Behavior of debris flows monitored on test slopes of Kamikamihorizawa Creek, Mount Yakedake, Japan

Abstract

Kamikamihorizawa Creek on the slopes of Mount Yakedake, Nagano Prefecture, was selected as an observation site for debris flows due to the high frequency of debris flow and instrumented with monitoring equipments in 1970: seven years after the last phreatic explosion of this volcano. The monitoring system was improved by adding speedometers, stage meters, seismometers and so on. Over the last 30 years, data were obtained from 88 debris-flow events that contained more than 200 episodes of debris-flow surges. The data indicated the following common features related to debris flows. 1) High-intensity rainstorms with a short duration of about 10 min were more likely to trigger debris flows than storms with large rainfall for longer durations. 2) The threshold of rainstorm intensities for debris flows increased due to the changes in hillslope hydrology. 3) The hydrograph of debris-flow surges showed mass and boulder focusing on the front due to mechanical processes. 4) Pulsation and superelevation were common features that were significant as hazard-inducing factors. 5) Monitoring of seismic signals from debris flows is useful for hazard warning as well as for estimating debris-flow hydrographs. 6) Evaluation of hazard-prone locations downstream in the fan is possible by analyzing the interaction of the flows and the fan morphology. 7) Moderation of debris-flow hydrographs is achievable using new net dam or flat-board breakers in the lower reaches of gullies.