Floods in Nebraska on small drainage areas, magnitude and frequency

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Flood hazard information is needed for small streams as well as for large ones. This report explains methods of defining the magnitude and frequency of floods in Nebraska on uncontrolled and unregulated streams which have about 300 square miles or less of drainage area contributing to surface runoff. Composite frequency curves defined for two flood regions express a ratio of floods with recurrence intervals ranging from 1.1 to 25 years to the mean annual flood. Curves for 10 hydrologic areas were defined to show the relation of the mean annual flood to the contributing drainage area. A flood-frequency curve can be drawn from these two sets of curves for any site in the State within the range of drainage area and recurrence interval that is defined by the base data and not materially affected by the works of man. The two sets of curves are based on all available pertinent data from records of 5 or more years' duration. This report includes a tabulation of maximum flood peaks at gaging stations used and at a number of miscellaneous sites which have less than 300 square miles of contributing drainage area. INTRODUCTION When loss of life is not a factor, it is generally not economically sound to design structures in or across streams for the maximum flood that may occur. Economic considerations will dictate the choice of a design frequency. An evaluation of these economic factors is beyond the scope of this report. It should be noted that the recurrence interval of a flood does not imply any regularity of occurrence. For an example, at any site, two 2 5-year floods may occur in consecutive weeks or such a flood may not occur in a period of 50 years. The purpose of this report is to describe methods by which the magnitude and frequency of floods may be determined for most sites in Nebraska for which the drainage area is less than 300 square miles. The report was prepared in the Lincoln office of the U.S. Geological Survey, under the direction of Floyd F. LeFever, district engineer, Surface Water Branch, in cooperation with the Nebraska Department of Roads. Financial assistance in the preparation of the report was given by the Bureau of Public Roads.