Disaster mechanisms and hazard assessment of rapid and steep-gully debris flows based on FLO-2D simulation

The kinematic evolution and precise hazard zonation of rapid debris flows in steep gullies remain challenging due to complex topographic and hydrological conditions. Taking a typical catchment in the North Tianshan Mountains as a case study, this research investigates the dynamic disaster-triggering mechanisms and spatial hazard distribution of such events by integrating field surveys, laboratory testing, and FLO-2D hydrodynamic modeling. Results indicate that formation is primarily governed by the regional geological setting and rainfall intensity. Highly fractured rock masses provide abundant source material, while high-gradient topography facilitates rapid initiation and transport. Short-term intense rainfall acts as the decisive trigger, with the kinematic evolution characterized as a "source enrichment-dynamic triggering-path conduction-accumulation" disaster chain arising from multi-factor coupling. Quantitative reconstruction of the 2024 event demonstrates that the model achieves verification accuracies of 89.01% for maximum flow depth and 87.84% for deposition area, confirming its reliability for this specific gully type. Multi-scenario hazard assessments reveal that flow depth, velocity, and hazard footprints expand significantly with increasing rainfall return periods. Under a 100-year scenario, the maximum flow depth reaches 5.83 m, the peak velocity is 6.80 m/s, and the high-hazard zone covers 2.28 × 10⁴ m², posing severe threats to downstream settlements. By providing a validated, high-precision hydrodynamic framework, this study offers a robust scientific basis for multi-scenario hazard zonation and the design of engineering mitigation strategies in vulnerable mountainous terrains.