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Dan Mathews - 2 - August 1, 2006 <br />Storm durations for SEDCAD can vary from 0.2 hours to 24 hours. SEDCAD was used to <br />model the Roadside upper diversion ditch with the 1.75 inch 1.5 hour event. Other than <br />rainfall, the SEDCAD model is the same as the model in the permit located in Appendix 14- <br />6, beginning on page 14-6-1. Section B-B, the ditch segment that failed, had a projected <br />22.3 cfs peak flow for the rainfall event. Section B-B is designed to handle an 8.8 cfs flow <br />based on the 2.2 inch 100 year 24 hour event. Failure of ditch Section B-B would be <br />expected with the 22.3 cfs modeled flow. Similar results would be expected if other <br />damaged areas were modeled. A copy of the SEDCAD run is enclosed. <br />The Operator assessed the storm damage at the mine site on July 24~'. The most critical <br />area to repair was the Roadside refuse disposal area upper diversion ditch because if not <br />repaired, flow in the ditch would bypass pond 9. The bypass was corrected on the morning <br />of July 26~' and work is ongoing to repair the upper division ditch. The operator intends to <br />use flow fill, low strength concrete, to anchor the riprap on the steep slope. To the extent <br />possible, the flow fill will be placed between the rocks so the roughness (Mannings 'n') of <br />the channel will not be altered. <br />The upper diversion ditch at the Cameo No. 2 Refuse pile will be repaired next. The ditch <br />will be reconstructed as designed. The operator intends to install sediment traps above the <br />ditch where possible to help reduce sediment deposition in the diversion ditch. Remaining <br />repairs will be performed around the mine site as needed. <br />Please let me know if you have any questions. <br />Sincerely, <br />~~~ <br />J. E. Stover, P.E. <br />Consulting Engineer <br />Enclosures <br />cc: DMG Denver <br />