Eperimental observations on the effect of flexible vegetation on flow velocity and turbulence fields

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; Ghisalberti and Nepf, 2002). 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) and a DOP2000 ares used to measure the local flow velocities, for different vegetation layer configurations and varying the discharge. By using the measured data, the turbulent intensity distribution is examined and the spectral analysis is operated.