The URL can be used to link to this page

Home My WebLink About2024/03/25 - 2nd Stormwater Peer Review - Minor Modifications - - %�. Horsley Wiften Group 0 SUStainable, Ehvirownental Solutions 112 W'at r `tr t-61 Floor-Boston,MBA 02109 .......... 5 -2 1 ho,r 1 ten March 25, 2024 Ms. Jean Enright, Planning Director North Andover Planning Board 120 Main Street North Andover, Massachusetts 01845 Ref: Second 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 second 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 in response to our review letter dated March 7, 2024: • Letter to Jean Enright, Re: Initial Stormwater Peer review, prepared by VHB, dated March 11, 2024 (5 pages); • Stormwater Management System Operations and Maintenance Manual, Corner Development Project at Merrimack College, 315 Turnpike Street, North Andover, MA, revised March 11, 2024 (54 pages); • Exploration Location Plan, Figure 2, prepared by GeoEngineers, with Log of Boring G EO-107; and r � [f n. t- I, " ii tte n G ro u p �IM Horsley W'Itten Group, [iris. Town of North Andover March 25, 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 March 11, 2024 (6 Sheets) including: o Cover Sheet C0.0 o Legend and General Notes C1.0 o Site Preparation and Erosion Control Plan C2.0 o Grading and Drainage Plan C4.0 o Utility Plan C5.0 o Site Details C6.3 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. The following comments correlate to our initial peer review letter dated March 7, 2024. Follow up comments are provided in bold font. 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. March 25, 2024: The Applicant has revised Sheet C2.0 and added a callout for the removal of the existing pavement. HW has no further comment. 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. March 25, 2024: The area drain adjacent to Tree 4 has been called out to be removed on Sheet C2.0. HW has no further comment. 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. March 25, 2024: The Applicant has clarified which portions of the wooded fence will be removed. HW has no further comment. 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. March 25, 2024: The Applicant has clarified Erosion Control Note 4 on Sheet C1.0, specifying stabilization shall occur within 7 days. HW has no further comment. I I .. i ii�,,��a II. ii�� i�� C .�je III . II ii �� Town of North Andover March 25, 2024 Page 3 of 4 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. March 25, 2024: The Applicant has added a note regarding dewatering to Sheet C2.0 and Sheet 4.0. HW has no further comment. 4. The following comments relate to the grading and drainage design depicted on the Grading and Drainage Plan (Sheet C4.0): 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. March 25, 2024: The Applicant has adjusted the rim of the trench drain to be at a low point. HW has no further comment. 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. March 25, 2024: The Applicant has adjusted the inverts. HW has no further comment. 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. March 25, 2024: The Applicant has adjusted the rim elevation to C109. HW has no further comment. 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. March 25, 2024: The Applicant has clarified the chosen inverts for the outlet to Subsurface Infiltration System A. HW has no further comment. I I .. i lii ii '��a.�° II. ii� � \ �� i�� C .�� III . II ii �� Town of North Andover March 25, 2024 Page 4 of 4 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. March 25, 2024: The Applicant has revised the OCS with Weir detail on Sheet C6.3 as suggested. HW has no further comment. 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). March 25, 2024: The Applicant has provided an additional soil log and has clarified that the ESHGW elevation beneath subsurface infiltration system B is at elevation 215 feet. Providing the required 2 feet of separation to groundwater. HW has no further comment. 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. March 25, 2024: The Applicant has stated that the sewer inverts can be adjusted. However, the plan does not appear to have been revised. HW defers the design of the sewer structures to the Applicant's engineer. HW has no further comment. 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. March 25, 2024: The Applicant has revised the O&M Plan as suggested. HW has no further comment. Conclusions HW is satisfied that the Applicant has adequately addressed our comments. Please contact Janet Bernardo at 508-833-6600 or at jbernardo@horsleywitten.com if you have any questions. Sincerely, HORSLEY WITTEN GROUP, INC. � /' Y Ilhlu!1Rn Wl w'J,vf Janet Carter Bernardo, P.E. Principal I I .. i ii�,,��a. ii�� i�� C .�je III . II ii �� III