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This study employed numerical simulations to investigate the three-dimensional hydrodynamic structure of a channel obstructed by submerged rigid canopies. After validating the model using existing experimental data, a series of numerical experiments varying the canopy density and the blocking ratio were conducted. The results indicate that submerged canopy, different from emergent canopies, triggers the generation of three-dimensional coherent vortices. When canopies expand laterally, the scale, location, and evolution characteristics of cross-sectional secondary flows vary. The direction of secondary circulation at the canopy top of the fully covered vegetation channel is affected by the river width/depth ratio. Three-dimensional coherent vortices are jointly controlled by vertical and transverse Reynolds stresses. When the channel is seriously blocked, the degree of momentum exchange in the mixing layer does not change significantly. The boundary friction effect dominates the momentum exchange process near the wall. An equation coupling the drag length scale and blocking ratio with the vertical coherent vortex penetration length is proposed, indicating that the penetration length increases exponentially with both variables. Higher canopy density exhibits a stronger resistance to the vertical coherent vortex penetration length. In the process of canopy lateral expansion, the evolution time of the outer vortex to scale stability is longer than that of the inner vortex. The significance of research on the three-dimensional vortex structure of rigid submerged vegetation is established based on previous studies and existing conclusions. [ABSTRACT FROM AUTHOR]

Large eddy simulation (LES) was conducted to better understand the evolution of the flow field and vortex structures near a single spur dike. The fixed-bed boundaries used in the simulations were reconstructed from the topography measured during the experiments at several representative times. The numerical results showed that the presence of a scour hole increases the passage area for the flow near the bed such that more fluid in the upper layer is transformed into the downflow, resulting in a stronger near-wall jet. This enhanced jet weakens the rotational strength of the primary horseshoe vortex (HV1), causing it to be dominated by the backflow mode. With further development of scouring, HV1 is able to regain its strength and stability. In addition, a spiral flow with an approximately vertical axis of rotation is gradually formed owing to topographic changes. The low-pressure core near the bed inside this spiral flow results in the development of excess pore water pressure in the sand layer, which could decrease the effective shear stress in the sand layer and cause upward seepage. [ABSTRACT FROM AUTHOR]