Experiments are reported on the dynamics of a bed of particles sheared by a viscous Couette flow in an annular channel, with emphasis on the distributions of particle velocities, durations and lengths of the small saltation flights, and surface density of the moving particles. The velocity distributions are shown to decay approximately exponentially, with mean value, $U_{p}$, equal to $0.1\,\gamma d$, where $\gamma$ is the shear rate and $d$ is the particle diameter. The duration of the flights does not depend on the shear rate, and is equal to 15 times the settling time $d/V_{S}$, where $V_{S}$ is the Stokes settling velocity. Starting from an initially loosely packed bed, the surface density of the moving particles, $N_{p}$, was observed to decrease slowly over several days, unlike their velocity which remains constant with time. This decay is related to the increase of the threshold shear rate for particle motion, and corresponds to rearrangement of the particles near the bed surface (armouring). When the stationary state is reached, $N_{p}$ depends linearly on the shear rate, so that the particle flow rate, $Q_{p} \,{=}\, N_{p}U_{p}$, is a quadratic function of the shear rate. Two theoretical models are proposed to account for these observations. In the first one, the erosion and deposition rates are modelled using the two hydrodynamic time scales: the inverse shear rate $\gamma^{-1}$ for the erosion rate, and the settling time $d/V_{S}$ for the deposition rate. This model accounts for the linear dependence of $N_{p}$ on the shear rate. The second model was developed to capture the slow decrease of $N_{p}$, by considering the trapping of moving particles into troughs of the bed. This trapping model does recover the main features observed experimentally, although the characteristic time for the decrease of $N_{p}$ still remains too short. Our observations are, finally, compared to existing numerical and experimental studies on turbulent flows.