Monitoring tools for direct debris flow measurements using sensors such as load cells were first installed in Japan in the Arimura River on Sakurajima Island, Kagoshima Prefecture, where volcanic activity is severe, and numerous debris flows have occurred due to falling ash after eruptions. This system, which is collectively referred to as a DFLP system and is equipped with load cells and pressure sensors, collects information on debris flow characteristics such as specific weight and volumetric sediment concentration. Another DFLP system was installed at Kamikamihorizawa Creek, on the eastern side of Mt. Yakedake, Nagano Prefecture, in November 2014, where significant sediment deposition and large numbers of debris flows have been observed. The present study reports on debris flow surges monitored by this system on August 29, 2019. More than five surges were monitored using the DFLP during 20-minute period, and the rainfall intensity for a 10-minute period just before those events was 12mm, resulting in an accumulated depth of 56mm. During the debris flow surges, both stable and continuous sediment concentration measurements were performed. Using the flow discharge calculated from closed-circuit television video images, the time-averaged sediment concentration and relative mass density were calculated as 0.470 and 1.73, respectively. The equilibrium sediment concentration of coarse sediment particles for the 8°bed slope upstream from the system was estimated at 0.160 by the previous knowledge and 0.201 by the DFLP, with the higher DFLP value attributed to the presence of fine sediment particles in the mud phase.

When it rains over a large area, numerous mountain streams and rivers often lead to debris flows and greater river flows. Since these flows overlap at the stream and river confluences, damaged areas around the confluences may be expanded because of a flow bottleneck formed by debris flow deposition around them. To reasonably predict the processes, we require a numerical model that considers how the deposition and flood propagation of these flows occur at the confluences based on their various characteristics. Therefore, we developed a numerical model named “the multiple inflow model with debris flows and river floods.” In the developed model, the downstream ends of several 1-D calculation areas for mountain streams are connected on a 2-D calculation area for a confluence area at any selected points. With the developed model,we performed calculations to reproduce the debris flow disaster in the Nachigawa River plain, which induced by 2011 Typhoon Talas in Wakayama Prefecture, Japan. During this event, the debris flows that simultaneously flowed into the river from several mountain streams contributed to the deposition and flooding around the streams and river confluences, and the damaged areas around them expanded. The calculated result indicates that we can estimate reasonably the deposition and flood propagation of the debris flows and river flood around the confluences and their downstream areas. This also indicates that the developed model helps us to investigate how multiple debris flows inflowing from mountain streams contribute to the disaster, and develop more efficient countermeasures for these inflows.