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r Chapter 4 - Best Management Practices for Erosion and Sediment Control i Silt fences composed of a wire support fence and an attached synthetic filter fabric slow the flow rate significantly but have a higher filtering efficiency than burlap . Both woven and non-woven synthetic fabrics are commercially available . The woven fabrics generally i display higher strength than the non-woven fabrics . When tested under acid and alkaline water conditions , most of the woven fabrics increase in strength . There are a variety of reactions among the non-woven fabrics . The same is true of testing under extensive i ultraviolet radiation . Permeability rates vary regardless of fabric type . While all of the fabrics demonstrate very high filtering efficiencies for sandy sediments , there is considerable variation among both woven and non-woven fabrics when filtering the finer silt i and clay particles . Design Criteria i 1 . No formal design is required for many small projects and for minor and incidental applications . For channel flow applications refer to FDOT Standard Index 102 , i Chart 1 (Plate 4. 066) for guidance on recommended spacing . 2 . Filter barriers shall have an expected usable life of 3 months . They are applicable in ditch lines , around drop inlets, and at temporary locations where continuous construction changes the earth contour and runoff characteristics and where low or moderate flows (not exceeding 1 cfs) (0 .03 m3 / sec.) are expected . 3 . Silt fences, because they have much lower permeability than burlap filter barriers, have their applicability limited to situations in which only sheet or overland flows are expected . They normally cannot filter the volumes of water generated by channel flows , and many fabrics do not have sufficient structural strength to support the weight of water ponded behind the fence line. Their expected usable life is 6 r months . Construction Specifications i Materials 1 . Synthetic filter fabric shall be a pervious sheet of propylene , nylon , polyester, or polyethylene yarn . Synthetic filter fabric shall contain ultraviolet ray inhibitors and stabilizers to provide a minimum of 6 months of expected usable construction life r at a temperature range of 0° F to 120° F (- 17C to 49C) . 2 . Burlap shall be 10 ounces per square yard (340 g/m2) fabric. i 3 . Posts for silt fences shall be either 4 inch ( 10 cm) diameter wood , or 1 . 33 pounds per linear foot (2 kg/m) steel with a minimum length of 5 feet ( 1 . 5 m) . Steel posts shall have projections for fastening wire to them . 4 . Stakes for filter barriers shall be 1 " x 2" (2 . 5 x 5 cm) wood (preferred) , or equivalent metal with a minimum length of 3 feet (90 cm). 4-23