Investigation of 20 August 2019 Catastrophic Debris Flows Triggered by Extreme Rainstorms Near Epicentre of Wenchuan Earthquake

A strong earthquake could trigger a large number of co-seismic landslides and induce large amount of loose materials on steep slopes and in the gullies. Under strong rainfall conditions, these loose materials could induce devastating debris flows, which will endanger the resettled population and destroy the re-built infrastructures. From 19 to 20 August 2019, fourteen debris flows were triggered by extreme rainstorms near the epicentre of the Wenchuan Earthquake. Among the fourteen incidents, three of them produced debris flow dams, which changed the course of the Minjiang River and resulted in flooding at different parts of the reconstructed Miansi town. In addition, sixteen casualties, twenty two missing persons, and destruction of four main roads were reported. In this paper, one typical catchment, named as “Dengxi gully”, near the epicentre of the Wenchuan earthquake (Sichuan Province, China) was chosen as a case study for remote sensing analysis, field investigation of landslide evolution and debris flow development before and after the catastrophic events. The debris flow in the study area was initiated in four stages: (a) generation of a large amount of loose materials from the Wenchuan Earthquake; (b) run-off erosion from co-seismic landslide material on hilly slopes and repeated mobilizations in steep channels over the years; (c) development of high intensity localised rainfall events; (d) wash out of accumulated materials in gully by the flood. The study of “8.20 debris flows” can provide a benchmark for analysis of long-term evolution of debris flows in order to identify potential continuing hazards in the earthquake-affected areas and make proper engineering decisions.