Date of Award

12-1-2025

Document Type

Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Zhixiong Shen

Second Advisor

Shaowu Bao

Third Advisor

Till Hanebuth

Abstract

Rivers have been essential in the success of humankind. This reliance has created a vested interest in studying the rivers' environmental history through using floodplain deposits, with the assumption that coarser sediments correlate to higher discharge floods. However, shifting hydrological connectivity and hydrodynamics can complicate this correlation in primary repositories of flood deposits, oxbow lakes, by altering the texture of sediments delivered to them. Studying these complications is essential for quantifying paleoflood magnitude using the sediment record, but has been difficult, as the currently used index of hydrological connectivity relies on water stage, which does not reflect the sediment flux in the floodplain. Furthermore, hydrodynamics during a flooding event is nonuniform—termed here as backflow conditions—and can alter sedimentation patterns. This study uses a 2D unsteady-state hydraulic model of a reach of the Pee Dee River in South Carolina, along with a novel index of hydrological connectivity, the flow competency index, to identify the effects of hydrodynamics and morphodynamics on flood sedimentation in five oxbow lakes. In addition, a separate bathymetric scenario was created to show how a shift in hydrological connectivity will change these two factors. This study finds that the flow competency in the oxbow lakes is positively correlated with discharge, supporting the use of oxbow lake sediments for paleoflood hydrology studies. However, the correlation could be altered by shifting hydrological connectivity, potentially compromising sediment-based flood-magnitude estimates in oxbow lakes with greatly weakened connectivity. This study also finds that the backflow conditions could dominate floodplain hydrology in regions with poorly developed levees. Since sediments carried by backflow often show little grain size change, deposits in the backflow-dominated floodplain are not suitable for paleoflood hydrology studies. The use of flow competency and the identification of backflow conditions can help identify regions holding the best available paleoflood record.

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