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• 50-year runoff event. For the 100-year runoff event, it is neces- <br />sary to extrapolate the curves used in the analysis. At low alti- <br />tudes where peak runoffs are due to rainfall, extrapolation can <br />lead to serious errors. Aowever, the Trout Creek basin is at a <br />high elevation and peak flows are due to snowmelt. Extrapolation <br />of the 100-year event is adequate for these conditions. <br />The procedure consists of: (1) locating the drainage basin ' <br />on a map that is divided into hydrologic areas and flood-frequency <br />regions, (2) then the ratio of discharge to the mean annual flood <br />for the frequencies required is found on a graph showing this <br />ratio for a flood-frequency region, (3) the mean annual flood is <br />found on another graph using parameters of drainage area and mean <br />altitude for the hydrologic area and (4) the ratio of discharge to <br />• mean annual flood is multiplied by the mean annual flood and the <br />frequency discharge is the result. The Trout Creek drainage basin <br />is located in flood-frequency region A and hydrologic area 3. <br />~ Table 1 lists the resultant 10-year, 25-year and 100 year peak <br />i discharges. <br />Table 1 <br />Discharge for Trout Creek at Apex Mine #2 <br />I <br />~ Frequency Magnitude ' <br />I 10-Year 565-cfs <br />25-Year 680-cfs <br />100-Year 820-cfs <br />The flood plain and flood flow parameters have been determined <br />using the D.S. Army Corps of Engineers HEC-II step-backwater <br />computer program. Mannings 'n' roughness values were determined <br /> <br />134- <br />- ~- - -- <br />