| dcterms.abstract | Open channels cutting through vegetation are common features in wet- lands. Shear flows develop at the interfaces between the open water and veg- etation canopies due to the strong flow resistance applied by the plants. Such shear flows are unstable, due to the Kelvin-Helmholtz instability, inducing wave-like fluctuations, coherent vortices, as well as turbulent eddies, hence enhancing the mixing and exchanges between the open water and vegetation canopies. In this thesis, we develop a numerical model, applying a semi- implicit scheme to the shallow water equations, to study the shear instability in open channels that are partially filled with emergent vegetation. The work is inspired by the laboratory experiments in (White and Nepf, J. Fluid Mech. vol 593, 2007, pp1-32). The objective is to test the key experimental finding that the effects of vegetation can be represented using a lumped-parameter model of drag force with an enhanced vegetation resistance coefficient, with- out resolving the detailed flow structure due to individual plants inside the canopy. We first apply the numerical model to simulate the flow adjacent to a vegetation channel bank. We give a detailed analysis of the spatial and tem- poral development of the shear instability, including the Reynolds stress dis- tributions and the frequency analysis of velocity fluctuations. The numerical results are shown to be in good agreement with the experimental data, in par- ticular confirming the two-layer scaling of the shear layer at the interface. We then apply the numerical model to study the wake formation behind isolated vegetation patches, and the subsequent development of von Ka ?rm ?an vortex street. Here we show that the simple lumped-parameter model of vegetation resistance works well when the vegetation density is relatively high. How- ever, in the case of sparse vegetation, the model fails to predict the rate of wake recovery, presumably due to the lack of representing the small scale ed- dies due to vortex shedding from individual plants that persist downstream in the wake. Future works to improve the numerical model and applications for environmental transport and mixing are discussed. Key words: shallow water equations, shear instability, open channel flows, vegetated channel bank and vegetation patches. | |
