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

Abstract

Free‐floating aquatic plants, or macrophytes, often grow in dense mats, and their feathery, unanchored roots form canopies below the water surface that can affect water flow and water quality. This study examines the physical interactions of an invasive species of free‐floating macrophyte, water hyacinth ( Eichhornia crassipes ), and the surrounding water to better understand how free‐floating macrophyte root canopies affect hydrodynamics. Experiments in an open‐channel flow chamber were conducted to examine the flow fields through and around root canopies. Acoustic Doppler velocity measurement revealed that water flow was deflected around the root canopy and was reduced within the canopy. Interestingly, the vertical pattern of the velocity in the roots was similar to what has been observed in leaf canopies of terrestrial and submerged aquatic vegetation. In these cases, an unstable velocity profile (one with an inflection point) gives rise to water column turbulence, which was observed at distances >50% of the length of the root canopy, culminating in a large wake region immediately downstream. Although mixing caused by turbulence increased outside the root canopy, it did not increase within the canopy, and so there was limited exchange of water between the root canopy and the open water. Because of this, we expect the time that water resides within the root canopy to be dominated by the slow mean flow velocity, rather than turbulence. This result helps to explain why the root canopies of free‐floating macrophytes can suffer from low levels of dissolved oxygen (hypoxia) and poor water quality, and suggests ways in which to improve the operational efficiency of natural water treatment systems relying on these types of plants. 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.