Direct Debris Flow Measurements using DFLP system at Kamikamihorizawa Creek

Abstract

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.