Observations of mean and turbulent flow structure in a free-floating macrophyte root canopy

Free-floating macrophytes often grow in dense mats, and their feathery, unanchored roots form canopies at the water surface that can affect stream hydraulics and water quality. This study examines the physical interactions between a common species of free-floating macrophyte, water hyacinth (Eichhornia crassipes), and surrounding water flow to better understand the hydrodynamic effects of free-floating macrophyte root canopies. Experiments in an open-channel flow chamber were conducted to examine flow fields through and around root canopies. The presence of the root canopy in the channel caused deflection of flow around and reduced velocities within the canopy. Increased Reynolds stress and turbulent kinetic energy were observed beyond 50% of canopy length, culminating in a large wake region immediately downstream. Vertical profiles of mean streamwise water velocity beyond 50% of canopy length exhibited inflection points, suggesting mixing layer development analogous to mixing layers in leaf canopies of terrestrial and aquatic vegetation. The vertical turbulent structure exhibited sweeps, coherent vortices, and increased mixing efficiency along the canopy edge. Although turbulent mixing increased outside the root canopy, limited turbulent exchange between the root canopy and the open water was observed. This implies low momentum flux across the canopy–water interface, and therefore we expect residence time in the root canopy to be dominated by horizontal advection.