Direct Debris Flow Measurements using DFLP system at Kamikamihorizawa Creek

Observations of debris flows have been reported in numerous studies over many previous decades [e.g., Arattano , 1999 ; Rickenmann , 1999 ; Suwa et al ., 2019], and systematic research into the mechanics of these flows, including detailed investigations in flume tests, has been carried out for approximately 60 years [e.g., Daido , 1971 ; Rickenmann , 1999]. Previous studies of debris flow mechanisms have examined non-uniform-sized debris in mountain torrents via flume tests and field observations as part of efforts to clarify those mechanisms. However, it is still difficult to grasp the vertical and longitudinal component segregation of sediment particles and the vertical profiles of velocity and volumetric concentration that are related to pressure and stress. In Japan, the Disaster Prevention Research Institute of Kyoto University (DPRI) and the Matsumoto Sabo Office of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) began debris flow monitoring and observations at Kamikamihorizawa Creek on the eastern slope of Mt. Yakedake in 1970 [e. g., Suwa et al ., 1973 ; Okuda et al ., 1980]. Heavy rainfalls have caused numerous debris flow events since the last phreatic explosion of this volcano in 1962. Those observations, which continued until the 2000s, resulted in the collection of vast amounts of data on phenomena such as the longitudinal spreading velocity of the frontal part, temporal changes of surges, the relationship between the flow peak discharge rate and the volume, and other factors. At Sakurajima Island, debris flow observations were carried out from the 1970s to monitor the debris flow occurrences that followed volcanic eruptions [e.g., Osaka et al ., 2014], as well as at Kamikamihorizawa Creek, where sediment-water mixture measurements of debris flow bodies were carried out manually with a sampling box to evaluate sediment concentrations. However, little information regarding sediment Original Article