A field survey of water-sediment impacts on composite disaster of the "7·20" flash flood and debris flow in Hanyuan County, Southwest China

Rainstorm-induced flood hazards in mountainous areas often result in complex cascading effects by interacting with environmental and human systems. However, traditional studies typically categorize them simply as clearwater floods or debris floods/flows, overlooking their evolutionary characteristics and compound impacts. This study presents a novel classification-based approach to investigate the formation and destructive mechanisms of a catastrophic composite disaster of flash flood and debris flow in the Dayao Gully (DYG) catchment in Hanyuan County, Sichuan Province, China. The event resulted in 14 fatalities, 25 missing persons, and extensive infrastructure damage. Through comprehensive field investigations and multi-method analysis, three distinct disaster zones were identified with different magnitudes and impacts: (1) a clearwater flood disaster region with minimal geomorphological changes under a 5-year return period rainfall; (2) a debris flood disaster region triggered by a 30-year return period rainfall, leading to intense sediment transport with a total deposit volume of 52, 511 m³; and (3) a sediment-induced flood disaster region characterized by significant riverbed aggradation and infrastructure destruction due to sediment-induced blockage effects. The results reveal that the cascading characteristics of this composite disaster were primarily driven by intense rainfall, enhanced sediment transport motivated by supra-critical shear stress, and interactions with human infrastructure (e.g., bridges and buildings). This classification-based approach provides a quantitative assessment of spatial characteristics of cascading flood disasters, offering new insights into their evolutionary characteristics and highlighting the necessity for targeted disaster mitigation strategies in sediment-prone mountainous regions.