Extreme climate and human activities contribute to low-frequency, large-scale catastrophic debris flow: a case study in the Heishui Gully

Rainfall is an excitation factor for rainfall-type debris flows, and high-frequency rainfall commonly does not directly trigger large-scale catastrophic debris flows. A low-frequency, large-scale catastrophic debris flow occurred in Heishui Gully (HSG), Pingwu County, Sichuan Province, southwestern China on 12 July 2022. In this study, the HSG debris flow was used as a case study to reveal the formation mechanisms of low-frequency and large-scale catastrophic debris flow triggered by high-frequency gentle rainfall by calculating and analysing a total of five indicators, including the Standard Precipitation Index (SPI), the Freeze-Thaw Index (FTI), the empirical relationship between earthquakes and landslides, the hypsometric integral (HI) and human activities. The study results indicate that high-frequency gentle rainfall is a direct factor in the debris flow outbreak in HSG. Long-duration extreme drought conditions and extreme freeze-thaw erosion provided sufficient soil conditions for debris flow formation. Frequent earthquakes and mature erosional geomorphologic evolution have indirectly contributed to the debris flow formation. Simultaneously, human activities downstream of HSG amplified the risk of debris flow disaster. The results reveal the contribution of extreme climates and human activities to low-frequency, large-scale catastrophic debris flows initiated by high-frequency gentle rainfall, which is significant for accurately assessing and predicting debris flow occurrence.