Particle entrainment by bursting phenomena in open-channel flow over rough bed

In natural gravel-bed rivers, fine sediment deposition contributes to the bed morphology and entrapment of plant seeds often leads to the evolution of vegetated areas. Therefore, it is important to understand the transport mechanisms of fine particles between and over large-grain gravels. The present work explores influence of the bursting phenomena on transport of fine grain particles between roughness elements through laboratory experiments. The rough bed consists of hemispherical elements fixed to the flume bed to mimic a natural gravel riverbed. First, two-components of flow velocity within and over the rough bed were measured by particle image velocimetry (PIV) combined with refractive-index matching (RIM). This visualizing method reveals the complex flow structure in a valley of the rough bed; especially, the formation of a horseshoe-shaped vortex at the upstream side of a roughness element. The linear stochastic estimation (LSE) of the conditional velocity field given an upward flow event near the channel bed demonstrates that the horseshoe vortex is temporarily enhanced after a high-momentum flow passes over the roughness crest and a sweep impinges onto the roughness surface. Next, the simultaneous analysis of particle trajectories and flow velocities was performed for some particle entrainment events between roughness elements. Most upward particle motions in the near-bed region are initiated by the upward flow induced by sweep and terminate below the roughness height. On the other hand, some of the particles lifted by sweep are transported beyond the roughness height by a following ejection event.