Stable patterns of downstream fining and coarsening in mixed bedrock-alluvial rivers

Abstract:

Recent studies on the long term evolution of large and low slope mixed bedrock-alluvial rivers showed that 1) alluvial-bedrock and bedrock-alluvial transitions can be stable features in fluvial systems, and 2) the morphodynamics of these transitions depend on the magnitude of the bedrock slope S_b relative to the alluvial equilibrium slope S_a. These studies, however, were performed for the case of uniform bed material and they did not provide any information on the longitudinal sorting patterns that can be expected in the mixed bedrock-alluvial reaches. We thus derived and implemented a one-dimensional mathematical formulation that is able to model bedload transport, erosion and deposition of non-uniform material in mixed bedrock-alluvial rivers. Our numerical results reveal that a stable M2 backwater curve is associated with an alluvial-bedrock transition and a pattern of downstream fining, while a stable pattern of downstream coarsening forms and stabilizes in the presence of an M1 backwater curve and the associated bedrock-alluvial transition. Our numerical model is based on a rather simplifying assumption to compute bedload transport in the mixed bedrock-alluvial reach: we multiply the bedload transport capacity of the flow for the alluvial cover fraction of the reach. Further, the alluvial cover fraction of the reach is assumed to be a linear function of the average local bed elevation. To validate our bedload formulation in the mixed bedrock-alluvial reach we are performing laboratory experiments at the University of South Carolina with non-uniform material in the sand range. The experiments are currently in progress in a 20 cm wide, 8 m long and 0.9 m deep sediment feed flume. We present the laboratory scale validation of the bedload transport model and the comparison between the numerical results and experimental data at mobile bed equilibrium for the experiments with alluvial-bedrock and bedrock-alluvial transitions.