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Home My WebLink About2024/11/15 - 3rd Extension of Order of Conditions 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 15, 2024 Mr. Robert Douglas Director of Conservation Andover Conservation Commission 36 Bartlett Street Andover, MA 01810 Re: Third Peer Review— Extension of Order of Conditions Stormwater Management Plan Merrimack College, Andover, MA MassDEP File No. 090-0750 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 third technical peer review of the application for a new Order of Conditions to extend the duration of the Merrimack College Stormwater Management Plan originally issued on October 5, 2001. The application package was prepared by VHB, Inc., on behalf of Merrimack College (Applicant). The intention of the request is to extend the framework established by the previous Order of Conditions that allowed Merrimack College to track development projects within the Berry Brook watershed using credits under an established stormwater management bank (Stormwater Bank). Under Special Condition 5.2 of the original 2001 Order of Conditions, the Stormwater Bank was created to support a 30-year timeframe for full build-out of the Merrimack College Master Plan. Under Special Condition 5.3, the Stormwater Bank was originally established with a credit of 62 cubic feet per second (cfs) from which the 100-year peak runoff rates are allowed to increase. As of the most recent July 18, 2018 Amended Order of Conditions, 14.63 cfs of credit remains. Materials Reviewed The following additional documents and plans were received by HW for review in response to our November 7, 2024, review letters: • Letter to Andover Conservation Commission, Response to Comments— Second Peer Review, Merrimack College Lecture Hall, prepared by VHB, dated November 13, 2024 (5 pages); • Letter to Andover Conservation Commission, Response to Comments— Second Peer Review, Extension of Order of Conditions, prepared by VHB, dated November 13, 2024 (3 pages); r � 'W ilte n.col M @ H o f- ,I ,W ii tte n G ro u p �IM Horsley Group, [it . Andover Conservation Commission November 15, 2024 Page 2 of 4 • Stormwater Management System Operations and Maintenance Manual, Lecture Hall at Merrimack College, prepared by VHB, Inc., Revised November 13, 2024 (57 pages); and • Stormwater Management Report, Lecture Hall at Merrimack College prepared by VHB, Inc., dated September 12, 2024, revised November 13, 2024 (264 pages). Technical Review Based on a review of the above-listed materials, HW understands that the Stormwater Bank was established and approved under the original 2001 Order of Conditions to give credit to Merrimack College for the installation of two detention basins on campus—the Football Field and Elm Street Detention Basins —which accept municipal runoff from offsite subwatersheds south of Highland Road and southeast of Route 125. These stormwater management practices were designed and constructed in response to a 1996/1997 study of the Berry Brook watershed conducted on behalf of Andover by Charles Fuller. The two detention basins were proposed in order to alleviate upstream flooding without installing stormwater management practices in locations intended for development per the Merrimack College Master Plan. Per Sasaki's June 2001 letter to the Andover Conservation Commission Administrator, the two practices reduced the total peak runoff rate from the built-out College Master Plan conditions of the site by 62 cfs relative to the 2001 existing conditions for the 24-hour, 100-year storm event. The 2001 Order of Conditions was issued to establish a credit of 62 cfs from which increases in peak runoff rates caused by future development associated with the College Master Plan could be deducted. The timeframe of the College Master Plan was understood to be 30 years (i.e., 2001-2031). Requests for Amended Orders of Conditions to extend the original 2001 Order of Conditions are allowed at five-year intervals up to six times. The 2018 Amended Order of Conditions represented the fifth such amendment; granting this current 2024 request would therefore represent the sixth and final Amended Order of Conditions. As of the 2018 Amended Order, 14.63 cfs credit remains in the Stormwater Bank. The Applicant is not requesting to use any of the remaining credit for the proposed Lecture Hall project that is included in the 2024 NOI Application for an Amended Order and described in a separate stormwater peer review letter dated October 28, 2024, updated November 15, 2024, prepared by HW (DEP File No 090- 1441). The following comments correlate with our second peer review letter dated November 7, 2024. Follow up comments are provided in bold underlined font where applicable. 1. The methodology for estimating the 24-hour, 100-year precipitation depth has advanced since the original 2001 Order of Conditions to include methods for incorporating the impacts of climate change on increasing rainfall depth. In order to account for climate change in current and future proposals to develop the college, HW recommends that the Commission include the following condition: "Estimates of the 24-hour, 100-year design storm depth and peak flow rate shall be calculated using the NOAA14 PLUS methodology." November 7, 2024: In its response letter, the Applicant has stated that it "will continue to use NOAA14 PLUS methodology in stormwater design for future projects." However, the Applicant's Stormwater Management Report for the A m~ � � m I ��m I,i� � �o� �� ,��i 'III i ��i�� i e ii. III III ..�.. �� ..� �III� �.. III III ..III III �. III� � �� . �III �. � III ° Andover Conservation Commission November 15, 2024 Page 3 of 4 Lecture Hall utilizes the less conservative, standard NOAA14 values for rainfall. HW also notes that the 2-year rainfall depth used as the design storm in the flow monitoring letter(3.4 inches) is larger than the design depth used in the Lecture Hall project's stormwater analysis (3.17 inches). HW's suggested condition of approval stands. November 15, 2024: The Applicant has revised the HydroCAD model for the Lecture Hall to use the higher of the rainfall precipitation data between the Original 1999 Stormwater Plan and the current NOAA Atlas 14 for the project site. HW's suggested Special Condition is noted below. "Values of the design storm depths for the 24-hour storm events analyzed shall be determined using the higher value between the NOAA14 methodology, the Original 1999 Stormwater Plan, or current data provided by MassDEP, whichever is highest." 2. HW notes that the Applicant is within 7 years of the 30-year College Master Plan end date and is applying for the final of six Amended Orders of Conditions. HW recommends that the Applicant indicate whether the College Master Plan is anticipated to be fully executed by the end of the 30-year timeframe. The Commission may consider whether to allow additional extensions beyond the sixth Amended Order of Conditions if the Applicant does not anticipate completion of the College Master Plan. November 7, 2024: The Applicant has indicated that it is indeterminate whether the Stormwater Management Plan will be fully executed by the end of the 30-year timeframe. The Commission may choose to continue to receive requests for Amended Orders of Conditions at five-year intervals as a condition of approval. November 14, 2024: Suggested Special Condition, 3. Per the original 2001 Order of Conditions, Special Order 5.4 states that "Merrimack College is responsible for the construction, maintenance, and monitoring of the detention basin sites so that the optimal performance of the basins is maintained." HW recommends that the Applicant provide information on the Operation and Maintenance (O&M) schedule and routine at the detention basins and verify that O&M occurs at appropriate intervals. The Applicant should also provide information about stormwater O&M activities throughout the campus, such as roadway sweeping and cleaning of catch basins and drain manholes that flow to the two detention basins. November 7, 2024: The Applicant has provided an O&M Plan for the project and has included responsible parties and maintenance logs within the plan. The Commission may choose to request maintenance records annually as a condition of approval. November 14, 2024: Sug-gested Special Condition, 4. Per Special Order 5.4, HW recommends that the Applicant provides documentation of ongoing flow monitoring at the inlets and outlets of the detention basins. HW recommends that the Applicant verify that the detention basins are performing as designed. A m~ � � m I ��m I,i� � �o� �� ,��i 'III i ��i�� i e ii. III III ..�.. �� ..� �III� �.. III III ..III III �. III� � �� . �III �. � III ° Andover Conservation Commission November 15, 2024 Page 4 of 4 November 7, 2024: The Applicant has provided a flow monitoring report summarizing rainfall since the previous monitoring report, which was provided on June 13, 2019. Based on the study period, the stormwater basins appear to be successfully attenuating peak flow as well (if not better) than originally designed. The Commission may choose to request monitoring reports annually as a condition of approval. November 14, 2024: Suggested Special Condition. 5. HW recommends that the Commission and Applicant verify whether the adjacent neighborhoods currently experience drainage issues that are attributable to the drainage design on and around Merrimack College. If drainage issues can be improved by the Applicant, the Commission may consider requiring action by the Applicant as a condition of approval. November 7, 2024: The Applicant has provided the 1999 (existing, per original Order of Conditions), 2000 (short term proposed) and 2020 (full build proposed) design flow values, as well as a flow monitoring report which demonstrates that the basins provided in the original stormwater design are maintaining or improving peak discharge rates within the Berry Brook watershed. HW agrees that the constructed stormwater basins are functioning as originally proposed. If one of the reasons the Commission permitted the construction of the basins in 2001 and create the stormwater bank for Merrimack College was in response to flooding concerns in the adjacent neighborhood, HW recommends that the Applicant and the Commission discussion how the basins have improved those issues. November 14, 2024: The Applicant has provided documentation detailing the reduction in peak flows to the adjacent neighborhoods. No further action is requested. Conclusions HW is satisfied that the Applicant has adequately addressed our comments. Please contact Janet Bernardo at ibernardoCcD-horsleywitten.com or at 857-263-8193 if you have any questions. Sincerely, HORSLEY WITTEN GROUP, INC. w � l 4 Janet Carter Bernardo, P.E. Principal ��. it �� i �� m ^:w �� l�l m m .. III V �u l ... III ��..,m� ��..�� iii��..�� iii III ��.��.iii III��. iii�� .��III��.III iii��iii iu��...� ��III�� III III��. �� III��. �� iii �� ..��iii ��III ��iii��. � iii� � �� III � ��