), which was attached to the report, designat ing a removal rate of 77% for tributary impervious areas less than 0.45 acres to an STC 900 unit (the site tributary area, including off site impervious areas is 0.39 acres). At one point, circa November 2010, the MADEP also required a calculation of WQ flow rate for proprietary products such as a Stormceptor. The bypass rate for the STC 900 is 0.64 cfs, the site WQF using the DEP guidance is 0.5 cfs. . This calculation was also included in the report. Since the WQF is less than the bypass rate of 0.64 cfs for the unit as well as the fact that the impervious tributary area of 0.39 acres is less than the 0.45 acres of impervious area contained in the Massachusetts issued STEP fact sheet, we chose the 77% removal rate cited in the fact sheet as the removal rate for the unit. In addition, the vendor’s PCSWMM model also indicates that the STC 900 is adequate. As noted in the attached TARP Tier I comparison results, the verified removal rate is 75%. Using this verified removal rate of 75% still results in 80% removal of TSS. It is important to note that in its file number review, the Massachusetts DEP did not mention that the Stormceptor unit is credited for only 50% and, in fact, had no comments at all. 4. It is not clear where the roof drainage from the front of the building will go, although it appears this flow was included in the area tributary to the Stormceptor unit. To the maximum extent possible, roof downspouts should be directed away from impervious areas to enhance infiltration of the clean roof runoff and minimize the flow across the pavement. Response: The front of the building will not have gutters or downspouts so the roof area tributary to the proposed catch basin and Stormceptor was included in the calculations. The rear of the roof will discharge onto pervious surfaces in keeping with the comment. 5. Based on the log for Test Pit TP-l, at the proposed drywell location, the entire to-foot excavation was in fill, possibly associated with the old railroad bridge abutment. Evidence of trash disposal and layers of ash were also noted. Not only is this material unsuitable for infiltration, but it may suggest the need for further investigation of the site for potential hazardous wastes. Test Pit TP-2 had significantly less fill and may be a more suitable location for infiltration. I note that while three test pits are shown on the plan, the log for Test Pit TP-3 was not included in the Stormwater Report. Response: The test pit was in an area that was used for burning household trash which was the 26 Main Street page 3 of 4 custom back in the day. There was no evidence of hazardous waste in the pit nor was there any encountered in the other 2 test pits. Test pit 3 must have been omitted in the copy, as the original does have it. A copy of test pit 3 data is attached. 6. The proposed drywell location is also only about 12 ft from a steep embankment. For most subsurface infiltration structures, the DEP Handbook calls for a minimum setback of 50 ft from slopes greater than 15 percent to prevent breakout of the infiltrated flow. While 50 ft may be more than is needed for a small shallow drywell, I believe that 12 ft is inadequate. Response: The drywell has been removed. 7. It is not clear from the design detail whether the drywell cover is to be solid or grated. If grated, I recommend a beehive grate to prevent clogging. Response: The drywell has been removed. 8. The design detail for the level spreader indicates that the upgradient area is to be grassed. The flared end section should be sized in accordance with the projected discharge velocities onto the turf, and a design detail provided on the plan. Response: The discharge velocities for t he 15-inch pipe are: 1-year V=1.54 fps, 2-year V=1.91 fps, 10-year V=2.74 fps and the 100-year isV= 3.40 fps. These velocities are less than 4 fps for grass. However. the turf in the level spreader has been reinforced with an HDPE anchored cell to enhance resistance to outlet velocity. A detail has been added to the plans, as suggested. 9. I have the following comments on the O&M Plan:  The plan should include a schedule and log form listing the long-term maintenance tasks, and should be a stand-alone document separate from the Construction Period O&M/Sediment Control Plan. I find it useful to include a simple sketch plan showing the locations of the BMPs to be maintained. Response: A simple sketch of the BMP locations has been added, as suggested.  Item #3 in the O&M Plan should be re-worded to say that the catchbasins should be cleaned when the sediment level is within two feet of the outlet pipe; this is easier to measure than the depth of sediment accumulated. Response: The wording has been revised, as suggested.  The Stormceptor unit should also be cleaned a minimum of once per year, with a vacuum truck. Response: The Stormceptor unit requires that it be cleaned when 8” of sediment has accumulated. The rate of accumulation is a function of the specific site and will determine the removal time. The maintenance schedule as obtained from the manufacturer is cited below:  The unit should be inspected post construction and prior to being placed in service for proper placement and for any damage.  Inspect the unit every six months during the first year of operation to determine the oil and sediment accumulation rate.  In subsequent years, inspections can be based on first year observations, or at least once per year. 26 Main Street page 4 of 4  Cleaning of the unit is required once the sediment depth reaches 15% of the unit’s storage capacity (generally takes one year or longer). For the Stormceptor STC 900, the depth of sediment at 15% capacity is 8 inches.  Inspect the unit immediately after any oil, fuel or chemical spill. The unit is cleaned using a standard catch basin vacuum truck and usually takes less than two hours. A licensed waste management company should remove the contents particularly when waste products consist of oil, fuel and/or chemical spills.  Periodic inspection of the drywell and the outlet sump and level spreader should be included in the O&M plan. Response: The drywell has been removed. Inspection of the level spreader was included in the O & M plan. Note 5 has been expanded to require repair and sediment removal, as required. 10. The Long-Term O&M Plan indicates that snow will be plowed to just off the edge of pavement. Based on the site configuration, it appears that much of the snow would get plowed off the eastern end of the parking area, within the wetland buffer zone and down gradient of the treatment processes. Snow storage in this area should be specifically prohibited, and alternative locations on the site designated. Response: The end of the drive is well over 50 feet from the wetland. There is also ample room for snow storage on the southerly side of the drive. Available snow storage area along the drive has been depicted on the plan. Should you have any questions concerning this correspondence, feel free to contact us. Sincerely, ANDOVER CONSULTANTS, INC. James S. Fairweather II, P.E. Project Engineer Enclosures cc Eggleston Environmental MADEP-NERO Jeffco, Inc.