Free-floating macrophytes are unique in that they live at the air-water interface, leading to the development of two distinct structural assemblages, or canopies: leaf canopies comprising above-water structures and root canopies comprising submerged structures. Certain species are considered invasive weeds, owing to characteristics such as high growth rates aided by asexual reproduction, formation of dense floating mats that out-compete other plant species, and unanchored root systems that allow dispersal by passive drifting. Invasions of these weeds harm native ecosystems and impede human activities. This research examines physical interactions between free-floating macrophytes and surrounding air and water flows to better understand the fluid-dynamic effects of free-floating macrophytes and the transport mechanisms that govern ecological dispersal.
Laboratory and field experiments were performed to address these goals. In the laboratory, experiments were conducted on leaf and root canopies of the free-floating macrophyte Eichhornia crassipes (Mart.) Solms to both measure flow-induced forces and observe surrounding flow fields. For a given raft geometry, forces and drag coefficients in water exceeded those in air. Over similar Reynolds number (Re) regimes, water drag coefficients decreased with increasing Re while air drag coefficients were relatively constant. Force-velocity relationships indicate root canopies reconfigured by streamlining in higher flow velocities while leaf canopies did not. Root canopy streamlining is further explained through biomechanical testing: the major vegetative structures of Eichhornia crassipes (roots, stolons, and petioles) had similar moduli of elasticity but second moments of area were three orders of magnitude smaller in roots compared to stolons or petioles, leading to significantly lower flexural rigidity in roots. Flow interactions with the root canopy differed for an individual plant compared to a raft assemblage. These results suggest that water currents are the dominant mechanism for Eichhornia crassipes dispersal.
Based on flow field observations in the laboratory, the presence of Eichhornia crassipes rafts caused deflection of air and water flows around the canopy structures and increased turbulence in both fluids. In both air and water, increased Reynolds stress and turbulent kinetic energy were observed beyond 50% of canopy lengths, culminating in large wake regions downstream. As upstream water velocity increased, the distance to fully-developed conditions decreased and turbulence levels increased for root canopies. In water, vertical profiles of mean streamwise velocity beyond 50% of root canopy length featured inflection points, suggesting mixing layer development; the vertical turbulent structure featured sweeps, coherent vortices, and increased mixing efficiency along the root canopy edge. These findings are analogous to mixing layers seen in submerged aquatic vegetation canopies. Although turbulent mixing was increased outside the root canopy, limited turbulent exchange was observed between the root canopy and the open water. This implies low momentum flux across the canopy-water interface; therefore in root canopies having similar structure to Eichhornia crassipes, residence time is expected to be dominated by horizontal advection. In air, the spatial development of the mean streamwise velocity profile generally agreed with a model of flow adjustment developed for terrestrial vegetation canopies. As leaf canopy length increased, turbulence levels increased, particularly in the downwind wake region. Comparing the flow fields in water and air for one particular raft, the root canopy induced a greater flow acceleration and generated a larger-intensity wake region that extended further downstream. These results suggest the fluid-dynamic effects of the root canopy exceed those of the leaf canopy.
Field experiments were performed in a tidal channel to observe free-floating macrophyte transport under varying water velocities and nearly-constant wind velocities. A free-floating macrophyte raft was equipped with a global positioning system and an acoustic Doppler velocimeter to measure raft position and relative water velocity. Results indicate that water currents dominated raft transport during ebb and flood tides, and that wind dominated transport during slack tide. Raft and water velocities were correlated during ebb and flood tides and anticorrelated during slack tide. During ebb tide, wind opposed one component of the raft velocity, reducing its magnitude compared to water velocity. In contrast, during flood tide, wind was aligned with one component of the raft velocity, leading to raft velocities that exceeded water velocities. These field observations corroborate the laboratory drag force measurements, suggesting water currents, when present, are the dominant dispersal mechanism for free-floating macrophytes. However, wind plays an important secondary role and must be considered along with ecosystem geometry. This research builds upon existing vegetation canopy studies and provides the foundation for a predictive model of free-floating macrophyte dispersal based on physical processes.
