Loading...
HomeMy WebLinkAbout08/04/2022 - 1st VHB Response - - fl0 4 m, i August 4, 2022 Ref: 11625.25 Janet Carter Bernardo, P.E. Associate Principal Horsley Witten Group, Inc. 294 Washington St Boston, MA 02108 Re: Initial Stormwater Peer Review Merrimack MAC Storage Building North Andover, Massachusetts Applicant: Merrimack College 315 Turnpike St North Andover, MA 01845 Dear Ms. Bernardo: On behalf of the Applicant, VHB has received comments issued by the Horsley Witten Group regarding the review of the Stormwater Management Report and Site Plan for the above-referenced project.We appreciate the effort associated with this review and offer the following responses. Horsley Witten Group comments are presented below in italics, followed by V H B's responses in bold type. 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 250 Stormwater Management and Erosion Control regulations (Stormwater Regulations), and Chapter 765 Stormwater Management and Erosion Control Bylaw(Bylaw). In accordance with North Andover Stormwater Regulations,Article V,Applicability,§250-09. B. if a project will disturb less than 43,560 sf but is part of a larger common plan a land disturbance permit is required. In accordance with North Andover Stormwater Regulations,Article V11, Stormwater Management Plan ,§250-21. B. a project is required to comply at a minimum with the most recent version of the MSH. Therefore, we have used the MSH 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. 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 is proposing an overflow stone spillway located approximately 50 feet from the resource area. The velocity from the overflow weir is less than 2 feet per second(fps). It appears that no new Engineers I Scientists I Planners I Designers 101 WaIlhirut Stireet Ill: ,,, 13)ox 9151,Wa-tertowri,Massachusetts a sett 471 III617.9241770 IIIF 617.9 24.22 8 6 www.vhb.mii-n Ref: 11625.25n August 4, 2022 Page 2 stormwater conveyances will discharge untreated stormwater directly to wetlands or waters of the Commonwealth. RESPONSE: N/A b. The Applicant has provided a stormwater narrative. However, it is not clear that the plan set matches the narrative. For example, the narrative describes complying with Standard 1 by providing outlet pipes with flared end sections, however it appears that the proposed pipes discharge into the infiltration basin, so the statement is not relevant to the wetlands. The narrative also describes a filter strip associated with the infiltration basin. However, the filter strip is not obvious on the plan set. HW recommends that the Applicant clarify the proposed design and confirm that the narrative is consistent with the site plans. RESPONSE: Outlet pipes noted were intended for the ones discharging into the proposed infiltration basin, ultimate overflow prior to Wetland 1 is the overflow stone Swale as noted in the report.The previous filter strip has been replaced with a sediment forebay to provide pre- treatment for the infiltration basin. Please refer to report section Mass DEP Standard 1, and planset pages C-3.00 and C-5.02 for design and detail. c. The Applicant has described the proposed stormwater basin as an infiltration basin. However, the Applicant has not included an exfiltration rate as part of the HydroCAD model. The geotechnical report indicates that the soil should not be expected to drain well. HW recommends that the Applicant clarify the type of stormwater practice proposed. RESPONSE:An exfiltration rate has not been included in the HydroCAD model to be conservative. See report section Mass DEP Standard 2 for clarification and Appendix B for HydroCAD model. 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. The Applicant has provided the Existing and Proposed Drainage Conditions figures. The Applicant has indicated that there is one design point(DP-1) at wetland series WF1. However, it appears that under existing conditions a portion of the site may flow towards wetland series WF2. Under proposed conditions the some area may continue to flow towards wetland series WF2 though it is difficult to confirm with the grading provided. HW recommends that the Applicant confirm the existing and proposed drainage areas. RESPONSE:Wetland 2 has been added to all drainage figures as DP-2. See revised/new Tables 2- 5 for related information, and Figures 2 and 3 for revised existing and proposed drainage areas including DP-2, Wetland #2. b. In accordance with §250-22 B. (6) a summary of pre- and post-development peak rates and volumes of stormwater demonstrating no adverse impacts should be provided as part of the narrative. The Applicant has provided the peak flows but not the peak volumes for the 2-year, 10 year,25 year, and 100 year storm events. HW recommends that the Applicant add the peak volumes to Table 4 in the narrative. Ref: 11625.25n August 4, 2022 Page 3 RESPONSE: Peak volumes have been provided in report section Mass DEP Standard 2, Table S. The analysis shows a decrease in runoff volumes for all storm events to DP-1, and a de-minimis change in runoff volumes for all storm events to DP-2. c. HW recommends that the Applicant revise the HydroCAD model assuming all pervious lands in the site are in good hydrologic condition in accordance with the North Andover Stormwater Regulations§250-23 E. (17). RESPONSE: HydroCAD model was modified to assume all pervious lands are in good hydrologic condition. See Tables 2 and 3 for curve numbers; report section Mass DEP Standard 2, Tables 4 and 5 for revised peak discharge rates and volumes; and Appendix B for HydroCAD. d. HW recommends that the Applicant use precipitation values equal to or greater than the values provided by National Oceanic and Atmospheric Administration (NOAA)Atlas 14 for the 24-hour storm events, as outlined in §250-23 E. (19). RESPONSE: NOAA Atlas 14 storm event precipitation values were checked and revised. See Appendix B for HydroCAD model revision and report section Mass DEP Standard 2, Tables 4 and 5 for corresponding peak discharge rates and volumes. e. As noted above the Applicant references a filter strip in the narrative but it is not obvious on the plan set. HW recommends that the Applicant provide a detail of the stormwater basin and clearly indicate the location and size of the filter strip or revise the narrative accordingly. RESPONSE:The previously proposed filter strip has been removed from the design, and replaced with a sediment forebay for pretreatment of the infiltration basin. Revised details are included on planset page C-5.02. f. Per§250-23 E. (25) Stormwater basins shall be sized to accommodate the 100 year storm event with a minimum of one foot of freeboard. The Applicant has documented that the proposed stormwater basin provides 0.02 feet of freeboard during a 100 year storm event. The proposed basin is approximately 1.0 foot deep. The Applicant may want to reconsider the stormwater practice proposed and consider installing a bioretention area instead of a detention basin. RESPONSE:The infiltration basin was re-sized to accommodate other responses and additionally re-graded to provide a berm along all edges of the basin except the pavement in front of the Storage Building to help provide 1.0 foot of freeboard based on the new conditions. Bioretention Areas were evaluated however based on the anticipated groundwater conditions the soil media profile would not have sufficient separation to the ESHGWT. See Appendix B for HydroCAD model revision; report section Mass DEP Standard 2;and Appendix D, Water Quality Volume Calculations. g. The Applicant has indicated a 4-inch perforated pipe that is approximately 90 feet long to be installed approximately one foot below the surface, along the north side of the proposed storage building. The Applicant has also provided an Underdrain detail on Sheet C5.00. The Underdrain detail does not include Ref: 11625.25n August 4, 2022 Page 4 a pipe diameter or a depth of cover. It is not clear if the 4-inch perforated pipe is considered an underdrain, or a perimeter drain that will capture the roof runoff that sheet flows off the back of the building. HW recommends that the Applicant provide a detail for the 4-inch perforated drainpipe and clarify where the underdrain will be installed. HW further recommends that the Applicant add cleanouts to the 4-inch perforated pipe. RESPONSE:The 4-inch perforated pipe is considered to be the underdrain that will capture roof runoff that sheet flows off the back of the building. The underdrain detail has been edited to show proper layout and pipe diameter.A cleanout has been added to the eastern side of the underdrain. See planset page 3.00 and 5.01 for revised design and detail; report section Mass DEP Standard 2 for clarification. h. On the Proposed Drainage Conditions Figure 3 and the Layout and Materials Plan (C2.00) the Applicant has included a Drip Strip around three sides of the proposed building. HW recommends that the Applicant provide a detail for the Drip Strip and include its function in the Stormwater Report. RESPONSE: Detail for the drip strip and underdrain has been revised.The function of the drip strip is for stabilization of roof runoff from the back side of the building. See report section Mass DEP Standard 2 for function and planset page C-5.01 for detail. i. HW recommends that the Applicant confirm that no runoff from the existing Merrimack Athletic Center will flow towards the proposed infiltration basin. RESPONSE: Confirmed, no runoff from the existing Merrimack Athletic Center will flow towards the infiltration basin. Refer to report section Mass DEP Standard 2 for explanation. 3. Standard 3 requires that the annual recharge from post-development shall approximate annual recharge from pre-development conditions. a. The Applicant has noted that the proposed project is providing recharge to the maximum extent practicable due to poor soils and high groundwater. Per§250-23 E. (74) The Planning Board may alter or eliminate the recharge volume requirements if the site is situated in unsuitable material. The regulations state that nonstructural practices including filter strips that treat roof top or parking lot runoff should be implemented to the maximum extent practicable. It appears that the Applicant has proposed a filter(drip) strip to capture the roof runoff however it has not been explained, detailed, or sized adequately. HW recommends that the Applicant revisit the design and provide additional information regarding the strip (drip)filter proposed. RESPONSE:The previously proposed filter strip was intended along the edge of pavement for pretreatment for the infiltration basin, but has been replaced with a sediment forebay.The drip strip along the back of the proposed storage building is intended to stabilize the ground surface for roof runoff directed off the back of the building. See report section Mass DEP Standard 2 and planset page C-5.01 for detail. Limited design modifications. Ref: 11625.25n August 4, 2022 Page 5 b. The Applicant has provided a geotechnical report detailing that the soil should not be expected to drain well. HW defers to the Planning Board the elimination of the recharge requirements. RESPONSE: N/A 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 a TSS work sheet claiming 80% TSS removal for an infiltration basin. HW notes that to receive 80% credit pretreatment is required. HW recommends that the Applicant provide adequate pretreatment for the roadway runoff discharging into the infiltration basin. HW further notes that a dry detention basin does not receive any TSS removal credit.As noted previously the Applicant may choose to revisit the stormwater practice proposed. RESPONSE:A sediment forebay has been provided as pre-treatment for the infiltration basin. Refer to report section Mass DEP Standard 4 for revisions; planset pages C-3.00 for design modification and C-5.02 for related details; and Appendix D, TSS Removal Worksheets. b. The Applicant has provided a flared end section detail on Sheet C5.00. However, the detail does not include any dimensions. HW recommends that the Applicant add the applicable dimensions for the stone apron and pipe diameter. RESPONSE: Detail and dimensions have been added to project plans. Supporting calculations have been added to Appendix A of the report. Refer planset page C-5.01 for detail revisions and C-3.00 for design. c. The Applicant has noted in its narrative that the proposed stormwater system has been designed to treat one-half inch of water quality volume. Per.§250-23 B. (1)(b) new developments are required to retain the volume of runoff equivalent to one inch multiplied by the total post-construction impervious surface. Per .§250-23 C. (1)(b) redevelopment projects are required to retain the volume of runoff equivalent to 0.8 inches multiplied by the total post-construction impervious surface. Per,§250-23 C. (2) when both new development and redevelopment are proposed for a project site, the redevelopment and new development work shall be conducted per the applicable regulations. HW recommends that the Applicant review the proposed design and confirm compliance with ,§250-23 Design and Performance Criteria. RESPONSE: Infiltration basin is now designed to treat 0.8-inches of water quality volume for all re-developed impervious surfaces and 1-inch of water quality volume for all new development impervious surfaces. See report section Mass DEP Standard 4; Appendix D, Water Quality Volume calculations for new calculations; and planset page C-3.00 for revised basin size. 5. Standard 5 is related to projects with a Land Use of Higher Potential Pollutant Loads (LUHPPL). a. The proposed Athletic Storage Building is not considered a LUHPPL. HW recommends that the Applicant confirm that the school campus in the vicinity of the Storage Building is not considered a LUHHPL per the anticipated traffic volume in the area. Ref: 11625.25n August 4, 2022 Page 6 RESPONSE: Confirmed, the project site nor any areas tributary to the site or proposed BMPs are considered a LU H H PL. See report section Mass DEP Standard 5. 6. Standard 6 is related to projects with stormwater discharging into a critical area, a Zone 11, 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. RESPONSE: N/A 7. Standard 7 is related to projects considered Redevelopment. a. The proposed project is considered a mix of new development and redevelopment. The Applicant is required to comply with §250-23 C. (2) of the North Andover Stormwater Regulations. HW recommends that the Applicant confirm compliance with the recently amended stormwater regulations. RESPONSE:The infiltration basin for the site is now sized for both new development (areas of new impervious) and re-development (areas of existing impervious). See report section Mass DEP Standard 4 and 7 for acknowledgement;Appendix D, Water Quality Volume calculations for revised calculations; and planset page C-3.00 for revised basin design. 8. Standard 8 requires a plan to control construction related impacts including erosion, sedimentation, or other pollutant sources. a. The Applicant has proposed a staked silt fence with a straw wattle around the limit of work as shown on the Grading, Drainage, and Erosion Control Plan (Sheet C-3.00). Other erosion control devices are noted in the Stormwater Report(Appendix D, Section F) including hay bales barriers, catch basin protection, and construction entranc%xit. HW recommends that the Applicant include locations for inlet protection and the construction entrance on Sheet C3.00. RESPONSE: Catch basin inlet protection and a stabilized construction entrance/exit has been added. Refer to planset page C-3.00 for revisions and C-5.02 for related details b. HW recommends that the Applicant provide a detail for the inlet protection, noting it should be provided in any catch basin within 100 feet of the limit of work. RESPONSE:An inlet protection detail has been added on planset page C-5.02. Inlet protection has been placed at both catch basins on the south end of the parking lot. c. HW recommends that the Applicant clearly delineate the project's limit of work on all sheets. RESPONSE: See revised plans for limit of work. Limit of work modified due to basin re-sizing. d. HW recommends that the Applicant extend the silt sock/silt fence barrier line along the northern and western edge of the existing bituminous parking lot so that it contains any runoff from construction Ref: 11625.25n August 4, 2022 Page 7 activity between the proposed storage building area and the proposed stockpile area. HW further recommends that the Applicant add an erosion control barrier around the laydown and stockpile area. RESPONSE:The silt sock/silt fence barrier line has been extended around the perimeter of the parking lot and a silt sock has been added around the perimeter of the stockpile area. Refer to planset page C-3.00 for revisions. e. It appears that the storage building is proposed within an existing tree line. HW recommends that the Applicant locate any trees within the limit of disturbance with a diameter greater than 10 inches per §250-23.A. (7). HW recommends that the Applicant confirm whether the critical root zones (CRZs) of any large trees will be impacted by the proposed development,construction activity over CRZs will potentially damage the root systems of trees and cause long-term degradation of the tree's health. HW further recommends that any trees in the area of the limit of work proposed to be protected are clearly marked in the field and a tree protection detail is added to the plan set. RESPONSE: Upon consensus of the stormwater management design, we would like to propose a field verification walk to identify existing significant trees within the basin limits and evaluate the proposed design to minimize impacts. f. HW recommends that the Applicant add a note stating,prior to any land disturbance activities commencing on the site, the developer shall physically mark limits of no land disturbance with tape, signs, or orange construction fence, so that workers can see the areas to be protected. The physical markers shall be inspected daily. RESPONSE: Note has been added to sheet C-1.00. g. HW recommends that the Applicant add a note stating, permanent vegetation and erosion control structures, as necessary, shall be installed preferably immediately after construction is completed but otherwise no later than the first full spring season immediately thereafter. RESPONSE: Note has been added to sheet C-1.00 and in report section Mass DEP Standard 8. h. HW recommends that the Applicant add a note stating, all temporary erosion and sediment control measures shall be removed after final site stabilization. Disturbed soil areas resulting from the removal of temporary measures shall be permanently stabilized within 30 days of removal. RESPONSE: Note has been added to sheet C-1.00 and in report section MassDEP Standard 8. 9. Standard 9 requires a Long-Term Operation and Maintenance(O& M) Plan to be provided. o. In the Long-Term Maintenance/Evaluation Checklist, Section E.4 of Appendix D, HW recommends adding a row for vegetative areas with an inspection frequency of semi-annually. RESPONSE: Row has been added in section E.4 of Long-Term O&M Plan in Appendix D. Ref: 11625.25n August 4, 2022 Page 8 b. In the Maintenance Checklists, Section E.5 of Appendix D, under Infiltration Basins, HW recommends changing the text to read "Inspect twice per year"instead of"Inspect once per year." RESPONSE: See Infiltration/Detention Basins in section E.5 of Long-Term O&M Plan in Appendix D for revision. c. The Applicant has provided a standalone O&M Plan in Appendix D of the Stormwater Report. HW recommends that the Planning Board reference this document and require a signed O&M Plan as a condition of approval. RESPONSE: Noted.Additionally to highlight, it is understood that snow storage from plowing of exterior areas shall not be located within the 25-ft no disturb zone and per request of the Conservation Commission, also not located in the vicinity of the proposed storage building. We would like to highlight and distinguish that the plans have been revised to identify the location of proposed storage for the Zamboni ice removed from the interior of the existing MAC Building, shown adjacent to the proposed MAC storage building.This area is proposed in a location where the ice melt will flow overland to the new sediment forebay and infiltration basin. 10. Standard 10 requires an Illicit Discharge Compliance Statement be provided. a. HW recommends that the Planning Board include a condition of approval requiring an Illicit Discharge Compliance Statement signed by the property owner prior to land disturbance. RESPONSE: Noted, a signed illicit discharge statement has been provided by the property owner and is included in the revised Stormwater Report. 11. Additional Comments: a. The North Andover Stormwater Regulations were amended in 2022. The Applicant has noted it has demonstrated compliance with §250-27,A-F, which refers to the previous regulations. HW recommends that the Applicant review the amended regulations and confirm compliance. RESPONSE:As noted in the responses herein, the design has been revised to address the recently amended North Andover Stormwater Regulations. b. The Applicant has included details for Heavy Duty Flexible Pavement as well as Standard Duty Flexible Pavement. HW recommends that the Applicant indicate where the two different pavement types will be used. RESPONSE: See planset page 5.01 for detail revision.Will only utilize Standard Duty Flexible Pavement. c. The Utility Plan includes a proposed electric/telephone (E/T) service and a proposed gas service. It does not include water or sewer. HW recommends that the Applicant confirm all applicable utilities have been provided. Ref: 1162 5.2 5n August 4, 2022 Page 9 RESPONSE:All applicable utilities have been provided. No water or sewer required to support the project. d. The Applicant has not included any landscaping. HW recommends that the Applicant review§250-23.A. (2) and provide any proposed landscaping for the Storage Building or justi fy why this is not applicable. RESPONSE: Proposed landscaping includes lawn and New England Wetland Conservation Mix for the infiltration basin and stabilizing areas disturbed during construction.The Project maintains minimum of 45'vegetated buffer between the site and Route 114. Thank you again for your time and consideration.We look forward to ongoing coordination of the proposed project with you as noted. If there are any additional questions/comments in the meantime, please feel free to contact me at o koejjge h.c m or 617-607-0094. Sincerely, VHB Jeffrey Koetteritz, P.E. Project Manager