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

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Q = C iAWhere,Q = design flow (cfs)C = Rational Run<strong>of</strong>f 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 Ro<strong>of</strong> 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)Ro<strong>of</strong>top 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 Ro<strong>of</strong> Drains inCommercial/Multi-family Development ProjectsArea (maximum ro<strong>of</strong> size) Length (min travel distance) Depth <strong>of</strong> 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 <strong>of</strong> 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 <strong>City</strong> <strong>of</strong> Portland <strong>Stormwater</strong> Management <strong>Manual</strong>(2004) and <strong>City</strong> <strong>of</strong> San Jose policy apply 100 sq. ft <strong>of</strong> 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 <strong>of</strong> 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 <strong>of</strong> the tree canopy. The resulting area is considered the area effectively managed by theexisting interceptor trees.<strong>Stormwater</strong> <strong>Quality</strong> <strong>Design</strong> <strong>Manual</strong> for the <strong>Sacramento</strong> and South Placer Regions<strong>May</strong> <strong>2007</strong> Appendix D-4 Page 10

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