Abstract
The Ryugamizu cliff facing the Kagoshima Bay is well known as an hazardous area for slope failures in Japan. Slope failures and debris flows have frequently occurred on the cliff during every rainy seasons. The Quaternary volcanic rocks, pyroclastic rocks and sedimentary rocks composing the cliff form a complicated distribution and structures. Based on drilling log data of 200 meters depth on the plateau, the rock mass composing the cliff can be understood as a combination of permeable volcanic rocks with systematic joints and poor permeable siltstone intercalated by them. All strata dip gradually southwestern direction with plunging structure against the cliff. Lateral changes of thickness and lithofacies are especially remarkable in poor permeable strata.
Based on these geological structures, most of rainwater fellen on the plateau may flow southwestward. However, if the rain is heavy, uplift of groundwater table due to rain water may bring about gush of water from the cliff which is one of triggering causes for the occurrence of slope failures and debris flows.
