Evaluation of Selected Methods for Estimating Flood Peaks for Small Watersheds in South Dakota

The need for information about floods on small streams in South Dakota has long been recognized. For example, the magnitude and frequency of flood flows need to be considered in the design of highway structures such as bridges and culverts, in land-use planning, in establishing rates for flood insurance, in formulating emergency evacuation plans for flood-prone areas, and other planning aspects. In many instances, the project sites may be at locations where very limited flood or hydrologic data are available. Consequently, methods of flood prediction for these ungaged drainage basins have been developed. The earliest methods for estimating peak flow used empirical formulas that involved various physiographic characteristics of the drainage basin. Today, unit hydrograph, flood routing, and flood frequency analyses are used whenever possible to predict flood flows. The empirical procedures remain in wide use where adequate data is not available for the more reliable methods. The method selected to determine peak flows and their frequencies depends on the following factors: 1. The desired objective. 2. The available data. 3. The area and characteristics of the watershed. 4. The importance of the project and time available for analysis. A basic consideration in the design of bridges and culverts is the estimation of the rate of runoff expected during peak flow periods. The most widely used methods for determining the rate of runoff have been either a hydro-graph synthesis approach using rainfall-runoff models, or empirical equations relating hydroclimatic and physiographic properties to the peak flow with a selected return period. Since the peak flow alone is required for the design of most minor highway drainage structures, only the empirical methods have been considered in this study