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Bed Surface Patchiness in Gravel-Bed Rivers


Gravel-bed rivers commonly display distinct sorting patterns on their beds. Visually, this heterogeneity often appears to form an organization of textural patches or facies. Patches can be distinguished into &ldquofree patches,&rdquo which are zones of sorted material that move freely, such as bedload sheets; &ldquoforced patches,&rdquo which are areas of sorting forced by topographic controls; and &ldquofixed patches&rdquo of bed material rendered immobile through localized coarsening that remain fairly persistent through time. The local bed surface grain size, and therefore bed surface patchiness, exerts considerable influence on local bed mobility, bedload transport rates, hydrodynamic roughness, and benthic microhabitats. Despite the ecological and morphodynamic importance of bed surface patchiness, we lack accurate and objective methods to delineate patches, and have little understanding of how patches form, evolve, and interact.

This dissertation investigates the mechanisms responsible for the formation of bed surface patches. First, two sets of flume experiments are used to explore how fixed and free patches respond to reductions in sediment supply, and to show that sediment supply is a primary control on the distribution of fixed patches and the dynamics of bedload sheets. A near-field scale flume experiment is then used to show how bed topography, the flow field, and the sediment transport field interact to form forced patches. Size-selective, cross-stream sediment transport is shown to be a critical mechanism responsible for the development of forced bed surface patches. A high-resolution dataset of spatial grain size distributions collected during this experiment is then analyzed with several clustering methods to develop an objective method of delineating patches. These analyses suggest that rivers have a tendency to form a finite number of patch types that become distributed throughout the reach. Lastly, a two-dimensional morphodynamic model simulating flow, mixed-grain size sediment transport, and bed evolution is presented and used to show that forced patches interact with the evolving bed and, through their effects on the flow field, have a considerable impact on morphologic evolution.

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