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HomeMy WebLinkAbout2026/07/10 - Response to 1st Stormwater Peer Review - 492 Sutton Street July 10,2026 Ms.Jean Enright,Planning Director McC L U R ETNA Planning Department Town of North Andover 120 Main Street North Andover,Massachusetts 01845 Re: Response to Initial Stormwater Peer Review Comments 492 Sutton Street North Andover,Massachusetts Dear Ms.Enright and Board Members: On behalf of the Applicant,Lawrence Municipal Airport,McClure Engineering Company(McClure)is in receipt of the Horsley Witten Group's(HW)Initial Stormwater Peer Review Letter dated July 6,2026,regarding the Special Permit Application for the proposed improvements at 492 Sutton Street in North Andover,Massachusetts.This Letter provides McCLure's response to each of the comments provided by HW.For ease of reference,each comment from the HW review Letter is restated below in the same numbering sequence used by HW,followed by McCLure's response. 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. 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. 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. Response:Acknowledged. 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. Response:The report has been revised to modify the total disturbance area,which should be reported as 59,300 sf(1.36 ac). 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 sift 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. 225 Friend Str-eet,SuEte 805, Boston, M1assachusetts,02114 TeL 617.466.6765 1 mccLurevision.com Response:The Tc has been adjusted for Pre-Development Subcatchment EX-2 and Post- Development Subcatchment PR-3.1 to more accurately represent the flow path and time of concentration calculation. The Tc values and resulting peak flows have been updated accordingly. c. HW has reviewed the Applicant's 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. Response:Acknowledged. 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. Response: The table has been corrected to display the actual Total Net Change(%)as the total change in flow divided by the existing flow. 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. Response: The table has been corrected to display the actual Total Net Change(%)as the total change in volume divided by the existing volume. 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. Response: The Grading&Drainage Plan(Sheet C105)has been updated to show the pipe from AD-3 to the existing Catch Basin EX-4 on Drawing C 105. 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. Response:A note has been added to Drawing C509 requiring removal of existing fill beneath the stormwater basin and vegetated detention areas,and replacement with clean fill as specified. LWM Administration Building—Response to Peer Review Comments Page 2 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. Response: The Post-Development Watershed Plan has been revised to clarify the impervious areas contributing to each BMP. The numbers in Appendix G(Standard 3)have been updated accordingly to match. 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. Response: Stage-Storage Tables have been provided in Appendix G. The previous version of the report had erroneously placed Stage-Discharge Tables in the Report instead of Stage-Storage tables. The correct tables are now in Appendix G following the Recharge calculations. 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. Response:A Groundwater MoundingAnalysis has been prepared for all three(3)infiltration practices and is provided in the revised materials. Additional supporting documentation has also been provided for each input in the calculation. See"Mounding Analysis Supporting Documentation"in Appendix K. d. HW recommends that the Applicant add the location of test boring B-4 to Drawings EX-1 and C105. Response: The location of test boring B-4 has been added to the existing conditions survey file and is now visible on Sheet C 105. We respectfully request that the EX-1 survey file remain as-is given that it is a stamped survey and was prepared for the Owner prior to this phase of the project. Borings not identified on the plan by that surveyor are still shown in the boring maps and on the remainder of the plan sheets in the plan set. 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. Response: The drawdown calculations for all three recharge BMPs have been revised to show only the"provided"recharge volume. The discrepancy in the Stormwater Basin volume below the lowest outlet has been corrected. LWM Administration Building—Response to Peer Review Comments Page 3 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. Response: The BMP calculations have been revised to include the recharge volume provided by the Stormwater Basin in the total recharge volume provided. 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. Response:The third-party documentation has been provided in the revised report in Appendix L to support the use of the Turret as pretreatment to the detention areas. 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. Response:Acknowledged. 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. Response:Acknowledged. 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. Response:Total Phosphorus Loads and Pollutant Removals were based upon the MA MS4 General Permit data. The calculations have been updated to reference the relevant tables of the MS4 Attachments,and the reference materials are included in Appendix G. 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. Response: The site is not anticipated to generate more than 1,000 vehicle trips per day. A Trip Generation Table has been added to the Stormwater Report.An estimate of 640 tips per day is LWM Administration Building—Response to Peer Review Comments Page 4 calculated. This number is conservative,as the trip generation values for the airport use is based upon flights per day. There are multiple access points for the airport and its various hangars and FBOs,so this project location,parking areas,and Terminal Road access is anticipated to have less trips per day than the calculated number for the entire airport operation. 6. Standard 6 is related to projects with stormwater discharging into a critical area,a Zone II,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. Response:Acknowledged. 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. Response:Acknowledged. 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. Response:Acknowledged. b. HW recommends that the Applicant include a stockpile location and confirm it is surrounded by erosion controls. Response:Sheet C107has been revised to include a stockpile location required to be surrounded by erosion controls. c. HW recommends that the Applicant includes inlet protection on catch basins within 100 feet of the construction entrance. Response:A note has been added to Sheet C 107 indicating the requirement to provide inlet protection on catch basins within 100 feet of the construction entrance. 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. Response:Drawings C102 and C107 have been revised to clarify the limit of work,including the Contractor StagingArea. LWM Administration Building—Response to Peer Review Comments Page 5 e. HW recommends that the Applicant clearly illustrate on Drawing C102 the emergency access route that must remain accessible throughout construction. Response:Sheet C102 has been revised to include the emergency access route that must remain accessible throughout construction. 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 manufacturer's guidelines for the proprietary structures,an estimated budget,and a simple sl<etch 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. Response:Acknowledged. 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. Response:Acknowledged. 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. Response:The text has been revised to include the reference to Chapter 195,Article 8,Part 1 of the Zoning Bylaw. McClure appreciates HW's review of the Special Permit Application and the opportunity to respond to these comments. Revised plans and supporting calculations reflecting the responses are submitted under separate cover with this letter. Please do not hesitate to contact us if the Planning Board or HW have any questions regarding these responses. Sincerely, MCCLURE ENGINEERING COMPANY 1,4 Lindsey DiTonno,PE Project Manager/Development Team Leader LWM Administration Building—Response to Peer Review Comments Page 6