Risk assessment for flash flood disaster considering water-sediment synergistic effect in alpine and canyon areas

To improve the accuracy of flash flood risk assessment in alpine and canyon regions, this study focused on Baoxing County, Ya′an City, Sichuan Province as the research area. Through field investigations, key risk indicators were systematically evaluated for each assessment unit, including flood control capacity, basin concentration time, population size, magnitude of loose solid material and special channel morphology. The flood disaster risk classification for 96 assessment units was conducted following the scoring framework provided in the Technical Requirements for Dynamic Management and Grading of Flash Flood Hazard Zones in Sichuan Province. Additionally, in rich sediment areas, the potential for sediment deposition, triggered by sediment-laden flash floods passing through special channel reaches, and its consequent reduction in flood control capacity were incorporated to adjust and refine the risk ratings. The results indicated that without accounting for water-sediment synergy, 66.7% of the area were classified as low risk and 33.3% as medium risk. However, 63.6% of historical flash flood events occurred within zones initially rated as low risk, suggesting a discrepancy in risk representation under this model. After integrating the impairing effect of water-sediment interaction on flood defense capacity, 31.2% of the originally medium risk zones were upgraded to high risk, and 43.8% of the lowrisk zones were elevated to mediumrisk. Under this revised assessment, 68.2% of recorded historical flash floods fall within medium and high risk categories, demonstrating that the adjusted approach more accurately aligns with observed disaster patterns and enhances the reliability and practical relevance of flash flood risk evaluation.