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bed stress, channel shoaling, estuary, mixed tides, sediment transport, tidal asymmetry
1 online resource ( pages)
Rivers and estuaries provide numerous ecological, economic, and cultural resources. The value of these resources is greatly influenced by sediment transport processes, which can be affected by human activities and climate variability. A key driver of sediment transport in tidal rivers and estuaries is tidal asymmetry of velocity and bed stress, which can manifest from both non-linear tidal interactions and linear interactions among astronomical tidal constituents. In this study, an analytical framework is developed to examine and describe the dynamics of bed stress asymmetry in semidiurnal, diurnal, and mixed-tide estuaries (Chapter 1). While tidal velocity asymmetry has been previously analyzed, this study is the first of its kind to directly evaluate spatial and temporal variability of bed stress asymmetry in rivers and estuaries. Because erosion and subsequent sediment transport is fundamentally a function of bed stress, this study improves understanding of how tidal asymmetry influences residual (subtidal) sediment transport. The analytical framework is summarized by three non-dimensional numbers that scale and classify the dynamics of bed stress asymmetry and sediment transport in rivers and estuaries. The framework provides thresholds for ebb- and flood-dominant sediment transport, and illustrates that flood dominant sediment transport can prevail in ebb dominant velocity fields (and vice versa). Particles with low thresholds for transport can also exhibit different and even opposing transport to those with higher thresholds. Furthermore, sediment transport can be driven by diurnal tidal constituents in systems that are traditionally classified as semidiurnal, and vice versa. This can confound efforts to understand sediment transport patterns, but is clarified by the non-dimensional numbers developed herein. Because bed stress asymmetry is controlled by the relative velocity amplitudes and phases of tidal constituents, which vary over fortnightly, semiannual, interannual, and decadal cycles, residual sediment transport patterns in estuaries are constantly changing. As a result, morphological stability of estuaries and alluvial rivers is ephemeral and likely attained only in an approximate sense. Application of the framework to observations in real systems suggests that bed stress asymmetry can contribute to particle trapping, channel shoaling, and may even act as a filter or seasonal barrier for sediment moving through an estuary (Chapter 2). A particular case of bed stress asymmetry was also examined in fuller detail for ebb-dominant, convergent estuaries with large river discharge (Chapter 3). In such systems, opposing along-channel gradients in tidal and fluvial velocity establish a global bed stress minimum, which can also contribute to particle trapping/sorting and seasonal storage of sediments. Analytical expressions for along channel profiles of bed stress and the location of the bed stress minimum were developed to study how changes in river discharge and channel geometry might influence particle trapping and were found to agree well with AdH 2D numerical model simulations. Channel development and flow regulation can alter tidal bed stress asymmetry and thereby shift sediment transport thresholds, with implications for channel shoaling; sediment supply to estuary turbidity maxima, wetlands, and the coastal margin; and the concentration and distribution of sediment-borne contaminants. Many estuaries experience tidal asymmetry, so the framework developed herein can be applied to a wide range of systems.
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Hudson, Austin Scott, "Tidal Bed Stress Asymmetry and Sediment Transport in Estuaries" (2023). Dissertations and Theses. Paper 6557.
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