A general flood-frequency analysis has provided a method for estimating flood magnitudes and frequencies on South Dakota streams. Related flood data useful in planning and design also are included in the report.
Two distinct hydrologic regions are delineated within the State. The divisional boundary for these regions is, in general, the western divide of the James River basin. For each region, the 2-, 5-, 10-, 25-, 50-, and 100-year floods are related to basin and climatic characteristics by regression equations. Indices based on contributing drainage area size, elevation, and mean annual precipitation were found to be the most useful variables in estimation of South Dakota floods. Regional relationships based on these variables can be used to estimate floods of selected frequency at most ungaged sites where peak flows are not significantly affected by regulation or other manmade works. Equations and graphs presented are applicable to drainage basins with areas approximately from
0.1 to 4,000 square miles (0.3 to 10,400 square kilometres) in the Eastern Region and from 0.1 to 9,000 square miles (0.3 to 23,300 square kilometres) in the Western Region. Limitations on use of these equations are given and the accuracy of resulting estimates is discussed.
Flood characteristics are tabulated for 130 gaging stations having 10 or more years of record. These frequency data may provide the best estimates of floods at these gaged sites. Also, maximum flood peaks determined at 188 gaging stations and 52 miscellaneous sites are compared with regional flood relationships.
Individual relationships are presented for the main-stem portions of selected streams where significant regulation is a factor or where drainage areas exceed the limits of applicability of the regional relationships.
