Deep catastrophic landsides have triggered large]scale debris flows that have had serious impacts on humans. Therefore, it is important to predict the run]out process of debris flows and to identify debris flow hazard areas. Here we developed a new technique for simulation of large]scale stony debris flows. Debris flows have been modeled by mixture of solid and fluid phases. In this study, we assumed that fine sediment in debris flows behaves as fluid phase rather than solid phase. Based on this hypothesis, we developed new methods to evaluate key parameters to simulate large]scale debris flows, such as sediment concentration, fluid density, and representative particle diameter and modified the continuity equation for sediment. We also proposed a new process]based method for determination of hydrographs at the lower end of the landslide scar. We conducted detailed field surveys of the past debris flow in Atsumari river in Minamata city and used topographic data from LiDAR imagery, porosity measurements of soil and weathered bedrock and the grain size distribution of the debris flow sediments to test our model. Using these new data and methods, we conducted numerical simulations of the past debris flow, which reproduced well the observed erosional and depositional pattern.

Key wordsFdebris flow, deep rapid landside, numerical simulation, fine sediment, Atsumari river