Investigation of the 2019 Wenchuan County debris flow disaster suggests nonuniform spatial and temporal post-seismic debris flow evolution patterns

It is widely recognized that post-seismic mass movements amplify and decay. Previous studies have found that most post-seismic mass movement is concentrated in the first few years following an earthquake. A major debris flow occurred in Wenchuan County in 2019, 11 years after the 2008 Wenchuan earthquake, showing that there might be a different temporal evolution pattern. In Wenchuan Country, the area struck by the 2019 disaster was investigated to explore whether a pattern existed. Remote sensing and field surveys investigate the initiation processes, sediment supply, and triggering rainfall. The result shows that most of the active landslides occurred in high-elevation areas where vegetation cover was lacking, and nearly half of them were reactivated landslides. The debris flows were mainly initiated by run-off erosion of debris in steep channels, and more than half of the sediment supply was from deposition along the channels in some catchments. The spatial and temporal evolution of debris flows was analyzed by combining our investigation results with historical events. More debris flows occurred in the northern part of the study area, where the relative annual rainfall and coseismic landslide density were low. The average values of debris-active catchments’ size and relief are increasing, while the average values of their coseismic landslide density and the annual rainfall received were decreasing from 2008 to 2019. Larger catchments receiving less annual rainfall tend to have a more prolonged and enhanced mass movement.