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HomeMy WebLinkAbout2013-03-05 Planning Board Supplemental Materials (13) February 27, 2013 Ms. Judith Tymon North Andover Town Planner 1600 Osgood Street North Andover, MA 01845 Re: Regency Place PRD, 464 Appleton St. Stormwater Management Review Dear Ms. Tymon: We have received a copy of Eggleston Environmental's January 29, 2013 Stormwater Management Review for the above referenced project, and we offer the following response. To facilitate the review of this information, we have reproduced each of Eggleston Environmental's comments in Italics, and our response to each comment immediately follows. 1. In general, the proposed plan uses a low impact drainage design that largely maintains existing drainage patterns and takes advantage of the site’s permeable soils to treat and infiltrate runoff onsite. I support the requested waivers regarding pavement width and curbing on the proposed roadway, as they are consistent with the project’s size and low impact development approach. CSI response: No response necessary. 2. Pending confirmation of soil suitability, the proposed plan also provides adequate recharge and attenuation of peak runoff rates. Impacts to the runoff volume are not addressed in the Stormwater Report, but I do note that the volume of flow to the two isolated wetlands on the site, while small, would be approximately doubled under post-development conditions. This increase should be evaluated in the context of the total flow to the wetlands to determine whether it would have any impact on the hydrologic regime. CHRISTIANSEN & SERGI, INC. PROFESSIONAL ENGINEERS AND LAND SURVEYORS 160 SUMMER STREET, HAVERHILL, MA 01830 tel: 978-373-0310 www.csi-engr.com fax 978-372-3960 z Page 2 CSI response: The HydroCAD analysis indicates that the pre- and post- development runoff volumes to the isolated wetlands (Control Point #3 in the analysis) are equal for the 1- and 2-year storms. There will be a very slight decrease in runoff volume under post-development conditions for the 10- and 100-year storms. We have spoken to Lisa Eggleston about this comment and she has confirmed after double checking that her comment was in error. 3. The two test pits conducted on the site to date appear to verify the presence of loamy sands (HSG A) suitable for infiltration in the areas proposed for infiltration basins “A” and “C”, however they also indicate a relatively shallow depth to groundwater, with the estimated seasonal high groundwater elevation (ESHGW) approximately 3 to 4 feet below grade. (Both basins do provide the two feet of separation required). Per the DEP and Wetlands Bylaw regulations, both of which require soil testing in each area proposed for stormwater infiltration, additional tests are needed in the locations of Basins “B” and “D”. I am particularly concerned about the suitability of the soils in the vicinity of Basin “B”, as the NRCS soils map shows it to be in close proximity to an area of less permeable HSG C silt loam, and the elevation of the adjacent wetland suggests that the groundwater table may be within two feet of the bottom of the proposed basin. I also note that there are several large rock outcrops on the north side of the proposed roadway that may be indicative of shallow bedrock in the area. CSI response: We have requested and received permission from the Conservation Commission to dig the test pit in proposed Infiltration Basin B, which is located within the buffer zone. The test pits are scheduled to be performed on March 7th, after which the plans will be revised to include the results. 4. Confirmatory soil testing for the roof infiltration systems can probably be deferred until the time of system installation. The plan calls for a single drywell for each house. It should be confirmed based on building architecture that all of the roof runoff can be directed to a single location. CSI response: Prior to the submittal of the individual site plans for the construction of the houses, a test pit will be performed at the location of each roof runoff infiltration system. The results of the test pits and the pertinent elevations of the infiltration systems will be provided on the site plans. The infiltration systems have been proposed on the low end of the houses to ensure that all roof runoff can be directed to the systems. 5. 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 z Page 3 Standards require that a mounding analysis be performed to demonstrate that the systems will be fully dewatered within 72 hours. CSI response: The required mounding analysis will be performed and submitted. 6. The proposed infiltration basins should be treated as impervious areas in the HydroCAD analysis in order to avoid double-counting of the infiltration capacity. CSI response: The HydroCAD analysis will be revised to include the designation of the infiltration basin surfaces as impervious areas. 7. Per DEP criteria, the infiltration basins should be designed to have a minimum of one foot of freeboard during the 100-yr storm modeled. CSI response: The infiltration basins will be revised to provide a minimum of 1 foot of freeboard. 8. As proposed, runoff from the proposed roadway would undergo minimal pretreatment in the grassed drainage channels prior to being discharged to the infiltration basins. Since the basins are in HSG A soils with a rapid infiltration rate (hydraulic conductivity >2.4 in/hr), the DEP regulations call for pretreatment to remove 44% of the TSS load prior to infiltration. I suggest that at a minimum check dams be added to the steeper portions of the drainage channels to reduce flow velocities and allow filtering and/or settling of coarser solids. Sediment forebays should be added to the two infiltration basins to preserve their infiltration capacity over the long term. It may also be possible to incorporate sediment sumps in the ponding areas of the channels upgradient of each driveway culvert to provide additional pretreatment. CSI response: The plans will be revised to include sediment forebays in Infiltration Basins A and B. We do not feel that the scope of the project warrants the installation of permanent check dams in the grass channels. We have provided a narrative and calculations in the attached Analysis of Drainage Swale Hydraulics. 9. The proposed stormwater BMPs are designed for a ½-inch water quality volume. The North Andover Subdivision regulations call for stormwater BMPs to be designed for a 1-inch water quality volume, as do the DEP standards for infiltration in soils with a rapid infiltration rate. It does appear that the proposed infiltration structures have ample capacity to capture and infiltrate the larger water quality volume. The Stormceptor unit should be sized for the 1-inch water quality volume on an equivalent flow basis. z Page 4 CSI response: The Water quality Volume calculations will be revised to reflect the 1-inch requirement. Calculations will be provided for the proposed Stormceptor. 10. It is not clear that Infiltration Basins “C” and “D” on Lots 2 and 3 are really needed, as they only receive flow from the driveways and vegetated lawn areas on each lot. Would it be possible to achieve the same level of flow attenuation with a landscape berm or vegetated swale to capture and infiltrate the runoff, rather than concentrating the flow at a single location? Alternatively, could the basins be designed as rain gardens in order to better integrate them into the landscape and ensure their ongoing maintenance? CSI response: Infiltration Basins C and D are proposed to mitigate minor increases in peak runoff rates to the abandoned railroad right-of-way. We agree that constructing these BMPs for controlling runoff from single lots seems excessive, but strict compliance with the rules requires it. We will review the calculations and evaluate alternative ways of mitigating the peak rates. Rain gardens are much more expensive to construct and maintain, and are more likely to fail if they are neglected. 11. The TSS removal calculations for the runoff from the lower portion of the proposed roadway assume 80% TSS removal by the proposed Stormceptor unit, citing as the basis for the removal rate a Technology Assessment Report done for the Massachusetts STEP Program in 1998. Per the attached notice, that report was recalled by DEP on January 1, 2011, as part of its transition to a stormwater performance rating system using the Technology Acceptance Reciprocity Partnership (TARP) protocol. Since Mass DEP has yet to formally assign TSS removal efficiencies to any hydrodynamic separators under this program, the Stormwater Handbook (V.2, Ch.4) calls for Conservation Commissions to consider other performance evaluations conducted using TARP protocol. New Jersey is the only state currently reviewing and certifying technologies under the TARP protocol, through the New Jersey Department of Environmental Protection (NJDEP) and the New Jersey Corporation for Advanced Technology (NJCAT). The Stormceptor units are currently certified through that program at a TSS removal rate of 50%. CSI response: Revised TSS removal calculations will be provided. 12. If possible, the existing catchbasin on Blue Ridge Road through which the proposed Stormceptor would discharge should be taken offline to prevent accumulated solids in the catchbasin from being resuspended by the discharge. z Page 5 CSI response: The catch basin located across Blue Ridge Road from the proposed Stormceptor is in a location that is not conducive to bringing it off line. The catch basin is located at the low point in the roadway, in an area a few feet outside of the gutterline of the pavement. It would not be possible to convert this catch basin to a manhole and then add another catch basin that would be in the low point without removing the pavement from a significant area of the roadway and regarding such that the low point would be moved to the new catch basin. It should also be noted that the two deep-sump, hooded catch basins and Stormceptor will provide far better treatment than the one existing catch basin that is being replaced, so the stormwater flowing into the lower catch basin should be much cleaner. The developer will install of a hood in the existing catch basin to further reduce the likelihood of re-suspension. 13. I recommend against the use of filter fabric on the bottom of the proposed roof infiltration systems as it can be prone to clogging. CSI response: The details for the roof runoff infiltration systems will be revised. The filter fabric will be removed. 14. The spillway elevations listed in the table of Infiltration Basin Elevations on Sheet 12 are not consistent with the Table on Sheet 7, or with the HydroCAD analysis. Is it the design intent to raise the spillway elevations to fully contain the 100-yr storm? CSI response: The plans will be revised so that the tables on Sheets 7 and 12 match, and that they are consistent with the HydroCAD analysis. The spillway elevations for Infiltration Basins A and B are intended to be above the 100-year peak storage elevation. The spillways for Infiltration Basins C and D are intended to be below the spillway elevation; the spillways are designed to be active outlet control devices for the basins during extreme storm events. 15. As is indicated in the Stormwater Checklist, the proposed project entails the disturbance of more than one acre and will therefore require a Construction Stormwater Pollution Prevention Plan (SWPPP) filed under the EPA Construction General Permit. I recommend that the Construction SWPPP be prepared by or in conjunction with the contractor selected for project construction, and that the Conservation Commission and/or Planning Board have the opportunity to provide input prior to the plan being filed. CSI response: Copies of the SWPPP will be submitted to the Conservation Commission and/or Planning Board as conditioned. z Page 6 16. The Erosion and Sediment Control Plan on Sheet 9 is listed as providing guidance (versus requirements) for control measures to be implemented. The plan should reference the fact that the project is subject to the requirements of the Construction General Permit and must be implemented in accordance with the SWPPP. CSI response: The plan will be revised to incorporate the recommended notation. 17. The detail for the proposed construction entrance calls for 1 - 2-inch stone. I recommend a stone size of at least 2-inches to more effectively prevent tracking. CSI response: The detail will be revised to show an increased stone size. 18. The Erosion and Sediment Control Plan or Construction SWPPP should specifically address the protection of proposed infiltration areas during construction. The proposed infiltration basins should not be used as sedimentation basins during construction without full restoration of their infiltration capacity. CSI response: The plan and the Pollution Prevention and Operation and Maintenance Plan will be revised to incorporate the recommended notes. 19. The O&M Plan indicates that the owners of each lot will be responsible for maintaining the roof infiltration systems and the grassed swale on their lot, and that if and when the Town accepts the roadway and easements the maintenance (including mowing) of the infiltration basins, catchbasins and Stormceptor would be conducted by the Town. Consideration should be given to the establishment of a Homeowners Association to assume responsibility for the maintenance of the swales and any pretreatment structures, infiltration basins and Stormceptor, as these are not drainage system components that the DPW typically maintains. CSI response: Town Engineer Gene Willis has stated in his review of the project that the DPW does not have the personnel to maintain the Stormceptor and the stormwater system components that will lie outside of the right-of-way, so the O&M Plan will be revised to include proper reference to a Homeowners Association. 20. The infiltration basins on Lots 3 and 4 are not included in the proposed drainage easements. Since their function is to limit the rate of runoff to downgradient properties, some sort of deed restriction may be needed to ensure that the basins (or suitable alternatives) remain in place and that their storage and infiltration capacities are maintained by the individual homeowners. CSI response: Appropriate deed restrictions will be placed on the lots. The Definitive Plans and Stormwater Management Report will be revised as noted to address the comments made by Eggleston Environmental, Hancock Associates, and the Town Departments. Please contact me if you have any questions regarding this matter. Very truly yours, Christ' en & Sergi, Inc. ho- G. ristiansen • Page 7 Analysis of Drainage Swale Hydraulics for “Regency Place”, North Andover, Mass. Date: February 27, 2013 The roadside drainage swales have been designed to convey runoff from the roadway and portions of the front yards to Infiltration Basins A and B. The swales are proposed as part of a Low Impact roadway and stormwater management design. The swales have been designed as grass lined, shallow-flowing treatment and conveyance systems. Volume 2, Chapter 2 of the Mass DEP Stormwater Management Standards includes the following design considerations for Grass Channels: Design grass channels to maximize contact with vegetation and soil surface to promote greater gravity separation of solids during the storm associated with the water quality event (either ½ inch or 1-inch runoff). Design the channel such that the velocity does not exceed 1 foot per second during the 24-hour storm associated with the water quality event. Do not allow the water depth during the storm associated with the water quality event to exceed 4 inches (for design purposes). Make sure the selected design storm provides at least 9 minutes of HRT within the channel. Increasing the HRT beyond 9 minutes increases the likelihood of achieving the 50% TSS removal efficiency. The proposed grass swales have been designed to comply with these design considerations, as follows: 1. The swales have more than sufficient capacity to convey all storms up to the 100-year event (see Hydrologic Analysis in submitted Stormwater Management Report). 2. The swales have been designed to ensure that velocities of flow will be less than 1 foot per second for all storms up to the 10-year event (see attached Channel Calculations report). 3. The maximum depth of flow expected in any of the swales is 3.26 inches during the 10-year storm (see attached Channel Calculations report). 4. The maximum flow length through the drainage swales is about 330 feet. Using the maximum velocity of 0.9 feet per second, the greatest hydraulic CHRISTIANSEN & SERGI, INC. PROFESSIONAL ENGINEERS AND LAND SURVEYORS 160 SUMMER STREET, HAVERHILL, MA 01830 tel: 978-373-0310 www.csi-engr.com fax 978-372-3960 z Page 2 residence time (HRT) is 367 seconds, or 6.1 minutes. Since this is less than the 9 minutes HRT required by DEP, we have not taken any TSS removal credit for the swales. The HRT is a function of the swale length, and the proposed roadway, which has been shortened as part of the cluster subdivision concept, is too short to provide a 9 minute HRT. (We note that one of the goals of Low Impact Development is to shorten roadways and disturb less land area, yet when that is accomplished here the roadside drainage swales are too short to receive credit toward treatment.) Though no credit can be taken for TSS removal in the swales, they will certainly provide substantial TSS removal. The watershed areas contributing to the drainage swales will be very small; 0.55 acres will drain to the swale in front of Lots 1, 2, and 3, 0.57 acres will drain to the swale in front of Lots 4 and 5, and 0.15 acre will drain to the swale in front of Lot 6. This is the reason that the peak runoff rates and velocities are a low as they are. The applicant intends to install sod in the swales as soon as the roadway binder pavement is installed. This will provide immediate stabilization and will allow the grass to provide filtration for the runoff much sooner than would occur if seed were used. Temporary checks dams will be maintained in the drainage swales during construction (see Sheet 9 of the Plans). Due to the limited drainage areas and low velocities of flow expected in the swales, permanent check dams are not needed. Other reasons that we do not propose permanent check dams are that they could interfere with proper grass growth by periodically oversaturating the soils above the check dams, and because the check dams will be physical obstacles that could interfere with the consistent mowing that will encourage thick, healthy grass coverage through the lengths of the swales. There is also the risk that check dams may be damaged or removed by homeowners because they are unattractive or because they are a hindrance to lawn maintenance. We understand the need for permanent check dams in applications where long swales are proposed, where large watersheds contribute to swales, where poor soil conditions exist, and where high flow rates are expected. This project does not have any of these conditions. z Page 3 Channel Calculations The below calculations were performed using the Manning Equation for open channel flow for the segment of drainage swale in front of Lot 1. This swale segment receives the highest peak flow rate of any segment of drainage swale and matches the greatest slope of any segment of swale. All of the other swale segments will have lower peak rates of flow, lower peak velocities of flow, and lower peak depths of flow. Given Input Data: Shape ........................... Trapezoidal Solving for ..................... Depth of Flow Flowrate ........................ 0.7700 cfs Slope ............................ 0.0600 ft/ft Manning's n .................. *0.1500 Height ........................... 24.0000 in Bottom width ................. 36.0000 in Left slope ...................... 4.0000 ft/ft (V/H) Right slope .................... 4.0000 ft/ft (V/H) Computed Results: Depth ............................ 3.2647 in Velocity .......................... 0.9225 fps Full Flowrate .................. 16.7901 cfs Flow area ....................... 0.8347 sf Flow perimeter ............... 42.7303 in Hydraulic radius ............. 2.8128 in Top width ....................... 37.6323 in Area ............................... 7.0000 sf Perimeter ....................... 85.4773 in Percent full ..................... 13.6028 % * The Manning Coefficient of 0.150 is for grass channels where the depth of flow is less than the height of the grass blades (generally 4 inches or less).