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Consideration for filter barrier(filter/silt fences) <br /> No formal design of filter barrier is required. However, in order to assure that <br /> the barriers will function properly and that excessive flows do not develop <br /> which could damage the barrier,the following are general "rules of thumb" <br /> that should be considered. <br /> 1) Drainage area — the size of the drainage area should be no more <br /> than one-quarter acre per 100 feet of barrier length. <br /> 2) Slope length —the maximum slope length behind the barrier should <br /> be no more than 100 feet. <br /> 3) Gradient — the maximum gradient behind the barrier should be <br /> no greater than 50 percent (2:1) <br /> 4) Materials — sheets of burlap or synthetic filter fabric, such as <br /> propylene, nylon, polyester, or ethylene yarn, can be used. <br /> If synthetic materials are to be used, they should contain ultraviolet ray <br /> inhibitors and stabilizers to provide a minimum of six months of expected <br /> usable life at a temperature range of 0 degree (F) to 120 degrees (F) <br /> The burlap or filter should be cut to the length of the barrier to avoid the use of <br /> joints. When joints are necessary, splice the fabric only at the support post, with <br /> a minimum six inch overlap. <br /> 5) Posts —posts may be either wood or metal. <br /> 6) Spacing — post spacing will depend on filter fence materials used <br /> and conditions encountered. <br /> 7) Trench — a trench should be excavated along the line of posts <br /> and upslope from the barrier. The trench should be a minimum of <br /> four inches wide and four inches deep. The trench should be <br /> backfilled and compacted once the barrier is in place. <br />