Sediment Motion beneath Surges and Bores

Positive surges and bores can induce significant bed-load transport in estuaries and river channels. Based upon physical modelling, the present study investigated the sediment motion beneath bores on a relatively long gravel bed. The freesurface measurements at a series of locations showed that the bore shape varied during its upstream propagation. An ultrahigh speed camera captured the details of gravel motion at 1200 fps. Frame-by-frame analysis of slow-motion video movies demonstrated three basic modes of pebble motion: rotation, rolling and saltation. More complicated pebble motion was a combination of 2 or 3 basic modes. The synchronous measurements of near-bottom velocity and bed-load material trajectories highlighted the importance of the adverse longitudinal pressure gradient and transient flow recirculation on the inception of particle motion. Long durations of gravel motion also indicated that the weak negative flow under secondary waves played some role in maintaining the upstream transient sediment transport