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HomeMy WebLinkAbout2012-01-17 Planning Board Supplemental Materials (2) a< �a4"a:r .�-�. r -� �:_ .� �&, �'m �l — c •� � � a a�� � �s F'. 3oi:��or'�i m -:�s�t_ �� �i� q.,�' ';i" g�" '.�N ^� •�� w�� „¢'-,7t5 a#�c �i 'se s'•�a .� Orli. tllf. s TO I! 07=977777777277.r. W .ncs .4 � m �.,:� . � was ..i ^st. ;;,o•� '�'�t�`� a,,r r€,.�o�.' .i �8i ix,e"k�g x= dP�#�&a',k �&k-.:� a^ + '�: '� u�=' a �, S & �� ,g>' `�'Af•'�n,rg `9 0 �6`p.�zJ i.a$'s %3:e �-:na.'s nv��k'f :Fe, ,sd'c �f..'a�s 9, .y: n<.r��a%' d_„,5x4' ni 'B• „�. s_ r" �br�u':- -N'fi- a ,4�h' °i.� :mi ..� •ft�+r€ff'. 3 �� 's s,e �9$`4a ,�- 4xu ..s r � mss: Response to Peer Review Comments VHB Project No.: 11625.00 December 8,2011 Page 2 unchanged in the proposed conditions. Stormwater runoff will sheet flow dozen to the road and flow northerly towards the closed drainage system in Cullen Avenue. Revised hydrologic data, peak rates and runoff vohnnes associated with these.atpdates have been included in the attached stortnwater report revisions. As before,the results of the analysis indicate that there is no increase in peak discharge rates between the existing and proposed conditions far areas M-1B-1.1,M-IB-1.2,M- 113-1.3,M-IB-IA and A4-1B-3. The design does include increase in M-IB-3A, blit these are well within those approved by the Stormwater Master Plan. Comment: 2. It appears as if subcatchment M-113 1.1 and M-113 11.1 should be identical but the areas are different. An explanation should be provided. Response: Based on farther information front the existing/proposed plumbing plans for the Volpe Athletic Center, areas M-I B 1.1 and MAB 11.1 have been revised to be identical. Comment: 3. ESS reviewed the proposed renovations to the football field detention basin in 2005;were these renovations completed? Response: Yes, the renovations were completed. Comment: 4. From the information provided it is not clear how the existing drainage pipes are connected to the football field detention basin. An overview of the campus which clearly shows the storm drain pipe network from the Volpe Athletic Center to the detention basin should be provided for clarity. Response: Refer to attached Figure A—Overall Drainage Plan,outlining the storm drain pipe network between the Volpe Athletic Center and the Football Field Detention Basin. Comment: 5. An overview of the drainage pipe network which illustrates how the proposed drain pipes connect to the detention basin should also be provided for clarity. B D Response to Peer Review Comments VHB Project No.: 1.1625.00 December 8,2011 Page 3 Response: Refer to attached Figure A—Overall Drainage Plan,outlining the storm drain pipe network between the Volpe Athletic Center and the Football Field Detention Basin. Comment: 6. The applicant should verify that there is no curb in the parking area near the Rain Garden 2. It appears that runoff will sheet flow from the parking area over the vegetated filter strip before reaching the rain garden.The vegetated filter strip is considered stormwater pretreatment required for rain gardens to function efficiently. Response: Correct,there is no curb along the inajority of the south-eastern edge of the proposed parking area near Rain Garden 2 (as denoted EOP on the Layout and Materials Plan). The plans include a grass and gravel combination filter strip per the Massachusetts Stormwater Handbook, which requires a inininnan 8 inches of gravel followed by 3 to 5 feet of sod. Adjacent to the proposed edge of pavement,the design plans include a 12-inch gravel strip followed by 20 ft. of sod prior to sheet flowing into Rain Garden 1. Comment: 7. The applicant should confirm that Rain Garden 1 is capturing clean roof runoff and therefore additional pretreatment is not required. Response: The area tributary to Raiz Garden 1 (saibcatchment M-1B 11.313) is comprised of clean roof ninoff and a small portion of pervious area within the vicinity of the Raul Garden, therefore no pretreatment nor water quality measures are rewired. Based on discussions with the peer reviewer assigned per the North Andover Conservation Commission,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 the inaxinwin extent practicable. It is understood that dace to soil conditions, the infiltration systems will not ineet the requirements set forth by DEP,however they will provide additional means of water quality and,although minor,infiltration. Comment: 8. The Rip Rap Sizing Calculations found in Appendix A indicates three flared end sections (FES).The flows from Roof-North and Roof-South appear to have been obtained from the 25- year HydroCAD modeling computations.Applicant should clarify how the flow of 21.68 cfs was calculated for FES-1. r Response to Peer Review Comments VHB Project No.: 11625.00 December 8,2011 Page 4 Response: A flow rate of 21.68 cfs was used to size the rip-rap dimensions of FES-1 as it represents the full-flow capacity of the most restricting branch of the proposed drain line, following all upstream connections. Revised calculations have adjusted this full flow capacity to 25.62 cfs;refer to attached Rip-Rap Sizing Calculations. Comment: 9. The Drainage Diagram for Existing and Proposed Conditions indicates that Wetland 2 flows into the Football Field/Wetland Detention,the applicant should provide clarification on how this connection is made. Response: Wetland 2 runs southerly away from the Volpe Center and discharges to an intermittent stream on the other side of Cullen Avenue. The stream runs southwest along the athletic fields dozen and around the west side of the football field,eventually tying into the basin. Mapping information has been compiled from the Stormwater Master Plan,Merrimack College Plans of Record and MassGIS. Comment: 10. Under LID Measures in the MassDEP checklist for Stormwater Report,Bioretention Cells (Rain Gardens)could be checked. Response: As noted,in response to Connnent#7,the Rain Gardens are proposed in order to provide recharge to the niaxiinuin extent practicable as well as additional rueans of water gaca.lity. As they will not ineet the full requirements set forth by DEP dare to soil conditions, this box was not checked. Comment: 11. The rain gardens appear to be sized adequately;however,the detail should be revised to meet the requirements within the Stormwater Manual.T11e soil media should be a minimum of 30-inches thick;depth to ground water should be confirmed;there are no overflow pipes on the drainage plan,however there is one shown on the detail;the plants should be appropriate for rain gardens'under drains be incorporated or is stormwater expected to infiltrate through the media?VHB should provide additional detail for the rain gardens to verify they have been designed and will be constructed appropriately. Response to Peer Review Comments VHB Project No.: 11625.00 December S,2011 Page 5 Response: The detail has been revised to show a 30"mininunat thickness for bioretention soil media,and to exclude the over flowhcnderdrain to match the plans. Test pit data,pet formed by Soil Exploration Corp.,has been attached to this response letter in a report front Kevin Martin,P.E.dated November 4,2011. Soil information at the proposed Rain Garden locations indicate groandzvater at 4- ft. belozo existinggrade(Elev.239±). This groundwater elevation zvotdd allow for the 2-ft, separation required,however the seasonal high groundwater elevation has not been confirmed. Rain garden plantings have been proposed as shown on Sheat L-1—Planting Plan. Comment: 12. Calculations should be provided to verify the closed drainage system pipes have been sized adequately,specifically the pipe which is being called out to be replaced. Response: Refer to the attached,Storin Drain Calculations,documenting adequate sizing of the closed drainage system pipes. Per the Merrimack College Storinwa ter Master flan,a design flow of 22.77 efs would be required in the drain line running from Existing CB-2 to the F'ootbaIl Field Detention Basin. As capacities of the existing three segments of this drain pipe were 4.63 efs,10.10 efs and 3.32 efs,it has been upsized to account for the intended master plan development,as well as to provide some additional capacity. Comment: 13. On the site plans an area on the north side of the building is called out as Ice Storage Area. What is the purpose of this area which is located within the 100-foot buffer zone? Response: This area is utilized for the storage of excess material szvept off of the hockey rink by the zainboni. The plaits have been revised to include a proposed bitinninoas curb surrotinding the existing paveinent to ensure no sal face runoff firom this area is diverted to the existing wetlands. Comment: 14. The Long Term Pollution Prevention Plan(LTPPP)does not discuss street sweeping;in earlier submissions for Merrimack College street sweeping was part of normal operations and maintenance.The LTPPP should incorporate procedures already incorporated in the college's routine maintenance program.Has Merrimack College reviewed the LTPPP and accepted it? r r Response to Peer Review Comments VHB Project No.: 11625.00 December 8,2011 Page 6 Response: The LTPPP has been reviewed with Merrinnack College and it has been noted that the cam pus is swept 3 tunes per year,typically around early April,mid-Map and August. Comment: 15. During construction where will soil stock piles be located?Applicant should verify that stock piles will be placed outside the 100-foot buffer zone and surrounded by erosion controls. Response: The Erosion and Sedimentation Control Manual shall be submitted along with the Storunwater Pollution Prevention Plan (SWPPP) includedfor the National Pollutant Discharge Elin inatioun Systei n(NPDES)Permit,which documents that soil stock piles shall reunain outside of the 100 foot buffer zone. The proposed silt fence barrier shown on sheet C-3---Grading,Drainage and Erosion Control Plan shall provide additional protection of the existing Wetlands. Comment: 16. An existing chain link fence is located around the tennis courts adjacent to Wetland 2.Will the sides of the fence adjacent to the wetland remain? Response: The existing chain link fence around the tennis courts adjacent to Wetland 2 shall be removed in its entirety. Disturbed areas adjacent to Wetland 2 shall be re-vegetated with New England Conservation/Wildlife mix per Sheet L-1—Planting Plan. Comment: 17. A portion of the proposed construction will occur within the 25-foot no disturb zone, including work around the tennis courts and placing the rip rap at the discharge points of the roof drains.The applicant should describe how construction will occur within the 25-foot no disturb zone to avoid any impact to the wetlands. Response: Work within the 25 foot no disturb zone consists of additional outlet protection at existing drain,plantings along the northeast edge of the existing building,as well as the removal and re- vegetation of existing impervious areas(tennis courts). This work is proposed within the 25 foot buffer zone in order to provide additional erosion protection for existing outfalls and enhance the areas adjacent to the existing Wetland 2. Construction activities within the 25 foot no disturb zone shall be Iinnited to these areas and protection of the existing Wetlands shall be as required in the NPDES Perunit per MassDEP.Additional protection of the existing Wetlands is provided per the proposed silt fence barrier. I� I r II I 2 i( N I - WWI S.=7=a,&-Aww., r low,._ jot m RIO= ■ e IAY ` N N'