Analysis of the time-averaged properties of sediment motion in a local scour process

We examined the kinematics of sediment particles in the proximity of a vertical plate attached to the side wall of a pressurized duct. The laboratory phenomenon investigated was archetypal of local scour near bridge abutments in covered rivers. We conducted a clear-water scour experiment where we measured the temporal evolution of the local erosion depth; the latter was found to follow a logarithmic trend. The concentration and velocity of the moving particles were measured on the basis of digital movies taken at various evolution stages of the process. The spatial pattern of the sediment motion showed that the action of the principal vortex system becomes progressively more evident with the general lowering of the sediment transport intensity detected as the scour hole increases in time. Furthermore, the phenomenology of local sediment transport was analyzed; key findings of the research include (1) the time-averaged values of the sediment transport rate are linked with those of concentration and velocity through relationships which are almost invariant, regardless of the experimental time and of the location within the scour hole; (2) intermittency and irregularity of the sediment fluxes increase as their mean intensity decreases; (3) the relative weight of the temporal fluctuations of concentration and velocity in the time-mean sediment transport activity is significant throughout the entire process. Potential implications of the present results for the modeling of local scour processes in pressurized-flow conditions are discussed. The considerations made are likely to have analogous applicability to local scour with a free surface, since the large-scale evolutions of the erosion depth in the two conditions may not be greatly different. Given the qualitative analogy of the present findings with earlier ones obtained for one-dimensional bed load, it is suggested that similar considerations may hold also for the small-scale modeling of a wider class of sediment transport processes.