A new procedure for estimating paleodischarges in alluvial rivers for quantitative paleoflood hydrologic analyses

Keywords: floods, risk, extreme, paleofloods
Session Type: Virtual Paper Abstract
Day: Friday
Session Start / End Time: 2/25/2022 08:00 AM (Eastern Time (US & Canada)) - 2/25/2022 09:20 AM (Eastern Time (US & Canada))
Room: Virtual 19

Abstract

Climate change is increasing the frequency of extreme floods globally. The societal and geomorphic impacts of extreme floods remain poorly quantified. Rarely do river basins have streamflow records with more than one, if any, observations of extreme floods. Consequently, quantitative paleoflood hydrologic (QPH) techniques have been used to reliably estimate the magnitude and frequency of extreme floods using sedimentary and tree ring records spanning millenia. These methods predominately have been used in bedrock river systems where channel dimensions can be assumed to remain consistent over time. Channel dimensions change over time in most alluvial rivers in temperate climates because of erosion and deposition. Significant increases in precipitation intensity are occurring and forecasted to further intensify in temperate regions of the U.S. Therefore, we developed a modified QPH procedure that permits the estimation of paleoflood discharges in alluvial channel settings based on flood deposit volume of d90 particle size, back-calculation of paleoflood height from a shear stress equation and d90, and 1-D HEC-RAS step backwater modeling. We applied this technique at two study locations: a terrace located on the main channel of the Tennessee River near Guntersville, AL and a floodplain of the French Broad River, a major headwater tributary. We compared the paleodischarges estimated using the modified QPH procedure to existing discharge estimates of extreme floods available for the Tennessee River Valley and a 150 year-long gauge record containing numerous historic floods. We discuss the potential for the modified QPH procedure to be applied in other alluvial river settings.