Comparative Analysis of Lateral Shear Layers Induced by Flexible and Rigid Vegetation in a Partly Vegetated Channel

Hydrodynamic processes in partly vegetated channels have been traditionally investigated by simulating vegetation with arrays of rigid cylinders. By contrast, natural riparian vegetation, generally flexible, presents a complex morphology that influences the dynamic and reconfiguration behavior, deeply affecting the flow structure. The aim of this study is to experimentally investigate the impacts of embedding natural plant features in the simulation of flow in partly vegetated channels, in comparison with the rigid cylinder representation.

Experiments were carried out with both reconfiguring plants of complex morphology and with rigid cylinders in two facilities. In this paper, the results gained on differences in the flow structure between the two setups are compared. Complex vegetation exhibited a strong reconfiguration behavior and pronounced dynamic motions, making a direct geometric comparison (e.g. using the frontal area per canopy volume) with rigid cylinders not possible. Therefore, we achieved the hydraulic similarity among the two sets of vegetated shear layers by matching both geometric and kinematic properties, comparing the normalized lateral distributions of velocity statistics. The reconfiguration-induced vegetative drag reduction exhibited by the natural-like vegetation was simulated by reducing the cylinder density. This resulted in defining three pairings of similar shear layers, each characterized by analogous velocity ratio and drag-density parameter: (i) unreconfigured or slightly reconfigured foliated vegetation ↔ dense array of cylinders; (ii) moderately reconfigured foliated vegetation ↔ sparse array of cylinders; (iii) strongly reconfigured foliated vegetation ↔ very sparse array of cylinders.

Morphology and flexibility of the vegetative obstruction played a minor role in the lateral distributions of normalized mean velocity and lateral Reynolds stress, which presented analogous trends mainly depending on the velocity ratio. On the other hand, different shear penetration within the vegetation was observed for flexible and rigid vegetation, with a systematically higher penetration found for natural-like vegetation (up to 0.75 of the shear layer width vs. 0.25 for rigid cylinders). All shear layers presented large-scale coherent structures with a Strouhal number of 0.032, but a higher efficiency of lateral transport of momentum was observed for the natural-like vegetation.

The investigated vegetated shear layers presented some universal characteristics independent of the vegetation features, mainly governed by the drag-density parameter and the resulting velocity ratio. However, the flexibility-induced mechanisms of natural vegetation were found to significantly affect the turbulent flow structure, markedly modifying the lateral exchanges across the interface. Further analyses are ongoing for the investigation of the turbulence anisotropy at the flow-vegetation interface.