2011-01-13_PERMIT FILE - C1981038A (6)

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2.05 APPLICATION FOR PERMIT FOR SURFACE OR UNDERGROUND MINING ACTIVITIES - MINIMUM REQUIREMENTS FOR OPERATION AND RECLAMATION PLANS. The design for a probable maximum thunderstorm exceeds the requirement for design of a 10-year event. Flows up to 550 cubic feet/second for PMTS would result. The diversion banks have been constructed using the most environmentally acceptable slope for the site specific locations; because of the steep nature of the original slope (1(h):l(v) and 1-1/2(h):1(v)) in most cases the ditch as constructed is 1(h):1(v). Given the existing steep slopes, any more gentle slope on the ditch would substantially increase the disturbed land (possibly three to four times as much acreage) and thereby increase tremendously the total sediment yields. The amount of material to be disposed of would also be much higher. An increase in sediment of up to three times more, and unnecessary visual scarring of natural landscape warrants the use of the steeper slope. Leaf, "Soil Loss Equation Study", can be found in the Diversions Appendix, Volume BA. The slopes will be stabilized by a continuing re-vegetation program. See Section 2.05.6(3) Recla- mation Plan. The channel follows the existing cut line of the fan bench level. This maximizes usage of previously disturbed areas and minimizes visual impacts. The channel was designed and constructed on gradients below~5 percent. This was done to minimize sediment transport along the channel. The bottom of the channel is also vegetated to control sedimentation. The low flow channel is heavily rip-rapped to further control sediment. Calculations on flow velocities and stability are in the Diversions Appendix in Volume 8A. It is unlikely that the probable maximum thunderstorm could totally be controlled. The channel would minimize destruction, though significant sediment would be generated with or without the channel. During the probable maximum thunderstorm, it could be expected that some damage would be sustained to the receiving watersheds. Visual observation has shown minimal erosion at the channel ends. The largest rainfall event to date has been 1.10 inches within a 24-hour period, and no erosion was observed. Because of the arid conditions, large rainfall events would lead to erosion in any watershed. Both channel ends enter the receiving drainage on bedrock surfaces and at high capacity out- lets. See calculations in Diversions Appendix. The channel bank has been analyzed and inspected for stability by Kenneth Medearis Associates of Fort Collins, Colorado, and Geo-Hydro Consulting, Inc. It appears to be stable, but for added safety the area above the bank will be monitored for tension cracks using the Operator's Slope Stability Monitoring Program and monitored with inclinometers and pore water pressure transducers. Additional stability information may be found in Phases I and II, Stability Analysis Appendices in Volume 6. ~~ Permit Renewal RN-o4 (4~9~04~ />1Q-106 - 25 - 01/03