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• • • • % • I x>«* <br />.if <br />i/fieri <br />./ .3>^ <br />y ^ 1 <br />- <br />-r*'i:—%..• * ■..* • • •» <br />*-‘r, /.' -T^V'"" • V f <br />v<i'■'*^^ , • * • * <br />’ 1 • / •. <br />'^S.V :>?.■•:''** <br />/yv <br /><5.3 SILT FENCE <br />DESCRIPTION AND PURPOSE <br />A silt fence is a temporaiy sediment bairicr <br />consisting of a inter fabric which is attached to <br />supporting posts trenched into the ground. SedLment- <br />laden runoff ponds uphill from the silt fence and <br />runoff is filtered as it passes through the fabric. <br />Silt fences are Intended to intercept and detain small <br />amounts of sediment from disturbed areas in order to <br />prevent sediment from leaving the site. Silt fences <br />can also prevent sheet erosion by decreasing the <br />velocity of runoff. <br />EFFECTIVENESS <br />The cfTccdvcntss of silt fences for trapping sediment <br />is mainly a function of the apparent opening size <br />(AOS) of the fabric. The AOS relates opening sizes <br />to those of standard sieves. <br />As the AOS value (and sieve number) gets larger, ihc <br />opening size decreases. The AOS of a filter fabric <br />should be large enough (openings small enough) to <br />effectively trap sediment However, it should also be <br />small enough to maintain an acceptable flow rare. <br />For most soils, a fabric with an AOS of 70 wiU trap <br />more than 90 percent of the sediment in runoff. <br />PLANNING CONSIDERATIONS <br />Silt fences should be used where; (1) the drainage <br />area is less than 2 acres. (2) the maximum <br />uncontrolled slope length above the silt fence is 150 <br />feet and (3) water reaches the silt fence as sheet <br />flow. StU fences usually should not be used in <br />chaiuids, waterways or other concentrated flow <br />paths. <br />Sill fences have a low pcrmeabUiiy to enhance <br />sediment trapping. This will create ponding behind <br />the fence, so they should be not be located where <br />ponding will cause property damage or a safety <br />haz^ The expect^ life of silt fences is generally <br />limited to 6 moruhs. <br />DESIGN RECOMMENDATIONS <br />1. Silt fences should be irrstalJcd on the contour <br />(vs. up and down a hill) and corrstrucied so <br />that flow catmoi bypass the ends. <br />* • <br />2. If the silt fence is longer than 600 feet, it <br />^uld be constructed in separate <br />independent units, with each unit having a <br />length less than 600 feet. <br />10/89 ©. < <br />>■ <br />6.3-1