Log velocity profile and bottom displacement for a flow over a very flexible submerged canopy

Abstract

Management of shallow lagoons and salt marsh environments usually requires use of numerical models in order to understand the hydrodynamic characteristics of these basins. In order for these models to give reliable results one has to have good knowledge of the interaction between the flow and the aquatic vegetation, which strongly alters the hydrodynamic characteristics. Many experimental and theoretical studies are available in the technical literature, but, because of the very different vegetation and flow characteristics, they do not yet allow one to have a clear and general knowledge of the phenomena, in terms of both flow resistance and velocity distribution. The latter is very important from the viewpoint of transport of sediment, nutrients and oxygen. Close inshore in the Mediterranean Sea, a plant which is widely present is Posidonia oceanica, organized in a very short stem and very long and flexible ribbon-like leaves, which covers sandy beds for vast expanses forming dense prairies. In the present paper, going on with previous studies concerning the interaction of a P. oceanica canopy with marine flow, the velocity profiles taken in a laboratory flume, whose bottom was covered by an artificial canopy reproducing a P. oceanica prairie, are analyzed. The experimental conditions involve leaves longer than the flow depth, which is a realistic situation in lagoon environments. The possibility is examined of describing the velocity distribution in the non-vegetated layer using the Prandtl-von Kàrmàn law referring to a suitable virtual bottom having virtual roughness, which jointly simulates the effects of the intense eddy activity in the mixing layer that forms near the top of the inflected vegetation. A relationship for the displacement of the virtual bottom from the actual bed as a function of some characteristic parameters of flow and vegetation is given, as well as the virtual roughness coefficient.