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IN <br />practices 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 />1,1.00 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 />