  We derive a two-layer depth-averaged model of sediment transport and
morphological evolution for application to bedload-dominated problems. The near
bed transport region is represented by the lower (bedload) layer which has an
arbitrarily constant, vanishing thickness (of approximately ten times the
sediment particle diameter), and whose average sediment concentration is free
to vary. Sediment is allowed to enter the upper layer, and so total load may
also be simulated, provided that concentrations of suspended sediment remain
low. The model conforms with established theories of bedload, and is validated
satisfactorily against empirical expressions for sediment transport rates and
the morphodynamic experiment of a migrating mining pit by Lee et al. (1993).
Investigation into the effect of a local bed gradient on bedload leads to
derivation of an analytical, physically meaningful expression for morphological
diffusion induced by a non-zero local bed slope. Incorporation of the proposed
morphological diffusion into a conventional morphodynamic model (defined as a
coupling between the shallow water equations, Exner equation and an empirical
formula for bedload) improves model predictions when applied to the evolution
of a mining pit, without the need either to resort to special numerical
treatment of the equations or to use additional tuning parameters.
