Investigation of the mobile granular layer in bed-load transport

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. While the Shields number controls incipient motion, it is not the most appropriate parameter for describing bed-load transport. The experimental observations suggest that the appropriate length-scale is the fluid height and that the proper control parameter is the dimensionless fluid flow-rate. A two-phase continuum model having a frictional rheology to describe particle-particle interactions can capture most of the experimental observations. Rheological constitutive laws having increasing degree of sophistication are discussed.