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

The designer must determine how many of the roof downspouts are to be disconnected, as a percentage oftotal roof downspouts. This determines the use multiplier for disconnected rooftops.The form is used to determine the area of impervious surface accounted for by rooftops, dependent on sitedensity (1-20 DU/A). This area is the maximum amount of impervious surface that can be effectivelymanaged with DRDs and comprises the reduction factor. Reduction factors for DRDs range from 0.08 to0.44 and are summarized in Table 1.Example:If a 20-acre residential site, 5 DU/A (23% rooftop impervious surface), includes the disconnection of allroof drains on 40% of houses with a setback of 12 feet (48% treatment) the Runoff Reduction measurewould result in:(0.48)(0.40)(0.23)(20 acres) = 0.044(20 acres) = 0.88 acres of effectively managed area (A c ).Divided Sidewalks (DS)Divided Sidewalks (DS) function to drain water runoff from sidewalks onto a strip of grass locatedbetween the sidewalk and the street. Divided Sidewalks are essentially a variation on DisconnectedPavement (DP) and the credits application method for DP was adapted from the Urban Storm DrainageCriteria Manual Volume 3 – Best Management Practices for Denver (2005). The landscaping strips areusually as wide as the sidewalks themselves, so the sidewalks are considered entirely treated. Sidewalksaccount for approximately 6% of the total area of most residential developments, so if all units usedivided sidewalks the development will treat 6% of total site runoff. If a development chooses to usedivided sidewalks on some areas, connector streets for example, the amount of credits applied will bescaled by the percentage of units using the design. A designer chooses the percentage of units using thedesign, and the number is multiplied by 0.04 to get the total credits obtained.If a designer uses divided sidewalks on 30% of units in a 200 acre development, the credit allowed: usemultiplier * reduction factor * total acreage(0.30)(0.04)(200 acres) = 0.012(200 acres) =2.4 acres of effectively managed area (A c ).Interceptor 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 for the central valley climate. The number of trees is multiplied by the creditto obtain an area reduced by interceptor trees. Credits may be applied for existing trees as defined inInterceptor Trees Fact Sheet. To calculate the credits allowed for existing interceptor trees, the designermust identify the square footage equal to one half of the existing tree canopy. The resulting area isconsidered the area effectively managed by the existing interceptor trees.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007 Appendix D-4 Page 5