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Chapter 4-Site-Specific /ndustriotorm Water BMPs <br />Filter Fence ........... <br />What Is It <br />A silt fence, also called a "filter fence," is a temporary measure for sedimentation control. It <br />usually consists of posts with filter fabric stretched across the posts and sometimes with a wire <br />support fence. The lower edge of the fence is vertically trenched and covered by backfill. A silt <br />fence is used in small drainage-areas to detain sediment. These fences are most effective where <br />there is overland flow (runoff that flows over the surface of the ground as a thin, even layer) or in <br />minor swales or drainageways. They prevent sediment from entering receiving waters. Silt fences <br />are also used to catch wind blown sand and to create an anchor for sand dune creation. Aside <br />from the traditional wooden post and filter fabric method, there are several variations of silt fence <br />installation including silt fence which can be purchased with pockets presewn to accept use of <br />steel fence posts. <br />Extension of fabric and wire <br />into the trench <br />Filter fabric k. <br />_ <br />- ti <br />I wire <br />Ill ????lil <br />=nll_ II, ?=j <br />FIGURE 4.20 FILTER FENCE DETAILS <br />(Modified from State of North Carolina, 1988; <br />and State of Wisconsin, 1988) <br />E <br />I When and Where to Use It <br />A silt fence should be installed prior to major soil disturbance in the drainage area. Such a <br />structure is only appropriate for drainage areas of 1 acre or less with velocities of 0.5 cfs or less <br />(Washington State, 1992). The fence should be placed across the bottom of a slope or minor <br />drainageway along a line of uniform elevation (perpendicular to the direction of flow). It can be <br />used at the outer boundary of the work area. However, the fence does not have to surround the <br />4-76