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<br />base the starting storage on the results of monthly operation studies. <br />Where reservoir conditions at the start of each flood are not essen- <br />tially constant as in the case of a reservoir operated for flood control <br />only, it is sometimes satisfactory to select a typical flood pattern for <br />inflows and one for local flows downstream and to route eight or ten <br />sizes of floods (ratios of the typical flood hydrographs) through the <br />reservoir and downstream, using techniques described in the preceding <br />chapter. The frequency of occurrence of each regulated flood is consi- <br />dered to correspond to the frequency of occurrence of the corresponding <br />unregulated flood. This rule is satisfactory as long as the pattern <br />used is reasonably typical of the various flood patterns that occur at <br />the location. Where different types of floods occur, such as snowmelt, <br />general rain floods and cloudburst floods, it would be necessary to <br />perform this operation for each type of flood. Separate frequency curves <br />of unregulated flows would be required for each type of flood. <br />Where reservoir conditions at the start of each flood can be mater- <br />ially different, the above set or sets of flood routings should be <br />repeated for each of various starting conditions. This would give a <br />frequency curve of regulated flows for each starting condition. These <br />must then be combined into a single frequency curve of regulated flows <br />as follows: <br />~. Determine from a monthly multipurpose operation study the pro- <br />portion of time that each starting condition (range of initial storage) <br />will prevail during the flood season. <br /> <br />4-02 <br />