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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 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 /> Design and • Dike and/or swale and channel must be stabilized with temporary or <br /> Installation permanent vegetation or other channel protection during construction. <br /> Specifications • Channel requires a positive grade for drainage; steeper grades require <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 lA rainfall distribution, for temporary facilities. <br /> Alternatively, use 1.6 times the 10-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 ll— Construction Stormwater Pollution Prevention 4-57 <br />