Outburst debris flow of Yusui Stream caused by a large-scale Silabaku landslide, Southern Taiwan

On August 7, 2021, a disastrous rainfall-induced landslide occurred at Mt. Silabaku in the Yusui Stream source area, Taiwan. The huge deposits from the Silabaku landslide severely damaged the Minbaklu Bridge, and minor collapses in the Yusui Stream disrupted the only major road, Expressway No. 20. More than 400 residents from the three adjacent villages were stranded for 19 days in the mountainous area. The analysis of rainfall data, seismic records, topographic and geologic data, and historical orthoimages of the slope conditions, landform evolution, and current status of the Silabaku landslide revealed that the landslide resulted from slate avalanche induced by heavy rains, with local failures during Typhoon Morakot in 2009. Based on the high-resolution topographic data of light detection and ranging and an unmanned aerial vehicle, the debris deposits accumulated along the Yusui Stream were estimated to be above 8 million m 3 . In addition, a fan volume of 2 million m 3 changed the flow line of the Laonong River, and the thick deposits pose a considerable threat to the safety of the nearby villages and infrastructures.