A hydrogeomorphological study on landslides and debris flows in volcanic areas

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We examined effective rainfall for occurrence of landslides and debris flows and their hydrogeomorphological processes and mechanism in the Shirasu (pyroclastic flow deposits from Aira volcano) area, southern part of Kyusyu, Japan. The results are as follows :1. The effective rainfall for occurrence of the landslide disasters in the Shirasu area in which a half-period of rainfall is 24 hours, is 200 mm in depth except for small scale ones. This critical value is an effective index for the prediction of landslide disasters caused by rainfall.2. The occurrence of debris flow originating from collapse of talus slope at the head of the Shirasu valley, is closely related to an increase of ground water level in the valley head slope and area of hydrological drainage basin before the deposition of Shirasu. This relation is effective for prediction of high potential sites for the source of debris flow.3. The ground water in the valley head slope has been supplied from rain water infiltrated through the deposits. It became clear that the ground water level responded to rainfall in a few hours from the hydrological observations on ground water level, soil water content, ground water discharge and rainfall. This suggests that the pipe and tunnel network for draining rain water is distributed in the deposits.4. The stability of talus slope at the valley head is sensitive to a change of the ground water level from the slope stability analysis by circular slide method.