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Home My WebLink About08/16/2022 - 2nd HWG Stormwater Peer Review - - %�. Horsley Wiften Group 0 SLIStainable, Ehvirownental Solutions 112 W'at r `tr t-61 Floor-Boston,MBA 02109 .......... 5 -2 1 ho,r 1 ten August 16, 2022 Ms. Jean Enright, Planning Director North Andover Planning Board 120 Main Street North Andover, Massachusetts 01845 Ref: Second Stormwater Peer Review Merrimack MAC Storage Building 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 letter report summarizing our second review of the Stormwater Management Report and Site Plan for the proposed Merrimack College MAC Storage Building at 315 Turnpike Street, North Andover, MA. The plans and stormwater report were prepared by VHB on behalf of Merrimack College (Applicant). The project proposes the construction of a 3,200 square foot (sf) storage building to house athletic equipment on 0.4 acres of land within the Merrimack College campus. The stormwater management for the proposed storage building includes a shallow infiltration basin. The project is considered a mix of new and redevelopment. The Applicant will increase the impervious surface by 1,700 sf of roof top and reduce the vehicular pavement area by approximately 1,500 sf. The proposed project is located within the 100-foot buffer zones from two wetland resource areas and will require an Order of Conditions from the North Andover Conservation Commission. The following additional documents and plans were received by HW in response to our July 21, 2022 initial peer review: • Letter to Horsley Witten Group, Inc., in response to the HW peer review of Merrimack MAC Storage Building, North Andover, MA, prepared by VHB, dated August 4, 2022 (9 pages); • Stormwater Report, MAC Storage Building, Merrimack College, North Andover, MA, prepared by VHB, revised August 4, 2022 (204 pages); and • Site Plans, MAC Storage Building, 315 Turnpike St., North Andover, MA, prepared by VHB, dated July 7, 2022, revised August 4, 2022 including: o Cover Sheet C0.00 o Legend C 1.00 o Layout and Materials C2.00 o Grading, Drainage, and Erosion Control Plan C3.00 o Utility Plan C4.00 o Details Sheet C5.01 and C5.02 o Existing Conditions Plan of Land SV-1 0 4 Views — Building 4 View r � [f n. t- I, " ii tte n G ro u p �IM Horsley W'Itten Group, [iris. Town of North Andover August 16, 2022 Page 2of9 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 250 Stormwater Management and Erosion Control Regulations (Stormwater Regulations), and Chapter 165 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 VII, 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. The comments below correlate with our July 21, 2022 initial peer review. Follow up comments are provided in bold font. 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 stormwater conveyances will discharge untreated stormwater directly to wetlands or waters of the Commonwealth. HW has no further comment. 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. The Applicant has revised the narrative to be consistent with the site plans. HW has no further comment. 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. The Applicant has not used an exfiltration rate for the infiltration basin to be conservative. HW has no further comment. Ilf iiroll , ,t "' O' I '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 3of9 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 same 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. The Applicant has provided new Existing and Proposed Drainage Conditions figures which show a portion of the site also flowing to WF2. HW has the following additional comments about the Existing Drainage Conditions Figure: i. The Time of Concentration (Tc) flow path for the roadway flows towards WF1. However, it appears that existing curbing separates the roadway from the grass area that drains to WF1. HW recommends that the Applicant explain how the runoff will reach WF1 or add a drainage boundary at the curb that separates the roadway from the grass area. i i. Unless there is a break in the existing curb, it appears that the runoff from a large portion of the existing pavement area flows to the catch basins located in the southwestern corners of the adjacent parking lot. HW suggests that the Applicant consider adding a third design point for this area which flows to the catch basins and clarify where the catch basins discharge to or justify the catchment areas provided. iii. It appears that the impervious portion of Drainage Area 2 flows in the easterly direction over the pavement. HW recommends that the Applicant verify that this portion of the pavement does flow to WF2. HW has the following comments about the Proposed Drainage Conditions Figure: i. It appears that the roadway portion west of the trench drain flows to the catch basins located in the southwestern corners of the adjacent parking lot unless there is a break in the curbing as noted above. HW recommends adding a third design point for this area which flows to the catch basins. HW also recommends that the Applicant add a drainage boundary at the existing curb that separates the roadway from the grass area. ii. It appears that the impervious portion of Drainage Area 13 flows in the easterly direction over the pavement similar to existing conditions. HW recommends that the Applicant verify that this portion of the roadway does flow to WF2. The existing and proposed conditions are very similar, so the final design point is not critical to the design. 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. Ilf iiroll , ,t "' O' I '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 4 of 9 The Applicant has added peak volumes in Table 5 in the narrative. This table indicates that peak volumes increase minimally to Design Point 2. The increase can be considered insignificant. No further action needed. 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). The Applicant has modified the HydroCAD model to assume all pervious lands in the site are in good hydrologic condition. HW has no further comment. 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). The Applicant has revised the HydroCAD model to use the NOAA Atlas 14 precipitation values. HW has no further comment. 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. The previously proposed filter strip has been removed from the design and replaced with a sediment forebay for pretreatment of the infiltration basin. HW has no further comment. 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. The Applicant has revised the size of the proposed infiltration basin and provided the freeboard calculations in Appendix D of the Stormwater Report. HW has no further comment. 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 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. The 4-inch perforated pipe is an underdrain that will capture roof runoff that sheet flows off the back of the building. The Applicant has revised the underdrain detail to include the layout and dimensions. The Applicant has also added a cleanout to the underdrain. HW has no further comment. Ilf iiroll , ,t "' O' I '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 5 of 9 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. The Applicant has provided a detail for the Drip Strip and explained its function in the Regulatory Compliance section (page 11) of the Stormwater Report. HW has no further comment. i. HW recommends that the Applicant confirm that no runoff from the existing Merrimack Athletic Center will flow towards the proposed infiltration basin. The Applicant has confirmed that no runoff from the existing Merrimack Athletic Center will flow towards the proposed infiltration basin. HW has no further comment. 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. (14) 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. The Applicant has proposed an underdrain to capture roof runoff from the proposed storage building. It is equipped with a drip strip to help stabilize the ground. The Applicant has provided a detail for the underdrain and drip strip on Sheet C-5.01 of the Plan Set. HW has no further comment. 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. HW has no further comment. 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. The Applicant has added a forebay as pre-treatment to the infiltration basin, satisfying 80% TSS removal requirement. HW has no further comment. Ilf iiroll ,t "' O' I '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 6of9 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. The Applicant has added the dimension for the stone apron and pipe diameter to the flared end section detail on Sheet C-5.01. 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. The Applicant has provided the requested calculations and has provided the applicable water quality volume. HW has no further comment. 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. The Applicant has confirmed that the school campus in the vicinity of the proposed Storage Building is not considered a LUHPPL. HW has no further comment. 6. Standard 6 is related to projects with stormwater discharging into a critical area, a Zone ll, 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. HW has no further comment. 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. The Applicant has provided the requested calculations and documented compliance. HW has no further comment. Ilf iiroll , ,t "' O' I '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 7of9 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 entrance/exit. HW recommends that the Applicant include locations for inlet protection and the construction entrance on Sheet C3.00. The Applicant has added catch basin inlet protection for two catch basins and a stabilized construction entrance to Sheet C-3.00 and corresponding details to Sheet C-5.02. HW has no further comment. 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. The Applicant has provided an inlet protection detail to Sheet C-5.02 of the Plan Set. HW recommends that the Applicant include inlet protection for the trench drain during construction once it is installed. c. HW recommends that the Applicant clearly delineate the project's limit of work on all sheets. The Applicant has delineated the project's limit of work on the plan set. HW recommends that the Applicant include the construction entrance and laydown area within the limit of work. Furthermore, HW recommends that the Applicant clarify if the grading for contours 243 and 244 are being adjusted under proposed conditions. 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 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. The silt sock/silt fence barrier has been extended around the perimeter of the parking lot, and a silt sock has been added around the perimeter of the stockpile area on Sheet C-3.00 of the Plan Set. HW has no further comment. 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. The Applicant has proposed a field verification walk after the stormwater management design is approved. The Planning Board and/or Conservation Commission may choose to include this site walk as a condition of approval. Ilf iiroll , ,t "' 0 1 '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 8of9 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. The Applicant has added the suggested note to Sheet C-1.00 of the Plan Set. HW has no further comment. 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. The Applicant has added the suggested note to Sheet C-1.00 of the Plan Set and to the report section of MassDEP Standard 8. HW has no further comment. 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. The Applicant has added the suggested note to Sheet C-1.00 of the Plan Set and to the report section of MassDEP Standard 8. HW has no further comment. 9. Standard 9 requires a Long-Term Operation and Maintenance (O & M) Plan to be provided. a. In the Long-Term Maintenance/Evaluation Checklist, Section EA of Appendix D, HW recommends adding a row for vegetative areas with an inspection frequency of semi- annually. The Applicant has added a row for vegetative areas. HW has no further comment. 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." The Applicant has revised the text accordingly. HW has no further comment. 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. The Applicant is amenable to requiring a signed O&M Plan as a condition of approval. The Planning Board and/or Conservation Commission may choose to include receipt of the O&M Plan as a condition of approval. 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. The Applicant has provided a signed Illicit Discharge Statement. HW has no further comment. Ilf iiroll , ,t "' 0 1 '1 �� owl) 1 0 uilll U ....,III iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oc�l Town of North Andover August 16, 2022 Page 9of9 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. The Applicant has adjusted the Stormwater Report and design as requested. HW has no further comment. 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. The Applicant has revised the pavement detail on Sheet C-5.01 of the Plan Set to indicate that only Standard Duty Flexible Pavement will be used. HW has no further comment. 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. The Applicant has confirmed that no water or sewer are required to support the project. HW has no further comment. 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 justify why this is not applicable. The Applicant has verified that lawn and New England Wetland Conservation Mix will be added to any areas disturbed during construction. HW has no further comment. Conclusions HW recommends that the Planning Board require that the Applicant provide a written response to address the few remaining minor comments as part of the Board's review process. 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. Please contact Janet Bernardo at 508-833-6600 or at jbernardo@horsleywitten.com if you have any questions regarding these comments. Sincerely, HORSLEY WITTEN GROUP, INC. 'Viy '^IIt4Y" vyM, Janet Carter Bernardo, P.E. Veronica Seward-Aponte, E.I.T. Associate Principal Environmental Engineer Ilf iiroll ,1,t "11 01 I '1 �� owl) 1 0 uilll U „�i ,.�„ ....,III.III III III . Ill��iii�, �1 � �� ...:�� iii� l III iii.11l��lii e �.�.iii��iii ��m��III III� � d oaiil