Residence time in coastal canopies

Aquatic canopies provide important ecosystem services such as improved water quality, oxygen flux, sediment stabilisation and trapping and recycling of nutrients. The ecological health of coastal canopies and the significant ecosystem services they provide depends largely on the continuous exchange of dissolved and particulate materials across the canopy boundaries. In coastal environments, where flow is typically wavedominated, vertical mixing is believed to be the dominant process controlling residence time and, therefore, exchange. However, experiments have shown that wave-driven flows over rough boundaries, such as canopies, generate strong onshore mean currents (75% of the orbital velocity far above the canopy) near the canopy top. Since these currents can significantly influence canopy residence time, it is imperative to understand how the two processes of vertical mixing and horizontal advection can influence water renewal and, ultimately, residence time in wave-dominated canopy flows. This thesis presents predictive formulations for (i) vertical mixing and (ii) horizontal flushing, the two key mechanisms dictating water renewal and ultimately residence time in these environments. It is also examined how embedding realism (in the form of flexibility and buoyancy) in the model vegetation can influence flow and turbulent structure as well as residence time. Finally, through consideration of a Peclet number Pe (the ratio of diffusive to advective time scales), a framework for quantitative prediction of residence time in these environments is presented. It is found that two important mechanisms dominate vertical mixing under wavedominated conditions: a shear layer that forms at the top of the canopy and wake turbulence generated by the stems. By allowing a coupled contribution of wake and shear layer mixing, a predictive formulation for the rate of vertical mixing in coastal canopies across a range of wave and canopy conditions is presented. Results also reveal that flexibility can significantly alter the hydrodynamics of the flow, shear layer characteristics and near-bed turbulent intensities. These differences ultimately lead to a significant reduction in the rate of vertical mixing in flexible canopies when compared to the rigid analogues such that vertical diffusivity in flexible vegetation was always lower than the correspond ing rigid canopy (by up to 35%). A physical description of, and predictive formulation for, the mean current generated in wave-dominated flows over large benthic roughness (such as the canopies of seagrass, macroalgae and corals) is also presented. This model indicates that the magnitude of the wave-driven current increases with the above-canopy oscillatory velocity, the vertical orbital excursion at the top of the canopy and the canopy density. An extensive laboratory study, using both rigid and (dynamically-scaled) flexible model vegetation validated the accuracy of the proposed model. Results reveal that Pe depends heavily on wave and canopy properties and may vary significantly in real coastal canopies. Quantitative predictions for residence time in the limit of Pe \u3c\u3c 1 (mixingdominated exchange) and Pe \u3e\u3e 1 (advection-dominated exchange) are presented. The results of this study can have significant implications for a range of environmental, ecological and biochemical studies as well as numerical simulations. In particular, it enables an enhanced predictive capability for the residence time of ecologically-significant materials such as nutrients, seeds, pollen as well as contaminants and dredging plumes. Additionally, the greatly improved understanding in the hydrodynamics of oscillatory canopy flows achieved through this study can serve as a foundation for the numerical modelling of these environments. Ultimately, the results of this study are a step towards an improving management and protection of coastal canopies and their associated ecological communities

Material residence time in marine canopies under wave-driven flows

© Copyright © 2020 Abdolahpour, Ghisalberti, McMahon and Lavery. Coastal canopies (e.g., seagrasses, coral reefs, and kelp forests) are vitally important ecosystems that provide a range of ecological services (e.g., oxygen production, sediment stabilization and trapping, and recycling of nutrients). The long-term health, productivity, and survival of these canopies rely heavily on the residence time of ecologically-significant materials in these environments. Recent studies have shown that submerged canopies induce a strong mean current over the canopy top, even in purely wave-dominated environments. Thus, in addition to vertical mixing, the horizontal flushing of materials (resulting from these canopy-induced currents) will dictate rates of water renewal and, therefore, residence time in wave-dominated flows over submerged canopies. Building on this recently-improved understanding, this paper provides (for the first time) a framework for estimation of material residence time (Tres) and its variation with core system parameters, including both canopy and wave characteristics. This is done through consideration of a Péclet number (Pe) which is the ratio of mixing to advective time scales. Prediction of residence time for a wide and realistic range of marine canopies (and a correspondingly wide range of Pe) reveals that while Tres decreases with wave height and increases with water depth, it has a complex relationship with canopy density and height. Importantly, residence time can vary from orders of seconds to hours, depending on wave and canopy properties. This has considerable ecological implications for marine canopies through the direct impact on a range of chemical and biogeochemical processes within the canopy. The framework presented here represents a critical step forward in being able to predict residence time in coastal canopies and test the interacting set of factors that influence the residence time in real, complex systems

Production of ZnO:Al2O3 thin films by rf magnetron sputtering method and characterization

Bu çalışmada RF Magnetron Püskürtme Yöntemi ile silikon alttaş üzerine 200 °C'de Al2O3 katkılı ZnO (çinko oksit) ince filmler üretildi. Farklı Ar/O2 oranlarında büyütülen ince filmler daha sonra hızlı termal tavlama (RTA) sistemi ile bir dakika süresince vakum ve oksijen ortamında tavlandı. Biriktirme süresince argon-oksijen oranının ve farklı ortamlarda tavlama sıcaklığının filmlerin yapısal, optik ve yüzey özelliklerine etkisi XRD, PL ve AFM yöntemleriyle incelendi. ZnO:Al2O3/n-Si numunelerin kristal tanecik boyutu, örgü parametresi, maksimum pik yarı genişliği (FWHM) ve kristallografik analizleri X-ışını kırınım yöntemiyle belirlendi. Hazırlanan numunelerin optik özellikleri, fotolüminesans (PL) spektrumundan alınan veriler yardımı ile belirlendi. Atomik kuvvet mikroskobu (AFM) kullanarak yüzeyin atomik yapısı ve yüzey topografisi incelendi. Hazırlanan ZnO:Al2O3 ince filmlerin en optimal üretim ve tavlama şartları, optik ve yapısal özellikleri belirlendi. Hazırlanan ince filmlerin tavlamadan önce ve tavladıktan sonraki karakterizasyon sonuçları kıyaslandı ve tavlamanın numuneler üzerine olumlu etki yaptığı gözlendi.In this study, Al2O3 doped ZnO thin films were deposited on silicon substrates at 200°C by RF magnetron sputtering method. The thin films deposited at different Ar/O2 rates were annealed in vacuum and oxygen ambient by rapid thermal annealing (RTA) system during 1 minute. During deposition the change in argon-oxygen rate and annealing ambient and temperature of thin films on structural, optical and surface properties were investigated by XRD, PL and AFM methods. Crystal grain size, lattice parameter, full width at half maximum (FWHM) values and crystallographic analysis of ZnO: Al2O3/n-Si structure was carried out by x-ray diffraction method. Optical properties of prepared samples were investigated with the help of photoluminescence (PL) spectroscopy. Atomic surface structure and surface topography were measured by atomic force microscopy (AFM) method. According to optical and structural properties, the optimum deposition and annealing conditions of deposited ZnO: Al2O3/n-Si thin films were determined. The experimental results of prepared thin films were compared with before and after annealing procedure and it was concluded that the annealing procedure has positive effect on prepared samples

Predicting the occurrence of rogue waves in the presence of opposing currents with a high-order spectral method

International audienceWe discuss the dynamics of unidirectional random wave fields that propagate against an opposing current through laboratory experiments and direct numerical simulations of the Euler equations solved with a high-order spectral method. Both approaches demonstrate that the presence of a negative horizontal velocity gradient increases the probability of the occurrence of extreme and rogue waves in the course of their propagation with the emergence of a rapid transition from weakly to strongly non-Gaussian properties. Numerical simulations capture quantitatively well the statistical properties of laboratory observations and substantiate that underlying physics are associated to quasiresonant nonlinear interactions triggered by the background current

Vertical mixing in coastal canopies

The spatial extent over which meadows of submerged aquatic vegetation, such as seagrass, have an ecological and environmental influence is tightly limited by the exchange of water across canopy boundaries. In coastal environments, the process of vertical mixing can govern this material exchange, particularly when mean currents are weak. Despite a recently improved understanding of vertical mixing in steady canopy flows, a framework that can predict mixing in wave-dominated canopy flows is still lacking. Accordingly, an extensive laboratory investigation was conducted to characterize the rate of vertical mixing in wave-dominated flows through measurement of the vertical turbulent diffusivity (Dt,z) of an injected dye sheet. A simple model of coastal canopies, an array of wooden dowels of variable packing density, was subjected to waves with a wide and realistic range of height and period. Vertical mixing across the top of a submerged canopy is shown to be driven by both the shear layer that forms at the top of the canopy and wake turbulence generated by canopy stems. By allowing for an additive contribution from these two processes, we present a predictive formulation for the rate of vertical mixing in coastal canopies across a range of wave and canopy conditions. The rate of vertical mixing, and the dominant mixing mechanism, is highly dependent upon a Keulegan–Carpenter number (KC) that represents the ratio of the particle excursion length to the length scale that defines the canopy drag. This study enables a significantly enhanced predictive capability for the residence time of ecologically and environmentally significant species within coastal canopies

The importance of creating dynamically-scaled models of aquatic vegetation in the laboratory

Physical modelling of vegetated flows is an essential component of process -based investigations into vegetation ecohydraulics. The vast majority of research into vegetated flows has employed rigid model vegetation, so that the canopy’s geometry (i.e. its height and front al area) is invariant and easy to quantify. Here, we demonstrate that embedding realism (in the form of flexibility and buoyancy) in the model vegetation can have a profound impact on the hydrodynamics. Specifically, we compare rates of vertical mixing in two types of model canopy (with identical heights and frontal areas) subjected to oscillatory flow over a range of realistic wave heights and periods. The two types of canopy were: (1) a rigid canopy consisting of wooden dowels, and (2) an array of flexible, buoyant model plants designed to mimic a meadow of the seagrass Posidonia australis. Dynamic similarity between the model and real seagrass was achieved by matching the two dimensionless ratios of the dominant forces that govern plant motion (rigidity, buoyancy and drag). Results demonstrate a significant difference in flow structure between the two canopies and a significant reduction in the rate of vertical mixing in a flexible canopy, relative to the rigid analogue. Thus, while the use of dynamically -scaled vegetation models adds a layer of modelling complexity, it represents a step towards a more faithful recreation of flow and mixing in these systems

Residence time in aquatic canopies in wave-dominated flows

The large-scale ecological and environmental impact of coastal canopies is tightly limited by the exchange of water across their boundaries. In coastal environments, where the flow is typically wave-dominated, vertical mixing is believed to be the dominant process controlling residence time (Tres ). Recent experiments of wave-driven flows over rough boundaries, however, have revealed the generation of a strong onshore mean current (up to 50% of the orbital velocity far above the canopy) near the canopy top. It is therefore imperative to understand that these two processes, i.e. horizontal advection and vertical mixing, can control residence time in coastal canopies. Through consideration of a Peclet number (the ratio of diffusive to advective time scales), this study presents a framework for quantitative prediction of residence time in these environments. Results reveal that Pe depends heavily on wave and canopy properties and may vary significantly in real coastal canopies. Quantitative predictions for residence time in the limit of Pe \u3c\u3c 1 (mixing-dominated exchange) and Pe \u3e\u3e1 (advection-dominated exchange) are presented. For Pe ~ O ( 1 ), characterization of each process will be necessary in describing residence time in these systems

Ocean Currents Trigger Rogue Waves

International audienc