The erosion process at a bridge abutment may be affected by the flow constriction when the abutment occupies a significant part of the flume width. We devised a specific experimental campaign to investigate the effect of the obstruction ratio (i.e., the ratio between the abutment length and the channel width) on the erosion depth: we performed homogeneous series of clear-water scour experiments in each of which the obstruction ratio was the only parameter that varied. The experimental results are presented in light of a dimensionless framework. It was found that the effect of the obstruction ratio on the time development of local scour depth may not be large; nevertheless, significant effect can be observed, even for relatively small values of the parameter during the earliest phases. The latter are important, for example, for the step-by-step modeling of the erosion development under unsteady flow conditions. We propose a simple equation to quantify the scour enhancement due to the flow constriction in c...

This research presents results for the sediment kinematics in a clear-water scour experiment with a circular pier. Various process phases were considered, until the scour depth equalled almost twice the pier diameter. The results are consistent with typical literature descriptions of the flow field and with the findings of few previous works where the sediment motion at a local scour was investigated. The relationship between the time-averaged concentration and velocity of sediments in the scour hole was found to be almost space- and time-invariant. In addition, the link was analogous to those found in an earlier work on abutments, providing for the first time evidence that the transport mechanics are identical for the two obstacle types. It is therefore suggested that the present results support the phenomenological description and modelling of various local scour processes.

A study of weak bed load over a plane bed is presented with a particular focus on the motion and diffusion of individual particles that were tracked along their movements. Grain position, velocity and acceleration were measured with particle-scale resolution; ensemble statistics over a significant sample of particle trajectories were then computed, enabling unsteady phases of particle motion (corresponding to entrainment and disentrainment) to be identified. Based on the experimental findings, a physically-based conceptual model for particle kinematics consisting of an initial (entrainment) unsteady phase, a quasi-steady phase, and a final (disentrainment) unsteady phase is explored. The unsteadiness effects in particle dispersion due to entrainment and disentrainment, typically overlooked in previous studies, are explicitly quantified by analysing the time evolution of the particle position variance. Starting with a previously published model of particle diffusion, a refined model is proposed which explicitly accounts for the unsteadiness effects in particle motion.