Development Characteristics and Disaster-Causing Mechanisms of the "8·3" Catastrophic Flash Flood and Debris Flow in Ganzi, Kangding, Sichuan Province

Objectives: On August 3, 2024, a catastrophic flash flood and debris flow devastated Ridi Village in Guzan Town, Kangding City, Ganzi Prefecture， Sichuan Province. This disaster led to the collapse of a bridge on the Yakang Expressway between the Ridi No. 1 and No. 2 tunnels and destroyed several homes in the village, resulting in 23 people missing and 4 confirmed fatalities. Methods: To gather critical information and support emergency rescue operations, an in-depth analysis of the Ridi Gully debris flow was conducted using drone aerial photography and on-site investigations. The study examined the formation processes and underlying causes of the debris flow, with key dynamic parameters being calculated. These data were then used to simulate the debris flow's dynamic evolution using FLO-2D software, the simulation results' verification accuracy (Ω) reached 1.65. Results: The findings suggest that the debris flow was primarily triggered by internal factors, including the steep terrain of the watershed, the presence of weathered and fractured granite, and the abundance of loose material within the gully. External factors such as recent temperature increases, which accelerated snow and ice melt, along with sustained rainfall, created the conditions necessary for the debris flow to occur. The flow transported approximately 58×10⁴ m³ of solid material, with the largest boulders measuring up to 6 m in diameter, while most ranged between 20 cm and 1 m. The flow velocity exceeded 8 m/s, and the impact force of individual boulders was over 500 kilonewtons, with the total impact force in the flow path surpassing 100 tons, resulting in significant destructive power. Conclusions: Critical insights into the formation and mechanics of the Ridi Gully debris flow offer valuable guidance for emergency response and disaster mitigation in regions with similar geological hazards in the mountainous areas of southwestern China.