Numerical assessment of canopy blocking effect on partly-obstructed channel flows: from perturbations to vortices

The evolution and characteristics of large-scale vortices are critical for understanding temporal and spatial exchange of mass and momentum. Using a validated depth-averaged large-eddy simulation (DA-LES) model, this paper visualizes the vortex processes in a partly-obstructed canopy channel, and analyzes associated hydrodynamics. Particular interests are focused on the effect of canopy blocking ratio (canopy-to-channel width ratio) reflecting vegetation lateral colonization extent, which is poorly addressed by current knowledge. We find that two vortex evolution patterns including sequent sole along-canopy vortices (SCVs) and alternating hybrid along-canopy and along-wall paired vortices (HCWVs) correspond to small and large blocking ratios, respectively. Particularly for the latter, the outer length of along-canopy vortices decreases once along-wall vortices arise, with the vortex fields characterized by pronounced decreased wavelength and increased wave number. Hyperbolic tangent velocity profiles typically for shear layer flows with SCVs shift to parabolic ones as HCWVs dominate. The simulations imply that the key mechanism of SCVs shifting to HCWVs is that along-canopy vortices touch the main-channel sidewall when canopy width continuously increases, triggering vortices shedding off the sidewall. New insights into sediment deposition in the main channel are achieved as the hydrodynamics and vortex fields alter under different canopy blocking extents.