Hydrodynamics of discontinuous rigid submerged vegetation patches in open-channel flow

Abstract

We investigate the effects of discontinuous rigid submerged vegetation patches on flow turbulence. Two laboratory flume experiments are performed to validate the large eddy simulation (LES) model. The obtained LES data are in good agreement with the experimental data. They are also highly accurate in capturing the secondary peaks of the mean velocity near the channel bed. The coherent vortices, which are generated by the shear between the slower canopy flow and the faster overlying flow, are associated with the velocity inflection and maximum Reynolds stress around the interface. The mean velocity in the gap regions is evidently slower than that in the canopy regions. A high canopy density and Reynolds number are more conducive for the fully developed flow state of discontinuous vegetation patches. The velocity distinctly increases within the first two patches with a high canopy density. The velocity profile in the large gaps is more stable than that in the small gaps below the vegetation height, whereas the effect of patch distribution is not evident in the overlying flow layer. A spectral analysis shows that two vortex scales, namely, stem-scale and shear-scale vortices, influence the turbulence of flow through discontinuous vegetation patches. The power spectral densities are consistent with Kolmogorov theory for a −5/3 slope when the dominant eddy frequency exceeds 0.04 Hz.