Recently, landslides occurred more frequently worldwide due to extreme climatic and geological disasters, many of which blocked the river in mountainous areas, posing a great threat to residents by inundation and outburst flood. The top two failure modes of landslide dams are overtopping and piping. Most of the current studies only investigated landslide dam failures caused purely by overtopping. However, landslide dams are normally composed of poorly graded and unconsolidated sediments, which are highly susceptible to piping. It is recorded that some landslide dams have experienced local piping before it overtopped. Under such circumstance, the dam erodibility and internal structure change accordingly, which further affect the subsequent overtopping and flood routing process. Therefore, the role of piping in landslide dam failures is crucial, which should be seriously considered and needed to be urgently studied. In this study, a preliminary investigation of landslide dam failures induced by the combined influence of piping and overtopping is carried out by laboratory experiments and numerical simulations. Results show that with increasing particle loss by piping, the soil void ratio increases while the friction angle decreases, leading to a more porous dam structure and a dramatically increased dam erodibility. The more particles have lost before overtopping, the larger the peak discharge is, and the thinner the hydrograph becomes, implying that the piping amplifies the outburst flood. It is also found that when the landslide dam has suffered a more serious piping before it overtopped, the peak flood discharge becomes larger and the peak arrival time advanced obviously, nevertheless, the flood attenuates more sharply along the river reach. The outcome of this research will enrich our understanding of landslide dam failures under the combined influence of piping and overtopping, which can provide scientific evidences for risk management and disaster mitigation of landslide dams.
