JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. C2, 10.1029/2001JC000871, 2002
Mixing layers and coherent structures in vegetated aquatic flows
Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering,
Massachusetts Institute of Technology,
Cambridge, Massachusetts, USA
Abstract
[1] To date, flow through submerged aquatic vegetation has largely been viewed as perturbed boundary layer flow, with vegetative drag treated as an extension of bed drag. However, recent studies of terrestrial canopies demonstrate that the flow structure within and just above an unconfined canopy more strongly resembles a mixing layer than a boundary layer. This paper presents laboratory measurements, obtained from a scaled seagrass model, that demonstrate the applicability of the mixing layer analogy to aquatic systems. Specifically, all vertical profiles of mean velocity contained an inflection point, which makes the flow susceptible to Kelvin-Helmholtz instability. This instability leads to the generation of large, coherent vortices within the mixing layer (observed in the model at frequencies between 0.01 and 0.11 Hz), which dominate the vertical transport of momentum through the layer. The downstream advection of these vortices is shown to cause the progressive, coherent waving of aquatic vegetation, known as the monami. When the monami is present, the turbulent vertical transport of momentum is enhanced, with turbulent stresses penetrating an additional 30% of the plant height into the canopy.
Received 13 March 2001; revised 31 July 2001; accepted 24 August 2001; published 13 February 2002.
Keywords: vegetated flow, mixing layer, monami, seagrass, vortices, turbulence.
Index Terms: 1845 Hydrology: Limnology; 1890 Hydrology: Wetlands; 4211 Oceanography: General: Benthic boundary layers; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes.
Citation: Mixing layers and coherent structures in vegetated aquatic flows, J. Geophys. Res., 107(C2), 10.1029/2001JC000871, 2002.