Abstract
River vegetation radically modifies the flow field and turbulence characteristics. To analyze the vegetation effects on the flow, most scientific studies are based on laboratory tests or numerical simulations with vegetation stems on smooth beds. Nevertheless, in this manner, the effects of bed sediments are neglected. The aim of this paper is to experimentally investigate the effects of bed sediments in a vegetated channel and, in consideration of that, comparative experiments of velocity measures, performed with an Acoustic Doppler Velocimeter (ADV) profiler, were carried out in a laboratory flume with different uniform bed sediment sizes and the same pattern of randomly arranged emergent rigid vegetation. To better comprehend the time-averaged flow conditions, the time-averaged velocity was explored. Subsequently, the analysis was focused on the energetic characteristics of the flow field with the determination of the Turbulent Kinetic Energy (TKE) and its components, as well as of the energy spectra of the velocity components immediately downstream of a vegetation element. The results show that both the vegetation and bed roughness surface deeply affect the turbulence characteristics. Furthermore, it was revealed that the roughness influence becomes predominant as the grain size becomes larger.
Highlights
Vegetation exerts important effects on hydraulic resistance, turbulent structures, mixing processes and sediment transport in rivers [1,2,3,4,5]
Special interest should be devoted to works on vegetated flows with rough beds, since the interactions between fluid, vegetation and bed sediment allow for reaching better knowledge of the turbulence characteristics in real rivers, which have a crucial role in sediment transport
To investigate the time-averaged flow velocity field with respect to the approaching flow velocity in the spatial flow domain, Figure 5 shows the colormaps of the dimensionless time-averaged accelerated and decelerated flow fields upstream of the investigated
Summary
Vegetation exerts important effects on hydraulic resistance, turbulent structures, mixing processes and sediment transport in rivers [1,2,3,4,5]. For this reason, a large amount of both experimental and numerical researches has been devoted to the study of the impacts of vegetation on the flow characteristics, influencing mass and momentum exchange across the river section, together with geomorphology, water quality and aquatic biodiversity (e.g., [6,7]).
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