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Chapter 4 - Best Management Practices for Erosion and Sediment Control <br /> 4 . 06 SILT FENCE <br /> ( ES BMP 1 .06) <br /> Definition <br /> 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 /> Purposes <br /> 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 /> 1 . Below disturbed areas where erosion would occur in the form of sheet and rill <br /> erosion . <br /> 2 . Where the size of the drainage area is no more than 1 /4 acre per 100 feet (1 . 3 ha <br /> 1100 m) of silt fence length ; the maximum slope length behind the barrier is 100 <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 /> (cfs) (0 . 03 m3 / 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 orstandard 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 /> 4-21 <br />