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However, Trapper does not expect to use the same treatments throughout the length or reach of a <br />drainageway. Since the overall goal is to establish a defined but stable drainageway, treatment <br />applications will be site-specific. For example, in the lower reaches of the main channel where flows are <br />the greatest, it may be necessary to use extensive and numerous combinations of materials and methods <br />to obtain temporary stability while perennial vegetation becomes established. However, in the highest <br />reaches of the drainages where flows are low, little or no treatment may be necessary to achieve <br />vegetation establishment. <br />Check Dams. The majority of reestablished drainages at Trapper Mine utilize check dams. Small check <br />dams are incorporated to reduce channel erosion by directing channel flow while dissipating flow energy <br />and controlling channel depth, thus reducing the impact of melting snow and heavy rains. <br />Check dams are built from coarse riprap obtained from competent rock selected from the mine area. <br />Where possible, the rock should be angular, well graded, and properly placed to minimize excess voids. <br />Check dams are typically two to three feet in height, with a 1.5H:1 V upstream face and a downstream face <br />of 3H:1 V or flatter. Materials such as brush, straw bales and synthetic fiber are sometimes used to assist <br />in check dam construction. Rock placement is such that the dam structures are porous, allowing water to <br />flow through — dissipating water velocity while temporarily retaining runoff. The stronger the flow velocity <br />the larger the rock used in check dams. <br />Check dams are "keyed" into the sides of the reconstructed drainage. Lateral trenches are dug into the <br />sides of the drainage so as to prevent water flow from gouging soil out from the sides of the dams. Check <br />dams are also shaped such that water is channeled toward the center of the dam to prevent water flowing <br />around the edges. A flatter profile is created on the downstream side of check dams to reduce erosion <br />potential along the downstream edge. Check dams extend below the soil surface a minimum of 2.5 feet to <br />prevent water from undercutting the check dam structure (Figure 4.8-1a). Similarly constructed deflector <br />dams are sometimes used in specific situations (Figure 4.8-1b). Deflector dams may be utilized as <br />additional stability structures to control lateral erosional features or to enhance channel sinuosity as <br />needed. <br />Check dams are built in sequence (Figure 4.8-1 a). As a general rule the distance between check dams <br />will decrease as the slope of the drainage increases. The frequency of check dams is driven by the <br />gradient of the drainage channel. <br />Check dams are designed to create stable reclaimed drainages once vegetation is well established in the <br />drainage and on associated reclaimed hill slopes. As vegetation is established, typically within about three <br />years, the drainages take on a natural appearance, unlike totally rip -rapped channels that will never look <br />natural in the reclaimed landscape. The combined effect of incorporating check dams into an overall <br />watershed sediment and erosion control program that also includes the use of contour ditches, small <br />stock ponds, dozer basins and downstream sediment ponds creates a stable watershed that ameliorates <br />runoff impacts until permanent vegetation is firmly reestablished. <br />4-183c <br />ApR 01 2015 <br />Revision: /OR _o <br />Approved: <br />