Debris flow is composed of mixed‐size gravel, and the grain size distribution and concentration of sediment strongly affects the flow behavior. To predict more accurate debris flows, we need a numerical model that considers mixed‐size gravel and can describe sediment sorting. We propose a more accurate method for predicting mixed‐size debris flow behavior, from torrents to alluvial fans. We combined a debris flow model that can describe sediment sorting with an integration model, and proposed an integrated model for mixed‐size gravel. Applying this model, we simulated debris flow on a virtual watershed under different conditions of inflow sediment distribution. Our results showed that an integrated model for mixed‐size gravel provide a more rational description of debris flow in torrents and alluvial fans than models of uniform‐sized gravel.

Key words：debris flow, numerical simulation, size segregation, change of distribution, 1‐ and 2‐dimensional integration model