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

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.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Civil and Environmental Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2006

Author dc:creator

• White, Brian L., 1975-

Advisor dc:contributor.advisor

• Heidi M. Nepf.

• Civil and Environmental Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/34376
• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Identifier URI

http://dspace.mit.edu/handle/1721.1/34376

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/34376