Sediment Supply and Transport Processes in the Initiation Area of Debris Flows, Ohya Landslide, Japan

Debris flows in mountain streams and ravines cause severe natural hazards due to their high velocity, large volumes, and immense destructive power. In catchments with high sediment supplies and steep channels, debris flows are generally caused by channel mobilization. In these catchments, there is a possibility that changes in quality and quantity of channel deposits influence the initiation of debris flows, because channel deposits constitute the main material of the flow as well as determine the hydrological initiation conditions. However, temporal changes in quality and quantity of deposits in steep channels have not been evaluated because of difficulties related to field observations. Here we examine changes of channel deposits in the initiation zone based on field observations in upper Ichinosawa catchment of the Ohya landslide in Japan. In spring 1998, a monitoring system was installed consisting of video cameras, ultrasonic sensors, capacitive water depth probes and water pressure sensors to discriminate the occurrence of debris flows and assess the rainfall-runoff processes of the catchment. Channel deposits of upper Ichinosawa catchment were periodically photographed to estimate their volume and diameter. Such measurements indicate that sediment supply by freeze-thaw increases the volume of channel deposits and decreases the diameter of bed surface material, whereas occurrence of debris flows decrease the volume of channel deposits and increase the diameter of bed surface material. Influence of other sediment supply processes (landslide and surface erosion) and bedload and suspended sediment transport on changes in volume and diameter of channel deposits is small compared with freeze-thaw and debris flow processes. Physical analyses indicate that sediment supplies and transport processes are largely influenced by gradients of hillslopes and channels. Field observations show that volume and diameter variations in channel deposits cause changes in rainfall-runoff processes. These seasonal changes in channel deposits may be important for estimating volume and timing of debris flows.