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C> <br />fi 2. where the size of the crainage area is no more than 1/4 acre per 100 <br />fj feet of silt fence length; the maximum slope length behind the barrier <br />is 100 feet; and the maximum gradient behind the barrier is 50 percent <br />(2:1). <br />' 3. In minor swales or ditch lines where the maximum contributing drainage <br />area is no greater than 2 acres.. <br />4. Under no circumstances should silt fences be constructed in live <br />streams or in scales or ditch lines where flows are likely to exceed <br />1 cubic foot per second (cfs). See Design Criteria for further <br />clarification. <br />Planning Considerations <br />G Laboratory work at the Virginia Highway and Transportation Research Council <br />(VH & TRC) has shown that silt fences can trap a much higher percentage of <br />suspended sediments than can straw bales. Silt fences may be preferable to <br />straw barriers in many cases. While the failure rate of si-It fences is lower <br />than that of straw barriers, there have been instances in which silt fences <br />were improperly installed. The installation methods outlined here can <br />improve performance. <br />Filter barriers are inexpensive structures composed of burlap or standard <br />weight synthetic filter fabric stapled to wooden stakes. Flow rates through <br />burlap filter barriers are slightly slower and filtering efficiency is <br />significantly higher than for straw bale barriers (see Table 1.06a). <br />Table 1.06a <br />�9 FLiOW RATES AND FILTERING EFFICIENCIES OF <br />VARIOUS SEDIMENT FILTER MATERIALS <br />Material Flow Rate (gal./sq. f� t fminv3 Filter Efficiency () <br />traw <br />Burlap (10 oz. fabric) 2.4 84 <br />Synthetic Fabric 0.3 (Avg.) 97 (Avg.) <br />Source: Va. Highway and Transportation Research Council <br />Silt fences composed of a wire support fence and an attached synthetic <br />i ter a ric slow the flow rate significantly but have a higher filtering <br />efficiency than burlap. Both woven and non -woven synthetic fabrics are <br />commercially available. The woven fabrics generally display higher strength <br />than the non -woven fabrics. when tested under acid and alkaline water <br />conditions, most of the woven fabrics increase in strength. There is a <br />variety of reactions among the non -woven fabrics. The same is true of <br />testing under extensive ultraviolet radiation. permeability rates vary <br />reoardless of fabric type. while all of the fabrics demonstrate very high <br />r <br />6- 316 <br />