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the design of flood- control structures, the largest flood of the type to <br />which the watershed is subject, which can reasonably be expected to occur <br />during the useful economic life of the improvements, subject to the further <br />limitation of cost which can be justified by the economic benefits provided. <br />39, Bear Creek and its tributaries are subject to floods which are <br />characterized in general by rapid rise and fall, high peak flow and rela- <br />tively small volume of run -off. These features become more pronounced on <br />smaller drainage areas. <br />40. Typical flood hydrograph,- A study of past floods in the Bear <br />Creek Basin indicates that$ while all conform to the general character- <br />istics given in the preceding paragraph, the details of each vary with the <br />location of the storm center and the extent and duration of the storm. In <br />consideration of the area within the cloudburst zone the following values <br />were adopted as being typical of floods on the main stream at Morrison, <br />above the mouth of Mt. Vernon Creek; <br />Distribution of Run -off <br />Hours Percent of total storm run -off <br />1 2 <br />2 7 <br />3 22 <br />4 37 <br />5 16.5 <br />6 8.5 <br />7 4.5 <br />8 2.5. <br />The indicated percent of the total runoff was estimated to pass Morrison <br />in successive hours. It was further estimated that the maximum hourly <br />flow would be 42 percent of the total and that the maximum instantaneous <br />rate, if continued for 1 hour, would pass 50 percent of the total run- <br />off. <br />41, Design floods.- A study of the available discharge records at <br />Morrison and Idledale was made with a view toward determining flood flows, <br />for the design of protection works, which would satisfy the criteria given <br />in paragraph 38. The effective area contributing to flood run -off at <br />- 13 - <br />