Alluvial morphodynamics of low-slope bedrock reaches transporting
non-uniform bed material
Research on bedrock rivers primarily focused on bedrock incision and, to
the best of our knowledge, morphodynamic models have not yet considered
the variability of sediment grain size and the presence of small scale
bedforms in low-slope (slope < 0.005) bedrock reaches.
Further, very few models can quantify spatial and temporal changes in
the fraction of channel bed covered with alluvium (alluvial cover)
within these reaches. Here we present a novel formulation of alluvial
morphodynamics of low-slope bedrock reaches transporting non-uniform bed
material. The formulation is implemented in a one-dimensional model and
validated against laboratory experiments on bedrock reaches downstream
of stable alluvial-bedrock transitions, where the flow accelerates in
space. The validated model is used to study the alluvial morphodynamics
of bedrock reaches upstream of stable bedrock-alluvial transitions.
Equilibrium results show that the interactions between flow, sediment
transport and non-erodible bedrock surface result in a flow decelerating
in the streamwise direction. The effects of this spatial flow
deceleration are 1) a streamwise increase in alluvial cover, and 2) the
formation of a pattern of downstream coarsening of bed surface sediment.
We then investigated the effects of sea level rise/fall on the location
of alluvial-bedrock and bedrock-alluvial transitions. In the case of sea
level rise, alluvial-bedrock transitions migrate downstream and
bedrock-alluvial transitions migrate upstream. Opposite migration
directions are expected in the case of sea level fall.