ANALYSIS OF VELOCITY FIELD AND TURBULENCE CHARACTERISTICS OF FLOW IN A VEGETATED LABORATORY FLUME

Abstract

Vegetation altering hydrodynamic conditions of an open channel flow controls the exchanges of sediment, nutrients and contaminants. Thus, the knowledge of the hydraulic characteristics of flow over vegetation is very important to support the management of fluvial processes. But, the analysis of the hydrodynamic conditions is complex because vegetation is flexible in varying degrees and it oscillates in the flow changing position. Furthermore, because of temporal changing of roughness due to natural vegetative growth, the response of vegetation to the flow can change in time. Many theoretical and experimental investigations have been performed in order to analyze both the mean flow and turbulence structure of open-channel flow (Nezu and Rodi 1986; Lemmin and Rolland 1997; Shvidchenko and Pender 2001). Recent experimental runs carried out in laboratory channels with flexible vegetation, realized by using artificial filaments (Kutija and Hong 1996; Ikeda and Kanazawa 1996), investigated some peculiar characteristics of flow turbulence structure and revealed the generation of periodic organized vortices whose center is located slightly above the top of the vegetation layer. In this paper the flow over real flexible vegetation is experimentally studied. A 2D-ADV (Acoustic Doppler Velocimeter) is used to measure the local flow velocities, for different vegetation concentrations and varying the discharge and the flume slope. The influence of both vegetation concentration and depth/vegetation height ratio on the measured velocity profiles is analyzed. The hyperbolic tangent profile of a pure mixing layer proposed by Ghisalberti and Nepf (2002) is tested and the local flow velocity measures are used to evaluate the mixing-layer parameters. The spectral analysis is operated in order to verify the formation of turbulence structures inside the vegetated layer. A new expression of the turbulent intensity distribution allowing the interpretation of experimental data both inside and above the vegetation layer is also presented.