Turbulence in partly vegetated channels: Experiments with complex morphology vegetation and rigid cylinders

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

A comprehensive framework tool for performance assessment of NBS for hydro-meteorological risk management

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Shear layer over floodplain vegetation with a view on bending and streamlining effects

Publisher Copyright: © 2022, The Author(s).Abstract: Shrubby and woody vegetation growing on floodplains profoundly influences hydrodynamic and transport processes in riverine systems. Existing hydrodynamic research is mostly focused on conditions with aquatic plants and rigid model vegetation. To appreciate the different hydrodynamic impacts of submerged floodplain and riverbank vegetation, a novel flume investigation was carried out. We simulated conditions found in riparian environments in terms of vegetation density, plant structure and flexibility, and presence of a grassy understory. Four experimental cases were defined so that vegetation exhibited different degrees of bending and streamlining. Extensive set of velocity measurements allowed reliable description of the double averaged flow. Vegetation morphology, with the flexibility-induced streamlining and dynamic motion controlled the magnitude and distribution of the vegetative drag, shaping the shear penetration within the canopy. The flows were highly heterogeneous, thus calling for spatially averaged approaches for the flow field investigation. The relative importance of dispersive momentum fluxes was high in the canopy bottom region where both Reynolds and dispersive stresses were small. The contribution of dispersive fluxes to momentum transport decreased with increasing reconfiguration. The results revealed the shear layers over floodplain vegetation to be dynamically similar to other environmental flows over porous obstructions. However, the velocity-dependent vegetative drag and deflected height introduced additional complexity in the flow simulation. Altogether our findings implied that accurate description of vegetated floodplain flows can be achieved only when plant morphology and flexibility are appropriately described in drag models. Article highlights: A novel experimental setup with flexible woody plants and grasses was used to model the hydrodynamics of vegetated floodplains.Plant morphology and flexibility controlled the vegetative drag, affecting key shear layer features, including the shear penetration.The spatially heterogeneous flows had higher dispersive stresses at the canopy bottom, where the total fluid stress was small.Peer reviewe

Vegetated channel flows: Turbulence anisotropy at flow–rigid canopy interface

This laboratory study aimed at investigating the mean and turbulent characteristics of a densely vegetated flow by testing four different submergence ratios. The channel bed was covered by a uniform array of aligned metallic cylinders modeling rigid submerged vegetation. Instantaneous velocities, acquired with a three-component acoustic Doppler velocimeter (ADV), were used to analyze the mean and turbulent flow structure. The heterogeneity of the flow field was described by the distributions of mean velocities, turbulent intensities, skewness, kurtosis, Reynolds stresses, and Eulerian integral scales. The exchange processes at the flow–vegetation interface were explored by applying the turbulence triangle technique, a far less common technique for vegetated flows based on the invariant maps of the anisotropic Reynolds stress tensor

Impact of reconfiguration on the flow downstream of a flexible foliated plant

Publisher Copyright: © 2022 Marco Maio et al., published by Sciendo.This paper explores the impacts of reconfiguration and leaf morphology on the flow downstream of a flexible foliated plant. 3D acoustic Doppler velocimetry and particle image velocimetry were used to experimentally investigate the hydrodynamic interaction between a foliated plant and the flow, testing two plants with different leaves morphology under different bulk flow velocities. The model vegetation was representative of riparian vegetation species in terms of plants hydrodynamic behavior and leaf to stem area ratio. To explore the effects of the seasonal variability of vegetation on the flow structure, leafless conditions were tested. Reconfiguration resulted in a decrease of the frontal projected area of the plants up to the 80% relative to the undeformed value. Such changes in plant frontal area markedly affected the spatial distributions of mean velocity and turbulence intensities, altering the local exchanges of momentum. At increasing reconfiguration, the different plant morphology influenced the mean and turbulent wake width. The leafless stem exhibited a rigid behavior, with the flow in the wake being comparable to that downstream of a rigid cylinder. The study revealed that the flexibility-induced reconfiguration of plants can markedly affect the local distribution of flow properties in the wake, potentially affecting transport processes at the scale of the plant and its subparts.Peer reviewe

Nature-Based Solutions (NBSs) Application for Hydro-Environment Enhancement. A Case Study of the Isar River (DE)

In mountain areas, natural hazards, e.g., flooding, snow avalanches, droughts, and landslides are triggered by climate change, anthropization, and economic development. Nature-Based Solutions (NBSs) are attracting increasing interest as they are able to couple technical solutions against natural hazards with ecological and socio-economic resilience. On this matter, the four-year H2020 Innovation Action “PHUSICOS—According to Nature” (Grant Agreement nr. 776681) project aims to assess the effectiveness of NBSs and hybrid solutions to hinder hydro-meteorological events in rural and mountainous areas in Europe. Among the ongoing activities within the project, a multi-criteria tool was implemented to assess the effectiveness of NBSs measures from the technical, environmental, and socio-economic perspectives. In this work, the preliminary application of the assessment tool to the Isar River (DE) PHUSICOS concept case is discussed, with reference to an ex-post analysis of the flood risk management plan, comparing the performances of the implemented NBS project scenario against a potential grey solution

Experimental and Numerical Assessment of Water Leakages in a PVC-A Pipe

Nowadays, in the definition of effective approaches for the sustainable management of water pressurized systems, the assessment of water leakages in water supply and distribution systems represents a key aspect. Indeed, the large water volumes dispersed yearly provoke relevant environmental, technical and socio-economic costs. Worldwide, many water systems show alarming levels of water losses, due to both the poor sealing of joints and the presence of cracks, enhanced by a high pressure level greater than that strictly required for assuring a proper service level to users. With the aim of analysing the correlation between pressure and leakages, in this work the results of an experimental and a numerical Computational Fluid Dynamics (CFD) investigation are provided and discussed. With reference to a drilled PVC-A (Polyvinyl Chloride-Alloy) pipe, a new-generation plastic material for water systems use, an experimental investigation was first carried out at the Laboratory of Hydraulics of the University of Naples Federico II, aimed at assessing the leakage-pressure relation for transversal rectangular orifices. A CFD model was then implemented and calibrated with experimental results, to different geometric configurations of the orifice, with the aim of assessing the dependence of the orifice geometry and orientation on the calibration of leakage law parameters

Assessment of NBSs effectiveness for flood risk management : The Isar River case study

Nature-based solutions (NBSs) are increasingly implemented to mitigate natural risks in urban and rural contexts, from coastal to mountainous areas. Nevertheless, the lack of quantitative approaches to assess NBSs' effectiveness limits their technical, social and cultural acceptance. Within the PHUSICOS project (EU H2020 Innovation Action; Grant Agreement nr. 776681) a comprehensive assessment framework tool (AFT) has been developed to fill this gap. This paper presents an ex-post analysis with the PHUSICOS AFT applied to the Isar River case study. The restoration of the urban reach of the Isar River, in the city of Munich, was implemented in the early 2000s and represents a successful example of ecosystem and user-friendly flood risk management plan. The performance of the NBS measures implemented to manage the flood risk and improve the ecological status of the river (NBS scenario) is assessed in comparison with an alternative scenario with traditional hard engineering measures (grey scenario, GS). Results underscore the NBS as a competitive alternative. The ex-post analysis shows the potential of the PHUSICOS AFT for NBS performance assessment, providing guidance on indicator selection, stakeholders' management and performance assessment. The application discussed here is expected to aid professionals and researchers involved in the design, implementation, monitoring and evaluation of NBSs.Peer reviewe

COMPARATIVE ANALYSIS OF LATERAL SHEAR LAYERS INDUCED BY FLEXIBLE AND RIGID VEGETATION IN A PARTLY VEGETATED CHANNEL

Hydrodynamic processes in partly vegetated channels have been often investigated by simulating vegetation with arrays of rigid cylinders. By contrast, natural riparian vegetation is generally flexible and presents a complex morphology that influences the dynamic and reconfiguration behavior, deeply affecting the flow structure. The aim of this study is to investigate the impacts of embedding natural plant features in the experimental simulation of flow in partly vegetated channels, in comparison with the rigid cylinder representation. Experiments were carried out with both reconfiguring vegetation made of bushes and grasses, and with rigid cylinders in two facilities. The results on flow structure gained in the two setups are compared. The lateral distributions of normalized mean velocity and lateral Reynolds stress of the considered hydraulically similar vegetated shear layers were observed to depend mainly on the differential velocity ratio and the overall vegetative bulk drag, observing analogous distributions for the two vegetation models. Nevertheless, different shear penetration within the vegetation was observed for flexible and rigid vegetation, with a systematically higher penetration found for natural-like vegetation. The flexibility-induced mechanisms of natural vegetation were found to significantly affect the turbulent flow structure, markedly modifying the lateral exchanges across the interface