The Azijue debris flow induced by rainfall on 30 August 2020 in Ganluo County, Sichuan Province, southwestern China

At about 8:00 p.m. local time on 30 August 2020, a large-scale debris flow with an estimated volume of 3.2 million m3 struck Azijue Village in north-eastern Ganluo County, Sichuan Province, China. The debris flow killed three residents, buried two villages, damaged a local middle school, crushed three bridges, and interrupted a significant transportation line (the Chengdu–Kunming Railway) for 14 days. This catastrophic debris flow was successfully detected by local residents 2 h before the final large-volume deposits arrived, which provided enough time for most of the residents to evacuate ahead of the deadly geohazard. To characterise the evolution process of the Azijue debris flow, a detailed field investigation was conducted with the aid of remote sensing technology. The results demonstrated that the debris flow, which had a high viscosity and large density, was part of a geohazard chain. Localised precipitation in a high-relief mountainous area primarily dominated by a subtropical monsoon climate was the main triggering factor for the debris flow. Thick ancient diluvial deposit in the debris flow gully was one of the main environmental factors that facilitated the rapid mass enlargement and long-runout movement of the debris flow. The catastrophic downstream loss of lives and infrastructure can be attributed to a major secondary landslide that originated at the end of the second sub-horizontal diluvial platform and initiated by the combined effect of continuous rainfall, top overloading, and toe cutting by the rapid and dense debris flow.