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EXHIBIT 7, ITEM 20, PART D <br />TEMPORARY CHANNEL DESIGN <br />For the three sediment ponds associated with the South Taylor Mine Plan, two temporary ditches <br />will be needed. These are the Section 28 Collector Ditch and the Section 29 Collector Ditch as <br />presented on Map 41 A. These ditches have an overall length of about 3,174 feet and 2,624 feet <br />respectively. Section 29 Collector Ditch will have an elevation change of less than 50 feet with a <br />maximum slope of 2.0 percent. Section 28 Collector Ditch will have an elevation change of <br />approximately 134 feet, with an average slope of 6.3 percent. <br />Design and construction of these ditches will be similar to the existing side channel ditches <br />which are currently in place at the Colowyo Mine. As described in Exhibit 7, Item 20, Part A, <br />the design storm generated a peak flow of 37 cfs. Although not all of this flow would report to <br />the side collector ditch, this peak flow is the basis for the design. Given the size of the drainage <br />area involved, the 10 year, 24 hour event as described by Rule 4.05.3 is the appropriate design <br />basis. <br />These structures will be designed to be trapezoidal in cross section with a 12 foot bottom width <br />and 3H:1 V sideslopes and a total depth of not less than 4.0 feet. <br />Those portions of the ditch alignment not constructed in competent rock shall be designed with <br />gradual slopes to conduct the peak flow under non-erosive (less than 5.0 feet per second) <br />conditions. Based on the design flow shown above and the dimensions provided, and assuming a <br />Manning's roughness of 0.035, this means a channel slope of less than 2.0 feet per 100 feet. <br />Under these conditions, no erosion protection is needed for Section 29 Collector Ditch; however, <br />due to varying conditions for Section 28 Collector Ditch, it will have erosion protection installed <br />as needed along the steep portions of the ditch. <br />~ ~a7 ~o~ <br />Exh. 7-20~-1 Revision Date: 3/29/07 <br />Revision No.: PR-02 <br />?2 75 <br />