Forecasting landslide hazard by the 3D discrete element method: A case study of the unstable slope in the Lushan hot spring district, central Taiwan

• A potential landslide was analyzed and forecasted using the 3D discrete element method.
• Scenario-based runout and deposition areas can be efficiently forecasted by the method.
• The discrete element method (PFC3D) is limited but useful for simulating landslides.
• The critical friction coefficient is a crucial factor for future applications of PFC3D.

Catastrophic landslides and related phenomena are commonly facilitated by the subtropical climate with frequent typhoons and recurrent earthquakes in Taiwan's mountainous areas. One area susceptible to potentially catastrophic landslides is located at Lushan in central Taiwan, which is famous for its hot springs and tourism. The northern slope above the hot spring district slips gradually and frequently due to heavy rainfall. For safety reasons, the slip-affected area has been under constant borehole monitoring by government agencies, and a controversial public debate has arisen over permanent evacuation of the hot spring district. In this study, we attempt to simulate possible scenarios of catastrophic slope failure using slip geometries derived from the monitoring data, and assess potential landslide impact areas by a discrete element method using the PFC3D code. In the worst case scenario, the Lushan hot spring district is predicted to be destroyed by debris in 20 s. Besides, the planned regional emergency refuge is rapidly jeopardized by flooding resulted from landslide-dammed lakes. This study addresses catastrophic slope failure under heavy rainfall conditions given a range of friction coefficients and varied continuity of the failure surfaces. It is noted that the PFC3D code has limitations in modeling all complex mechanisms of landslide, particularly in modeling loss of material shear strength due to increases in pore pressure. Nevertheless, the numerical simulation results by the 3D discrete element method provide scenario-based runout paths, particle velocities and landslide-affected areas, which are useful information for decision support and future landslide hazard assessment.