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• reducing peak flows and trapping sediments. Care should be taken at the inlet to ensure a gradual or moderate slope into the pool. Creating a steep angled drop into the pool may cause headcutting upstream and sloughing or accelerated erosion, which can significantly decrease the holding capacity of the structure. In locations where or during periods of time when erosion rates and sedimentation are high, sediment traps must be maintained frequently. Use, placement, functional life expectancy, and ability to perform maintenance should be considered when using sediment traps. For small impoundments where human safety and health risks are not of concern, generic designs that are modified to take into account local site conditions may be utilized for non-critical structures. 2.1.3.4 Sedimentation Basins A professional engineer familiar with sedimentation ponds will be utilized in site specific designs for sedimentation basins or other large impoundments that present risks to the environment or to public health and safety or that are used for final water treatment purposes. A professional engineer certification is normally required by regulation for sedimentation basin and other large impoundment designs. Sedimentation basins primarily serve to trap sediments. Secondarily they reduce or abate water pollution by providing a collection structure for the storage of chemical contaminants, debris, and other processing wastes. The designs of dams, spillways, and drainage facilities should be in accordance with standard engineering principles appropriate for the class and kind of structure being considered. Dewatering devices should be incorporated where permanent pools are unacceptable, or where adequate containment volumes for runoff must be provided. The ability of an impoundment to accomplish runoff treatment goals is directly related to basin specifications and construction procedures. The efficiency and life of an impoundment are directly related to the scale of up-gradient disturbance activities and to the types of erosion and runoff control measures being employed. Therefore, basin design should be performed on a site-specific basis taking into account local environmental factors. A designed catchment basin or impoundment may be incised, formed by an above- ground embankment, or formed by some combination thereof (Figure D-4). Typical sedimentation basins are 5 to 20 acre-feet in capacity. Sedimentation basins are the preferred treatment method for areas that remain disturbed for extended periods of time. Typically they are used to treat runoff from facilities areas, but they may be used to treat runoff from any type of disturbance area. However, because of their size, construction costs, and the additional disturbance area associated with their construction, sedimentation basins are normally impractical for use in treatment of small areas. Preliminary Response Plan A-l3 Yankee Gulch Sodmm Minerals Project American Soda, L.L.P.