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.

International audienceWe present reproducible experimental measurements for the onset of grain motion in laminar flow and find a constant critical Shields number for particle erosion, i.e., c = 0.12± 0.03, over a large range of small particle Reynolds number: 1.5 10 −5 Re p 0.76. Comparison with previous studies found in the literature is provided