Investigation of the slope-type debris flow disaster chain triggered by a landslide-induced road blockage in Yingpan Township, Shuicheng, Guizhou, on July 27, 2023

On July 27, 2023, at approximately 04:00, a debris flow, with an estimated volume of 25,000 m³, struck Lanhua Village in the southern part of Shuicheng District, Guizhou Province, China. This event led to the burial and damage of 15 houses in Lanhua Village. Remarkably, residents successfully observed this debris flow 30 min before its peak arrival, enabling sufficient time for evacuation prior to the onset of this fatal geological disaster. To elucidate the evolutionary process of the Lanhua Village debris flow, a detailed field survey was conducted utilizing unmanned aerial vehicle (UAV) technology. The findings suggest that extreme heavy rainfall served as the primary trigger for the debris flow. Landslides, comprising residual slope deposits at the top of the slope and induced by rainfall, resulted in the obstruction of Highway X244 and the blockage of culverts beneath the road. These blockages contributed to the formation of a peak cross-basin flood, exceeding 7 m³/s, which acted as the driving force initiating the debris flow. The thick layer of colluvial deposits covering the slope surface, with approximately 23,000 m³ of material mobilized by flood erosion, constituted the primary source contributing to the rapid expansion and movement of the debris flow. The topography, characterized by steep upper slopes and gentler lower slopes, was a contributing factor to the debris flow deposits spreading into residential areas, resulting in disaster. The event in Lanhua Village exemplifies a “landslide-blockage-flood diversion-erosion-debris flow” disaster chain.