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Home My WebLink About2024/03/07 - 1st Stormwater Peer Review - Minor Modifications - - %�. 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 March 7, 2024 Ms. Jean Enright, Planning Director North Andover Planning Board 120 Main Street North Andover, Massachusetts 01845 Ref: First Stormwater Peer Review Corner Development Project at Merrimack College — Minor Modifications 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). This submission represents minor modifications to plans that were previously approved by the North Andover Planning Board on December 5, 2023. The plans were modified to accommodate plans by MassDOT to widen Andover/Elm Street and enable pedestrian connections along the Route 114 Corridor. 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 63,460 sf of roof top and pavement area within a watershed area of 377,520 sf, an increase of approximately 52%. The following additional documents and plans were received by HW: • Memorandum to Jean Enright, Re: Corner Development Project at Merrimack College, Site Plan Review Narrative, dated February 20, 2024 (8 pages); • Stormwater Management System Operations and Maintenance Manual, Corner Development Project at Merrimack College, 315 Turnpike Street, North Andover, MA, revised February 20, 2024 (54 pages); • Stormwater Management Report, Corner Development Project at Merrimack College, prepared by VHB, revised February 20, 2024 (255 pages); and r � [f n. t- I, " ii tte n G ro u p �IM Horsley W'Itten Group, [iris. Town of North Andover March 7, 2024 Page 2 of 4 • Site Plans, Corner Development Project at Merrimack College, 315 Turnpike St., North Andover, MA, prepared by V H B, dated August 17, 2023, revised February 20, 2024 (14 pages) 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 o Landscape Plan 1-1.0 o Landscape Details L2.1-1-2.2 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. 1. The following comments relate to the Site Preparation and Erosion Control Plan (Sheet C2.0): a. The existing concrete path that cuts diagonally across the site is not called out for removal. HW recommends that the Applicant confirm whether the path is to remain, and revise the plans to clarify that the path is to be removed if not. b. The catch basin adjacent to Existing Tree 4 (along Medina Drive) is not called out for removal, but does not appear to be active in the proposed Grading and Drainage Plan (Sheet C4.0). HW recommends that the Applicant clarify if this existing catch basin is to remain, or revise the plans as needed. c. A callout stating "Remove existing wooden post and rail fence" is shown on the north side of the site, but does not point to anything specific. HW recommends that the Applicant revise or remove this callout. 2. The Applicant notes on Sheet C1.0 that earth materials shall be "exposed for a minimum of time before they are covered, seeded, or otherwise stabilized to prevent erosion" (Erosion Control Note 4). HW recommends that the Applicant specify a maximum time of 14 days of exposure before earth material stockpiles are seeded and stabilized. 3. The Applicant has included a Dewatering Filter Bag detail on Sheet C6.1, but does not make reference to dewatering in the notes on Sheet C1.0. HW recommends that the Applicant clarify whether dewatering is anticipated, and revise the plans to include a note on dewatering if so. 4. The following comments relate to the grading and drainage design depicted on the Grading and Drainage Plan (Sheet C4.0): s S.. �,� �i � I !. �I � �� .......... .......... .......... .. .�I : I' II � � I �� Town of North Andover March 7, 2024 Page 3 of 4 a. A spot grade is covered by the trench drain at the inner corner of Proposed Building 2. HW recommends the Applicant revise the plans to make the spot grade visible. Relatedly, the adjacent trench drain does not appear to be at a low point, and it seems possible that water will pond along the edge of the building. HW recommends that the Applicant revise the grading in this area to prevent ponding. b. At structures C110 and D101, proposed outlet inverts are higher than proposed inlet inverts, which appears likely to cause backing up of the drainage system. HW recommends that the Applicant revise the drainage design to allow constant flow. c. The rim elevation of structure C109 appears to be 1 foot above the proposed grade. HW recommends the Applicant confirm whether this design is intentional and revise the plans if not. d. The proposed Subsurface Infiltration System A is not connected to an outlet control structure (OCS), and the outlet pipe is set to an elevation that promotes limited storage/infiltration (i.e., the outlet is only 0.1 feet above the bottom of the infiltration system). The Applicant may consider raising the outlet or proposing an outlet control structure to maximize the storage and infiltration capacity of System A. 5. On the Site Details (Sheet C6.3), the OCS with Weir detail shows two orifices in the weir in the section view and only one orifice in the plan view. In the HydroCAD model, only one orifice is modeled. HW recommends that the Applicant clarify the number of orifices proposed for each OCS and revise the plans/HydroCAD model as necessary for consistency. 6. The proposed Subsurface Infiltration Systems are within 2 feet of the estimated seasonal high groundwater (ESGHW) table and contribute to attenuating peak flows during the 10-year storm. HW recommends that the Applicant provide a mounding analysis to verify that the proposed basins will not experience mounding. Further, HW recommends that the Applicant modify the design of System B to provide a minimum of 2 feet of separation between the bottom of stone (currently 217.0) and the ESGHW elevation (216.0). 7. As shown on the Utility Plan (Sheet C5.0), the inlet to SMH A202 from Proposed Building 1 is 1.8 feet above the outlet of the SMH. To reduce the potential of scour in the manhole, HW recommends that the Applicant consider lowering the inlet invert to 0.1 feet above the outlet invert. 8. The Operation and Maintenance (O&M) Plan specifies an inspection rate for the subsurface infiltration systems of at least once per year. HW recommends that the Applicant revise this rate to twice per year, per MSH Volume 2, Chapter 2. . �..........µ��.......... i l ..........� � �,� �i � I !. �I � �� ..�II : II µ� I �� Town of North Andover March 7, 2024 Page 4 of 4 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 them of 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. Sincerely, HORSLEY WITTEN GROUP, INC. ..... µ Janet Carter Bernardo, P.E. Jonas Procton, E.I.T. Associate Principal Water Resources Engineer � u �.iii� III iii��,.��:�iii � III'........ iii III�..� lii �..�� . ..� II �� II� .......... .......... .......... ..�� III