Experimental observations on the motion of bedload particles

A detailed set of experimental observations on bedload transport was carried out to study the dynamics of sediment particles in a free surface laboratory flume, in the case of low values of the applied Shields stress and on an arbitrarily sloping bed with local values of the longitudinal and transversal inclinations up to 20°. The experimental data concern the average motion characteristics of saltating particles, which are described in terms of the applied Shields stress, the flow velocity profile, the particle velocity profile, the local bedload discharge and the deviation angle, due to the effect of gravity on laterally sloping beds, between the average direction of particle velocity and the fluid bed shear stress. While the effects of gravity on the direction and intensity of bedload transport are presented in Francalanci & Solari (2007); in the present work, results about the average characteristics of the motion of saltating grain are presented. The results show the relation between the average particle velocity within the saltation layer, the average saltation height and the dynamic friction coefficient with the applied Shields stress. In particular, the average particle velocity is shown to be in good agreement with the experimental data from other authors, and appears to increase with the Shields stress; the saltation height, which is directly related to the thickness of the bedload layer, is in the order of 1.5-3 times the particle diameter and slightly increasing with the Shields parameter; the dynamic friction coefficient, estimated through a balance between average resistive and active forces based on the measurements of the velocity profile across the bedload layer of both the fluid and the particles, show a relation not only with the applied Shields stress but also with local bed inclination.