抄録
Owing to the large quantities of volcanic ash that falls continuously on basins, it is generally known that
debris flows can be easily triggered by even a small rainfall in such circumstances. The process of
occurrence of debris flow is explained that the infiltration capacities of slopes are reduced because of the
accumulation of ash, and therefore, rainfall events induce large quantities of surface runoff and subsequent
increase in erosion. The authors established the observation slope at Sakurajima Volcano, one of the most
active volcanos in Japan. Surface runoff was observed on the bare slope on which volcanic ash accumulates
continuously by eruptions. The purpose of observation is better understanding the relationship between the
amount of ash fall and the rainfall threshold for debris flow occurrence. The rainfall conditions necessary
for the occurrence of surface runoff were investigated over an observation period that included periods of
relatively high ash fall rates and periods with relatively low ash fall rates. Results reveal that there is no
evident difference in rainfall intensity that causes surface runoff or in the apparent infiltration capacity of
the slope in the case of short-term temporal changes in volcanic ash fall. It was also revealed that if a
no-rain period lasts for a long time, the amount of rainfall loss from the onset of rains to the occurrence of
surface runoff will increase to some extent.
