Date of Award

Fall 2018

Document Type

Open Access Dissertation

Department

Civil and Environmental Engineering

First Advisor

Enrica Viparelli

Abstract

The quantification of the changes in channel geometry and sand load that have occurred on the Missouri River after engineering works and that characterize the Altamaha River in the fluvial-tidal transition zone is the main objective of this research. Dam construction and channelization on the Missouri River resulted in flow regulation, sediment shortage by dam reservoir trapping, channel straightening and narrowing. Quantifying the morphodynamic response of the Lower Missouri River to these engineering projects at short and long-time scales is a difficult problem that we approach with the analysis of the available field data and with the aid of a one-dimensional morphodynamic model. The results show that the peak discharge and suspended sand load have reduced remarkably downstream of Gavins Point Dam, i.e. the lowermost dam on the Missouri River mainstem, and have recovered gradually to pre-dam levels downstream of the junctions with major tributaries. The analysis of mean daily flows reveals no observable change in high flow magnitude and frequency downstream of the Missouri River – Grand River confluence. Model results show no significant change in mean annual sand load in the downstream-most part of the study reach for short and long-term simulations. Recent studies on river morphology in the fluvial tidal transition zone show significant changes in channel geometry and sediment size over relatively short distances with the cross-sectional area and the sediment size decreasing the oceanward direction. The collection and analysis of field data in the fluvial- tidal transition zone of the Altamaha River presented herein provides greater insight into the characteristics and causes of these spatial changes. Stream power and Shields number were used to explain the observed changes, which occur over a distance ranging from 1 to 4 km. The analysis showed a sharp decline in stream power and Shields number at the boundary between the fluvial and tidal dominated reaches. This decline is thought to be responsible for the observed changes in channel geometry and sediment size.

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