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Cha2ter . 4 - Best Management Practice I s for Erosion and Sediment Control <br /> r <br /> 4. 06 SILT FENCE <br /> (ES BMP 1 .06) <br /> Definition <br /> Ow A temporary sediment barrier consisting of a filter fabric stretched across and attached to <br /> supporting posts and: entrenched . There are two types . . The silt fence is a temporary linear <br /> filter barrier constructed of synthetic filter fabric, posts , and , depending upon the strength of <br /> the fabric used , wire fence for support. The filter barrier is constructed of stakes and <br /> burlap or synthetic filter fabric. <br /> r <br /> Purposes <br /> ,r, 1 . To intercept and detain small* amounts of sediment from disturbed areas during <br /> construction operations. <br /> 2 . To decrease the velocity of sheet flows and low-to-moderate level channel flows. <br /> Conditions When Practice Applies <br /> • <br /> 1 . Below disturbed areas where erosion would occur in the formof sheet and rill . <br /> erosion . <br /> ` 2. Where the size of the drainage area is no more than 114 acre per 100 feet (1 .3 ha <br /> 11.00 m) of silt fence length ; the maximum slope length behind the barrier is 100 <br /> w <br /> feet (30 m) ; and the maximum gradient behind the barrier is 50 percent (2: 1 ) . <br /> 3 . In minor swales or ditch lines where the maximum contributing drainage area is no <br /> greater than. 2 acres (0 . 8 ha) . <br /> 4. under no circumstances should silt fences be constructed in live streams or in <br /> swales or ditch lines where flows are likely to exceed one cubic foot per second <br /> (crs)(0 .03 M31 sec.) . See Design Criteria for further clarification . <br /> � Planning Considerations <br /> Silt fences can trap a much higher percentage of suspended sediments than can straw <br /> bales and may be preferable to straw barriers in many cases. While the failure rate of silt <br /> fences is lower than that of straw, barriers, this failure rate is still due mainly to improper <br /> installation . The most effective application is to install two parallel silt fences spaced a <br /> minimum of three feet apart. The installation and maintenance methods outlined here can <br /> improve performance . <br /> .. Filter barriers are inexpensive structures composed of burlap or standard weight synthetic <br /> filter fabric stapled to wooden stakes. Flow rates through burlap filter barriers are slightly <br /> slower and filtering efficiency is significantly higher than for straw bale barriers. <br /> w <br /> 4-21 <br />