Interaction between driftwood deposition patterns and debris flow fan development

Intense rainfall events in mountain regions result in pronounced geomorphic activity including landslides, debris flows, sediment transport, and large woody debris. This study investigated the factors that affect wood deposition patterns during debris flows and the influence of wood on sediment deposition. We conducted flume experiments to simulate debris flow fans with driftwood and divided the experiments into plain and confluence sets. The plain set considered the condition where the debris flow tributary met a mild terrain, whereas the confluence set considered where the tributary met a mainstream. The results revealed that wood with roots was liable to form a log jam on the tributary, and small logs could travel long distances. In the confluence set, the mainstream flow caused asymmetric expansion of the debris flow fan and drove the small wood logs drifting downstream. In addition, wood logs may affect the sediment morphology, particularly in the plain set, which has no mainstream effect. In the plain set, driftwood constricted the development of fans in both volume and area. The wood also deflected the path and deposition of debris flows, causing an uneven fan surface. Our experiment results were consistent with the Japanese historical events, including the asymmetrical fans expansion in the confluence environment and the correlation between the travel distance and the size of the driftwood. This study provides valuable insights into the deposition patterns of debris flows with driftwood in mountain regions, filling the gap left by previous researches in driftwood-influenced debris flow deposition on fans.