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• <br />• <br /> <br /> <br />Table G-3. Design Rationale and Parameters for the <br />Mesa Gravel Pit Sedimentation Pond, <br />Routt County, Colorado <br />Impoundment Design Criteria: <br />A <br />Primary Requirements <br />1. Provide a minimum storage volume greater than three years sediment <br />accumulation plus temporary storage of the runoff volume resulting <br />from a 10-year, 24-hour precipitation event. <br />2. No discharge from the impoundment during a 10-year, 24-hour precipita- <br />tion event. <br />3. Sediment will be removed as necessary to ensure sediment storage <br />capacity over the design runoff volume. <br />4. Spillway capacity sufficient to pass peak flows from 25-year, 24-hour <br />precipitation events. <br />B. Pond Design <br />1. Runoff determination <br />a. Curve Number: <br />NOTE: Mesa Impoundment is designed based on the "worst case" <br />assumption that the entire 91.6 acre watershed is contributing <br />runoff to the pond. The following land conditions are assumed: <br /> Hydrologic <br /> Soil % Total Weighted <br />Soil Condition Group CN* Area CN <br />Undisturbed Land 8 71 23.6 16.8 <br />Reclaimed Land B 71 69.9 49.6 <br />Active Mining & Reclamation B 81 6.5 5.3 <br />TOTAL 71.7 <br />*CN (Ref. Haan & Barfield, 1978, pp. 67, 68) <br />Use CN = 72 <br />• <br />b. Determine 10-yr., 24-hr. precipitation (P10) <br />P10 1.6 in. (Ref. NOAA Atlas 2, Vol. III) <br />G-6 <br />