A PALEOFLOOD INVESTIGATION FOR REGIONAL IMPROVEMENT OF PEAK-FLOW FREQUENCY ESTIMATES FOR THE BLACK HILLS OF WESTERN SOUTH DAKOTA

The Black Hills region of western South Dakota has a history of exceptionally large floods, such as the catastrophic flood of June 9-10, 1972, in which at least 238 people perished from flooding in the Rapid City area. Annual peak flow maxima associated with the June 9-10 storm were recorded for 24 stream gages, of which 14 were record peaks at the time and 11 still remain as peaks of record. Many of the 1972 flood peaks, as well as peaks from several other notable events, are about an order of magnitude larger than the next largest peaks in available systematic peak-flow records. Regional knowledge of peak-flow characteristics is confounded by the overwhelming abundance of exceptional high outliers in short-term data sets. In 2007, the U.S. Geological Survey (USGS), in cooperation with the South Dakota Department of Transportation (SDDOT), completed a reconnaissance-level study that demonstrated the applicability of paleoflood-hydrology techniques to improve peak-flow characterization for low-probability events. Study results are available at: http://www.state.sd.us/Applications/HR19ResearchProjects/oneproject_search.asp?projectnbr=SD2005-12.

In 2008, the USGS began a full-scale study involving four primary drainages (Spring, Rapid, Boxelder, and Elk Creeks) in cooperation with SDDOT and various other local, State, and Federal agencies. The study relies primarily upon stratigraphic analysis of flood slackwater deposits and radiocarbon dating of entrained organic matter to develop high-resolution chronologies of multiple large flood events extending back several millennia. Discharge estimates based on hydraulic analyses are used to develop frequency analyses for paleoflood sites. Preliminary results indicate that over the last 2000 years, multiple floods have had peak discharges approaching or substantially exceeding 1972-scale discharges in all four drainage basins.