Long travel distance of landslide-induced debris flows

Abstract

Large-scale landslides often induce debris flows and cause serious damage to humans. These events typically have water contents in the landslide mass less than 60% and sediment concentrations more than 40%. In spite of high sediment concentrations, landslide-induced debris flows can runout long distances. For large-scale stony debris flows, many previous studies have suggested that coarse gravels behave as a solid phase, whereas fine particles with interstitial water can behave as a fluid phase. We hypothesized this fine sediment might be one of the key processes controlling the long travel distances of landslide-induced debris flows. Here we assumed that the maximum diameter of the fine sediment behave as a fluid phase should vary depending on the friction velocity of the debris flow and the settling velocity of sediments. We conducted detailed field surveys for four landslide-induced debris flows and applied our numerical simulation model to describe the travel distance of the debris flows. Our results show that, if we set the ratio of the friction velocity of debris flow to the settling velocity of sediments around 1 to 4, the simulated travel distance agreed well with our studied four debris flows. We also confirmed that, while the total volume or mean sediment diameter of debris flows varied between study cases, the variability of ratios was small. We believe that our new method and the information it provides, may be helpful for predicting the future risk from the landslide-induced debris flows.