Vegetation-generated turbulence in combined wave-current canopy flows

Laboratory experiments are conducted in combined waves and current to measure turbulence structure and intensity in arrays of wooden cylinders, a model for submerged rigid aquatic vegetation. For dense canopies, drag leads to velocity reduction within canopy. The velocity gradient at the top of canopy generates a shear layer which results in coherent vortices traveling downstream. The canopy-scale vortices control the transport of mass and momentum and penetrate into the canopy with a length of δ_e. In pure current, near-bed TKE (turbulent kinetic energy) is elevated as turbulence is transported from the shear region to the bed. TKE increases as the vortices develop downstream. In combined wave-current conditions, the turbulence within canopy is mainly set by the waves as the mean current is small compared to wave velocity. The near-bed turbulence is not significantly elevated by the shear-generated vortices and can be predicted using existing empirical model of stem-generated turbulence. As the ratio of wave velocity to current speed increases, the shear layer diminishes and δ_e, as well as the magnitude of Reynolds stress, decreases. In pure waves, no shear layer is observed and stem-generated turbulence dominates the turbulence structure and intensity within canopy.