In the presence of vegetation in open channel flows, various physical processes, such as sediment transport, may be dominated by large-scale eddies, of which mechanisms are not well understood at present. In this study, we aimed to explore vegetation-affected turbulence from the perspective of energy spectral analysis. First, we conducted a series of laboratory experiments of open channel flows with submerged vegetation by varying the flow rate, water depth, and vegetation density. With flow velocities measured using the particle image velocimetry (PIV) technique, energy spectral analyses were then performed over several representative locations in the flow field. The results show that the Kelvin–Helmholtz (KH) vortices dominate the flow in the surface layer, while the shedding wake controls the flow in the vegetation layer, particularly downstream of individual vegetation stems. The normalized frequency of the KH vortices increases for flows with dense vegetation, of which the peak value, when normalized as the Strouhal number, has an average of 0.21. Furthermore, by applying Taylor's frozen turbulent hypothesis, it is shown that both the scale of the KH vortices and the penetration depth reduce when the vegetation becomes dense. Within the vegetation layer, the minimum of the peak streamwise wavelength is observed to be related to the shedding wake, while its maximum scales with the size of the penetrating KH vortices.

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.

The logarithmic law of the wall is a foundational element in turbulence modeling. Its classical derivation, rooted in Prandtl’s mixing-length theory, is predicated on the existence of a constant shear stress layer. This paper demonstrates that this foundational assumption is not strictly satisfied for two-dimensional, uniform open-channel flows, where the Reynolds shear stress profile exhibits a distinct peak near the bed and never forms a true constant stress zone. We present a novel mechanistic derivation that circumvents this inconsistency. By reframing the bed shear stress as the time-averaged momentum flux from discrete, wall-coherent eddy impacts, we recover the log law through a mechanistic framework. Our model starts from the physical definition of the Reynolds stress at the bed, employs kinematic scaling for the velocity fluctuations, and incorporates the geometric constraint of eddy size. This approach does not require a constant stress layer and provides a more physically defensible explanation for the emergence and robustness of the log-law, directly linking it to the underlying structure of wall turbulence. The derivation resolves the long-standing paradox between the theory’s assumption and the empirical reality in open channel flows.