Initiation and runout characteristics of debris flow surges in Ohya landslide scar, Japan

Abstract The characteristics of debris flows (e.g., velocity, discharge, kinematic energy) are highly dependent on surges incurring abrupt changes to flow height, velocity, and boulder concentration. Therefore, understanding the initiation and runout characteristics of surges is essential when planning debris flow mitigation. Monitoring performed using 10 time-lapse cameras (TLCs) in Ohya landslide, central Japan, where debris flows occur frequently due to mobilization of storage (i.e., talus cone and channel deposits), allowed us to obtain data on a series of surge processes, from initiation to termination, which occurred during each debris flow event. We also analyzed temporal changes in the spatial distribution of storage in the debris flow initiation zone, associated with sediment supply from hillslopes and evacuation of sediment by the occurrence of debris flows, through periodic measurements of topography using unmanned aerial vehicles (UAVs). Debris flow surges were mainly induced by repetitive mass movement of storage through the erosion of channel deposits by overland flow, sliding of channel deposits, and sediment and water supply from channel banks and tributaries. Development of a spontaneous wave on the flow surface was not an important formation process of surges in the Ohya landslide. Many debris flow surges initiated at channel sections with deep storage (>2 m in depth), located