Insight on how bed configuration affects properties of bed load motion

Bed-load sediment transport is most usually modelled by relationships in which the solid discharge is a function of the shear stress, but such formulae may lead to unreliable predictions. Particle-scale relationship between turbulence and bed-load sediment motion has thus become one of the most important topics in sediment transport research. In this respect the bed configuration, affecting flow field, is one of the elements on which particle motion may depend. In this paper some results of a wider experimental campaign are presented. The latter was performed with the aim of examining the characteristics of sediment motion for variable bed configuration. The full experimental campaign comprehended mobile-bed configurations and fixed-bed ones. A new technique, based on image processing, was applied to movies recorded from above during the experiments to identify and track each moving grain. The cases experimented in this work proved some differences in the characteristics of sediment transport for fixed and movable underlying bed, even if the bulk flow properties were the same. Such differences should be accounted for when comparing experimental results from different sources.