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the coarse riprap. This presents water from undercutting the structure. Check dam wings <br />should extend upward to tie into the surrounding sideslopes. <br />Part 1 <br />Determining Maximum Channel Flow Rate without Erosion Protection <br />The first step in this analysis is to determine the maximum allowable flow rate for a variety <br />of representative grass -lined trapezoidal channels without check dams or other erosion <br />protection. <br />Using SEDCAD, several channel slopes and dimensions were modeled to determine the <br />maximum allowable flow rate in a given grass -lined channel. By gradually increasing the <br />inputs such as curve number and drainage area, the flow rate for a given channel was <br />increased until the flow velocity became higher than allowable. For vegetated channels, <br />SEDCAD allows a variety of vegetative covers to be modeled. All channels were <br />assumed to be grass -lined with a vegetative cover consisting of smooth brome, a <br />perennial sod forming grass commonly found on reclaimed lands. <br />Figure 1 summarizes the results of this initial effort. Trapper Mine typically uses a <br />trapezoidal channel with a bottom width of 20 feet. Note that for channel slopes of less <br />than 5.0 percent, SEDCAD allows a maximum flow velocity of 7.0 feet per second. <br />Similarly, for channel slopes of between 5 and 10 percent, SEDCAD allows a flow velocity <br />of 6.0 feet per second. Finally, for channel slopes in excess of 10 percent, SEDCAD <br />limits the maximum flow velocity to 5.0 feet per second. These velocity limits account for <br />the discontinuities in Figure 1 between 5.0 and 5.5 percent channel slopes and again <br />between 10.0 and 10.5 percent channel slopes. <br />This portion of the SEDCAD modeling is essentially a simple solution to the Manning <br />equation, where frictional forces as a result of low flow depths through the vegetation in <br />a relatively wide channel overcome the energy gained by changes in elevation. Using <br />SEDCAD allows for a further refinement, where a specified maximum flow velocity for <br />each slope condition is specified. Table 1 presents the same information in tabular <br />format. The individual one page SEDCAD results are presented in Appendix G. <br />Table 1, Summary of permissible flow rates in grass -lined drainages based on SEDCAD <br />modeling for trapezoidal channels of various widths and slopes, cfs <br />