NEW INSIGHTS IN PALEOFLOOD FREQUENCY ANALYSES AND QUATERNARY CLIMATE WITH A 11,000-YEAR PALEOFLOOD RECORD

Extensive research conducted by Dr. Ralph Klinger and others in the Western United States demonstrates the efficacy of using Quaternary paleofloods to improve extreme flood frequency analyses. This body of work informed updates to federal guidelines in flood frequency estimation and provides a conceptual framework for similar work in other U.S. regions and beyond. We present a paleoflood study from the Clinch River in eastern Tennessee, USA, which developed an 11,000-year Quaternary flood history and new flood frequency models to inform the probabilistic flood hazard assessment of Norris Dam. Stratigraphic evidence from a slackwater deposit in a rock shelter and a 6-meter-long sediment core from an adjacent floodplain constitute the paleoflood record. We used radiocarbon-dated charcoal from the rock shelter and optically stimulated luminescence ages in a Bayesian age-depth model of the floodplain core to constrain ages. We sampled the floodplain core at a 1 cm resolution to measure particle size using a Bettersizer laser granulometry and a 5 cm resolution for organic content and magnetic susceptibility. We identified 78 previously unknown extreme floods on the Clinch River, including a late Holocene flood with 2.5 times the magnitude of largest flood in the known record. The steady accumulation rate in the floodplain revealed a cyclical pattern of increased flood magnitude approximately every 2500 years. We incorporated paleofloods in Bayesian flood frequency analyses and selected perception thresholds based on insights from the alluvial Quaternary history. Paleoflood analyses revealed a heavy-tailed flood distribution, resulting in flood frequency curves that were more positively skewed than the observed record. Consequently, estimates for rare annual exceedances probability were larger than previously estimated. This study not only provided critical insight into extreme flood hazards impacting Norris Dam but also increased regional knowledge of the Quaternary climate with the longest paleoflood record in the Southeastern United States.