Mixing Layers and Coherent Structures in Vegetated Aquatic Flows

Submerged aquatic vegetation can dramatically alter the drag, turbulence and diffusivity characteristics of flow in aquatic systems. As a result, the diffusion and advection of contaminants and particulates are greatly influenced. However, modeling efforts generally treat submerged vegetation merely as a source of drag. This study explores the idea that flow through submerged aquatic vegetation resembles that of a mixing layer and can not be regarded simply as a perturbation of the bottom boundary layer. A dynamically accurate experimental model of a submerged eelgrass canopy was created in a laboratory flume. The appropriateness of the mixing layer analogy was examined, with specific emphasis on the generation of large, coherent vortices above the vegetation. The vortices result in a strongly oscillatory flow and are responsible for the coherent waving phenomenon (monami) observed in terrestrial and aquatic vegetation. High frequency velocity records enabled the examination of the periodicity and turbulence characteristics of the vortices. These structures represent a very prominent feature of the flow and have the potential to dominate transport in vegetated areas. Thesis Supervisor: Heidi M. Nepf Title: Associate Professor of Civil and Environmental Engineering