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<br />reliability criteria, for use in directly establishing safe releases. <br />This approach is valid even if the index flows are only a small fraction <br />of the total local flows. A simple method is as follows: <br />a. Plot observed hydrographs for the index station and corre- <br />sponding hydrographs of local runoff at the damage center for as many <br />historical floods as possible. A schematic representation of a local <br />flow area and an index area is shown in fig. 2.01a. <br />b. Determine a time of flood wave travel, t, from the reservoir <br />to the damage center. Shift the local flow hydrographs a time period <br />t earlier. This is illustrated in fig. 2.01b. <br />c. Shift the index-flow hydrographs so that peaks are coincident <br />with the peaks of the translated local flow hydrographs. The length <br />of time an index hydrograph is shifted is the "time of advance warning," <br />T, as illustrated in fig. 2.01b. If T is negative, the index. flow <br />occurs too late to provide a good warning, but index flow can still be <br />used to some advantage in the same manner. Adopt a "representative" <br />value of T for subsequent steps in the procedure. <br />d. For each flood, plot simultaneous values at frequent intervals, <br />of the two translated hydrographs from steps "b" and "c". The two <br />translated hydrographs are shown dashed in fig. 2.01b, and plotted <br />points are shown in fig. 2.01c. Draw a line enveloping the highest <br />values of damage-center local flows so that the line is smooth and passes <br />through the origin, as in fig. 2.01c. <br />e. Construct a "release curve" from this "envelope curve" by <br />plotting the difference between the target flow and the envelope value <br />of local flow against the index flow, as in fig. 2.01d. Fig. 2.01d can <br /> <br />2-04 <br />