The Wuxie debris flows triggered by a record-breaking rainstorm on 10 June 2021 in Zhuji City, Zhejiang Province, China

Deep into the night of 9 June 2021, a high-intensity and short-duration rainstorm, the largest rainfall ever recorded in Zhuji, struck the entire city. The abrupt rainfall triggered several debris flows, which killed four residents and closed a highly ranked national scenic area for over three months. The debris flows, triggered by record-breaking rainfall, occurred in a rhyolite porphyry area that was not recognised as an area prone to frequent debris flow. Thus, detailed field investigations were conducted on three debris flow gullies. It was demonstrated that an extreme rainfall event, complex topographic profiles and abundant diluvium in the gullies were the main contributing factors in the disastrous damage. In particular, steep eluvial slopes with a high gradient of over 30 degrees failed under heavy rainfall, which was one of the major factors in the initiation of debris flows. In addition, geometric properties, such as the curved travelling path, natural cliff and cross-sectional shape of the gullies, interacted significantly with the deep and lateral erosions of diluvium in the gullies, which promoted mass enlargement and the long-runout movement of the debris flows. Furthermore, reasonable countermeasures made some contribution to the reduction and mitigation of this kind of geo-disasters.

Automated summary

On 9 June 2021, Zhuji experienced the largest rainfall ever recorded, triggering several debris flows that caused casualties and the closure of a scenic area. The extreme rainfall event, complex topography, and abundant diluvium were the main contributing factors to the disastrous damage. Steep eluvial slopes and geometric properties of the gullies interacted significantly with diluvium erosion, resulting in mass enlargement and long-runout movement of the debris flows. Reasonable countermeasures played a role in reducing and mitigating such geodisasters.