Identification of coherent structures downstream of patches of aquatic vegetation in a natural environment

Abstract

Velocity measurements from experiments performed downstream of patches of submerged aquatic vegetation were analysed. Two patches of different species, i.e., P. crispus and M. spicatum, were characterised by the same size. Measurement points covered the water depth in the centre and at the sides of the patches from 0.1 m to 0.85 m downstream. Conditional quadrant analysis was modified and applied to the data to study turbulence structures propagating from the mixing layer formed at the interface between the vegetation canopy and flow above. These structures were responsible for the majority of momentum transport and tended to vary with patch distance, height and plant physical characteristics. The stiffer plant, i.e., M. spicatum, produced a higher difference between the contributions of even events, i.e., sweeps and ejections, and odd events, i.e., outward and inward interactions. The changes in the detected strong bursting events across measurement profiles indicated that two main kinds of organised motions occurred immediately behind the plant patches. Strong sweep motions carried water flow into the low-velocity region below the vegetation height. Strong ejections, which had the longest time of the occurrence and had the highest contributions to the u′w′¯ Reynolds stress, pushed flow towards the water surface. Both kinds of extreme events produced the same Reynolds stress in an instant, but ejections had a higher frequency of occurrence. The presented results show how the observed changes in ejection and sweeps distribution reflect the model where vegetation canopy-induced vortices transform into the dual-head hairpin vortices downstream of the inflection point.