Abstract:
In this study, we use multiple paleoflood hydrologic techniques to develop a chronology of flood events that pre-date stream gauge data for South Sauty Creek, a tributary of the Tennessee River in north Alabama. Paleoflood hydrology uses physical evidence of flooding to reconstruct the timing and magnitude of floods that occurred prior to historical and instrumental data. South Sauty’s gorge setting makes the stream highly prone to large floods, which as recently as 2020 resulted in loss of life. Streamflow data only begin in 2011, providing limited data for understanding the large floods generated by this stream. Tree core samples were collected from primarily oak (Quercus) trees with flood impact scars in the riparian zone, and dated using standard dendrochronology techniques. We developed a 247-year flood chronology, with the earliest dated flood in 1758 C.E. Dated tree scar heights correspond to stages associated with flows 34%-65% greater than discharges for the December 2019 high flow, and 17% - 28% greater than the 25-year event that occurred in 2015. The highest tree scars thought to be from the December 2015 flood modeled result in discharges consistent with a 50-200 year flood event. Additionally, sediment entrainment equations based on the Shield’s parameter were used to determine the minimum water height necessary to move the 10 largest imbricated cobbles located in channel adjacent to the tree-sampling site. We use HEC-RAS 5.0.6 to model the discharge associated with the stage that transported the imbricated boulders. Transportation of 80% of the measured cobbles is associated with the largest flows on record or greater, up to 50 times greater than the modeled 2015 event. Future work will expand the data set to include higher tree scars to isolate the dates of larger flood events based on inundation mapping of the floodplain.
