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<br />revegetated, 30-70 percent cover, and hydrologic soil <br />Group B. The difference between the original SCC model <br />and the model estimated by the Division was that SCC <br />used a curve number of 61. The assumption was that the <br />watershed would be reclaimed, have 70 to 100 percent <br />cover, and would fall under the category of held for <br />release. Seneca concurs with the Division that for <br />design purposes, the 30 to 70 percent cover factor <br />would be more realistic, especially for the first few <br />years after construction of the reclaimed channel is <br />completed. <br />71. SCC has decided to use a riprap channel for the <br />postmining drainage channel as opposed to the original <br />vegetated channel design. Most of the reason for this <br />change was the increase in peak discharge from 4.77 cfs <br />in the original design to a flow of 21.23 cfs now <br />because of using the higher curve number. The <br />Simons/OSM method for steep slope channel design was <br />selected as opposed to using the PADER method for the <br />riprap channel. Additional input for the riprap <br />channel included using a bottom width of 12 feet with <br />5:1 side slopes, and a slope of 13.80 percent which was <br />provided in Table 20-3. <br />The SEDCAD+ results calculated a D50 rock riprap size <br />of 6.00 inches. From information provided in the <br />SEDCAD+ manual on Page 6-14, the depth of the rock is <br />the channel and the largest rock size to be used is <br />1.25 times the D50 size calculated. The riprap to be <br />placed in the channel will consist of 3 to 8 inch rock <br />and be laid in at a thickness of 9 inches. For further <br />protection of the channel bed, a layer of Mirafi 70oX <br />geotextile fabric will be installed in the entire <br />drainage channel prior to placement of the rock riprap. <br />A copy of the Mirafi 700X brochure which includes <br />product specifications, is included in Attachment 20-1 <br />at the end of the channel design information. <br />One final statement should be made that the velocity <br />value output in the Simons/OSM method of 9.89 fps is <br />simply based on Q=VA and it may not be applicable for <br />very shallow flows (0.17 ft). The SEDCAD+ manual <br />states this fact on Page 6-13. The reason for this is <br />that uniform flow does not exist, Manning's <br />relationship is not valid, and an estimate of the <br />actual velocity cannot be easily determined. <br />Tab 22 <br />77b. The typographical error in the student's t formula <br />(Page 22-1-1) has been corrected and a revised page is <br />included. New index tabs are also included for <br />Appendices 22-1 and 22-2. <br />7 <br />