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Home My WebLink About2024/10/28 - 1st 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+ October 28, 2024 Mr. Robert Douglas Director of Conservation Andover Conservation Commission 36 Bartlett Street Andover, MA 01810 Re: Initial 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 initial 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 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 documents and plans were received by HW for review: • Letter to Andover Conservation Commission, regarding Notice of Intent, Merrimack College Stormwater Management Plan, Per Original Order of Conditions, DEP File #090-0750, prepared by VHB, dated September 17, 2024 (2 pages); • WPA Form 3— Notice of Intent, 175 Haverhill Street and 315 Turnpike Street, Andover, MA, prepared by VHB, Inc. on behalf of Merrimack College, dated September 16, 2024 (11 pages); • Merrimack College Lecture Hall Site Plans, prepared by VHB, Inc., dated September 12, 2024 (12 pages); Andover Conservation Commission October 28, 2024 Page 2 of 4 • Stormwater Management Report, Lecture Hall at Merrimack College prepared by VHB, Inc., dated September 12, 2024 (177 pages); • Merrimack College Master Drainage Study and Hydrologic Analysis, SA#71897.01, 315 Turnpike Street, North Andover, MA, prepared by Sasaki Associates Inc. on behalf of Merrimack College, dated April 26, 1999 (8 pages); • Letter to Andover Conservation Commission Administrator, regarding Merrimack College — Stormwater Management Plan SA#71897.01, prepared by Sasaki Associates Inc. on behalf of Merrimack College, dated June 13, 2001 (2 pages); • WPA Form 5— Order of Conditions MassDEP File No. 90-0750, Merrimack College, 315 Turnpike Street, North Andover, MA, from Andover Conservation Commission, filed April 26, 1999, issued October 5, 2001 (14 pages); and • WPA Form 5—Amended Order of Conditions MassDEP File No. 090-750 5tn Amendment, Merrimack College, 315 Turnpike Street, North Andover, MA, from Andover Conservation Commission, requested June 5, 2018, issued July 18, 2018 (16 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. HW has the following comments: 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. 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 ^: ^: � III V � �^. II. . Ilf ..�.. ��' ��'��..� ��iiil��. iii�Ilf .�.iii�III � lii� � �� . �III �.III .�iii��Iliµ iu,.��:��III� � III III. III �. �iii� �� °�� .L Ilf iii � � III .......... III lii III dU ilia I.......11l III III III iii iii.� :;m�' Co� III g Andover Conservation Commission October 28, 2024 Page 3 of 4 HydroCAD model. The subcatchment areas, surface materials, curve numbers, times of concentrations, depths of precipitations, and flow patterns appear reasonable. 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. 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 TB-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. 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. 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 complies with the local regulations. 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. 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. 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. . I'....... �. � a.. li� � ii: � � � ii � � � � . � .......... III lii III............e kHia I.. Co� III ge( ' Andover Conservation Commission October 28, 2024 Page 4 of 4 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. 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. 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 feasibale. 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. 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. Conclusions HW recommends that the Conservation Commission require that the Applicant provide a written response to address these comments as part of the Commission'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. r Y v f Ij kp d Janet Carter Bernardo, P.E. Josephine Gustavesen Principal Environmental Scientist � . i i . V L Ilf ^ ii� � I �i� ��µ� �.� �i �.� �f i� � i � �� III .......... III lii III Ck ilia I.......11l III III III iii iii iii i ck,Co� III qge