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Home My WebLink About2024/11/07 - 2nd Stormwater Peer Review - - Horsley Witten Group "I'll,...................."I rf MPo ustainable,Environmental Solutions 112 ftiter Shur -60 bcw w osioni,MA 02109 8,57-263-8193 hursleywitterl.com if+ November 7, 2024 Mr. Robert Douglas Director of Conservation Andover Conservation Commission 36 Bartlett Street Andover, MA 01810 Re: Second Peer Review of the Stormwater Design Merrimack College Lecture Hall Andover, MA MassDEP File No. 090-1441 Dear Mr. Douglas and Commission Members: The Horsley Witten Group, Inc. (HW) is pleased to provide the Andover Conservation Commission with this letter summarizing our second peer review of the construction of the Merrimack College Lecture Hall. The plans were prepared by VHB, Inc., on behalf of Merrimack College (Applicant). The Applicant proposes to construct a lecture hall building, with associated walkways, landscaping, stormwater management system, and utility infrastructure. It does not appear that there are any wetland resource areas within 100 feet of the proposed development. The Applicant proposes the construction of stormwater management system comprised of a series of catch basins to direct stormwater runoff to a subsurface infiltration system and subsequently to the existing stormwater basins currently managing stormwater runoff from the college campus and incorporated in the college master plan. HW has provided a second letter, dated October 28, 2024, updated November 7, 2024, regarding extending the 2001 Order of Conditions for the existing stormwater basins (DEP File No. 090-0750). A portion of the runoff from the Lecture Hall site is directed to a water quality unit prior to entering the subsurface infiltration system. The following additional documents and plans were received by HW for review: • Letter to Andover Conservation Commission, Response to Comments— Initial Peer Review, Merrimack College Lecture Hall, prepared by VHB, dated October 30, 2024 (5 pages); • Letter to Andover Conservation Commission, Response to Comments— Peer Review, Extension of Order of Conditions, prepared by VHB, dated October 30, 2024 (4 pages); • Merrimack College Lecture Hall Site Plans, prepared by VHB, Inc., dated September 12, 2024, revised October 30, 2024 (12 pages); • Stormwater Management System Operations and Maintenance Manual, Lecture Hall at Merrimack College, prepared by VHB, Inc., dated September 12, 2024 (57 pages); Andover Conservation Commission November 7, 2024 Page 2of5 • Stormwater Management Report, Lecture Hall at Merrimack College prepared by VHB, Inc., dated September 12, 2024, revised October 30, 2024 (178 pages); and • Letter to Andover Conservation Division, Merrimack College Detention Basin Flow Monitoring, prepared by VHB, Inc., dated September 12, 2024, (9 pages). Stormwater Review HW has reviewed the documents listed above and has the following comments concerning the stormwater management design in accordance with the Massachusetts Stormwater Handbook (MSH) dated February 2008, and the Town of Andover Stormwater Management and Erosion Control Bylaw and Regulations dated February 10, 2009 (Stormwater Bylaw). In accordance with Article 195-8.14 information required with a Site Plan Review application includes a Stormwater Management Plan. The Stormwater Management Plan has been submitted for review and comments have been addressed in a separate letter. We have used the Massachusetts Stormwater Standards as the basis for organizing our comments. However, in instances where the additional criteria established in §250-23 of the North Andover Code requires further recommendations; we have referenced these as well. The following comments correlate with our initial peer review letter dated October 28, 2024. 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 has analyzed the pre- and post-development stormwater runoff at design point DP-1 A-1 A. DP-1 A-1 A is located in Parking Lot B, adjacent to the project area. Under existing conditions, the parking lot sheet flows into the existing stormwater drainage system for Merrimack College. Under proposed conditions, impervious cover will increase from 3,150 sf to a total of 21,350 sf, accounting for roof cover and paved areas. Runoff from the majority of impervious cover is directed to either a water quality unit and then a subsurface infiltration system or to the subsurface infiltration system directly. Based on Figure 4 of the Stormwater Report, runoff from proposed concrete sidewalk will sheet flow to the Parking Lot B Drainage System. No further action requested. November 7, 2024: No action is requested. 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. HW has reviewed the Pre-Development and Post-Development Drainage Plans and the HydroCAD model. The subcatchment areas, surface materials, curve numbers, times of concentrations, depths of precipitations, and flow patterns appear reasonable. November 7, 2024: In the Merrimack College Detention Basin Flow Monitoring Letter, the 2-year rainfall depth used (3.4 inches) is larger than the design depth used in the Lecture Hall project's stormwater analysis (3.17 inches). HW notes that the Applicant appears to have used higher values in the original Merrimack College Stormwater Management Plan and prior design projects. HW recommends the Applicant use consistent methodology for all proposed projects within the College campus, including the Lecture Hall Design. µ . Ilf ..�.. �� ..� ��iiil �.. iii�Ilf .�.iii�III �. lii� � �� . �III �.III iii��Iliµ iu,.��:��III� � III III. III �. �iii� �� °�� iiil � Ilf iii� � III .......... III IIL........... kHr I.......11l:III III III iii iii i ck, ° o III IIIege Andover Conservation Commission November 7, 2024 Page 3of5 b. The Applicant has conducted four soil borings within the parcel for the Lecture Hall. The borings are comparable primarily indicating glacial till, with groundwater only observed in the deepest boring at 16 feet below ground surface. The Applicant has used an exfiltration rate of 0.52 inches per hour (iph) for the subsurface infiltration system. HW recommends that prior to construction the Applicant conducts additional soil testing within the footprint of the subsurface infiltration system to confirm the infiltration rate and depth to seasonal high ground water is appropriate. November 7, 2024: The Applicant agreed to conduct additional soil testing within the footprint of the subsurface infiltration system. HW recommends that the Applicant provide the soil testing data to the Conservation Commission prior to significant land disturbance to support the infiltration rate and estimated seasonal high groundwater (ESHGW) values used in the design. 3. Standard 3 requires that the annual recharge from post-development shall approximate annual recharge from pre-development conditions. a. VHB performed a Frimpter adjustment to determine the estimated seasonal high groundwater table (ESHGWT) based on conditions found at soil test pit T13-4. HW recommends conducting additional soil testing within the footprint of the subsurface infiltration system prior to construction. HW further recommends that the Applicant provides documentation to the Andover Conservation Commission confirming that the design as presented is appropriate or provides a modified design as necessary. November 7, 2024: As stated above, HW recommends that the Applicant provide the additional soil testing data to the Conservation Commission prior to significant land disturbance. 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 states the Water Quality Unit has a TSS Removal Rate of 50%. The Applicant shows a CDS hydrodynamic separator as the water quality unit in the Site Details, Sheet 8 of 8. HW recommends that the Applicant justify the use of the 50% TSS Removal Rate and provide documentation from a third- party reviewer supporting the listed TSS removal rate. November 7, 2024: The Applicant has provided a "CDS Estimated Net Annual Solids Reduction" Sheet in Appendix D of the Stormwater Management Report, revised October 30, 2024. Based on the calculations, the water quality unit provides approximately 90% efficiency. The Applicant has used a TSS Removal Rate of 50% in the TSS Removal Calculation Worksheet to be conservative. HW recommends that the Applicant provide documentation from a third party peer reviewer of the CDS Hydrodynamic separator that indicates that the TSS removal rating for the proprietary product is greater than 50%. b. Per §250-23 B new development sites are required to retain the volume of runoff equivalent to, or greater than 1.0 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. The Applicant has provided calculations documenting that the system can provide water quality for 0.5 inches of runoff. HW has confirmed that the system is adequately sized to retain the 1.0 inch of runoff. However, we recommend that the Applicant revise the calculations for documentation purposes, confirming it ^: . Ilf ..�.. ��'��..� ��iiil �.. iii�Ilf .�.iii�III �. lii� � �� . �III �.III iii��Iliµ iu,.��:��III� � III III. III �. �iii� �� °�� iiil � Ilf iii� � � III .......... III IIL........... iir I.......11l'':III III III iii iii i ck, ° o III IIIeg Andover Conservation Commission November 7, 2024 Page 4 of 5 complies with the local regulations. November 7, 2024: The Applicant has updated the Stormwater Management Report to reflect the required water quality runoff depth as required by local regulations. No further action is requested. 5. Standard 5 is related to projects with a Land Use of Higher Potential Pollutant Loads (LUHPPL). a. The proposed use is not considered a LUHPPL, therefore Standard 5 is not applicable. No further action requested. November 7, 2024: No further action is requested. 6. Standard 6 is related to projects with stormwater discharging into a critical area, a Zone II, 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. No further action requested. November 7, 2024: No further action is requested. 7. Standard 7 is related to projects considered Redevelopment. a. The project at 315 Turnpike Street is considered new development, and therefore, Standard 7 is not applicable. No further action requested. November 7, 2024: No further action is requested. 8. Standard 8 requires a plan to control construction related impacts including erosion, sedimentation, or other pollutant sources. a. The proposed project will be disturbing greater than one acre of land. A SWPPP is required by EPA for land disturbance of greater than 1 acre. The Applicant has provided recommended erosion controls to be included in a SWPPP in Appendix E of the Stormwater Report. The SWPPP should include source control and pollution prevention measures, stormwater practices to address erosion and sedimentation, stabilization measures, and procedures for operating and maintaining the proposed stormwater practices. The plan should also identify the parties responsible for implementing the plan. HW recommends that the Conservation Commission require receipt of a final signed SWPPP a minimum of 14 days prior to land disturbance. November 7, 2024: HW acknowledges that the Applicant intends to provide the finalized SWPPP to the Conservation Commission and the Planning Board at least 14 days prior to land disturbance. The two boards may choose to require receipt of the SWPPP as a condition of approval. b. The Applicant has provided a Site Preparation and Erosion Control Plan, Sheet 2 of 8, that includes silt socks at the back of curb openings along Aherne Road. HW recommends that the Applicant include an additional silt sock at the curb opening across from the construction exit. November 7, 2024: The Applicant updated the Site Preparation and Erosion Control Plan (Sheet C2.0) to include an additional silt sock at the curb opening across from the construction exit. No further action is requested. ^:� a a. '":��'":1� ":� i '":�� ^: ^: °:��, ^:� '.a. . III'....... ..�.. �� ..� ��iiil��. �� iii�III'....... .�.iii�III � lii� � �� . �III �.III .�iii��Iliµ iu,.��:��III� � III III. III �. iii� �� °�� iiil�I III'....... iii� � III .......... III IIL........... kHr I..III III III III iii iii.� m� ° o III IIIege Andover Conservation Commission November 7, 2024 Page 5 of 5 9. Standard 9 requires a Long-Term Operation and Maintenance (O & M) Plan to be provided. a. In the Stormwater Management Report, under Standards 4 and 9, the Applicant has noted that a Long-Term O&M Plan is included under a separate cover as part of the O&M Plan. HW did not receive the Operation & Maintenance Plan for review. We recommend that the Applicant provide a complete O&M Plan to the Conservation Commission that includes the entire campus if feasible. The O&M Plan should include the responsible parties, descriptions of the various stormwater practices, clear descriptions on how they should be maintained and the frequency of inspections. A simple schedule, with a simple sketch to illustrate where the practices are located on the site and a budget, so that the property owner understands what to expect. November 7, 2024: The Applicant has provided an O&M Plan for the Lecture Hall project and has included responsible parties and maintenance logs within the plan. HW recommends that the Applicant provide a description and delineation of public safety features and an estimated operations and maintenance budget. The Commission may choose to request maintenance log records annually as a condition of approval. In the Response to Comments Letter, the Applicant stated the intent to prepare and provide a campus-wide O&M Plan as part of the Order of Conditions Peer Review. 10. Standard 10 requires an Illicit Discharge Compliance Statement be provided. a. The Applicant has provided a signed Illicit Discharge Compliance Statement in Appendix F of the Stormwater Report. No further action requested. November 7, 2024: No further action is requested. Conclusions HW recommends that the Conservation Commission requires that the Applicant provide a written response to address the few remaining comments as part of the permitting process. 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. Josephine Gustavesen Principal Environmental Scientist CC: Jean Enright— North Andover Planning Board I::% i, ": Ilfl �..��..� ��..�� iii��..�� iii�Ilf�� iii�III��. � "Ii �� .��III ��.III �iii��iii��iu,.��...��III�� III III � .�� III �. �� iii� ��°�� iii�� Ilf�� iii � �� III .......... III lii III............e k ilia I.......11l III III III iii iii iii i c lii C oIII III ege J(' :BSI: