2008-12-04_PERMIT FILE - C1994082A (4)

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February 1999 Yoart Mine * 13-12 Drainage and Sediment Control Plan •9 5.0 DITCH AND CULVERT DESIGN Haul Road Ditches Runoff from haul roads located within the mining disturbance area is routed to sedimentation ponds. The road-side ditches associated with these haul roads are designed to safely convey flow from the 10-year, 24- hour storm event. The steep slope sections of the roadside ditches may experience erosive velocities during the design storm event. Routine maintenance of the road and ditches will repair any erosion if and when it occurs. Armoring the ditches with large rock riprap is not practical from an operational standpoint. Material from the road is constantly being moved into the ditches from the movement of large haulage equipment. Therefore, the ditches need to be maintained continuously. Rock riprap placed in the ditches would last only a short time before it was covered with road material. As shown on Exhibits 13-11.3, Proposed Haul Boad B Extension and 13-11.4, Proposed Haul Roadr C d~ D, culverts will be installed at several points along the haul roads to limit watershed areas, and thereby, reduce peak flows and velocities. In addition, rock riprap will be placed below the culverts to control velocities and control erosion prior to the runoff reaching the down-gradient sedimentation ponds. In addition, water bar runouts may be constructed at select points along the haul roads to reduce flow velocities in the haul road ditches. All ditches associated with haul roads located outside the mine disturbance area (e.g., Haul Road A) will designed to safely convey flow from the 10-year, 24-hour storm event and limit erosive velocities. However, this runoff will not be routed to a sedimentation pond. Sediment control measures such as rock riprap and straw bales will be installed, as needed. Design inputs and results for proposed haul road ditches are shown on the SEDCAD4 printouts located in Appendix 13-12.2, Haul Road Ditch Design SEDCAD4 Results. A summary of the ditch designs is shown on Figure 13-12.1, Ditch Design Schedule. Diversion Ditches Three diversion ditches have been installed at the Yoast Mine. Ditch No. 2 conveys runoff into Pond 010. The ditch layout is shown on Exhibit 13-12.1A, Permanent Channel and Pond Watersheds. Ditch 13A conveys runoff to Pond 013, while Ditch 14A conveys runoff to Pond 014. Ditch layouts are shown on Exhibit 13-12.1, Yoast Mine Watersheds. The diversion ditches are designed to safely pass the peak flow from the 10-year, 24-hour storm event. The ditch dimensions were determined using Ma.nning's Equation for open channel flow. The diversion ditches will be grass-lined or armored, depending on flow velocities. The design for Ditch No. 2 is presented in Attachment 13-2, Diversions. Design demonstrations for Ditches 13A and 14A are included in the corresponding pond designs in Appendix 13-12.1, Sediment Pond SEDCAD4 Results (Ditch 13A is with Pond 013; Ditch 14A is with Pond 014). A summary of the ditch designs is presented on Figure 13-12.1, Ditch Design Schedule. Haul Road Culverts The culverts are designed to safely pass the peak flow from the 10-pear, 24-hour storm event. In general, haul road culverts were sized using the U.S. Bureau of Public Road nomography. Culvert design inputs and results are included in Appendix 13-12.2, Haul Road Ditches SEDCAD4 Results. Design culvert locations are shown on Exhibits 13-11.3 and 13-11.4; permanent culvert locations are shown on Exhibit 20-2, Postmining Topography Map. The permanent feature demonstration for culvert EX-2 is included with Channel YPM-16/YPM-20 SEDCAD demonstration (see Appendix 13-12.3). The design (and permanent feature) demonstration for culvert EX-5 is included with Channel YPM-22 SEDCAD demonstration (see Appendix 13-13.3). Haul road culvert designs are summarized on Figure 13-12.2, Culvert Schedule. -_ Montgomery Watson Mining Group * 165 S. Union BouAvarr~ Suite 460 * I.nkemoo 0 ora - ~ 37 Revised 08/08