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i <br />Summary <br />Temporary diversions at Trapper Mine are designed to accommodate the peak flow <br />from a 2 -year, 24-hour event as required by Colorado Mined Land Reclamation <br />Division regulations. <br />The standard design is trapezoidal with <br />ratio of 0.1. Side .slopes are IH:IV <br />assumed ditch gradient of 5%. <br />Assumptions: <br />a flow depth (D) to base width (B) <br />and dimensions are designed with an <br />- No watershed area greater than 640 acres. <br />- Average slope of watershed - 14%. <br />- Roughness coefficient (n) - .035. <br />- Trapezoidal configuration in barren earth. <br />- 1H:IV sideslopes. <br />- Worst case curve numbers: <br />0-40 acres - all active mining, CN = 87. <br />41-100 acres - 2/3 active mining, CN = 80. <br />101-640 acres - watershed equally represented by 5 types of area <br />shown on Table II -3, Approved Curve Numbers at <br />Trapper Mine; CN = 75. <br />Typical Cross Section <br />B <br />Figure II -1. Temporary diversion design assumptions and calculations. <br />Table II -1 summarizes ditch dimensions for various watershed sizes. peak flow <br />velocities are not excessive at a 5% gradient. Most temporary diversions can <br />be constructed at gradients less than 5%. Due to the temporary nature of the <br />diversions and the fact that diversion outflow is routed through our sedimen- <br />tation system, erosion caused by higher flow velocities (steeper gradients) is <br />not considered significant. <br />REVISED Hrb i I ' � <br />