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;. <br /> <br />Mr. Dan Mathews <br /> <br />-2- <br />October 9, 1984 <br />interpreting the rules to require storaye of a ten-year <br />2~:-I;our storm, with discharge to be made when adequately <br />settled. Using that assumption, a slide gate was installed <br />to dewater through the primary spillway at an adjustable <br />rate. Astage-storage relationship graph is not relevant <br />to this design. The emergency spillway is in place v;ith a <br />width of 10'> side slopes of lv:lh, and a dischar•ye slope <br />of 10."G. Assuming the worst case peak flow from the emergency <br />spillway, a 25-year 24-hour storm upon a full pond, the water <br />depth is calculated as follows: <br />Peak flow for a 25-year 24-hour storm is calculated using <br />the method presented on page G-6 of Figure 2.05.3(4)(12)-1 <br />of the Permit, and using the acreages for the sediment ,. <br />pond on Table G-1 of the same figure. <br />(P - 0.2 S)2 P = Accumulated precipitation, <br />Q = 2.6" (from Table 2.05.3(4)(b)-2) <br />p+0.8S <br />S = (1000/CN) - 10 <br />Q74 = 0.67 <br />Q88 = 1.47 Q = Runoff in inches <br />Peak flow for the 1.573 acres of CN 74 = 4.2 cfs/inch <br />of runoff X 0.67 inches of runoff = 2.81 cfs <br />Peak flow for the 23.334 acres of CN 88 = 34 cfs/inch <br />of runoff X 1.47 inches of runoff = 49.98 cfs <br />Total peak flow = 2.81 + 49.98 = 52.79 cfs <br />Using the Planning Equation, a deptlr of 0.6' will handle <br />this flow down the l0 grade of the existing spillway. <br />If there are a;ry fur•tlier- questions about these replies to your <br />comments on Stipulation 1, please feel free to contact me. <br />Very truly yours, <br />LMR:KES <br />enc. <br />cc: OSM <br />~~ Larry M. Reschke <br />Mine Engineer <br />