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submitted a design for the gully repair as part of TR-36, which was subsequently approved by <br />• Colorado Division of Mining and Geology (now known as Colorado Division of Reclamation, <br />Mining and Safety). <br />Recent observations indicate that the meandering channel design and construction (TR-36 design) <br />has not totally stabilized the gullied area. Consequently, SCC has prepared a new design for the <br />upper PM-8 area (the TR-41 design). The two meandering flow paths have been removed and <br />replaced with V-ditches constructed across contour. The spacing of the V-ditches limits down-slope <br />flow distances, which should reduce velocities and erosive forces. The V-ditches will then convey <br />the collected run-off to Channel SIIPM-8. <br />The locations of the V-ditches are shown on Exhibit 7-19, Postmining Drainage Control, as well as <br />Exhibit 13-2A, Postmining Topography and Permanent Features. The SEDCAD4 demonstration is <br />presented in Appendix A on pages 7-12-73 Through 7-12-84B. The V-ditch design is summarized <br />in the following sections. <br />Hydrology <br />The upper PM-8 area has been divided into five subwatersheds (see Exhibit 7-22). Drainage <br />areas range from 8.7 acres to 31.4 acres. Recent vegetation monitoring in the general vicinity <br />shows that average vegetative cover varies between 30% and 70%, which the SCS (1986) <br />indicates is a fair cover condition. The SCS suggests that a herbaceous type cover rated "fair", <br />• with a "B" soil hydrologic condition corresponds to a curve number of 71 (CN=71) and, <br />consequently, what SCC used in the V-ditch design. This curve number reflects a more <br />conservative design, which SCC believes is warranted in this area due to the recent erosional <br />forces (circa 2004 - 2006) that plague this area. <br />Reclaimed <br />Soil Type: B <br />Ground Cover Type: Herbaceous <br />Ground Cover: Fair <br />CN: 71 <br />V-ditch #I collects run-off from upper part of the watershed area. This ditch will be constructed <br />across contour at a gradient of approximately 5%. The contributing drainage area is <br />approximately 31.4 acres. The SEDCAD4 demonstration (see Appendix A, pages 7-12-73 <br />through 7-12-84B) estimates the 10-year, 24-hour storm event will generate a peak run-off of <br />2.2 cubic feet per second (cfs). <br />V-ditch #2 is a new ditch that traverses across contour at a gradient of approximately 5%. The <br />contributing drainage area is 27.3 acres. The SEDCAD4 demonstration estimates the 10-year, <br />24-hour storm event will generate a peak run-off of 1.7 cfs. <br /> <br />TR-51 7-12-10 Revised 09/09