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 often investigated by simulating vegetation with arrays of rigid cylinders. By contrast, natural riparian vegetation is generally flexible and presents a complex morphology that influences the dynamic and reconfiguration behavior, deeply affecting the flow structure. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels, in comparison with the rigid cylinder representation. Experiments were carried out with both reconfiguring vegetation made of bushes and grasses, and with rigid cylinders in two facilities. The results on flow structure gained in the two setups are compared. The lateral distributions of normalized mean velocity and lateral Reynolds stress of the considered hydraulically similar vegetated shear layers were observed to depend mainly on the differential velocity ratio and the overall vegetative bulk drag, observing analogous distributions for the two vegetation models. Nevertheless, different shear penetration within the vegetation was observed for flexible and rigid vegetation, with a systematically higher penetration found for natural-like vegetation. The flexibility-induced mechanisms of natural vegetation were found to significantly affect the turbulent flow structure, markedly modifying the lateral exchanges across the interface.