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runoff for a 25-year 24-hour duration storm yielding 3 inches of precipitation. <br /> The use of a 25-year frequency storm for purposes of sizing the culverts <br /> was recommended by the Colorado State Highway Department. <br /> Using the SCS graphical method ofdetermining peak discharge, CN = 80, <br /> and 1 .25 inches of runoff, the peak discharges calculated for ditch sections <br /> 3-A, 4-A and 4-E were 110, 155 and 130 cfs respectively. These discharge <br /> values and a minimum gradient of . 1 ft/f t were applied to a Bethlehem Steel <br /> Company nomigraph to determine that 36 inch diameter culverts will be <br /> sufficient to pass 1 .25 inches of runoff. <br /> DECANT SYSTEM <br /> The proposed decant system for tailing pond No. 4, consisting of two, <br /> 10—inch diameter welded steel pipes, will be sufficient as an aid in passing <br /> impounded storm water from direct rainfall on the pond and rainfall runoff <br /> from the drainage area between the pond and the two diversion ditches. <br /> The storm water from the design storm stored on pond No. 4 is subject <br /> to the drawdown criteria established by MESA. According to that criteria, <br /> decants must be capable of evacuating, within ten days, 90% of the volume <br /> of water stored during the design storm. <br /> The total area of the pond and the drainage area between the pond <br /> and the diversion ditches is approximately 54 acres. The amount of water <br /> that could be impounded on the pond from a probable maximum thunderstorm <br /> rainfall of 8.5 inches is 38 acre-ft. Assuming no losses due to infiltration <br /> r <br /> or evaporation, the two decants must pass a combined total of 3.42 acre- <br /> ft per day to remove 90% of the impounded water within ten days. This <br /> is equivalent to 0.86 cfs/decant. Upon completion of new milling operations, <br /> 23 <br /> t <br />