SEDIMENTARY SIGNATURE OF HOLOCENE FLOOD DEPOSITS IN UPPER REACHES OF THE TENNESSEE RIVER

Flood risk analyses are typically based on stage and flow monitoring data, as well as historic reports of major floods. In much of the United States, these records extend back only to the mid-late 1800s. The lack of earlier data reduces the ability to assess flood risk for high magnitude, low frequency flood events, which are relevant for facilities like nuclear power plants and hydroelectric dams. Researchers at the University of Tennessee (UT) carried out a desktop survey and field study to assess the potential for using paleoflood sediments to aid in determining frequency and stage of major Holocene floods along the upper Tennessee River and major tributaries. This is part of a larger effort, funded by the Electric Power Research Institute, the Nuclear Regulatory Commission, and the Tennessee Valley Authority, which covers different sections of the Tennessee River and involves teams of investigators from five different universities and the US Geological Survey. Phase I investigations by the UT team include development of a GIS-based desktop tool to survey areas that were likely to have been inundated by major floods and have potential for preservation of sediments from these floods. Likely settings for deposition and preservation of paleoflood deposits include areas of channel widening, severe channel bends, hydraulic shadows behind obstacles, alcoves and caves in bedrock walls, back-flooded tributary mouths and valleys, and terraces. Field investigations included visits to 23 sites, located in 5 study areas, over a length of approximately 150 km of river. Investigations at individual sites often included mapping of soil profiles and collection of samples for laboratory analysis of soil horizons and texture. Preliminary evaluation of the data indicates that 7 of the 23 sites had multiple paleosols, each with overlying flood deposits. One site, located downstream of Watts Bar Dam, had at least 12 sequences of paleosol/flood deposits. Calibrated ages of charcoal fragments collected in paleosols and flood deposits at 4 of the sites indicate time depths of 300 to 2700 years, which is a time range that is highly relevant to flood risk analysis. Planning is underway for additional field and desktop investigations.