Experimental observations of 3D flow alterations by vegetation under oscillatory flows

Abstract

This study presents observations from a series of experiments on an oscillatory tunnel, using a three-dimensional, volumetric particle image velocimetry system to investigate the effect of a single plant morphology on flow alterations. Three synthetic plants, mimicking three species representative of riverine, tidal, and coastal vegetation communities are investigated under various combinations of wave period and orbital excursion. The study allows to investigate the temporal and spatial distribution of the velocity field past the submerged plants with high spatial resolution. It shows that even a detailed characterization of plant morphology, represented by obstructed area or patch porosity, is not enough to accurately parameterize variations in instantaneous velocity, turbulent kinetic energy, bed shear stresses, and coherent flow structures. The study shows that bending and swaying of the plant generates eddies at multiple scales, at various locations and orientations with respect to the stem, branches, and leaves, which may be overlooked with point measurements or even 2D PIV, and can significantly enhance or dampen forces at the bed driving sediment transport processes in sparse vegetation patches.