6009 FLEMING ST 2021-06-09 - Laserfiche WebLink

• Popach Short Plat—2 Lots area so any potential upstream ground water in this area is expected to be negligible and mainly be contributed from site runoff. • Proposed finish grade elevations of new permeable driving surfaces has been installed at or slightly above existing grades to maximize subsurface soil depths and separation from restrictive layers. 12"minimum gravel storage depths have been proposed below the permeable driving surfaces which is based on the assumption • that a 12" cubic section of rock storage with an approximate 35%void space would be able to provide approximately 0.35 cubic feet of runoff storage per cubic foot. Assuming a typical 100 year 24-hour storm event results in approximately 3.5- inches of runoff,this would equate to an equivalent volume of water equal to about • 0.30 cubic feet per square foot of driving surface. Providing a 12" gravel base rock storage depth for the new permeable driving surfaces should provide enough storage volume to contain a 100 year 24-hour storm event. Additionally, with an estimated 0.25 in/hr design rate for typical sandy loam type soils, it's estimated that the 100 year 24-hour storm event would be able to infiltrate this storm event in an • approximately 14 hours. Stormwater BMP's for new roof runoff has been proposed through above ground bio-retention planter boxes per BMP T5.14B and T7.30. Per 2014 DOE SWDM, bioretention areas shall be sized with a minimum horizontal area of 5% of total impervious area draining to it. With an estimated roof area of 480 sf. for the new • garage on lot 1, a minimum 24 sf. bioretention planter box shall be installed. For the future roof area on lot 2 (estimated at 1,750 of roof area), a minimum of an 88 sf. bioretention planter box shall be installed (if receives all roof runoff) or potentially two planter boxes at least 44 sf in size to receive each half of roof runoff. Each new planter box to be installed with an overflow wier near the top of each box to • provide an overflow path in the event that the planter boxes are clogged or the system fails in some way. Minimum Requirement # 6: Runoff Treatment Due to the site proposing less than 5,000 sf. of new pollution generating impervious • surfaces, and less than a 3/4 Acres of forest to lawn conversion, runoff treatment is not required. Minimum Requirement # 7: Flow Control Due to the site proposing less than 5,000 sf. of new and/or replaced effective impervious area and less than a 0.10 cfs. increase in the existing 100 year storm runoff(based on 1 hour time step modeling), flow control is not required. Refer to site development summary and WWHM flow calculations below for additional • information. WESTERN ENGINEERS,INC. 13 PROJECT: 15-1463-A •