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HomeMy WebLinkAbout2026/07/28 - 3rd Stormwater Peer Review - 492 Sutton Street Horsley W*itten Grou Sustainable V i� ;,l ,� i ' V Vn li a Solutions 112 t r Street,® 'Floor Boston,MA 02109 July 28, 2026 Ms. Jean Enright, Planning Director Planning Department Town of North Andover 120 Main Street North Andover, Massachusetts 01845 Re: Third Stormwater Peer Review 492 Sutton Street North Andover, Massachusetts Dear Ms. Enright and Board Members: The Horsley Witten Group, Inc. (HW) is pleased to provide the North Andover Planning Board with this report summarizing our third review of the Watershed Special Permit Application for the proposed improvements at 492 Sutton Street in North Andover, MA. The plans were prepared by McClure Engineering Company on behalf of the Lawrence Municipal Airport (Applicant). The Applicant has noted that the Lawrence Municipal Airport property contains over 400 acres of land. The project proposes the construction of a new 7,069 square foot (sf) administration building and the reconfiguring of the parking lot, as well as utility, stormwater, and landscaping improvements. The stormwater improvements include the addition of a stormwater infiltration basin, two vegetated detention ponds, and one manufactured cartridge filtration unit. It appears that the proposed actions will increase the total impervious area on the site by approximately 7,360 sf. The total land disturbance is somewhere between 49,300 sf and 53,500 sf. Based on the Resource Area Map in the Stormwater Management Report, there are no wetland resource areas or buffer zones within 100 feet of the project area. The following additional documents and plans were received by HW in response to our second peer review letter dated July 24, 2026: • Letter to Planning Department, regarding Response to Second Peer Review Comments, prepared by McClure Engineering Company, dated July 28, 2026 (8 pages). Stormwater Management Design Peer Review HW offers the following comments concerning the stormwater management design as per the Massachusetts Stormwater Handbook (MSH) dated February 2008, and the North Andover Chapter 165 Stormwater Management and Erosion Control Bylaw (Bylaw), and Chapter 250 Stormwater Management and Erosion Control regulations (Stormwater Regulations) adopted June 21, 2022. �������� ���i���� c o�������� Town of North Andover July 28, 2026 Page 2 of 10 In accordance with §195-8.14.E.(8)All applications for site plan review shall include the submittal of a stormwater management plan prepared in accordance with the latest version of the Massachusetts Stormwater Handbook and additional criteria established herein and demonstrating full compliance with the Massachusetts Stormwater Standards and the North Andover Stormwater Management and Erosion Control Regulations promulgated under Chapter 165 of the Town Bylaws (Stormwater Management and Erosion Control Bylaw). We have used the Massachusetts Stormwater Standards as the basis for organizing our comments. However, in instances where the additional criteria established in §250-22 of the North Andover Code requires further recommendations; we have referenced these as well. The following comments correlate with our second peer review letter dated July 24, 2026. Follow-up comments are provided in bold underlined font as needed. 1. Standard 1: No new stormwater conveyances (e.g., outfalls) may discharge untreated stormwater directly to or cause erosion in wetlands or waters of the Commonwealth. a. The Applicant has evaluated the pre-development site at two Design Points (DP). i. DP-1 is an existing catch basin located on the northeast side of the existing Administration Building. Within the existing parking lot there are several catch basins that pipe the stormwater that falls within the project area to DP-1. ii. DP-2 is the remaining project area that is not piped to DP-1 but flows offsite to the north towards the airfield. Stormwater from half of the existing administration building flows towards DP-2. July 24, 2026: No further action is requested. b. The Applicant has evaluated the post-development site at the same two design points. Additional catch basins and stormwater practices have been incorporated to manage the stormwater from the reconfigured parking lot and the proposed roof runoff prior to being piped to DP-1. The total area and the impervious cover that overflows offsite towards DP-2 has been reduced under proposed conditions. July 24, 2026: No further action is requested. c. The Applicant is not proposing any new untreated conveyance systems that outfall into a wetland. It appears that the Applicant has complied with Standard 1. July 24, 2026: No further action is requested. Town of North Andover July 28, 2026 Page 3 of 10 2. Standard 2: Stormwater management systems shall be designed so that post- development peak discharge rates do not exceed pre-development peak discharge rates. a. Within Section 3 of the Stormwater Management Report, the Applicant states that the total proposed land disturbance area is approximately 49,300 sf. However, the Limit of Work on the Permit Plans appears to be closer to 53,500 sf. HW recommends that the Applicant confirm the total anticipated land disturbance for documentation purposes. July 24, 2026: The Applicant has revised the total proposed land disturbance area to be 59,300 sf. No further action is requested. b. The Applicant has submitted a Pre-Development Drainage Map. EX-2 shows a time of concentration (Tc) flow path that passes through an area with an existing silt fence. HW recommends that the Applicant revisit the Tc flow path for EX-2. HW has reviewed the other Tc values and has no other comment. July 24, 2026: The Applicant has updated the Pre-Development Drainage Map and the associated calculations as suggested. HW has reviewed the revised Tc path and calculation and finds it to be reasonable. No further action is requested. c. HW has reviewed the Applicants HydroCAD model, including the routing diagram, surface areas, curve numbers (CN), and precipitation depths and found the values to be reasonable. No further action is required. July 24, 2026: No further action is requested. d. In Section 2.4 of the Stormwater Management Report, the Applicant has provided summary tables illustrating that the Proposed Design Storm Peak Flows are less than the Existing Peak Flows. HW agrees that the peak flows are reduced under proposed conditions for the 2-, 10-, and 25-year events. During the 100-year event the peak flow to DP-1 increases slightly. However, the overall reduction flowing north has been reduced. The Applicant has included a Net Change column with the Table in Section 2.4. HW agrees that the Applicant has reduced the peak flows but does not agree that the total percentage net change is listed correctly. July 24, 2026: The Applicant has updated the calculations associated with the revised Tc flow path and the calculation error previously highlighted has been corrected. The 2-year peak flow towards DP-1 experiences a minimal increase (0.03 cubic feet per second). The overall peak flow to the north is decreasing. No further action is requested. Town of North Andover July 28, 2026 Page 4 of 10 e. The Applicant has provided tables showing that the Proposed Design Storm Volumes are reduced compared to the Existing Design Storm Volumes. There is a minimal increase of volume during the 100-year storm event to DP-1. However, the total reduction from the project area is reduced over existing conditions. Like the peak flows, HW agrees that the Applicant has reduced the peak volumes, but we do not agree that the total percentage net change is listed correctly in Section 2.5 of the Stormwater Management Report. July 24, 2026: The Applicant has updated the calculations associated with the revised Tc flow path and the calculation error previously highlighted has been corrected. The 100-year proposed volume in the HydroCAD model appears to be 1.716 af. However, in the table, the proposed volume is listed as 1.713 af. HW recommends that the Applicant review the runoff volume results and correct if necessary. HW notes that the total increase volume during a 100-year storm event to the catch basin listed as DP-1 is 0.015 of(653 cf). The total volume directed towards the north is reduced by 0.041 of(1,786 cf). It is HW's opinion that the minor increase in volume to the existing catch basin is insignificant. July 28, 2026: The Applicant has provided the updated table. No further action is requested. f. The Applicant has proposed Area Drain AD-3 in Vegetated Detention Area 02 (VDA- 02). It does not appear that the outfall pipe is shown on the plan view indicating which structure the outfall pipe is directed to. HW recommends that the Applicant includes the pipe from AD-3 to the existing catch basin EX-4 on Drawing C105. July 24, 2026: The Applicant has updated the plans to show the pipe between AD-3 and EX-4. No further action is requested. g. HW notes that the borings provided in the Stormwater Management Report indicate several feet of fill was observed beneath the proposed stormwater basins. HW recommends that the Applicant includes a note on Drawing C509 stating that all fill material below the Stormwater Basin or below the Vegetated Detention Areas will be removed and replaced with clean fill meeting the design criteria. July 24, 2026: The Applicant has added a note on Drawing C509 stating that all fill material below the Detention Area or Stormwater Basin will be removed to the depth of native soils and replaced with clean borrow material. No further action is requested. Town of North Andover July 28, 2026 Page 5 of 10 3. Standard 3 requires that the annual recharge from post-development shall approximate annual recharge from pre-development conditions. a. The Applicant has provided recharge calculations in Appendix G of the Stormwater Management Report. HW could not confirm the impervious area listed for the Stormwater Basin or Vegetated Detention Area 01. The values listed in Appendix G are not consistent with the HydroCAD model. HW recommends that the Applicant confirm the impervious area directed towards each of the stormwater basins. It is HW's opinion that the water surface is not considered impervious for the recharge or water quality calculations. However, using a CN of 98 for the water in the HydroCAD model is reasonable. July 24, 2026: The Applicant has provided impervious areas for the Stormwater Basin and the Vegetated Detention Areas. HW has been able to confirm the impervious areas for the recharge calculations. No further action is requested. b. The Applicant has provided recharge calculations. However, no stage storage calculations have been provided to verify the volume available within the basins below the overflows. HW recommends that the Applicant provides the stage storage HydroCAD print out. July 24, 2026: The Applicant has provided the stage storage calculations to verify the volume available within the basins below the overflows. No further action is requested. c. The Applicant has included the test boring logs for the borings conducted within the footprints of the stormwater basins. It appears that the Applicant has provided more than 2 feet but less than 4 feet of separation between the bottom of the Basins and the estimated seasonal high groundwater (ESHGW) elevations. Mounding analyses are required in compliance with Volume 3, Chapter 1, page 28 of the MSH. HW notes that the Applicant has included one mounding analysis in Appendix K of the Stormwater Management Report. However, there are three infiltration practices that should be evaluated. Furthermore, HW recommends that the Applicant includes documentation for the various inputs used in the Hantush spreadsheet. July 24, 2026: The Applicant has provided the requested mounding analyses for the detention areas and the background information for the Hantush spreadsheet. In the Mounding Analysis Supporting Documentation, the Half Length is (y) and the Half Width is (x). However, in the Mounding Analysis calculations, the Half Length is (x) and the Half Width is (y). Additionally, sometimes the Supporting Documentation Half Width is listed as the Half Length within the calculations, and the Half Town of North Andover July 28, 2026 Page 6 of 10 Length is used in the place of the Half Width. HW recommends that the Applicant clarify which values represent the Half Length and which values represent the Half Width. July 28, 2026: The Applicant has revised the mounding calculations adjusting the x and y inputs appropriately. As presented groundwater should not rise up into the infiltration systems. No further action is requested. d. HW recommends that the Applicant add the location of test boring B-4 to Drawings EX-1 and C105. July 24, 2026: The Applicant prefers not to alter the EX-1 survey file, as it is a stamped survey that was provided to the Owner prior to this phase of the project. The boring information has been added to Sheet C-105. No further action is requested. e. The Applicant has provided draw down calculations. However, the volume utilized in the calculation should be the volume stored below the outfall and not the required recharge volume. HW recommends that the Applicant revised the calculations in accordance with Volume 3, Chapter 1, page 26 of the MSH. HW notes that the Applicant has included a calculation utilizing the provided recharge volume. However, the volume listed for the Stormwater Basin is not consistent with the volume listed under the recharge calculations provided previously. July 24, 2026: The Applicant has revised the draw down calculations. HW has reviewed the revised calculations and finds them acceptable. No further action is requested. f. HW appreciates that the Applicant has not included the recharge volume from the Stormwater Basin in the total provided recharge. However, it is HW's opinion that this volume can be included for total recharge. July 24, 2026: The Applicant has included the recharge volume from the Stormwater Basin as part of the total provided recharge. No further action is requested. 4. Standard 4 requires that the stormwater system be designed to remove 80% Total Suspended Solids (TSS) and to treat 1-inch of volume from the impervious area for water quality. a. The Applicant has provided third party documentation for the TSS removal for the Contech Jellyfish Filter. HW notes that the Applicant has used a Rain Guardian as pretreatment for the Vegetated Detention Areas. HW agrees that the Rain Guardian does provide pretreatment. However, we recommend that the Applicant provide the third-party documentation to be included in the project file. Town of North Andover July 28, 2026 Page 7 of 10 July 24, 2026: The Applicant has provided third-party documentation for the TSS removal of the Rain Guardian in Appendix L. No further action is requested. b. The Applicant has provided the TSS removal calculations for the various treatment trains prior to the stormwater reaching the existing catch basin at DP-1. HW agrees that including the Jellyfish structure provides a minimum of 80% TSS removal for the proposed development. July 24, 2026: No further action is requested. c. The Applicant has provided water quality calculations to document whether the proposed development is retaining 0.5 inches of runoff over the impervious area. The Applicant has included the two Vegetated Detention Areas to calculate the water quality volume. The Stormwater Basin has additional volume. However, it is managing very little impervious area, so the Applicant has not included the Stormwater Basin in its total water quality volume. HW agrees that the volume retained in the Stormwater Basin should not be included in the water quality volume calculation. The Jellyfish is providing additional means to improve water quality for this redevelopment. No further action is requested. July 24, 2026: No further action is requested. d. The Applicant has included Total Phosphorous Reduction Calculations. It is not clear how the SCM Load was calculated. HW recommends that the Applicant document how the SCM Load (P) values provided were determined. July 24, 2026: The Applicant has provided Total Phosphorous Reduction Calculations and has provided sources on the percent cumulative phosphorous load reduction. HW finds the cumulative phosphorous load reduction for all values reasonable. No further action is requested. 5. Standard 5 is related to projects with a Land Use of Higher Potential Pollutant Loads (LUHPPL). a. The proposed administration building with a 45-space parking lot is not considered a LUHPPL assuming the site will not generate more than 1,000 vehicle trips per day. HW does not believe that Standard 5 is applicable. However, we recommend that the Applicant confirm that the anticipated vehicle trips per day are less than 1,000. July 24, 2026: The Applicant has provided an analysis estimating the number of vehicle trips per day. A conservative estimate of 640 trips per day has been provided. No further action is requested. Town of North Andover July 28, 2026 Page 8 of 10 6. Standard 6 is related to projects with stormwater discharging into a critical area, a Zone H. or an Interim Wellhead Protection Area of a public water supply. a. The proposed development is not discharging near or into a critical area, Zone II or an IWPA area, Therefore Standard 6 is not applicable. No further action is requested. July 24, 2026: No further action is requested. 7. Standard 7 is related to projects considered Redevelopment. a. The Applicant is increasing the impervious area by approximately 7,400 sf. The proposed project is a mix of new and redevelopment. The Applicant is required to comply with the Stormwater Management Standards for the new development and is allowed to comply to the maximum extent practicable for the previously developed portions of the site. The Applicant is also required to improve existing conditions. On page 11 of the Stormwater Report the Applicant has outlined how it is improving existing conditions. HW agrees that the proposed stormwater improvements and Long-Term Operation & Maintenance Plan are an improvement. No further action is requested. July 24, 2026: No further action is requested. 8. Standard 8 requires a plan to control construction related impacts including erosion, sedimentation, or other pollutant sources. a. The proposed project will be disturbing greater than one acre of land, therefore the Applicant is required to obtain a Construction General Permit and prepare a Stormwater Pollution Prevention Plan (SWPPP) in accordance with the EPA NPDES Program. The SWPPP should include source control and pollution prevention measures, stormwater practices to address erosion and sedimentation, stabilization measures, and procedures for operating and maintaining the proposed stormwater practices during construction. The plan should also identify the parties responsible for implementing the plan. The Planning Board may choose to require a signed SWPPP a minimum of 14 days prior to land disturbance as a Condition of Approval. July 24, 2026: The Planning Board may choose to require receipt of a signed SWPPP a minimum of 14 days prior to land disturbance as a Condition of Approval. b. HW recommends that the Applicant include a stockpile location and confirm it is surrounded by erosion controls. July 24, 2026: The Applicant has revised Sheet C107 to provide a stockpile surrounded by erosion controls. No further action is requested. Town of North Andover July 28, 2026 Page 9 of 10 c. HW recommends that the Applicant includes inlet protection on catch basins within 100 feet of the construction entrance. July 24, 2026: The Applicant has added a note to Sheet C107 to provide inlet protection on catch basins within 100 feet of the construction entrance. No further action is requested. d. HW notes that on Drawings C102 and C107, the limits of work around the proposed stormwater basin should include the Contractor Staging Area shown on Drawing C004. July 24, 2026: The Applicant has delineated the limits of work around the proposed stormwater basin, including the Contractor Staging Area. HW finds the delineated limit of work reasonable. e. HW recommends that the Applicant clearly illustrate on Drawing C102 the emergency access route that must remain accessible throughout construction. July 24, 2026: The Applicant has illustrated the emergency access route on Sheet C102. The Planning Board may choose to confirm with the Fire Department that the route is acceptable. 9. Standard 9 requires a Long-Term Operation and Maintenance (O & M) Plan to be provided. a. The Applicant has provided a stand-alone Long-Term O&M Plan. The O&M Plan includes a description of each stormwater practice with an inspection log, the manufacture's guidelines for the proprietary structures, an estimated budget, and a simple sketch illustrating where each of the stormwater practices is located within the project area. The Planning Board may choose to require receipt of a Long-Term Operation and Maintenance Plan signed by the property owner as a Condition of Approval. July 24, 2026: The Planning Board may choose to require receipt of a Long- Term Operation and Maintenance Plan signed by the property owner as a Condition of Approval. 10. Standard 10 requires an Illicit Discharge Compliance Statement be provided. a. The Applicant has provided an unsigned Illicit Discharge Statement. The Planning Board may choose to require receipt of an Illicit Discharge Statement signed by the property owner as a Condition of Approval. July 24, 2026: The Planning Board may choose to require receipt of an Illicit Discharge Statement signed by the property owner as a Condition of Approval. Town of North Andover July 28, 2026 Page 10 of 10 11. Within the Permit Plans in the Overall Site Plan, the Applicant states "Relief is requested from Section »»> of the North Andover Bylaw." HW recommends clarifying which section of the North Andover Bylaw relief is requested from. July 24, 2026: The Applicant has clarified in the plans that relief is requested from Section 195-8, Part 1 of the North Andover Bylaw. No further action is requested. Conclusions HW is satisfied that the Applicant has adequately addressed our comments. The Applicant is advised that provision of these comments does not relieve him/her of the responsibility to comply with all Town of North Andover Codes and By-Laws, Commonwealth of Massachusetts laws, and federal regulations as applicable to this project. We appreciate the opportunity to assist the Town of North Andover with this project review. Please contact Janet Bernardo at 508-833-6600 or at ibernardo(c-D-horsleywitten.com if you have any questions regarding these comments. Sincerely, HORSLEY WITTEN GROUP, INC. Janet Carter Bernardo, PE Ariel Shramko, EIT Principal Staff Engineer