2002-06-26 CC Packet - Laserfiche WebLink

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. Sediment~CtQntrol -Sill: f1"ences 1~M,uirements General Planning and Siting In operation, the fence generally becomes clogged with fine particles, which in turn reduces the flow rate. This causes a pond to develop more quickly behind the fence. The designer should anticipate ponding and provide sufficient storage areas and overflow outlets to prevent flows from overtopping the fence. Since silt fences are not designed to withstand high standing water, locate them so that only shallow pools can form. Tie the ends of a silt fence into the landscape to prevent flow around the end of the fence before the pool reaches design level. Often a crescent shape will perform better than the traditional straight line. Provide stabilized outlets to protect the fence system and release storm flows that exceed the design storm. Deposition occurs as the storage pool forms behind the fence. The designer can direct flows to specified deposi- tion areas through appropriate positioning of the fence or by providing an excavated area behind the fence. Plan deposition areas at accessible points to facilitate routine cleanout and maintenance. Show deposition areas in the erosion and sedimentation control plan. A silt fence acts as a diversion if placed slightly off the contour. This may be used by the designer to control shallow, uniform flows from small, disturbed areas and to deliver sediment-laden water to deposition areas. Silt fences serve no function along ridges or near drainage divides where there is little movement of water. Confin- ing or diverting runoff unnecessarily with a silt fence may create erosion and sedimentation problems that would not otherwise occur, as well as add costs. . Install silt fences on the contour (as opposed to up and down a hill) and construct so that flow cannot bypass the ends. Ensure that the drainage area is no greater than 1/4 acre per 100 feet of fence. . Make the fence stable for the IO-year peak storm runoff. By design, ensure that the depth of impounded water does not exceed 2 feet at any point along the fence. Types The following three types of silt fences are used for different circumstances. For details on each type of fence, see Table 1. . Heavy Duty. Use at locations where extra strength is required, such as near water bodies; on areas with unstable wetland soils, steep slopes, highly erodible soils or high runoff; and on areas that are inaccessible to equipment. Preassembled. For light-duty applications, to protect temporary construction or to supplement the other types of silt fence. This type is installed with plow-type equipment with preattached stakes spread at 6- to 8-foot intervals. Machine-Sliced Installation: Appropriate for general use during site grading and to protect critical areas. Preferred for most sites due to the effective installation method. 3-106 Minnesota Urban Small Sites 8M? Manual