Among natural hazards, debris flows are fast gravitational particulate flows involving multi-phase media. The physical understanding of the mechanisms that trigger the stability, the mobilization, the flow and the arrest of these saturated and weakly consolidated sediment masses is a scientific challenge with important implications in the domains of the geomorphology, the environment, and the rock mechanics.

While the physics of rock avalanches is mainly controlled by solid interactions between particles and mud flows are mainly dominated by fluid forces, debris flow involve complex interactions between the solid and the fluid phases. These transient flows, mobilize important volumes, occur in very short time scales, less than 10 000 seconds, with velocities that can be as fast as 10 meters per second, imply strong inertial forces, and exhibit a remarkable mobility which remains unexplained. The solid phase concentrations are not that different from those of the static sediment masses and can exceed 60 %. While these materials can sustain shear stresses up to a certain threshold, during the flow they behave like a multiphase fluid over characteristic time scales of several seconds. This fluid behaviour implies: for slow flows, competing effects between volumetric dissipation, associated with the viscosity of the interstitial fluids, and surface dissipation, associated with contacts between solid particles, as well as segregation and sedimentation processes; for fast flows, turbulent scales of dissipation, and dynamic collisions between solid particles, associated with the notion of granular temperature, as well as non local interactions between the fluid and the solid phases that can trigger density waves phenomena.

The modelisation requires to take into account the multiphase nature of these flows. Unfortunately, we do not have today clear physical models for such inertial regimes. A better physical understanding of the dynamics of debris flows should allow the evaluation of the runout distances and of the capacity for the flow to over pass local topography. An important aspect here is to characterize the dynamics at both local and global scales. This requires the analysis of velocity fluctuations and their correlations; of the fluid-solid interactions like viscous drag and non-local hydrodynamic interactions; of the thickness fluctuations of the mobilized layer during the flow.

The aim of our current research is to get new physical insights in the dynamics of debris flows from a combined experimental and numerical approach. This especially since the violence of debris flow phenomena makes field measurements quite difficult even though textures of the deposit are useful observations. In view of the complexity of the mechanisms involved in debris flows, it is important to first consider simple physical models at the scale of the particles even if such analysis and its integration into phenomenological models through internal variables remains a challenging problem. Experimental and numerical analysis require measurements of physical variables at both local and global scales.

In the experimental approach, we are considering a bidimensional set-up, an inclined Hele-Shaw cell, and a tridimensional set-up, an inclined plane channel, for dense polydisperse suspensions. By controlling the viscosity of the interstitial fluid, different flow regimes can be studied. Segregation and sedimentation mechanisms can also be quantified during experiments. The experimental analysis will make use of ultrasonor and light measurements, of pressure sensors and digital camera visualisation.

In the numerical approach, we are developing direct simulation methods, in 2D and 3D, at both the discrete and the continuous scales. At the discrete scale, the method takes into account a continuous fluid phase, with viscous and inertia effects, and a solid phase made of discrete particles in which interactions between particles and fluid as well as particle collisions are explicitly formulated. This allows to study the complex interactions between fluid pressure and granular temperature. At the continuous scale, we try to develop models that take into account inertia effects and solid-fluid phase interactions within a shallow-water approximation.

Internals collaborations:

This work is done in close collaboration with the laboratoire des systèmes géologiques of C. Jaupart and the volcanic observatories.

External collaborations:

B. Maury and O. Pironneau, Laboratoire d'Analyse Numérique, Paris VI;
B. Perthame, Département de Mathématiques et Applications of the ENS (ULM);
A. Vincent, Laboratoire de Physique and CERCA, Montréal University (Canada);
S. Douady and B. Andreoti, Laboratoire de Physique Statistique de l'ENS (ULM);
Ph. Gondret, FAST laboratory, Paris VI/Orsay University.