Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Enrica Viparelli

Abstract

The Congaree River planform is characterized by an abrupt increase in channel sinuosity in the downstream direction. Such a change is not typical of alluvial meandering rivers but has been observed in mixed bedrock-alluvial rivers. This motivated the first study of this dissertation, specifically designed to determine if the Congaree River is indeed a mixed bedrock-alluvial river in its low sinuosity (upstream) reach. To test this hypothesis, sonar-based measurements of channel bed and sublayer elevation were performed from Columbia to the Congaree River-Wateree River confluence. The sublayer is here defined as the first surface visible in the sonar measurements underneath the bed surface. Field measurements revealed the presence of numerous sublayer exposures on the channel bed and pictures of the bed surface confirm that this sublayer correspond to Cretaceous deltaic deposits. These deposits are significantly less erodible than alluvium and can thus be considered a bedrock surface. Further, data reveal that 99.5% of the Congaree River is mixed bedrock-alluvial with a relatively thin (smaller than 4 m) alluvial cover of the shallow bedrock that decreases downstream and is preferentially exposed in high curvature bends. In other words, channel sinuosity is highest where the alluvial cover is thinnest, and the initial hypothesis had to be rejected. The second part of the dissertation is centered on the study of floodplain sedimentation in the downstream part of the Congaree River floodplain located in the Congaree National Park. The study was designed to test current floodplain sedimentation models based on the sharp distinction between channel bed material (sand) and floodplain material (wash load or mud). These models predict 1) high sedimentation rates close to the main channel with an exponential decay toward the floodplain interior, and 2) coarse sediment (sand) trapping in levees close to the main channel with mud being deposited in the floodplain interior. Sedimentation rates measured with sediment tiles, grab samples and results of a previous study were used to characterize floodplain deposits in high and low relief areas, close to the Congaree River main channel and in the floodplain interior. High sedimentation rates and sand collected in the floodplain interior clearly show that traditional floodplain sedimentation models fail to describe sedimentation in wide, complex floodplains such as the Congaree River National Park, while capturing levee formation close to each channel. The analysis of relief maps and hydrodynamic modeling results of floodplain flow revealed that floodplain channels play a critical role in conveying water and sand far from the main channel during overbank and sub-bankfull inundation. Data shows that floodplain sedimentation rate has no relation with floodplain relief. Further, sand is preferentially found in high velocity areas connected to the main channel through floodplain channels. Finally, the comparison between sand from the Congaree River main channel and the floodplain reveals that floodplain material (mud) is a mixture of wash load and of bed material transported in suspension above the floodplain channel bottom.

Rights

© 2024, Mahsa Ahmadpoor

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