Caroppi, Gerardo (2018) Turbulence in partly vegetated channels: Experiments with complex morphology vegetation and rigid cylinders. [Tesi di dottorato]

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Vegetation is a fundamental feature of riverine ecosystems, playing a variety of valuable ecological and biological roles. Concurrently, the presence of vegetation and its interaction with the flow alter the mean and turbulent flow field, with implications on flow resistance, water conveyance and transport of mass and energy. The proper understanding of these vegetation-influenced processes is essential for solving the existing and future river management challenges, concerning both societal needs and ecosystem requirements. The objective of this thesis is to provide new insight on the flow-vegetation hydrodynamic interaction with a specific focus on partly vegetated channels, a configuration representative of natural settings. Indeed, in natural watercourses, vegetation is generally found along river margins, partly obstructing the river cross-section and laterally interacting with the flow. Riparian vegetation presents a complex morphology and, owing to its flexibility, exhibits a dynamic and reconfiguring behavior under the flow forcing. In the analysis of flow in partly vegetated channels, these flow-influencing characteristics have been generally neglected, simulating vegetation with rigid cylinders. In the current study, two main experimental campaigns were performed to investigate the turbulent structure of the flow in partly vegetated channels, simulating vegetation with natural-like plant stands (PN) and with rigid cylinders (PR). The PN tests aimed at investigating the effects of plant morphology, reconfiguration and dynamic motions on the turbulent flow field. Furthermore, the effects of seasonal variability of plants on flow structure were explored. Results showed that plant morphology and reconfiguration play a key role in the vegetated shear layer dynamics, significantly affecting the exchange processes across the vegetated interface. The PR test series was performed to investigate the effects of vegetation density on the turbulent flow structure. The results showed that, for rigid vegetation, the density directly affects the shear layer features, governing the onset of large-scale coherent structures. Finally, the impacts of embedding natural plant features in the simulation of partly vegetated flows were explored by comparing the shear layers induced by complex morphology vegetation (PN) and by rigid cylinders (PR). In addition, an existing model for velocity prediction was tested against the experimental results, showing the need to improve existing models for taking into account the peculiar hydrodynamic behavior of natural vegetation.

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