Evolution of the large landslide induced by Typhoon Morakot: A case study in the Butangbunasi River, southern Taiwan using the discrete element method

In 2009, Typhoon Morakot delivered high intensity rainfall and triggered many sites of large landslide, which projected areas were larger than 10,000 m2 from the satellite image, especially in southern Taiwan. According to DTM analysis, the material volume of the large landslide in Butangbunasi River was more than four times larger than that of the nearly famous landslide disaster in Hsiaolin village. Nevertheless, the different movement of these two large landslides led to different disaster impacts. Based on literatures and field works, the large Hsiaolin landslide was a catastrophic landslide, which had high velocity and less energy loss in the kinematic process. The source area of the large landslide in the Butangbunasi River, on the other hand, was composed of several landslide events, and the kinetic energy dissipated when the landslide materials attained to riverbed in sequence. In this study, we divided the Butangbunasi River large landslide into five slope units, namely A, B, C, D and E landslide events, to recognize the mechanism of the large landslide, and the simulation by using the Particle Flow Code 3D (PFC3D), which is based on the discrete element method, provided the kinematic process of the large landslide in the Butangbunasi River. It is noticed that most of the initial energy dissipated during the movement which the fist happening landslide event counteracted the crash of the follow-up events, and the residual energy gradually declined by the collision interaction of landslide blocks until the end of the large landslide. The result helps to reconstruct the evolution of the large landslide from small-scale landslide events and contributes to predict the affected region of the disaster. It also reveals that even though the impact of the large landslide is reduced by sequent small-scale landslide events, there still remain secondary disasters, such as landslide dam, which show a high potential risk in the river valley and midstream.