Owing to the large quantities of volcanic ash that falls continuously on basins, it is generally known thatdebris flows can be easily triggered by even a small rainfall in such circumstances. The process ofoccurrence of debris flow is explained that the infiltration capacities of slopes are reduced because of theaccumulation of ash, and therefore, rainfall events induce large quantities of surface runoff and subsequentincrease in erosion. The authors established the observation slope at Sakurajima Volcano, one of the mostactive volcanos in Japan. Surface runoff was observed on the bare slope on which volcanic ash accumulatescontinuously by eruptions. The purpose of observation is better understanding the relationship between theamount of ash fall and the rainfall threshold for debris flow occurrence. The rainfall conditions necessaryfor the occurrence of surface runoff were investigated over an observation period that included periods ofrelatively high ash fall rates and periods with relatively low ash fall rates. Results reveal that there is noevident difference in rainfall intensity that causes surface runoff or in the apparent infiltration capacity ofthe slope in the case of short-term temporal changes in volcanic ash fall. It was also revealed that if ano-rain period lasts for a long time, the amount of rainfall loss from the onset of rains to the occurrence ofsurface runoff will increase to some extent.
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