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Channel stability in alluvial gravel-bed streams MacKenzie, Lucy

Abstract

Alluvial fans, conic depositional landforms that develop where headwater streams outlet into a main valley, are desirable locations for development in mountainous regions worldwide. Alluvial fans may become hazardous during high flow events; the relatively high gradient and the abundance of loosely packed sediment allows alluvial fan channels to undergo rapid morphodynamic change, putting bordering infrastructure at great risk. Despite these hazards, our understanding of channel dynamics on alluvial fans remains limited. Through physical modelling, this thesis investigates the processes contributing to channel stability in such environments. Using three sets of paired experiments, I show that channel stability is mediated by the mobility of the largest grains in the channel. Pairs of experiments were identical in all regards (i.e. discharge, sediment supply, gradient, median grain size), the only difference was a slight increase in the proportion of large grains found in the bed material, while the median sediment size for the experimental pairs remained essentially the same. Overall, I found that channels with bed material containing fewer large grains experienced two to four times as much erosion and deposition across a range of discharges and rates of sediment supply. These findings contradict the conventional models of channel stability, that use the median grain size to represent the bed surface; these models commonly assume that streams undergo morphodynamic change once that grain size is mobile. My experiments demonstrate that channel stability is linked to the mobility of the largest grains, not the median size. Using high resolution models of the bed surface, I show that the channels containing the greater proportion of large grains tend to be more stable due to their increased frequency at the bed surface. Based on these results, I propose a three phase model of channel stability wherein the thresholds between stable/dynamically stable and dynamically stable/unstable are governed by the thresholds of entrainment and full mobility of the largest grains, respectively. This new model of channel stability could improve our capacity to predict when catastrophic events may occur in steep, alluvial channels.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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