Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 2. Controls on the Formation of Alluvial Patches

Abstract

AbstractUnderstanding the development and spatial distribution of alluvial patches in mixed bedrock‐alluvial rivers is necessary to predict the mechanisms of the interactions between sediment transport, alluvial cover, and bedrock erosion. This study aims to analyze bedrock alluviation patterns using a 2D morphodynamic model, and to use the model results to better understand the mechanisms responsible for alluvial patterns observed experimentally. A series of simulations are conducted to explore how alluvial patterns in mixed bedrock‐alluvial channels form and evolve for different channel slopes and antecedent sediment layer thicknesses. In initially bare bedrock low‐slope channels, sediment cover increases with increasing sediment supply because areas of Froude‐subcritical flow enable sediment deposition, while in steep‐slope channels the flow remains fully supercritical and the model predicts so‐called runaway alluviation. For channels initially covered with sediment, the model predicts a slope‐dependent sediment supply threshold above which sediment cover increases with increasing sediment supply, and below which the bedrock becomes fully exposed. For a given sediment supply, the fraction of bedrock exposure and average alluvial thickness converge toward the equilibrium value regardless of the initial cover thickness as long as it exceeds a minimum threshold. Steep channels are able to maintain a continuous strip of sediment under sub‐capacity sediment supply conditions by achieving a balance between increased form drag as bedforms develop and reduced surface roughness as the portion of alluvial cover decreases. In lower‐slope channels, alluvial patches are distributed sporadically in regions of the subcritical flow.