A hydrogeomorphological study on landslides and debris flows in volcanic areas

研究了日本九州南部白苏（Aira火山火山碎屑流沉积）地区山体滑坡和泥石流发生的有效降雨及其水文地貌过程和机制。研究结果如下：1。白松地区半期降水为24 h的滑坡灾害发生有效降雨量除小规模外，深度均为200 mm。该临界值是预测降雨引起的滑坡灾害的有效指标。白露河谷头部坡面崩塌引发的泥石流的发生，与白露沉积前河谷头部坡面及水文流域面积地下水位的升高密切相关。该关系对泥石流高势源位置的预测是有效的。谷头坡的地下水是由雨水通过沉积物入渗补给的。通过对地下水位、土壤含水量、地下水流量和降雨量的水文观测，可以清楚地看出，地下水位在几个小时内对降雨有响应。这表明沉积物中存在排水管道和隧道网络。从圆滑动法分析的坡面稳定性来看，谷头坡面稳定性对地下水位变化较为敏感。

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.