Sediment kinematics in abutment scour

This paper presents the results of a clear-water scour experiment where the sediment kinematics around a rectangular abutment was measured at various evolution stages of the erosion process. Sediment activity was quantified by measuring the instantaneous concentration and velocity of the moving grains by image processing. The analysis was mainly focused on the region upstream of the abutment, where the maximum scour depth is measured. Experimental results on the kinematics of the grains were discussed and interpreted on the basis of the typical literature descriptions of the water flow field. The suitability of averaged quantities to describe the grain mechanics was evaluated by analyzing the instantaneous values of sediment concentration and velocity. We found that the grain kinematics can be effectively interpreted by means of the mean sediment pattern only at some locations and evolution stages: Typically the sediment activity becomes progressively more intermittent in the upstream part of the scour hole, reducing the similarity between the instantaneous and the average sediment patterns. The findings may have important implications for both the phenomenological depiction and the numerical modeling of local scour processes.