2025-05-05_PERMIT FILE - C1981010 (23)

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