Introduction The design of hydraulic structures for use in ungaged drainage basins requires some estimate of flood flows and their frequency of occurrence. Because no historical streamflow data exist for these drainages, floods are estimated either by regional frequency analysis or, with the help of digital computers, by parametric rainfall-runoff event simulation.
Computer models dealing with rainfall-runoff event simulation are commonly used today by engineers and hydrologists. These models are used to predict flood hydrographs given an input rainfall volume, distributed over time in some manner, and certain geomorphic basin parameters.
Studies exist in the literature documenting the effects of time distribution of rainfall on runoff hydrographs. The reader is referred to works by Wei and Larson (1971), Yen and Chow (1980), and Shanholtz and Dickerson (1964) as examples. Because this relationship between the time distribution of rainfall and hydrograph characteristics exists, the separate study of storm rainfall is essential for accurate flood prediction regardless of other variables that also influence the runoff process. Additionally, methods of constructing design storms are available and in wide use, but they are general in nature and assume storms occur with the same temporal distribution across much of the country. Because of the drastic climatic differences between the areas encompassed by existing procedures, it was felt their design curves are not likely to be representative of the actual time distribution of storms in semi-arid regions such as Wyoming. It was, therefore, decided to develop a new design storm construction procedure applicable to the State of Wyoming based on observed storm rainfall in Wyoming. This new design storm methodology is the topic addressed herein.
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