A computer simulation method is developed to predict the temporal variations of debris‐flow discharge, solids concentration, and vertical mean particle‐size distribution. A single finite step in the simulation is divided into two stages: In the first stage, a sediment bed composed of a well‐graded mixture is eroded by surface water introduced at the head of a channel, thereby generating a debris flow. In the second stage, larger particles in the debris flow move upward because of dispersive force and are then transported forward faster than smaller particles, which remain in the lower part of the flow because the velocity is slower in the lower layer. Another numerical simulation method is also developed to predict simultaneously the shape of the debris‐deposit area, the distribution of the deposit thicknesses within the area, and the three‐dimensional distribution of the mean diameters of the deposited particles. Both simulation methods are verified using laboratory experiments.