  The transport of dense particles by a turbulent flow depends on two
dimensionless numbers. Depending on the ratio of the shear velocity of the flow
to the settling velocity of the particles (or the Rouse number), sediment
transport takes place in a thin layer localized at the surface of the sediment
bed (bedload) or over the whole water depth (suspended load). Moreover,
depending on the sedimentation Reynolds number, the bedload layer is embedded
in the viscous sublayer or is larger. We propose here a two-phase flow model
able to describe both viscous and turbulent shear flows. Particle migration is
described as resulting from normal stresses, but is limited by turbulent mixing
and shear-induced diffusion of particles. Using this framework, we
theoretically investigate the transition between bedload and suspended load.
