Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Q = C iAWhere,Q = design flow (cfs)C = Rational Runoff coefficienti = rainfall intensity (in/hr)A = Area (acres)The equation is solved for hydraulic residence time, using various setback lengths and thefollowing assumptions. The setback lengths are that which will provide a 7 minute hydraulicresidence time.Commercial Disconnected Roof Drain Assumptions:W = 8 feet (recommended by local hydrologist as typical average)s = 0.01n = 0.3 (commercial planter strip, will contain some bushes and larger plants)Rooftop C = 0.9 (rational C) (corresponds to 100% imperviousness)T = 7 minutes (standard residence time for treatment)This results in the following setback/travel distance values for commercial sites.Table 4- Minimum Travel Distance for Disconnected Roof Drains inCommercial/Multi-family Development ProjectsArea (maximum roof size) Length (min travel distance) Depth of flow3,500 sq ft 21 feet 0.4 in5,000 sq ft 24 feet 0.5 in7,500 sq ft 28 feet 0.6 in10,000 sq ft 32 feet 0.7 inInterceptor Trees (IT)Interceptor trees can prevent and/or delay water from landing on an impervious surface. Much of theintercepted water runs down along the tree’s leaves and branches and evaporates, or runs down into theroot system. Properly located trees can reduce the effective impervious fraction by diverting rain thatwould otherwise fall on streets and sidewalks. The City of Portland Stormwater Management Manual(2004) and City of San Jose policy apply 100 sq. ft of credit for a deciduous tree and 200 sq. feet for anevergreen tree. Research results published by Q. Xiao (1998, 2000(2), 2003) provides evidence that thiscredit system is appropriate. The number of trees is multiplied by the credit to obtain an area reduced byinterceptor trees. Credits may be applied for existing trees as defined in Interceptor Trees Fact Sheet. Tocalculate the credits allowed for existing interceptor trees, the designer must identify the square footageequal to one half of the tree canopy. The resulting area is considered the area effectively managed by theexisting interceptor trees.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 10