Procedures are defined for estimating the magnitude and frequency of future flood peaks and flood volumes and for estimating the expected hydrograph shape of rainfall-induced runoff of small streams in South Dakota. The procedures are applicable to flood flows that are not significantly affected by artificial storage or other manmade activities. For 115 gaged sites within the State, the estimates are from the frequency curves defined from the gaging records. Flood-frequency estimates are made for ungaged sites by using regression relations that require evaluation of the size, main channel slope, and a soil-infiltration index for the drainage basin. Regression relations are considered applicable for estimating flood peaks at sites on basins draining 0.05 to 100 square miles and for estimating flood volumes at sites on basins draining from 0.05 to 15 square miles.
Limitations on use of the regression relations and the reliability of regression estimates are discussed. A method is indicated for using the estimating procedure and embankment ponding to design smaller highway culverts.
The procedures were developed from analysis of flood records of 123 gaged sites in South Dakota and adjacent States. For 66 sites, short-term records were extended on the basis of long-term climatic records and a rainfall-runoff model. Linear least-squares regression analyses were used with logarithmic transformation of variables to relate flood magnitudes as dependent variables to indexes describing the drainage basins as independent variables. A dimensionless hydrograph shape was found appropriate for rainfall-runoff hydrographs in South Dakota.
