Understanding 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 patterns of
bedrock alluviation 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, the model
predicts a linear relationship between sediment cover and sediment
supply because areas of 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 a linear relationship between bedrock
exposure and sediment supply develops, 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 so 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.