Gravitational effects on bed load transport at low Shields stress: Experimental observations

In the present study we investigate the bed load transport mechanism due to the action of a free surface water flow when the transversal and longitudinal slopes of the bed are not negligible and in the case of low values of the applied Shields stress. Such a problem, which has been recently tackled by Parker et al. (2003) by means of a theoretical formulation, does not appear to be fully addressed in the literature from an experimental point of view. To this aim, average motion characteristics and transport dynamics of bed particles over an arbitrarily inclined nonerodible bed are studied by means of an experimental activity based on different techniques. A laser Doppler and an image-based technique are developed here and employed to study the average properties of saltating particles, while volumetric sampling is applied to estimate the average bed load transport intensity at equilibrium. Results show the intensity and direction of average particle velocity and average bed load transport as a function of the applied Shields stress for various bed inclinations. It appears that the intensity of bed load transport increases with the applied Shields stress much faster than predicted by the linear formulations on horizontal beds, while its direction may largely deviate from the direction of the applied shear stress at the bed when the bed is transversally tilted. Experimental results are compared with the theoretical results of Parker et al. (2003) and show a good agreement.