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Design and Installation Specifications <br /> . Dike and/or swale and channel must be stabilized with temporary or permanent vegetation or <br /> other channel protection during construction. <br /> . Steep grades require channel protection and check dams. <br /> . Review construction for areas where overtopping may occur. <br /> . Can be used at the top of new fill before vegetation is established. <br /> . May be used as a permanent diversion channel to carry the runoff. <br /> . Contributing area for an individual dike or swale should be one acre or less. <br /> . Design the dike and/or swale to contain flows calculated by one of the following methods: <br /> Single Event Hydrograph Method:The peak volumetric flow rate calculated using a 10- <br /> minute time step from a Type 1A, 10-year,24-hour frequency storm for the worst-case <br /> land cover condition. <br /> OR <br /> o Continuous Simulation Method:The 10-year peak flow rate, as determined by an <br /> approved continuous runoff model with a 15-minute time step for the worst-case land <br /> cover condition. <br /> Worst-case land cover conditions(i.e., producing the most runoff)should be used for analysis <br /> (in most cases,this would be the land cover conditions just prior to final landscaping). <br /> Interceptor Dikes <br /> Interceptor dikes shall meet the following criteria: <br /> . Top Width:2 feet minimum. <br /> . Height: 1.5 feet minimum on berm. <br /> . Side Slope:2H:1V or flatter. <br /> . Grade: Depends on topography, however,dike system minimum is 0.5%,and maximum is <br /> 1%. <br /> . Compaction: Minimum of 90 percent ASTM D698 standard proctor. <br /> . Stabilization: Depends on velocity and reach. Inspect regularly to ensure stability. <br /> . Ground Slopes<5%: Seed and mulch applied within 5 days of dike construction(see BMP <br /> C121: Mulching). <br /> . Ground Slopes 5-40%: Dependent on runoff velocities and dike materials.Stabilization <br /> should be done immediately using either sod or riprap, or other measures to avoid erosion. <br /> . The upslope side of the dike shall provide positive drainage to the dike outlet. No erosion shall <br /> 2019 Stormwater Management Manual for Western Washington <br /> Volume II-Chapter 3-Page 330 <br />