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<br /> <br />Design Rationale and Parameters for the <br />• Mesa Gravel Pit Sedimentation Pond, <br />Routt County, Colorado <br />Impoundment Design Criteria: <br />A. Primary Requirements <br />1. Provide a minimum storage volume greater than three years sediment <br />accumulation plus temporary storage of the runoff volume resulting from a <br />10-year, 24-hour precipitation event. <br />2. No discharge from the impoundment during a 10-year, 24-hour <br />precipitation event. <br />3. Sediment will be removed as necessary to assure 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' assumption <br />that the entire 91.6 acre watershed is contributing runoff to the pond. The <br />following land conditions are assumed: <br />f <br />~J <br />SOIL CONDITION HYDROLOGIC SOIL <br />GROUP CN• % TOTAL AREA WEIGHTED CN <br />Undisturbed Land B 71 23.6 16.8 <br />Reclaimed Lantl B 71 69.9 49.6 <br />Active Minin & reclamation B 81 6.5 5.3 <br />TOTAL 71 .7 <br />•CN IRef. Haan & Barfield, 1978, pp. 67, 681 <br />Use CN = 72 <br />Revised May 1998 <br />