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silt fence
A silt fence, sometimes (misleadingly) called a "filter fence," is a temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment (loose soil) in stormwater runoff. Silt fences are widely used on construction sites in North America and elsewhere, due to their low cost and simple design.〔Sprague, C.J. (1999). "Assuring the Effectiveness of Silt Fences and Other Sediment Barriers." Proceedings of Conference 30, International Erosion Control Association, Nashville, TN. pp. 133-154.〕 However, their effectiveness in controlling sediment can be limited, due to problems with poor installation, proper placement, and/or inadequate maintenance.
==Design and installation==
Silt fences are often perimeter controls, typically used in combination with sediment basins and sediment traps, as well as erosion controls, which are designed to retain sediment in place where soil is being disturbed by construction processes (i.e., land grading and other earthworks).〔Commonwealth of Virginia. Department of Conservation and Recreation (VA DCR). Richmond, VA. (“Virginia Erosion and Sediment Control Handbook: Principles, Practices and Costs.” ) Chapter 2. 1992.〕
A typical fence consists of a piece of synthetic filter fabric (also called a geotextile) stretched between a series of wooden or metal fence stakes along a horizontal contour level. The stakes are installed on the downhill side of the fence, and the bottom edge of the fabric can be trenched into the soil and backfilled on the uphill side, although it is quite difficult to move the trenched "spoil" from the downside to the upside of the trench. The design/placement of the silt fence should create a pooling of runoff, which then allows sedimentation to occur. Water can seep through the silt fence fabric, but the fabric often becomes "blocked off" with fine soil particles (all sediment-retention devices have this challenge, and none of them "filter" storm water for very long) . A few hours after a storm event, the fabric can be "disturbed" in order to dislodge the fines, and allow clean water to flow through. Depending on the protected watershed and erosion, larger soil particles will settle out, ultimately filling the silt fence to the top of the structure; requiring another silt fence above or below it (creating a new ponding area), or for the silt fence to be removed, the sediment removed or spread out, and a new fence installed. The fence is not designed to concentrate or channel stormwater. The fence is installed on a site before soil disturbance begins, and is placed down-slope from the disturbance area.〔VA DCR. (“Virginia Erosion and Sediment Control Handbook: Silt Fence.” ) Chapter 3 - Standard and Specification No. 3.05. 1992.〕〔U.S. Environmental Protection Agency (EPA), Washington, DC (2012). ("Silt Fences." ) ''Stormwater Best Management Practice.'' Document no. EPA 833-F-11-008.〕
Sediment is captured by silt fences primarily through ponding of water and settling, rather than filtration by the fabric. Sand and silt tends to clog the fabric, and then the sediments settle in the temporary pond. 〔 Document No. EPA-821-R-09-010.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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