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<br />I <br />I <br />I <br />I <br />I <br />I <br />! I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />May 1 snowpack water equivalents in the two basins. This snowmelt peak was <br />then added to the rainfall portion of the flood peak estimated by one of two <br />methods as follows: The rainfall portion was calculated by multiplying the <br />rainfall portion of the Tri-County PMF peak by the direct ratio of the PM? in <br />each basin and then by either: (1) the direct ratio or (2) the square root of <br />the ratio of the total drainage areas of the two basins. The peak flow is <br />121,000 cfs by the square root method and 138,000 cfs by the direct method. <br />The average of approximately 130,000 cfs was adopted as the peak flow estimate <br />for the PMF. <br /> <br />The above adopted PMF estimate was then checked for general reason- <br />ableness by the following three methods: (1) estimate based on the ratio of <br />the Tri-County PMF to the 500-year flood; (2) estimate based on the placement <br />of the Tri-County PMF peak on the USBR peak discharge envelope curves used in <br />the Dcminguez Reservoir flood Study (see Fig. II-6); and (3) comparing the <br />estimate to that contained in the USBR Feasibility Report for the Bluestone <br />Project on the Colorado River. The following peak discharge values were <br />obtained based upon the above three methods, respectively: 125,000 cfs, <br />120,000 cfs, and 11 0, 000 cfs. The peak discharge of 130,000 cfs for the PMF <br />therefore appears conservati ve and has been selected for use in preliminary <br />design studies of the Una Project. The volume of the PMF was not estimated <br />since it would be very large and no routing effect could be obtained frcm the <br />Una Reservoir, considering the nature of the flood hydrographs. The spillway <br />will, therefore, be designed for the peak inflow. An approximation of the PMF <br />hydrograph at Una is compared to the Tri-County Project PMF hydrograph in <br />Fig. II-7. <br /> <br />G. RESERVOIR SEDIMENTATION AND OPERATION DURING FLOODS <br /> <br />1. Sediment Yield <br /> <br />Sediment inflow to the Una Reservoir consists of the yield from the <br />7,370 square mile drainage basin upstream frcm the dam site. The average <br />annual sediment yield of the basin was estimated based upon suspended sediment <br /> <br />II-15 <br />