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I I . May be used as a permanent diversion channel to carry the runoff. . Sub-basin tributary area should be one acre or less. I . Design capacity for the peak volumetric flow rate calculated using a 10-minute time step from a 10-year, 24-hour storm, assuming a Type 1A rainfall distribution,for I temporary facilities.Alternatively, use 1.6 times the 10-year, 1-hour flow indicated by an approved continuous runoff model. For facilities that will also serve on a per- I manent basis, consult the local government's drainage requirements. Interceptor dikes shall meet the following criteria: II . Top Width: 2 feet minimum. • Height: 1.5 feet minimum on berm. 1 . Side Slope: 2H:1V or flatter. . Grade: Depends on topography, however, dike system minimum is 0.5%, and max- I imum is 1%. . Compaction: Minimum of 90 percent ASTM D698 standard proctor. I . Horizontal Spacing of Interceptor Dikes: Average Slope SlopeLength Percent Flowpath I 20H:1 V or less 3-5% 300 feet (10 to 20)H:1 V 5-10`)/0 200 feet (4 to 10)H:1 V 10-25% 100 feet I (2 to 4)H:1V 25-50% 50 feet . Stabilization: depends on velocity and reach I . Slopes <5%: Seed and mulch applied within 5 days of dike construction (see BMP C121: Mulching (p.284)). I . Slopes 5 -40%: Dependent on runoff velocities and dike materials. Stabilization should be done immediately using either sod or riprap or other measures to avoid 1 erosion. . The upslope side of the dike shall provide positive drainage to the dike outlet. No I erosion shall occur at the outlet. Provide energy dissipation measures as neces- sary. Sediment-laden runoff must be released through a sediment trapping facility. • Minimize construction traffic over temporary dikes. Use temporary cross culverts for Ichannel crossing. Interceptor swales shall meet the following criteria: I . Bottom Width: 2 feet minimum; the cross-section bottom shall be level. . Depth: 1-foot minimum. 2014 Stormwater Management Manual for Western Washington IVolume II- Chapter 4-Page 332