Catastrophic debris flows triggered by the 20 August 2019 rainfall, a decade since the Wenchuan earthquake, China

Enhanced debris flow activity observed after the 2008 Wenchuan earthquake, Sichuan Province, SW China, is still intense more than a decade since the earthquake. A heavy rainstorm on 20 August 2019 caused catastrophic debris flows in the epicentral area of the earthquake. By remote sensing imagery interpretation and field investigation, 13 debris flows were identified. An empirical rainfall threshold I-D model (intensity-duration) of post-seismic debris flows is established. Changing rainfall thresholds, characteristics, and initiation mechanisms of the 2019 debris flows were analyzed, which are different from the events that occurred in previous years. We find that the 2019 debris flows were mainly initiated by concentrated runoff erosion of sediments stored in tributary channels of catchments, with little contribution from the coseismic landslides and new hillslope failures. Differently, the major sediment supply of debris flows in previous years was mainly from the reactivation of coseismic landslide deposits. By analyzing the triggering rainfall of all recorded debris flows after the Wenchuan earthquake, we find that the rainfall threshold in 2019 is about 1/2-4/5 of the pre-earthquake value, and the I-D model we built can predict the 2019 debris flows successfully. Considering that there are still a large number of coseismic landslide deposits in the mountain catchments and the rainfall thresholds are slowly recovering, the post-seismic debris flows may last for a long period.