Sediment sorting plays an important role in a beach morphological process. A granular material model is an effective tool to investigate mechanism of vertical sorting in a sheetflow layer, because a physical experiment in a sheetflow layer is quite difficult due to a high sediment concentration. In the present study, to simulate particulate flow in a sheetflow layer with a high resolution, the filtered incompressible Newtonian viscous fluid equation, having multiphase flow formulation based on the CIP-CUP method, is coupled with both DEM as a particle dynamics model and SSM as a turbulent flow model. Then, the proposed model is applied to a vertical sorting in a sheetflow layer to predict its internal structure from a viewpoint of computational mechanics.