Long‐Runout‐Landslide‐Induced Debris Flow: The Role of Fine Sediment Deposition Processes in Debris Flow Propagation

Abstract

AbstractLandslide‐induced debris flows can travel long distances. Many field studies, laboratory experiments, and theoretical studies have been conducted to clarify the mechanisms of long‐runout landslides. However, information concerning landslide‐induced debris flows is still inadequate despite several explanations proposed. Thus, we collected various forms of data, including video images and light detection and ranging data, for a landslide‐induced debris flow that occurred on 28 July 2015, in Fukaminato River, Japan, and clarified the debris flow behavior from initiation to deposition. We showed that the water content of the debris flow should only be approximately 30% immediately after landslides occurred at the site. However, a fluid phase‐dominated flow flowed over 1.2 km, expanding the damaged area. We inferred that the fine sediments behaved as part of the interstitial fluid in the debris flow and that, during debris flow deposition, some fine sediments were stored in the interstitial space of the riverbed and did not contribute to forming the skeleton of deposits. We conducted a numerical simulation to test the processes of fluid phase‐dominated flow propagation, focusing on the flow and deposition of fine sediments. Our simulation results suggest that fine sediments' flow and depositional processes significantly influence both the initiation of fluid phase‐dominated debris flow and the travel distance of landslide‐induced debris flow.