6708 CADY RD 2025-03-31 - Laserfiche WebLink

BMP T5.12: Sheet Flow Dispersion Purpose and Definition Sheet flow dispersion is the simplest method of runoff control.This BMP can be used for any imper- vious or pervious surface that is graded to avoid concentrating flows. Because flows are already dis- persed as they leave the surface,they need only traverse a narrow band of adjacent vegetation for effective on-site stormwater management. Applications and Limitations Use this BMP for flat or moderately sloping (< 15%slope)surfaces such as driveways, sports courts, patios, roofs without gutters, lawns, pastures; or any situation where concentration of flows can be avoided. Design Guidelines . See Figure V-3.2: Sheet Flow Dispersion for Driveways for details for driveways. • See BMP T5.10B: Downspout Dispersion Systems for dispersion trench design criteria. • Provide a 2-foot-wide transition zone to discourage channeling between the edge of the imper- vious surface(or building eaves)and the downslope vegetation.This transition zone may con- sist of an extension of subgrade material (crushed rock), modular pavement, drain rock, or other material acceptable to the Local Plan Approval Authority. • Provide a 10-foot-wide vegetated buffer for up to 20 feet of width of paved or impervious sur- face. Provide an additional 10 feet of vegetated buffer width for each additional 20 feet of impervious surface width or fraction thereof. For example, if a driveway is 30 feet wide and 60 feet long provide a 20-foot wide by 60-foot long vegetated buffer,with a 2-foot by 60-foot trans- ition zone. . The design must not result in erosion or flooding of downstream properties. • Runoff discharge toward landslide hazard areas must be evaluated by a geotechnical engin- eer or a qualified geologist. Do not allow sheet flow on or above slopes greater than 20%, or above erosion hazard areas, without evaluation by a geotechnical engineer or qualified geo- logist and approval by the Local Plan Approval Authority. • For sites with septic systems,the discharge area must be ten feet downgradient of the drain- field primary and reserve areas(WAC 246-272A-0210).A Local Plan Approval Authority may waive this requirement if site topography clearly prohibits flows from intersecting the drain- field. Runoff Model Representation Where this BMP is used to disperse impervious area runoff into an undisturbed native landscape area or an area that meets BMP T5.13: Post-Construction Soil Quality and Depth, the impervious area should be modeled as a lateral flow impervious area. Do this in WWHM on the Mitigated 2019 Stormwater Management Manual for Western Washington Volume V-Chapter 3-Page 692