2008-12-04_PERMIT FILE - C1994082A (12)

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• Attachment 13-12, Drainage and Sediment Control Plan, presents all input and output information. The design parameters are minimum requirements; the postmine channels are generally conservatively over-constructed (e.g., wider bottom width, larger rock riprap). Figure 13-12.3 (three sheets) summarizes the minimum postmining channel design requirements. The postmining channel profiles are shown on Exhibit 20-28, Posimining Channel Profiles. After the watershed has been graded and the location of the drainage channel determined, the channel is constructed to the required shape. All drainages are inspected for stability according to the rill and gully plan in Tab 22, Revegetation Plan. Maintenance or monitoring that may be necessary as a result of the inspections are submitted to the Division for review and, if necessary, approval in accoMance with the Rill and Gully Plan. The side slopes of the meanders of the postmine drainage channels are protected from erosion by revegetation with drainage seed mix and shrub plantings according to Tab 22, Revegetation Plan. Runoff during the spring of 2003 caused significant erosion of approximately 2,450 feet of natural channel that connects the eastern edge of the North Pit and Pond 011. In order to remediate the damage, SCC will reconstruct the channel (YPM-12) using arubber-tired backhoe and other small equipment to minimize additional disturbance. In general, the existing channel will be re-configured to have stable side slopes, a • 2-foot wide bottom, and a minimum of 0.3 feet of freeboard. Re-construction activities will require temporary disturbance of the land surface along the channel bank to allow access and construction. It is planned that the topsoil along side the existing channel will be stripped back far enough to allow the channel side slopes to be reconfigured and stabilized. Once a section of channel is reconstructed the topsoil will be re-spread and the backhoe will proceed working on the next section of channel. Equipment access will be via an existing road/trail that currently exists along the drainage bottom. All disturbed areas, including the reconstructed channel, will be re-seeded using the approved drainage seedmix. As shown on Exhibit 20-2B, Postmining Channel Profiles, grades along the length of Channel YPM-12 range from around 9 percent to about 25 percent. The predicted flow in the reconstructed channel (after construction of Pond 011A) will be approximately six (8) cubic feet per second (cfs) with velocities ranging from 4.5 to 5.8 feet per second (fps) on grades from 9 to 18 percent. The SEDCAD4 evaluation indicates that channel reaches with a bed slope less than 18 percent will be stable as a vegetated channel; reaches over 18 percent will be vegetated and have wood or rock check dams installed in • TR-37 10 Revised 02/08