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• • • • * • a <br />** •••’•!’ .r ."‘ •" • .T *!T* <br />*;*‘4’*•*•'•. -''•C .v^*’ *' * • ••■ <br />• • • I ^ ’• . • ••. <br />• * » * * “T *• ••* • • *. < • •. <br />• • . • • • < •*•' * *' . <br /><• . “ • .•••-.•.• ■•• . * <br />• •*. .^' • % .• • *• % • - r <br />-.••I.- •. - V>' •, -“T: >-'5 UjT: <br />6.3 SILT FENCE <br />DESCRimON AND PURPOSE <br />A silt fence is a temponiy sediment barrier <br />consisting of a IQter fabric which is attached to <br />supporting posts trenched into the ground. Sediment* <br />laden runoff ponds uphill from the silt fence and <br />runoff is filtered as it passes through the fabric. <br />Silt fences are iruended to intercept and detain small <br />amounts of sediment fimm distuit^ areas in order to <br />prevent sediment from leaving the site. Siltfences <br />can also prevent sheet erosion by decreasing the <br />vdodiy of runoff. <br />EFFECTIVENESS <br />The effectiveness 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 stairdard sieves. <br />As the AOS value (and sieve number) gets larger, the <br />opeiung size decreases. The AOS of a fUter fabric <br />should be large enough (opciungs small enough) to <br />effectively trap sedtmcnL However, it should also be <br />small enough to maintain an acceptable flow rate. <br />For most soils, a fabric with an AOS of 70 will trap <br />more than 90 percent of the sediment in runoff. <br />PLANNING CONSIDERATIONS <br />Silt fences should be used where: (I) the drainage <br />area is less than 2 acres. (2) the maximum <br />unoonoolled slope length above the silt fence is ISO <br />feet, arrd (3) water reaches the silt fence as sheet <br />flow. Silt fences usually should not be used in <br />channels, waterways or other concentrated flow <br />paths. <br />Silt fences have a low pcnneabOiiy 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 />hazard. The expect^ life of silt fences is generally <br />limited to 6 months. <br />DESIGN RECOMMENDATIONS <br />1. Silt fences should be installed on the contour <br />(vs. up and down a hill) and constructed so <br />that flow cannot bypass the ends. > <br />2. If the sill fence is longer than 600 feet, it <br />should be constructed in separate <br />indqrendeni units, with each unit having a <br />length less than 600 feet. <br />10/89 •••6.3-1