Application of Paleoflood Survey Techniques in the Black Hills of South Dakota

Flood-frequency analyses for the Black Hills area have large uncertainties because of several complicating factors, including large peak flows associated with the extraordinary 1972 storm; geologic influences; and potential influences of topography on precipitation patterns. The objectives of this study were (1) to apply paleoflood survey techniques as demonstrated in the recently completed South Dakota Department of Transportation (SDDOT) Research Project, SD2005-12; and (2) to the extent possible, discern differences in flood characteristics across elevation gradients within the study area. Comprehensive paleoflood investigations were conducted at 29 locations along six study reaches, including Spring Creek, two reaches of Rapid Creek, two subreaches of Boxelder Creek, and Elk Creek. Application of paleoflood investigations resulted in the following summary of major results and outcomes. (1) An overall conclusion is that high-resolution flood chronologies were obtained in many locations, and meaningful flood-frequency analyses were derived for the six study reaches. The results provide much better physically based information on low-probability floods than has been previously available, substantially improving estimates of the magnitude and frequency of large floods in these basins and reducing associated uncertainty. (2) Flood-frequency analyses using the PeakfqSA model were determined most appropriate and were used as the primary basis of comparison among results. (3) The paleoflood investigations provide a rich history of large floods locally extending back 2,000 years. (4) The eastern Black Hills are postulated as susceptible to intense orographic lifting associated with convective storm systems and also have high relief, thin soils, and narrow and steep canyons—factors favoring generation of exceptionally heavy rain-producing thunderstorms and promoting runoff and rapid concentration of flow into stream channels. (5) The flood-frequency estimates are most applicable to sites near the study reaches where most of the paleoflood investigations were conducted. (6) “Normalization” of quantile estimates for the study reaches allowed for extrapolation of low-probability flood recurrence within appropriate domains beyond the specific study reaches, especially along the eastern flanks of the Black Hills where the results are most applicable. (7) The long-term flood chronologies serve important purposes beyond incorporation in peak-flow frequency analyses. (8) Future research in the southern and northern parts of the Black Hills would provide additional beneficial information.