To deal with transitional flow from debris flow through sheet flow to bedload flow, a riverbed shear stress formula was developed by applying the existing theory of debris flow to the sediment layer and Manning's theory to the water layer. We also propose a new method for determining the transitional condition of the sediment transport region to a continuous transition of equilibrium concentration. Numerical simulations were performed using an existing model and our new model. Results for

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... ions where the riverbed gradient changed gradually showed that the existing model indicated irregular deposition deformation. Moreover, simulation results of flow depth and sediment concentration oscillated around transition boundaries. When discontinuities of riverbed shear stress and equilibrium sediment concentration coincided, numerical oscillations became larger. Our model yielded smooth and stable simulation results without these problems.