An integrated hydrodynamic model for flash flood and debris flow simulations

Climate change is forecasted to result in more frequent and intense storms, which in turn are likely to cause more flash floods and other hazardous processes in steep hilly and mountainous catchments. These flash floods are driven by complex and rapid overland flow responses to intense rainfall across these catchments. Where loose slope or valley-based deposits are available, flood water may mobilise these materials and transform into dynamic high-velocity, high-density debris flows that can pose significant threats to people, property, and infrastructure considerable distances away from the areas where these deposits are mobilised, exacerbating the already devastating situation caused by flooding. Hydro-dynamic models solving the full shallow water equations (SWEs) have shown great potential to reliably simulate the dynamics of overland flows and flash floods at catchment scales. However, simulating the transition from flash flood into debris flow is still technically challenging because of the difficulty of simulating erosion and deposition processes robustly. A reason is that the commonly used method for calculating erosion and deposition rate may suffer from singularity in the presence of vanishing velocity, which poses a major challenge for practical applications. In this work, we have developed a novel integrated hydrodynamic model for simulating flash floods and debris flows. Overland flows, change of debris concentration and bed elevation change are simulated simultaneously to model the transition between flash flood and debris flow. The overland flow processes are simulated by solving the full SWEs using a Godunov-type finite volume method. A novel method for calculating erosion and deposition rates is incorporated into the SWEs-based model to simulate the change of debris concentration and bed elevation change. The new method can maintain numerical stability and accuracy even in the presence of vanishing velocity. Therefore, the new model can effectively simulate the full process of rainfall-runoff-flooding turning into debris flows. Satisfactory simulation results have been obtained for both laboratory-scale and real-world test cases. The new model has the potential to be applied for flash flood/debris flow risk assessment and early warning.