Research on regional inundation risk based on a coupled flash flood and debris flow model——A case study of a typical area of Changping District in Beijing during the "23·7" rainstorm event

In mountainous areas under extreme rainstorm scenarios, the concurrent occurrence of flash floods and debris flows triggers disaster chain effects. Traditional single hydrological-hydrodynamic models often underestimate inundation risks due to their neglect of terrain alterations. Taking Changping District, Beijing, as the study area, this paper constructs a coupled simulation model for the flash flood-debris flow disaster chain based on TRIGRS, MASSFLOW, and VMR-TRITON. First, the applicability of the VMR-TRITON model in Changping District was validated using the flash flood event of August 9, 2024.Second, debris-flow-induced terrain deformation is incorporated into the hydrodynamic model by dynamically updating terrain elevation over time. Finally, the model was verified using Hantai Village, a severely affected area during the "23·7" catastrophic rainstorm. The results indicate that the average relative simulation error of the VMR-TRITON model in Changping District is approximately 18.98%, demonstrating good applicability. Compared with simulation results that ignore channel deposition, the coupled flash flood-debris flow model improved the relative error in the simulated inundation area of Hantai Village from -52.66% to -22.98%, significantly enhancing accuracy. This study confirms the necessity of considering debris flow silting effects in simulations and provides a scientific basis for the refined defense against flash flood disasters.