Grain-scale nonequilibrium sediment-transport model for unsteady flow

A one-dimensional (1D) finite-volume model is developed for simulating nonequilibrium sediment transport in unsteady flow. The governing equations are the 1D mass and momentum conservation equations for sediment-laden flow and the sediment continuity equation for both bed load and suspended-load transport. The Rouse profile is modified to consider the nonequilibrium transport of suspended sediment. The spatial lag between the instantaneous flow properties (e.g., velocity, bed shear stress) and the rate of bed load transport in unsteady flow is quantified by using an adaptation length, which is derived theoretically by applying the momentum principle in the bed load layer. This new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress. The nonequilibrium model is applied to simulate a series of laboratory dam-break flows over erodible beds, and the simulated results agree well with the experimental measurements.