Experimental study of grain saltation in open channel flows with simulated submerged vegetation

Saltation is the predominant mode of bedload sediment transport. However, current knowledge on sediment grain saltation is largely derived from studies conducted in purely shear open channel flows, with limited information available for vegetated flows. This study endeavors to explore vegetation-affected sediment grain saltation by conducting laboratory experiments in open channel flows with simulated submerged vegetation. Particle image velocimetry technique and particle tracking velocimetry system were used to capture the flow field information and track saltating grains over three representative planes, respectively. The results show that the stem-induced turbulence significantly intensified the variation in trajectories of saltating grains. The vertical movement of saltating grains is enhanced, while their streamwise velocity and travel distance are reduced. Due to the streamwise flow velocity that increases with increasing distance from the bed, the saltating grains tend to accelerate in the ascending phase but decelerate in the descending phase. At the same elevation, the streamwise grain velocity during the descending phase is greater than that in the ascending phase. However, this difference is mitigated by the wake generated by the vegetation stem. Furthermore, the probability distribution of grain occurrence measured between two adjacent streamwise rows of vegetation stems can be represented using an exponential function, suggesting that the grain saltation can still be modeled as a diffusion process. These results can provide an important step toward understanding the mechanisms behind sediment grain saltation under vegetated flow conditions.