Response of ancient landslide stability to a debris flow: a multi-hazard chain in China.

On June 17, 2020, a debris flow dammed lake was formed in Meilong gully, Danba County, China, and a subsequent outburst flood led to the reactivation of the Aniangzhai ancient landslide. This new type of multi-hazard chain (MHC) has attracted widespread attention. This study aims to introduce the formation and evolution processes associated with this novel MHC and develop proposed countermeasures based on field investigations and numerical simulations. We show that this type of MHC comprises five stages: (1) debris flow blocking a river; (2) formation of a barrier dam and lake; (3) lake outburst; (4) revival of an ancient landslide; and (5) flood-induced debris flow. Among these, stage (4) brings particular uncertainty to the risk assessment of the MHC. For translational landslides, the erosion associated with an outburst flood may continuously reduce their stability, while for rotational landslides, their stability may instead increase. By analyzing the formation conditions of the debris flow, criteria required for river blockage, deformation history of the Aniangzhai landslide, and changes in geomorphology, we conclude that the risk of an MHC at this locality may increase in the future. Both rigid dams and flexible barriers could be effective measures to address these risks. Our field investigations and remote sensing analyses indicate that other watersheds in the study area are also exposed to the risk of a similar MHC; this requires detailed future analysis. [ABSTRACT FROM AUTHOR]