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<br />CHAPTER 1. NATURE OF HYPOTHETICAL FLOODS <br /> <br />Section 1.01. Definition <br /> <br />Hypothetical floods consist of hydrographs of artificial flood flows <br />at one or more locations that can be used as a basis for flood-control <br />planning. design and operation decisions or evaluations. These artificial <br />floods represent classes of floods of a specified or implied range of <br />severity. They can be used as a direct basis of design or as representa- <br />tive flood magnitudes for use in evaluating the over-all effectiveness of <br />a project or system of projects in preventing flood damages at one or <br />more locations at or downstream of projects. <br />Hypothetical floods are ordinarily derived from rainfall or snowmelt <br />or both. with ground conditions that are appropriate to the objectives of <br />the study. but th~ can also be derived from runoff data alone. usually on <br />the basis of runoff volume and peak-flow frequency studies and represen- <br />tative time sequences of runoff. <br /> <br />Section 1.02. Need <br /> <br />Hypothetical floods are needed in order to analyze the effect of floods <br />of a specific magnitude and sequence where gaged historical data are inade- <br />quate to define the event being considered. In addition. hypothetical floods <br />are needed for assessing the effects of changes in river basin conditions <br />(including project construction. urban development. deforestation. etc.) <br />on downstream flood potential. <br /> <br />1-01 <br />