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<br />'! <br />~ <br />I: <br />, <br />~I <br />f! <br />:1 <br /> <br />CHAPTER 5. BALANCED FLOODS <br /> <br />Section 5.01. Definition and need <br /> <br />, <br /> <br />I <br />I <br /> <br />r <br />! <br /> <br />A balanced flood is one that is of equal severity for all possible <br />critical durations of project design. Severity is expressed in terms of <br />exceedence probability or exceedence frequency. <br />In the planning of a flood control project involving storage or in <br />the development of reservoir operation rules, it is not ordinarily known <br />what the critical duration will be. because this depends on the amounts <br />of reservoir space and release in relation to flood magnitude. When <br />alternative types of projects are considered, critical durations will be <br />different, and a design flood should reflect a degree of protection that <br />is comparable for the various types of projects. Accordingly, balanced <br />hypothetical floods are useful for planning. design and operation pur- <br />poses. <br /> <br />Section 5.02. Varying volume-duration relationShips of floods <br /> <br />Time patterns of floods that occur at any particular location are of <br />a great variety. Some floods have high peak flows and are of short dur- <br />ation. Others have the reverse characteristics, and, of course. short- <br />duration volumes can be small in relation to long-duration volumes in <br />SOMe floods and large in others. No one historical flood would ordi- <br />narily be representative of the same severity of peak flow and runoff <br />volumes for all durations of interest. <br />If a project is designed to regulate all floods of record, it is <br />likely that one flood ~ll dictate the type of project and its general <br />features, because the largest flood for peak flows is also usually the <br />largest-volume flood. Yet, there is information in the other floods that <br />can be used to obtain a better balance of the peak flow and volumes for <br />various durations of a design flood. It is important that this infor- <br />mation be used effectively. <br /> <br />5-01 <br />