Numerical simulation of aggradation and downstream fining

Rivers typically exhibit a tendency for grain size to become finer in the downstream direction. Data for a set of large-scale experiments on the aggradation of heterogeneous gravel have recently become available. These experiments show substantial downstream fining over several tens of meters. Here a decoupled numerical model for bed aggradation and downstream fining is developed in an attempt to test an existing gravel transport model against the experimental data. Generally good agreement is found between the predictions and the observations in the absence of all but trivial adjustments to the gravel transport model. The same transport relation does not perform as well for a corresponding case of uniform sediment. In all of the experiments the Froude number was close to unity, a condition which would suggest that a decoupled model might break down. Coupled and decoupled models for uniform sediment are thus compared for a case with Froude number very close to unity. They are also compared for cases in which the upstream water discharge, sediment feed rate and downstream water surface elevation vary strongly. The surprisingly good agreement between the two models suggests that concerns in the literature about the use of decoupled models may have been overstated.