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HomeMy WebLinkAbout2012-02-07 Planning Board Supplemental Materials (6) Enright, Jean From:Susan Harrington [sharrington@gcgassociates.net] Sent:Wednesday, January 25, 2012 12:05 PM To:'Lisa Eggleston' Cc:Tymon, Judy; Mike Carter Subject:RE: Fountain Drive Attachments:1154-DESIGN-REV1 DETAIL II_SHEET 6.pdf; 1154-DESIGN-REV1 SITE_SHEET 4.pdf; 1154-POST-REV1_HydroCAD Report.pdf; 1154-POST-REV1 POST.pdf Hi Lisa, In response to your comments I have attached preliminary PDF copies of revised plans. Sheets revised and attached are the Proposed Site Plan (Sheet 4), Detail Sheet II (Sheet 6 of 6), and Post Drainage Plan. Revised Post Drainage Hydro CAD analysis is attached. The following are your comments as they are addressed: Rain garden design should be based on the total area that drains to it (e.g. at least a portion  of the community building roof), not just the pavement areas modeled. - Drainage design now includes half of the Community Building roof. (Post drainage calcs attached) Additional pretreatment is needed for the pavement runoff into the rain garden. For the sheet  flow off the new parking lot you could add a stone trench or pea stone diaphragm, followed by the grassed channel, or a forebay in the rain garden. The piped discharge should also have a sediment forebay. – Pea stone diaphragm and sediment forebay are added to design plan and detail sheet. In order to provide adequate treatment, soil media in the rain garden needs to be at least 24  inches deep, and 30 inches if trees are proposed. –Soil media is revised to show 24” depth. Based on our discussion of water levels in the area it appears that there will be adequate  separation to groundwater beneath the rain garden; the Planning Board can condition that confirmatory soil tests be done prior to installation. – OK. Based on the type C soils, the design infiltration rate should be a maximum of 0.27 in/hr using  the Rawls table. The model uses a rate that converts to 0.42 in/hr over the bottom area shown on the plan. –New hydrocad calculations attached with summarys to show design infiltration rate of 0.27 in/hour is used. Plan should show an overflow outlet/swale from the rain garden, consistent with the way it is  modeled. As we discussed, the outlet should be located and/or graded so that the flow is distributed overland toward the catchbasin at the intersection of East Water St. and Davis St. – Proposed overflow weir is designed for Rain Garden to direct any overflow to grassed area where it will flow overland towards existing catch basin rather than flowing to East Water Street. The berm around the rain garden as shown on the plan is not consistent with the 3:1 side  slope shown in the detail. – Overall grading in Rain Garden area is refined on plan. Rain garden depth is revised from a 6” to 9” depth sloping up over 2.25’. I suggest naturalizing the shape of the rain garden so that it just looks like a landscape  feature. – Revised shape to follow contours. Snow storage for new parking areas should be addressed. – Locations shown on plan for new  parking areas as well as currently used location. 1 Where possible, I recommend directing the upgraded roof downspout/splash pads to flow to  pervious areas of the site and away from paved areas. – Note added to plan and detail sheet. Please contact me with any questions or comments regarding this preliminary review submission. Thank you, Susan Harrington From: Lisa Eggleston [mailto:lisa@egglestonenvironmental.com] Sent: Friday, January 20, 2012 4:06 PM To: sharrington@gcgassociates.net Cc: Judy Tymon Subject: Fountain Drive Hi Susan, Following up on our discussion this morning, here is a summary of my comments on the Fountain Drive project: Rain garden design should be based on the total area that drains to it (e.g. at least a portion  of the community building roof), not just the pavement areas modeled. Additional pretreatment is needed for the pavement runoff into the rain garden. For the sheet  flow off the new parking lot you could add a stone trench or pea stone diaphragm, followed by the grassed channel, or a forebay in the rain garden. The piped discharge should also have a sediment forebay. In order to provide adequate treatment, soil media in the rain garden needs to be at least 24  inches deep, and 30 inches if trees are proposed. Based on our discussion of water levels in the area it appears that there will be adequate  separation to groundwater beneath the rain garden; the Planning Board can condition that confirmatory soil tests be done prior to installation. Based on the type C soils, the design infiltration rate should be a maximum of 0.27 in/hr using  the Rawls table. The model uses a rate that converts to 0.42 in/hr over the bottom area shown on the plan. Plan should show an overflow outlet/swale from the rain garden, consistent with the way it is  modeled. As we discussed, the outlet should be located and/or graded so that the flow is distributed overland toward the catchbasin at the intersection of East Water St. and Davis St. The berm around the rain garden as shown on the plan is not consistent with the 3:1 side  slope shown in the detail. I suggest naturalizing the shape of the rain garden so that it just looks like a landscape  feature. Snow storage for new parking areas should be addressed.  Where possible, I recommend directing the upgraded roof downspout/splash pads to flow to  pervious areas of the site and away from paved areas. Feel free to contact me directly if you have any questions. As we agreed, I will anticipate receiving 2 electronic versions of your revisions to review before you produce a final set of plans for the Planning Board hearing on Feb 7th. Lisa D. Eggleston, P.E. Eggleston Environmental Please note our new mailing address: 32 Old Framingham Rd., Unit #29 Sudbury, MA 01776 tel 508.259.1137 fax 866.820.7840 lisa@egglestonenvironmental.com Please note the Massachusetts Secretary of State's office has determined that most emails to and from municipal offices and officials are public records. For more information please refer to: http://www.sec.state.ma.us/pre/preidx.htm. Please consider the environment before printing this email. 3