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Attachment 13-12, Drainage and Sediment Control Plan, presents all input and output information. The <br />design parameters are minimum requirements: the postmining channels are generally conservatively over - <br />constructed (e.g. wider bottom width, larger rock riprap). 13-12.3 (three sheets) summarizes the minimum <br />postmining channel design requirements. The postmining channel profiles are shown on Exhibit 20-2B, <br />Postmining Channel Profiles. <br />After the watershed has been graded and the location of the drainage channel determined, the channel is <br />constructed to the required shape. All drainages are inspected for stability according to the Rill and <br />Gully Plan in Tab 22, Revegetation Plan. Maintenance and monitoring that may be necessary as a result of <br />the inspections are submitted to the Division for review and, if necessary, approved in accordance with <br />the Rill and Gully Plan. The sideslopes of the meanders of the postmine drainage channels are protected <br />from erosion by revegetation with the drainage seed mix and shrub plantings according to Tab 22, <br />Revegetation Plan. <br />Runoff during the spring of 2003 caused significant erosion of approximately 2,450 feet of the natural <br />channel that connects the eastern edge of the North Pit and Pond 011. In order to remediate the damage, <br />SCC will reconstruct the channel (YPM-12) using a rubber -tired backhoe and other small equipment to <br />minimize additional disturbance. In general, the existing channel will be re -configured to have stable <br />sideslopes, a 2 -foot wide bottom, and a minimum of 0.3 feet of freeboard. <br />Re construction activities will require temporary disturbance of the land surface along the channel bank <br />to allow access and construction. It is planned that the topsoil alongside the existing channel will be <br />stripped back far enough to allow the channel sideslopes to be reconfigured and stabilized. Once a <br />section of the channel is reconstructed, the topsoil will be re -spread and the backhoe will proceed <br />working on the next section of channel. Equipment access will be via an existing road/trail that <br />currently exists along the drainage bottom. All disturbed areas, including the reconstructed channel, <br />will be resseded using the approved drainage seedmix. <br />As shown on Exhibit 20-2B, Postmining Channel Profiles, grades along the length of Channel YPM-12 range <br />from around 9 percent to about 25 percent. The predicted flow in the reconstructed channel (after <br />construction of Pond 011A) will be aprroximately six cfs, with velocities ranging from 4.5 to 5.8 peet per <br />second on grades from 9 to 18 percent. The SEDCAD evaluation indicates that channel reaches with a bed <br />slope less than 18 percent will be stable as vegetated channels; reaches over 18 percent will be vegetated <br />and will have wood or rock check dams installed in <br />TR -37 10 Revised 02/08 <br />