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Home My WebLink About2023/09/06 - 1st 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 September 6, 2023 Ms. Jean Enright, Planning Director North Andover Planning Board 120 Main Street North Andover, Massachusetts 01845 Ref: Initial Stormwater Peer Review Corner Development Project at Merrimack College 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 initial review of the Stormwater Management Report and Site Plan for the proposed Merrimack College Corner Development Project 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 77,600 square foot (sf) building situated along Turnpike Street and a 62,000-sf building situated along Andover Street on 4.0 acres of land within the Merrimack College campus. The proposed project also includes improvements to Medina Drive, pedestrian improvements, utilities, stormwater management, and landscaping. The stormwater management for the two proposed buildings and site improvements includes a closed drainage system with catch basins and area drains which discharge into three separate subsurface infiltration chamber systems. The subsurface chamber systems connect into the existing drainage system within Andover/Elm Street. The project is considered new development. The Applicant will increase the impervious surface by 60,710 sf of roof top and pavement area within a watershed area of 377,520 sf, an increase of approximately 50%. The following documents and plans were received by HW: • Memorandum to the North Andover Planning Board regarding the Site Plan Review for the Corner Development Project at Merrimack College, prepared by VHB, dated August 17, 2023 (36 pages); • Stormwater Management Report, Corner Development Project at Merrimack College, prepared by VHB, dated August 17, 2023 (292 pages); and • Site Plans, Corner Development Project at Merrimack College, 315 Turnpike St., North Andover, MA, prepared by VHB, dated August 17, 2023, including: o Cover Sheet C0.0 o Legend and General Notes C1.0 o Site Preparation and Erosion Control Plan C2.0 o Layout and Materials Plan C3.0 o Grading and Drainage Plan C4.0 o Utility Plan C5.0 o Site Details C6.1-C6.3 r � [f n. t- I, " ii tte n G ro u p �IM Horsley W'Itten Group, [iris. Town of North Andover September 6, 2023 Page 2of5 o Landscape Plan L1.0 o Landscape Details L2.0 o Lighting Plan SL1.0 o Existing Conditions Plan of Land Sv-1 (August 15, 2023) 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 the Chapter 250 Stormwater Management and Erosion Control regulations (Stormwater Regulations) adopted June 21, 2022. In accordance with Stormwater Regulations, Article V- Applicability, §250-9 activities entailing land disturbance of over 43,560 square feet must obtain a Land Disturbance Permit. In accordance with the Stormwater Regulations, Article VI - Stormwater Management Plan, §250- 21. B. The Stormwater Management Plan shall be designed to meet the most recent version of the Massachusetts Stormwater Standards. Therefore, 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. 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 analyzed the pre- and post-development stormwater runoff to one design point (DP-1). DP-1 is the closed drainage system across Andover/Elm Street on the northwest side of the Project Site. It is not clear on the plan set where the closed drainage system eventually discharges though it appears to be piped into a resource area. HW recommends that the Applicant clarify the outfall. If the outfall is a resource area, HW recommends that the Applicant confirm that erosion is not currently occurring. 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. 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 and the peak volumes for the 2-year, 10-year, 25-year, and 100-year storm events. HW recommends that the Applicant provide a revised table if needed based on the comments below. b. HW has reviewed the Existing and Proposed Drainage Conditions figures and the HydroCAD model. The subcatchment areas, surface materials, times of concentrations, and flow patterns appear reasonable. No further action required. c. The Applicant has included three subsurface infiltration chamber systems. HW notes that the Outlet Control Detail (OCS) for Subsurface Infiltration System B indicates that the structure is 4.0 feet diameter and illustrates the two 10-inch orifices in line vertically. The HydroCAD model includes Device 2 as a 6-foot weir with two 10-inch orifices set at the same horizontal elevation. HW recommends that the Applicant revise the HydroCAD model using a 4-foot weir or adjust the OCS detail. HW further recommends that the Applicant adjust the OCS detail to show the orifices beside each other to avoid "'." III �� .,�m II `�� ':�� own III�� iiilll�� iii�� Ilf III iiiilii iii��'�'�� III 10 :! °:, ": a i is a. ilI Ih� m��.�� 111�� iii ��1��:�� iii lll....l„i�. ��lii��.�� III ��iii�� iii�.��ii��;��m��111°��°"��:��III III��:�� "�i<�: Town of North Andover September 6, 2023 Page 3of5 confusion. d. The HydroCAD model for Pond 3, Subsurface Infiltration System B, utilizes an outlet device set at elevation 215.8. The Grading and Drainage Plan includes a callout for an outlet elevation at 216.2. HW recommends that the Applicant revisit this outlet and confirm the system has been modeled correctly. e. The Applicant has used 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). No further action required. 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 subsurface chamber systems can accommodate a 100-year storm event. The proposed chamber systems do not provide one foot of freeboard. HW recommends that the Planning Board provide direction to the Applicant on whether the subsurface chambers are required to provide a foot of freeboard. g. Per§250-23 E. (30) All drainpipes are to be reinforced concrete pipes and have a minimum diameter of 12 inches. The Applicant has proposed HDPE drainpipes with some being less than 12 inches. HW recommends that the Applicant review these criteria and revise the plans or provide justification to the Planning Board to allow the use of the different pipe material and sizes. 3. Standard 3 requires that the annual recharge from post-development shall approximate annual recharge from pre-development conditions. a. The Applicant has provided a Stormwater Evaluation Memorandum prepared by GeoEngineers, dated July 27, 2023. The geotechnical report recommended using an exfiltration rate of 0.52 inches per hour (iph). The Applicant has used 0.52 iph in the HydroCAD model and has provided the required 2 feet of separation to groundwater. No further action required. b. HW concurs that the Applicant has provided the required recharge. However, HW was not able to confirm the values listed in Table 5 of the Stormwater Management Report. For documentation purposes HW recommends that the Applicant review the values listed and confirm the depth proposed between the lowest outlet and the bottom of the system. It may be beneficial for the Applicant to provide the stage storage area print out from the HydroCAD model. If applicable, HW recommends that the Applicant update Table 5. 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 worksheet that lists the water quality units and the subsurface infiltration chambers to provide the 90% TSS removal per the Town's requirements. No further action required. b. Per §250-23 B. (1) the Applicant is required to remove 60% of the average annual load of Total Phosphorus. The Applicant has provided the Phosphorus Removal worksheets that indicate that the Applicant is removing 79% of the Phosphorus load via the III i'�IIL I II III.I.I I �ml.,.m lu lll .......... III lll i�.1I� .......... I iii ll �„I III I Ilf �II I I ili i'�a I,1 �I '�� I ..��. �� I�� .��l � �i��1I ��I ege doal<:. Town of North Andover September 6, 2023 Page 4 of 5 subsurface infiltration chambers. The calculations appear reasonable. No further action required. 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 or meet a combination of retention and treatment that achieves the above standards listed under §250-23 B. (1). It appears that the criteria have been met. No further action required. 5. Standard 5 is related to projects with a Land Use of Higher Potential Pollutant Loads (LUHPPL). a. The use of the site as dormitories within a college campus is not considered a LUHPPL. Standard 5 is not applicable. 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. 7. Standard 7 is related to projects considered Redevelopment. a. The project is considered new development. The Applicant will increase the impervious surface by 60,710 sf of roof top and pavement area within a watershed area of 377,520 sf, an increase of approximately 50%. Standard 7 is not applicable. 8. Standard 8 requires a plan to control construction related impacts including erosion, sedimentation, or other pollutant sources. a. The Applicant has provided a Site Preparation and Erosion Control Plan, sheet C2.0 with details on Sheet C6.1. The Applicant has also provided a narrative for Recommended Construction Period Pollution Prevention and Erosion and Sedimentation Controls in Appendix E of the Stormwater Management Report. b. 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. c. The Applicant is proposing to disturb more than one acre of land. In accordance with the EPA Construction General Permit, the Applicant is required to file a Notice of Intent with the EPA and prepare a Stormwater Pollution Prevention Plan (SWPPP). HW recommends that the Applicant provide the Town of North Andover with a copy of the SWPPP a minimum of 14 days prior to land disturbance. The Planning Board may choose to require receipt of the final SWPPP signed by the contractor as a condition of approval. 9. Standard 9 requires a Long-Term Operation and Maintenance (O &M) Plan to be provided. a. The Applicant has provided an Operation and Maintenance Plan in Appendix D of the Stormwater Management Report. The O&M Plan can be removed from the Stormwater Management Report and be a standalone document for use by the college. HW notes .......... i .......... �I I ��i i�� I i� � � �� : � � �� 11'e!I �� "�i�,P oil �� i �� ^ i � I ge d ai< Town of North Andover September 6, 2023 Page 5 of 5 that the town is listed incorrectly on the cover page. b. In the Long-Term Maintenance/Evaluation Checklist, Section E.4 of Appendix D, HW recommends that the Applicant add the water quality units. c. HW recommends that the Planning Board reference this document and require a signed O&M Plan as a condition of approval. 10. Standard 10 requires an Illicit Discharge Compliance Statement be provided. a. The Applicant has provided an Illicit Discharge Compliance Statement signed by the property owner. No further action required. Conclusions HW recommends that the Planning Board require that the Applicant provide a written response to address these 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. Janet Carter Bernardo, P.E. 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