Coupling Depth-Averaged and 3D numerical models to study debris flow: Saint-Vincent event

Debris flows are extremely rapid and unpredictable phenomena whose rheology is poorly understood. Moreover, human settlements are often located in areas prone to debris flows. The combination of these features makes debris flows hazardous phenomena. Barriers are usually installed in debris flow paths to mitigate risk. However, their design is still based on empirical methods. In order to base the design of barriers on a more reliable approach, the understanding of debris flows must be improved. Continuum numerical models have proved to be a helpful tool for studying debris flows. In particular, numerical models can predict the speed and the flow depth in debris flows paths, and roughly estimate the forces and the pressure acting on a mitigation structure. Currently, two main groups of continuum numerical models are available to study debris flows (i) depth-averaged (DA) models and (ii) three-dimensional (3D) models. Although DA models can study a real-scale event, they may over-simplify the flow-structure interaction. On the other hand, 3D models can be very reliable for studying flow-structure interaction but studying a whole phenomenon (from triggering to deposition) would require enormous computational resources. This work aims to show how the coupling of a DA and a 3D model allows an effective and performing analysis of a debris flow dynamics. The study is focused on the 2014 Saint-Vincent event (Aosta Valley, Italy).