Investigation of the Hsien-du-Shan rock avalanche caused by typhoon Morakot in 2009 at Kaohsiung county, Taiwan

The Hsien-du-shan rock avalanche at Kaohsiung County, Taiwan, was triggered by heavy rainfall brought by Typhoon Morakot in 2009. Heavy rainfall began on the day of August 6, and continued to the morning of August 9 when the slope failure and landslide occurred. In this post-event investigation, physical, mechanical, and hydraulic properties of the rocks/soils at this large landslide site are evaluated. The in situ hydraulic conductivity of the soils at the source area was found to be too low (k=1.48×10−6m/s) for rainfall to vertically infiltrate to the deep sliding surface, which was found to have a maximum depth of 85.6m. The post-failure simulations using discontinuous deformation analysis (DDA) suggest that the geometry of the sliding surface governs the failure behavior, and two continuous progressing failures likely occurred at the beginning of the Hsien-du-shan rock avalanche. Slope deformation was observed by the local residents before the Hsien-du-shan rock avalanche occurred as a geomorphologic precursor. In addition, water accumulation along the sliding surface governs the behavior of the sliding surface. Therefore, in the future, an early warning system of a rock avalanche can be developed by simultaneously applying high-resolution airborne LiDAR to detect slope deformation and using geo-electric measurements to image the infiltration of surface water to the sliding surface. ► We conducted DDA to simulate the post-failure behavior of the Hsien-du-shan rock avalanche. ► Two continuous progressing failures likely occurred at the beginning of the landslide. ► Physical and mechanical properties of local rocks/soils were investigated. ► The permeability of sliding materials is too low to infiltrate rainfall to the sliding surface.