PhD defense by Suzanne Lapillonne

Debris flows, i.e. rapid movements of a mixture of solid and fluid on steep terrain, pose significant risks to inhabited mountainous areas. Accurate modelling is crucial for effective risk mapping and to define relevant mitigation strategies in these regions. While there’s enthusiasm in the community for debris flow modelling, only few models have used surge-scale data from real field measurements for calibration and validation at various scales. This Ph.D. thesis aims to conceptualise a field-driven coupled solid-fluid model for alpine debris flow surges at the surge scale. The numerical modelling in this study employs the hybrid model DualSPHysics, which uses the Lagrangian Smoothed Particles Hydrodynamics (SPH) method for fluid modelling and fully couples it with the solid dynamics solver ProjectChrono. After an introduction of the state-of-the-art in debris flow research and a presentation of the numerical methods, the work is divided in three interlocked sections :

• First, we focus on field data exploration by presenting a protocol for the processing of debris flow surge data. The outputted data on debris flow surges, processed from the monitoring station of the Réal torrent (South East France) will then feed the numerical model.
• Secondly, a simplified model is introduced. The front of the surges are represented in 2D featuring polydisperse boulders and a viscous Newtonian fluid. The model is rigorously validated against experimental data and empirical considerations.
• Thirdly, the model increases in complexity by adding a non-Newtonian rheology to the interstitial fluid.

The work shows that such models are able to represent debris flow motion with field-like features.