On August 7, 2021, a devastating debris flow hit the Yusui stream in Taiwan, damaging the Minbaklu Bridge on the southern cross-island highway. The enormous volume of the debris flow was caused by a catastrophic rainfall-induced landslide at Mt. Silabaku, the source of the Yusui stream.
Historical aerial photos indicated that the main scarp of the Silabaku landslide was present for over 20 years before the incident. The slope remained stable until Typhoon Morakot in 2009. Over the following decade, several rainfall events further deteriorated the slope, which partially collapsed due to headward erosion along gullies. Concurrently, the main scarp began to spread, and the residual slope slipped downward.This study integrated multitemporal optical images to investigate the evolution of the Silabaku landslide and estimate the slope’s strain/strain rate trend, enabling evaluation of the urgency of failure. Landslide topographic features and volume were interpreted on the basis of high-resolution digital terrain model data.Finally, the relationship between intense rainfall events and the rainfall threshold for landslides was explored.
Key Words: Yusui stream, large-scale landslide, rainfall-induced landslide, unstable sediment, strain/strain rate