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Silt fence may be used for areas where sediment control is needed for inU.rmediate periods of <br />time (i.e., more than a year but not permanenfly). Silt fences will be placed on the down <br />gradient side of the disturbed area (Figure D-1 and Figure D-2). To the extent possible, silt <br />fence will be installed on the contour. However, silt fences ate ideal for installation along <br />perimeters that vary in contour when flow and sediment volumes are low. Care must be taken <br />to avoid directing any excessive amounts of flow to low points which may overload fence <br />capacrity and result in fence failure. Support systems for silt fence will be adequate to <br />withstand Ure anticipated flow volumes and sediment loads anticipated for the installation area. <br />Sediment fence will be UV resistant with fabric-tensioning strips. The bobtom of the fabric will <br />be keyed into the ground according to standard installation procedures. Silt fences will not be <br />used where excessively rocky soils prevent their proper mstallation. Silt fences will be <br />routinely inspected and maiotained following significant runoff events. The cost of <br />maintainmg silt fence vs. the cost of implementing mote costly control ma~sures will be taken <br />into consideration when determining which control measure to use. Silt fence may also be <br />used as a substitute for straw or hay bale check dams, since its effective life is usually longer. <br />Sediment Traps <br />Sediment traps are small incised dugouts or depressions that may be phtceri inteamittenUy along <br />or at the outlet of a water course or erosion feature (Figure D-3). They are effective in ponding <br />surface water, increasing the time of concentration, reducing peak flows and trapping <br />sediments. Care should be taken at the inlet to ensure a gradual or moderate slope into the <br />pool. Creating a steep angled drop into the pool may cause headc~utting upstream and <br />sloughing or accelerated erosion which can significantly decrease the hollding capactity of the <br />structure. In locations where or during periods of time when erosion rates and sedimentation <br />are high, sediment traps must be maintained frequently. Use, placeiment, functional life <br />expectancy and ability to perform maintenance should be considered Wfien using sediment <br />traps. <br />For small impoundments where human safety and health risks are not of concern, generic <br />designs that are modified to take into account local site conditions may be utilized for non- <br />critical structures. <br />Sedimentation Basins <br />A number of good resources are available for the design, sizing and constivction procedtues of <br />sedimentation basins, including SEDCAD University of Kentucky at Lexington (SIDCAD <br />4.0, 1998), the SCS National Engineering Handbook Section 19 (SCS, 1985), Faith Manual <br />(Department of the Interior, Bureau of Rechunation, 1974), and the SCS Engineering Field <br />Mamral (SCS 1984). A Qrofessional engineer familiar with sedimen~lation ponds will be <br />utilized in site specrfic destgms for sedimentation basins or other largt: impoundments that <br />present risks to the environment or to public health and safety or that ar: used for final water <br />treatmem purposes. A professional engineers certification is normally rrxluired by regulation <br />for sedimentation basin and other large impoundment designs. <br />Sedimentation basins primarily serve to trap sediments. Secondarily they reduce or abate water <br />pollution by providing a collection structure for the storage of chemical contaminants, debris <br />and other processing wastes. The designs of dams, spillways and drairtaf;e facilities should be <br />in accordance with standard engineering principles appropriate for th. class and kind of <br />structure being considered. Dewatering devices should be incorporated where perrttanent pools <br />are unacceptable or where adequate containment volumes for runoff must be provided. <br />L~2 <br />