Although the particle method is effective for high resolution simulation, it has a significantly large calculation time. Therefore, a coupling method of particle and grid-based simulations(PBS-GBS)for debris flows is developed in this study. In the connection part between the particle method(PBS)and the grid-based method, the flow rate, the transport concentration, and the water depth are converted. Numerical experiments were conducted under a constant gradient condition at 1 8degrees and

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... ion with a dam addition to it. A coupling method, PBS-GBS, and a standalone particle method, PBS, were used. Results of PBS-GBS show that sediment concentration and water depth in the connecting part are continuous. Moreover, even if the riverbed rises due to sediment deposition at the dam, the inflow wall particles increase their position accordingly, and the river bed position is also continuously connected. The temporal changes of flow rate and bulk concentration at the downstream end with PBS-GBS are almost equal to those with PBS. However, sediment discharge and transport concentration at the downstream end are lower when using the PBS method than those from the PBS-GBS method. This is because the grid-based method and the particle method have different evaluation methods of transport concentrations. In the particle method, the vertical distribution of the sediment concentration is calculated, therefore transport concentration is lower than bulk concentration. Result comparison of calculation time of PBS and PBS-GBS methods shows that the number of calculated particles decrease, and the calculation time is shortened by limiting the calculation region of the particle method. The calculation time is proportional to the number of particles. Therefore, PBS-GBS method is particularly effective in the case where the particle method is applied locally, such as evaluating the effect of the check dam.