Momentum and mass transport by coherent structures in a shallow vegetated shear flow

Abstract

In many aquatic systems, from tidal creeks with fringing mangroves to rivers and associated floodplains, there exists an interface between dense vegetation and a high conveyance channel. A shear flow develops across this interface and its dynamics influences the exchange of mass and momentum between the vegetation and the channel. This thesis describes an experimental study in a shallow laboratory channel with 1/3 of its width filled with circular cylinders, a model for emergent vegetation. The experiments reveal the formation of a shear layer with nearly periodic vortex structures. The vortices are documented with respect to their physical characteristics and their effect on mass and momentum exchange. Distributions of mean velocity and turbulent Reynolds stress show a two layer structure in the shear flow. An inner layer exists near the interface, with a width that establishes the penetration of momentum into the vegetation; an outer boundary layer exists in the main channel, where the vortices reside, with a width independent of the vegetation. In each layer the mean velocity distributions are self-similar. Results of a linear stability analysis suggest that channels with differential drag are conducive to the growth of Kelvin-Helmoltz shear instabilities. Indeed vortices are observed for all experimental conditions, and their passage frequency matches the most unstable frequency from linear theory. A typical vortex structure is educed by conditional sampling, and reveals strong crossflows consisting of sweeps from the main channel and ejections from the vegetation, leading to high Reynolds stress at the interface.

(cont.) The sweeps also maintain the coherent structures by increasing the shear at the interface and enhancing energy production. Finally, a model is developed for exchange between the vegetation and the channel in terms of the vortex size and passage frequency. The semi-empirical model describes both mass transfer coefficients and interfacial friction coefficients in data from a range of vegetated flows, and suggests that a constant proportion of the vortex volume is exchanged over each cycle. The exchange coefficient is used to quantify the flushing timescale of a vegetated layer, and is applied to the problem of overbank transport of suspended sediment between a river and its floodplain.