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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 1 A, 10-year, 24-hour frequency storm for the worst -case <br />land cover condition. <br />OR <br />• 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 />C 121: 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 ll - Chapter 3 - Page 330 <br />