Abstract. The influence of climate change on patterns has the potential to alter stability of partially saturated soil slopes. Changes in rainfall patterns have a strong influence on stability of partially saturated soil slopes, which recently have resulted in shallow landslides. In this paper, a comprehensive case study on the 2011 Umyeonsan (Mt.) landslides was highlighted. The incident involves the collapse of a soil slope and the debris flow under extreme-rainfall event, causing 16 fatalities and serious damaged to 146 housings. A fundamental study was carried out on the cause and mechanism of landslide/debris flow. An analytical method is developed for determining the failure mechanism of unsaturated soil slopes under extreme-rainfall, the effect of groundwater flow; the downward velocity of wetting front, and the upward velocity of groundwater level. Based on this, we propose the conceptual methodology of landslide design based on experimental tests and numerical analyses which consider the important mechanism of the combined effects of both groundwater flow and rainfall infiltration into the slope.