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The probable maximum inflow design flood to be used for the design of the <br />• subject improvements includes inflow from the probable maximum <br />thunderstorm, as well as snowmelt runoff and a small amount of ditch flow <br />from feeder ditches. The total probable maximum inflow design flood has <br />been determined to be 1272 acre feet, with minor additional flow <br />contributed from reservoir feeder ditches, totalling 9 acre feet for a <br />total of 1,281 acre feet, and a peak flow of 10,770 second feet. See Table <br />8. Similarly, the probable maximum general storm with both snowmelt and <br />feeder ditch inflow is shown on Table 8A and shows a maximum inflow of <br />2,064 acre feet and a peak flow of 2,850 second feet. <br />Spillway Sizing <br />• Flood volumes critical to spillway sizing would occur at such times as <br />the Minnesota Dam is being filled with maximum ditch flow, snowmelt is <br />occurring over the drainage area, and a probable maximum thunderstorm is <br />falling. As shown on the attached inflow-outflow hydro graph, Table 8, this <br />flow is anticipated to be approximately 10,770 cubic feet per second. If <br />surcharge storage impoundment is neglected, the resulting spillway must be <br />sufficiently large to pass the peak of this inflow design flood. Since the <br />excess storage capacity of the Minnesota Dam is limited to a total of 116 <br />acre feet between existing spillway crest elevation 45.9 and dam crest <br />elevation 51.4, see Tables 9 and 10, and the expected maximum thunderstorm <br />will add an approximate 830 acre feet to the Reservoir at its peak, it <br />follows that neither the existing spillway size nor a reasonable increase <br />in spillway size will permit sufficient outflow, see Table 11. To <br />• facilitate both an adequate and economical storage capacity and reasonable <br />sizing of spillway, it is proposed to raise the existing dam crest with at <br />VI-13 <br />