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February 1999 Yoast Mine " 13~ 12 Drainage and Sediment Control Plan ~ 7 <br />5.0 DITCH AND CULVERT DESIGN <br />Haul Road Ditches <br />• Runoff from haul roads located within the mining disturbed area will report to sedimentation ponds. <br />The road-side ditches associated with these haul roads will be designed to contain the flow from the <br />10-year, 24-hour storm event. As designed, the cut slope side of the haul road will serve as the <br />shallow slope of the roadside ditch. Although the slope shown on the ditch designs is lOH:1V, in <br />practice the slope will more typically be in the range of 15-20H:1V. This design will increase the <br />hydraulic radius of the ditch as well as provide for easy cleaning of the ditch by graders. <br />The steep slope sections of the roadside ditches will experience erosive velocities during the design <br />storm event. Routine maintenance of the road and ditches will repair any erosion if and when it <br />occurs. Armoring the ditches with large rip rap is not practical from an operational standpoint. <br />Material from the road is constantly being moved into the ditches from the movement of large <br />haulage equipment. Therefore, the ditches need to be maintained continuously. Rip rap placed in <br />the ditches would last only a short time. As shown on Exhibits 13-11.3, Proposed Haul Road B <br />Extension and 13-11.4, Proposed Haul Roads C & D, culverts will be installed at various points <br />along the haul roads to limit watershed areas, and thereby, reduce peak flows and velocities. In <br />addition, rip rap will be placed below the culverts to control velocities and control sediment prior to <br />the runoff reaching the down-gradient sedimentation ponds. <br />All ditches associated with haul roads located outside the disturbed area will designed to contain the <br />flow from the 10-year, 24-hour storm event and control erosive velocities. Sedimentation control <br />measures such as rip rap and straw bales will be installed, as needed. <br />• Design inputs and results for proposed haul road ditches are shown on the SEDCAD4 printouts <br />located in Appendix 13-12.2, Haul Road Ditches SEDCAD4 Results. A summary of the ditch <br />designs is shown on Figure 13-12.1, Ditch Design Schedule. <br />Diversion Ditches <br />All diversion ditches aze designed to safely pass the peak flow from the 10-yeaz, 24-hour storm event. <br />The ditch dimensions were determined using Manning's Equation for open channel flow. The <br />diversion ditches will be grass-lined or armored, depending on flow velocities. <br />Design results can be found in Appendix 13-12.1, Sediment Ponds SEDCAD4 Results. A summary <br />of the ditch designs is shown on Figure 13-12.1, Ditch Design Schedule. <br />Culverts <br />The culverts are designed to safely pass the peak flow from the 10-year, 24-hour storm event. The <br />culverts were sized using the U.S. Bureau of Public Roads nomograph. All culvert design inputs and <br />results can be found in Appendix 13-12.2, Haul Road Ditches SEDCAD4 Results. Culvert locations <br />are shown on Exhibits 13-11.3 and 13-11.4. All new or proposed culverts are summarized on Figure <br />13-12.2, Culvert Schedule. <br />6.0 POST MINING CHANNELS <br />All post mining channels were designed to safely pass the peak discharge from the 10-yeaz, 24-hour <br />. ecertt. Past Tuning rhaztlel dimensions aaere deterrrliaed zlsi~g ?vlanning's Egssarie~ fer open <br />channel flow. All channel design inputs and results can be found in Appendix 13-12.3, Post Mining <br />Revised 5/99 <br />Montgomery Watson Muting Group • 165 S. Union Boulevard, Suite 460 "Lakewood, Colorado 80118 "(303f 763-5140 <br />