Abstract
In a fundamental study the influence of coherent structures on the incipient motion of single fluvial grain was experimentally investigated. To fully characterize coherent structures, the three-dimensional velocity field must be known with high temporal resolution. Using a tr-3D PTV system (tr = time-resolved, PTV = particle tracking velocimetry) this could be achieved. The influence of hairpin vortices and counter-rotating longitudinal vortices (VLSM) on sediment transport has been postulated in several studies, but due to the missing 3D information, evidence has been lacking. In the present "Rolling Stones test series", the incipient motion of a single grain was investigated for smooth and rough beds. It could be shown for the first time that both hairpin vortices and VLSM trigger particle entrainment. Hairpin vortices could also be detected on rough beds, contrary to common hypotheses, and triggered particle movement. The long-term study should also find practical applications in the coming years and increase the accuracy of sediment transport calculations in rivers.