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Home My WebLink About2012-01-17 Planning Board Supplemental Materials Transportation Land Development Environmental • Services s ® imagination innovation I energy Creating results for our clients and benefits far our communities December 22,2011 _VLInasse_IIa—gen RrustUn, T1w- Mr.John Simons,Chairman North Andover Planning Board 1600 Osgood St. North Andover,MA 01845 Mr.Louis Napoli,Chairman North Andover Conservation Commission 1600 Osgood Street,Suite 2-36 North Andover,MA 01845 Re: Response to Stormwater Peer Review Comments DEP File#:242-1531 Volpe Athletic Center Expansion-Merrimack College North Andover,Massachusetts Dear Chairmen,Board Members and Commissioners, On behalf of Merrimack College,VHB provides the following information in response to the peer review comment letter prepared by Eggleston Environmental on October 26,2011. The Eggleston Environmental comments are listed below,followed by the response in italics. Comment: 1. Additional information regarding the scope of the 2005 modifications to the Stormwater Master Plan,as well as copies of the Certificates of Compliance referenced in the response letter should be provided to the Commission. Response: Based on further conversations with the College,it is understood that Certificates of Compliance have not been issued for the drainage improvements,as the Order remains open. It is understood that a sitewalk was conducted on November 16"`to observe the constructed stormwater master plan improvements. Please refer to the attached plans of the 2005 stormwater modifications at the "Athletic Game Field". 101 Walnut Street Post Office Box 9151 Watertown, Massachusetts 02471-9151 617.924.1770 ® FAX 617.924.2286 L:\11625.00\docs\letters\2011-12-21 EE Response to Volpe Center Comments#2..doc email: info@vhb.com www.vhb.com Stormwater Response to Comments Project No.: 11625.00 December 22,2011 Page 2 Comment: 2. In response to my comment#5,VHB has indicated that the plumber and architect will need to incorporate the stormwater drainage areas used in the design into the design of the building,and suggested that the submission of the plumbing plans be made a condition of approval.Instead of asking the Town boards or their agents to verify the design,I suggest that this be made a requirement of the as-built certification provided by a registered Professional Engineer at the completion of construction. Response: Agreed,this requirement can be added to the Order of Conditions. Comment: 3. The proposed plan calls for two rain gardens to be located along the eastern edge of the project site;rain garden#1 would receive runoff from a portion of the new roof area,and rain garden#2 would receive runoff from a portion of the new parking area.The primary purpose of the rain gardens is to provide some degree of recharge on the site,despite the relatively low-permeability soils,and to retain the runoff within the drainage area to the onsite wetlands.Rain garden#2 will also provide treatment of the pavement runoff.VHB has noted that the 2 ft of separation called for in the DEP guidelines may not be attainable on the site, and I concur that this may well be the case.The DEP guidelines also do not permit infiltration structures within 50 ft of the wetlands,and both rain garden locations are located within the 50-ft wetland no-build zone.However,given that this is a redevelopment project and the areas proposed for the rain gardens seem to be the only feasible locations for infiltration on the site,I do not believe that strict compliance with the DEP guidelines is either necessary or preferable.Unless they are totally below the normal groundwater level,the proposed rain gardens will provide some degree of recharge during low groundwater periods, and will help to attenuate runoff flows to the wetland.In addition,the vegetative uptake coupled with infiltration and/or filtration in rain garden#2 will provide water quality enhancement for the pavement runoff.If the groundwater is at or near the bottom of the basins they may need to be constructed and planted as pocket wetlands;while the groundwater recharge would be minimal they would still provide recharge to the onsite wetlands and water quality treatment for the pavement runoff. Response: Agreed,based on discussions with the peer reviewer,it was suggested that the application of Rain gardens be explored,with the understanding that they would be located in areas where minimal infiltration could be attained. Therefore, the Rain Gardens are proposed in order to provide recharge to , LA 11625 00\docs\letters\2011-12-21 EE Response to Volpe Center Comments#2.doc Stormwater Response to Comments Project No.: 11625.00 December 22,2011 Page 3 the maximum extent practicable. It is understood that due to soil conditions, the infiltration systems will not meet the requirements set forth by DEP;however they will provide additional means of water quality and infiltration. Test pit data,performed by Soil Exploration Corp.,has been attached to this response letter in a report from Kevin Martin,P.E.dated November 4,2011. Soil information at the proposed Rain Garden locations indicate groundwater at 4-ft. below existing grade(Elev.239±). This groundwater elevation would allow for the 2-ft.separation required,however the seasonal high groundwater elevation has not been confirmed. Comment: 4. It is not clear how the roof drainage from area 11.313 will drain to rain garden#1 as no roof drain connection is shown.If gutters or roof drains are not proposed for that part of the building addition I would suggest adding dripline trenches to further enhance infiltration. Response: Runoff from the portion of the building included in area 11.313 shall be collected in a roof drain and discharged via a downspout to Rain Garden 1. The proposed downspout location along with associated stone protection has been added to the plans. Comment: 5. Additional pretreatment,e.g. a filter strip,stone diaphragm,sediment forebay,or some combination thereof,is needed prior to the pavement runoff entering rain garden#2. Response: The plans have been updated to include a grass and gravel combination filter strip per the Massachusetts Stormwater Handbook,which requires a minimum 8 inches of gravel followed by 3 to 5 feet of sod. Adjacent to the proposed edge of pavement,the design plans shall include a 12-inch gravel strip followed by 20 ft. of sod prior to sheet flowing into Rain Garden 1. Comment: 6. The rain garden detail shown on Sheet C-6 is not consistent with the proposed design,which calls for an overflow Swale and no landscape drain.The detail should also specify the bioretention soil mix.Additional detail on the proposed plantings for the rain gardens will need to be provided;I do not believe they were included on the landscape plan submitted to the Planning Board.Final determination of the plantings should be based on the groundwater hydrology when the basins are excavated. e 0 L:\11625.00\docs\letters\2011-12-21 EE Response to Volpe Center Comments#2..doc Stormwater Response to Comments Project No.: 11625.00 December 22,2011 Page 4 Response: The Rain Garden detail on Sheet C-6 has been revised to reflect the design shown on the plans by removing the landscape drain. Sheet L-1—Planting Plan has been updated and provided,detailing proposed plantings at the Rain Gardens. Refer to Attachment A: Section 329500— Bioretention Areas for clarification on the bioretention soil mixture. This specification section shall be included in the project documents. Comment: 7. Per the attached 8/31/11 NJCAT certification letter,the proposed STC-900 units have an approved TSS removal rate of 50%for field(vs.laboratory)installations,not 75%as is used in the TSS calculations.Hence,the treatment train for the lower portion of the new parking lot, comprised of deep sump catch basins and the Stormceptor units,does not meet the 80%TSS removal called for in the DEP regulations.I believe this is offset at least to some extent by the net improvements in water quality provided by the proposed upgrades to the existing drainage system in Cullen Ave.The college does also sweep the streets and parking areas several times per year,which will help with TSS removal. Response: According to the manufacturer's sizing criteria,both STC-900 Units have been sized via the PCSWMM Stormceptor Sizing program which documents TSS Removal Efficiencies for WQU-A1 and WQU-A2of 91%and 80%,respectively. Sizing was based off of providing a minimum TSS Removal Efficiency of 80%for the required water quality volume(0.5"runoff depth)relative to each unit's tributary impervious area,per Standard 4 of the Massachusetts Stormwater Handbook. However,based on the NJCAT certification letter dated August 31,2011,the Stormceptor STC proprietary separators have been certified by NJDEP with a 50% TSS removal rate based on the criteria provided therein,therefore, the assumed TSS Removal efficiency rates have been adjusted. As shown on the attached, Weighted Average TSS Removal Calculations,a majority of the proposed parking lot(±6,745 s.f.of impervious area) is diverted via sheet flow runoff to Rain Garden 2,which, with the use of the gravel/sod filter strip shall achieve treatment train of 90%TSS Removal per MassDEP. The remaining portion of new impervious area associated with the new parking lot and additional parking along Cullen Avenue(±4,535 s.f.of impervious area) is collected in the closed drainage system and diverted to the STC-900 unit(WQU-A1)which shall achieve a treatment train of 64% TSS Removal per the updated NJCAT certification and consequently,MassDEP. By performing a weighted average of these treatment trains based on the impervious areas,a removal efficiency of 80% can be attained for the new impervious area proposed as part of the project. As noted in the comment above,the project treats existing parking/road areas as well as the proposed new lot. Overall,the Stormceptor STC-900(WQU-B1)treats approximately 1.5-acres of impervious AIRL L:\11625.00\docs\letters\2011-12-21 EE Response to Volpe Center Comments#2.doc Stormwater Response to Comments Project No.: 11625.00 December 22,2011 Page 5 area(±1.7-acres of total area), roughly 1-acre of which is the existing parking lot and roughly 0.33-acres of which is part of the proposed building expansion. Considering that the project treats a significant portion of existing impervious areas,a majority of the proposed impervious area is comprised of roof runoff,and as a weighted TSS removal efficiency of 80% can be attained for the additional impervious area requiring treatment, VHB believes that sufficient measures are proposed in order to meet the MassDEP requirements to the maximum extent practicable. In addition to the Eggleston Environmental review, the proponent has also received comments from the Town of Andover Conservation Commission's peer reviewer (ESS). The Andover correspondence and related plan updates are appended to this letter. Merrimack College is appreciative of the additional input provided on the project and we look forward to the opportunity to present these updates at the January 17th Planning Board meeting Conservation Commission meeting and the January 25th Conservation Commission meeting. Very truly yours, VANNASE HANGEN BRUSTLIN,Inc ristopher M.Lovett,�I�E,LEE AP Senior Project Mana �r Cc: Lisa Eggleston—Eggleston Environmental Jeff Doggett—Merrimack College—Chief of Staff Robert Coppola—Merrimack College—Director of Facilities Attachments: • Stormwater Improvements—Football Basin-Construction Drawings • Geotechnical Memorandum • Updated Site Plans • Andover Peer Review Correspondence e L:\11625.00\docs\letters\2011-12-21 EE Response to Volpe Center Comments#2.doc