Momentum and scalar transport in vegetated shear flows

Abstract

Environmental aquatic flows are seldom free of vegetative influence. However, the impact of submerged vegetation on the hydrodynamics and mixing processes in aquatic flows remains poorly understood. In this thesis, I present the results of laboratory experiments that describe the salient hydrodynamic and transport features of vegetated flows. Flume experiments were conducted with dowels and buoyant polyethylene strips used to mimic rigid canopies and flexible seagrass meadows respectively. Although traditionally treated as rough boundary layers, vegetated shear flows more closely resemble mixing layers. Specifically, vertical velocity profiles contain an inflection point, yielding the flow unstable to a street of Kelvin-Helmholtz vortices. These vortices dominate transport through the shear layer, such that the rate of mixing of both mass and momentum is shown to scale upon their size and rotational speed. However, mass is mixed approximately twice as rapidly as momentum. The spread of a scalar plume is shown to be a function of the number of vortex cycles experienced by the plume, irrespective of the canopy characteristics or flow speed. In contrast to mixing layers, the vortices in a vegetated shear layer grow only to a finite size, often not penetrating fully to the bed. This separates the canopy into an upper zone with rapid, vortex-driven transport and a lower zone where mixing occurs on the much smaller scale of the stem wakes. Vortex growth is shown to cease once the shear production of vortical energy is balanced by the drag dissipation of that energy by the canopy.

(cont.) The mixing length of momentum scales upon the final vortex size, allowing closure of a one-dimensional Reynolds-averaged Navier-Stokes model. Finally, canopy flexibility has a significant impact on the hydrodynamics of vegetated flows. The oscillating velocity field associated with the vortex street drives a coherent waving of the canopy, whose geometry changes rapidly over time. Using the height of a waving plant as an indicator of phase in the vortex cycle, synchronized velocity records show that the turbulence structure at the top of the canopy consists of a strong sweep at the front of the vortex, followed by a weak ejection at its rear.