Analysis of the temporal evolution of the sediment movement in the vicinity of a cylindrical bridge pier

A detailed analysis of the sediment movement in the vicinity of a cylindrical pier is presented. The experiments were conducted in a laboratory flume. The movement of the sediment grains was detected and evaluated by image processing techniques at several stages of the scouring process. The pictures of the sediment surface were captured with a CMOS-camera from above through a plexiglass plate which was slightly submerged on the water surface. 20% of the sediment grains were colored black to assure a significant contrast. The experiments were conducted for 20 h at 80% of the critical section averaged velocity. The results of the study comprise time-averaged moving directions and magnitudes, scatterplots of the displacement events and transport intensities at a high spatial resolution for all scouring stages. The temporal evolution of the main scouring agents (tangential velocity components and the horseshoe-vortex system) and their importance during the particular stages were evaluated. The development of the grain velocity magnitudes, the dispersion of movement events and the transport intensities are discussed.