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- 66 - <br /> <br /> <br />1 <br />1 <br /> <br />l' <br />~' <br />I <br />,.' <br />~' <br />1, <br />~_~ <br />1, <br />V <br />4. Determine composite hydrograph by summing the q values for each hydrograph <br />time as shown on the work sheet (Figure H.10). Therefore, q10 is 1279 cfs for <br />the gaging station near the mine site (total drainage area = 16.7 sq. mi.) <br />Method 3 Frequency Analysis <br />Frequency analysis for Fourmile Creek using data from discontinued USGS <br />gaging station near mine site. Yearly high-flow data were plotted for this station <br />(Figure H.3). The ten-year peak flow is estimated to be 200 cfs. <br />Discussion <br />Estimates of the 10-year peak flow using regression analysis and frequency <br />analysis agree very well (Table H.10). The SCS method estimate is significantly <br />high. The frequency analysis is the most reliable estimate since it is based on <br />actual data. <br />The relative importance of various parameters in the SCS method can be <br />discerned by comparing the peak flow estimates for Drainage Urea 1 (105 ac) <br />and Drainage Area 2 (75 ac). <br />In regression analyses, a purely statistical correlation of dependent parameters, <br />drainage area is generally highly correlated to peak runoff rate. Although DA <br />1 is 30 percent larger than DA 2, the SCS peak flow estimate is 10 percent smaller. <br />The reason is the importance given to slope in the calculation. Areas with relatively <br />steep slopes, such as DA 2, have short time of concentration and a high peak <br />flow. The values obtained using the SCS m•;thod are extremely conservative. <br />46 <br />