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4.2 Runoff Conveyance and Treatment BMPs <br />BMP C200: Interceptor Dike and Swale <br />Purpose Provide a ridge of compacted soil, or a ridge with an upslope swale, at the <br />top or base of a disturbed slope or along the perimeter of a disturbed <br />construction area to convey stormwater. Use the dike and/or swale to <br />intercept the runoff from unprotected areas and direct it to areas where <br />erosion can be controlled. This can prevent storm runoff from entering the <br />work area or sediment-laden runoff from leaving the construction site. <br />Conditions of Use <br />Design and <br />Installation <br />Specifications <br />Where the runoff from an exposed site or disturbed slope must be conveyed <br />to an erosion control facility which can safely convey the stormwater. <br />• Locate upslope of a construction site to prevent runoff from entering <br />disturbed area. <br />• When placed horizontally across a disturbed slope, it reduces the <br />amount and velocity of runoff flowing down the slope. <br />• Locate downslope to collect runoff from a disturbed area and direct it <br />to a sediment basin. <br />• Dike and/or swale and channel must be stabilized with temporary or <br />permanent vegetation or other channel protection during construction. <br />s Channel requires a positive grade for drainage; steeper grades requit-e <br />channel protection and check dams. <br />• Review construction for areas where overtopping may occur. <br />• Can be used at top of new fill before vegetation is established. <br />• May be used as a permanent diversion channel to carry the runoff. <br />• Sub-basin tributary area should be one acre or less. <br />• Design capacity for the peak flow from a 10-year, 24-hour storm, <br />assuming a Type 1 A rainfall distribution, for temporary facilities. <br />Alternatively, use 1.6 times the l 0-year, 1-hour flow indicated by an <br />approved continuous runoff model. For facilities that will also serve <br />on a permanent basis, consult the local government's drainage <br />requirements. <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 2:1 or flatter. <br />Grade Depends on topography, however, dike system minimum is <br />0.5%, maximum is 1 %. <br />Compaction Minimum of 90 percent ASTM D698 standard proctor. <br />February 2005 Volume 1l — Construction Stormwater Pollution Prevention 4-57 <br />