TEST USING SEDIMENTARY RECORDS TO QUANTIFY EXTREME PALEO-FLOOD: A CASE STUDY OF AN OXBOW LAKE IN SOUTH CAROLINA

Extreme flooding has become an increasing issue along the coasts for people’s health and infrastructure stability alike. As the effect of climate change continues to persist, the need to prepare for such events becomes imperative. To improve the understanding of climatic forcing in regards to extreme flooding, there is merit in searching beyond the instrumental records. There has been some work done in the past to correlate extreme flooding and its sedimentary traces preserved in floodplain depressions, such as oxbow lakes, based on the assumption that the coarser grain sediments in the sediment layers correspond with higher peak discharges brought on by flooding. In this study, we test this correlation using a core sample (1.24 m long) taken from an oxbow lake off the main channel of the Waccamaw River in South Carolina. Grain size of the core sediment was measured for every centimeter to investigate how that might correspond with extreme flooding of the Waccamaw River during the ~60-year life span of the lake. To calculate a reliable range of sediment coarseness, two separate parameters were used, the grain size at 90% cumulative frequency distribution, D90, and the most coarse component through end-member modeling (EMM). The variables were detrended and normalized to determine the z-scores, or standard scores, for each layer. The z-scores of both parameters depicting coarse sediments corresponded strongly to the peak annual discharge of extreme flooding events (> 9000 cfs) with a correlation coefficient (r²) of 0.45 for D90 and 0.77 for the EMM analysis. The confidence between these correlations can open up future research opportunities for more articulate paleo-flood records. An analysis of the peak annual discharge of the Waccamaw River over the last ~70 years indicates that the majority of the extreme discharges are caused by tropical cyclones, suggesting that the history of extreme flooding of this river may reveal how historical tropical cyclones have affected this region.