A hydrogeomorphological study on landslides and debris flows in volcanic areas

本文研究了日本九州南部Shirasu地区滑坡和泥石流发生的有效降雨量及其水文地貌过程和机制。研究结果如下：1.石拉苏地区滑坡灾害发生的有效降雨量除小尺度降雨外，其余均为24小时半周期的深度200 mm。该临界值是预测降雨引起的滑坡灾害的有效指标。雪拉苏河谷头坡面崩塌泥石流的发生，与雪拉苏沉积前河谷头坡和水文流域区域地下水位升高密切相关。这一关系对于泥石流源头高势点的预测是有效的。谷头坡的地下水是由沉积物中渗入的雨水提供的。通过对地下水位、土壤含水率、地下水流量和降雨量的水文观测发现，地下水位在几个小时内对降雨作出了响应。这说明雨水排放的管网和隧道网络分布在沉积物中。用圆周滑动法分析坡体稳定性时，谷头边坡的稳定性对地下水位的变化比较敏感。

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.