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N -~- maintain sufficient open sediment storage in the pond. Available sediment storage in the pond should be sufficient to contain either one year's sediment yield or the sediment yield from the 10-year, 24-hour rainfall event. 5) The applicant failed to adjust the peak flow estimate of the L5-year, 24-hour rainfall event for varying basin shape. Peak flow should be reestimated making both the slope and basin shape adjustments. (Refer to Appendix E in Peak Flows in Colorado.) 6) She emergency spillway for Pond E was sized using the Manning Equation for open channel flow. This methodology is incorrect. The applicant should demonstrate that the proposed design is adequate using the Broad - Crested Weir Equation. Collection and Diversion Ditches 1) Peak flow calculations for the ditches were not adjusted for varying slope or basin shape. Peak flows should be reestimated making both the slope and basin shape adjustments. 2) The calculated ditch velocities are for grass-lined channels. The ditches will be in place and functioning for some tune before adequate vegetation protection can be achieved. Therefore, the applicant should show that unprotected channel velocities (earthen channel) would not be erosive. Alluvial Vallev Floors Formal AVF determinations and findings have not yet been mane for Mine No. 3. Adjacent portions of both Fish Creek and Dliddle Creek are potential alluvial valley floors. The activities being approved in this technical revision (construction of 1 sediment pond and six diversion ditches) would not effect these potential alluvial valley floors. Most of the disturbance is within the Fish Creek drainage, and, as indicated on Map TR-3, it would be located outside of the area identified as unconsolidated stream laid deposits. Therefore, the proposed operations would not interrupt, discontinue, or preclude farming on the potential AVF. The sediment control plan would control runoff from the existing surface disturbance and would be expected to improve the existing quality of runoff leaving the minesite. No mine pit water or water from underground workings would be discharged through Pond E. Therefore, the activities considered in this technical revision would not cause material damage to the surface and ground water that supply the potential alluvial valley floors.