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Home My WebLink About2024/10/28 - 1st 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+ October 28, 2024 Mr. Robert Douglas Director of Conservation Andover Conservation Commission 36 Bartlett Street Andover, MA 01810 Re: 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 report summarizing our 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 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); r � 'W ilte n.col M @ H o f- ,I ,W ii tte n G ro u p �IM Horsley Witten Group, [it . Andover Conservation Commission October 28, 2024 Page 2of3 • 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 261) 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). 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, prepared by HW (DEP File No 090-1441). HW offers the following comments concerning the NOI Application for an Amended Order of Conditions:1441) 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 m ^: a.' � III �. I�. . Ilf ..�.. �� ..� ��iiil��. iii�Ilf .�.iii�III � lii� � �� . �III �.III .�iii��Iliµ iu,.��:��III� � III III. III �. �iii� �� °�� Ilf .�iii�III� � � III iii�i l iii l��..�� III .......... .......... .......... (IImolIIIge ° : ;° Andover Conservation Commission October 28, 2024 Page 3of3 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." 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. 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. 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. 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. Conclusions HW recommends that the Commission review the existing Order of Conditions and determine if additional conditions are warranted at this time. Please contact Janet Bernardo at ibernardo(cD-horsleywitten.com or at 857-263-8193 if you have any questions regarding these comments. Sincerely, HORSLEY WITTEN GROUP, INC. N Janet Carter Bernardo, P.E. Jonas Procton, P.E. Principal Water Resources Engineer .Vll.,l �� . . ..........Ilf i�I iii a. .......... II ii ��.�� . � � � . � II II �� . � �i l � li� � � �� i� .......... 11 'cil�ege 6�)'-1 1 III'll,GIII141,