Relative importance of slope material properties and timing of rainfall for the occurrence of landslides

Daizo Tsutsumi and Masaharu Fujita

Throughout Japan , almost all areas near hillsides or mountain slopes are threatened by landslides caused by heavy rainfall during the rainy and typhoon seasons. To mitigate potential disasters, many researchers have investigated landslides caused by heavy rainfall and have developed simulation models to predict landslide occurrence. However, because landslide mechanisms are complicated and involve many factors such as rainfall, surface and subsurface geomorphology, and soil physical properties, accurate prediction of landslides remains difficult by conventional simulation methods. Over the last several decades, many researchers have reported the existence of numerous preferential flow pathways for subsurface runoff, such as soil pipes or macro-pores in the soil layer. Simple infiltration analysis ignores those preferential flow pathways and therefore does not accurately simulate actual soil water flow, making it difficult to predict landslide occurrence accurately. For the last five years, we have investigated several landslide sites and have used experimental and modeling approaches to examine landslide occurrence. The results confirm that to improve precision of landslide prediction, simulation models should incorporate some important factors that affect landslide occurrence. This paper reports these issues as well as the results of the landslide investigations, and discusses the methods required to predict and mitigate landslide disasters.