Abstract: In this paper an approach to simulate bed load graing due to degradation in alluvial rivers with sand bed is presented. The approach is characterized by introducing a new equation for the evolution of the bed graing in an active layer. The equation permits to describe both the temporal and spatial variances of the bed graing in the active layer.Therefore,in the case of the degradation,the method has ability to simulate the process that the coarsing of the bed material initially occurred at the bed surface which progressively develops into the inner part of the river bed. Being a nonlinear partial differential equation which must be solved with the flow and sediment transport equations,a numerical methodology for the approach is also described in this paper. The simulations of the bed graing during degradation of the channel bed by the approach agree satisfactorily with the available experiments conducted in laboratory flumes. The approach can be useful in the numerical modeling of the fluvial processes.