A theoretical model based on a depth-averaged version of two-phase flow equations is developed to describe the initiation of underwater granular avalanches. The rheology of the granular phase is based on a shear-rate-dependent critical state theory, which combines a critical state theory proposed by Roux & Radjai (1998), and a rheological model recently proposed for immersed granular flows. Using those phenomenological constitutive equations, the model is able to describe both the dilatancy effects experienced by the granular skeleton during the initial deformations and the rheology of wet granular media when the flow is fully developed. Numerical solutions of the two-phase flow model are computed in the case of a uniform layer of granular material fully immersed in a liquid and suddenly inclined from horizontal. The predictions are quantitatively compared with experiments by Pailha, Nicolas & Pouliquen (2008), who have studied the role of the initial volume fraction on the dynamics of underwater granular avalanches. Once the rheology is calibrated using steady-state regimes, the model correctly predicts the complex transient dynamics observed in the experiments and the crucial role of the initial volume fraction. Quantitative predictions are obtained for the triggering time of the avalanche, for the acceleration of the layer and for the pore pressure.

The mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular-channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow-rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle-particle interactions. Different rheological constitutive laws having increasing degree of sophistication are tested and discussed.

We experimentally investigate how a layer of granular material fully immersed in a liquid starts to flow when suddenly inclined from a horizontal position. The flow is shown to strongly depend on the initial volume fraction, its initiation being dramatically delayed by a slight initial compaction. A model, which takes into account the dilatant behavior of the granular material and the coupling with the interstitial fluid, captures the main experimental features.