Aquatic Vegetation Canopies Research Articles

Longitudinal Mixing in Flows with Submerged Rigid Aquatic Canopies

The presence of dense submerged vegetation alters mixing characteristics in open channel flows as they cause differential velocities inside and above canopies. The prediction models for longitudinal mixing in the presence of submerged canopies often use the drag coefficient to represent the canopy, which limits the usability of the models when the canopy properties are not fully understood. Here, attempts were made to present a methodology which can be used for deriving the coefficient of longitudinal dispersion in the presence of submerged vegetation based on velocity measurements, using a mixing length approach to model turbulence. An experimental study was conducted in a large-scale laboratory facility to investigate the longitudinal dispersion characteristics in open channel flow with submerged aquatic vegetation canopies. Detailed velocity and solute tracer measurements were undertaken for a representative range of flow velocities. The velocity measurements were used for deriving turbulent shear stress, mixing length, and diffusivity using established theoretical and empirical relationships to derive the longitudinal dispersion. The longitudinal dispersion measured in two locations in the water column for the two canopy submergences was discussed based on the amount of vertical mixing and differential advection. The canopy with a smaller stem length (i.e., higher submergence ratio) has a higher vertical diffusivity, resulting in increased vertical mixing in the water column. The canopy with the higher stem length (i.e., lower submergence ratio) consists of minimal vertical diffusivity, causing the longitudinal dispersion measured above the canopy to be significantly high, even though the longitudinal dispersion measured inside the canopy is much lower. The mathematical model which was adapted for calculating the coefficient of longitudinal dispersion and the tracer results show good agreement, indicating that the N-zone model can accurately predict the longitudinal dispersion in submerged aquatic canopies when used with the presented methodology.

Distribution of emergent aquatic vegetation leaf inclination angle and its simulation

Leaf normal inclination angle distribution (LAD) determines the interception of radiation by leaves and the transmission of radiation by the vegetation canopy, which affects the distribution of incident photosynthetically active radiation on plant leaves and ultimately directly affects the net primary productivity of vegetation. However, the contact measurement method for leaf inclination angle is difficult to perform due to the complex growing environment of wetland aquatic vegetation. In this study, the method of measuring normal leaf inclination angles was improved for wetland aquatic vegetation based on horizontal digital photography combined with the application of a protractor. Phragmites australis and Typha orientalis, two typical emergent aquatic vegetation (EAV) species growing in wetlands, were used to evaluate the robustness of leaf inclination measurement methods. In addition, LAD values were measured and analyzed for their variations across seasonal and vertical gradients within the canopy. Finally, the applicability of the two simulation algorithms for leaf normal inclination was evaluated and compared. Analysis revealed that Phragmites australis LAD exhibited a variation with seasonal changes and vertical canopy height, which is beneficial for light transmission within the canopy. The LAD of Typha orientalis was always an erectophile pattern and changed only slightly with the seasons. The new findings provided additional insights and evidence on aquatic plant resistance and environmental adaptation. Comparing simulation models for EAV, the trigonometric algorithm outperforms other algorithms in accuracy and deviation. The results of this research provide a valuable reference for exploring radiation transfer within aquatic vegetation canopies and understanding vegetation growth.

Hydrodynamics and sediment deposition in turbidity currents: Comparing continuous and patchy vegetation canopies, and the effects of water depth

A flume experiment was carried out to improve understanding of interactions between turbidity currents and aquatic vegetation canopies and their landscape-scale consequences. It focussed on comparing hydrodynamics and sediment deposition in continuous canopies with those in vegetation patches, and on the effects of varying water depth – both of which are previously unreported. The currents’ particulate load was characterised as a mix of fine and coarse fractions. Varying canopy frontal densities, a, and water depths, H, were used. Fifteen runs were carried out with the flume fully vegetated, and a further ten with shorter vegetation patches. In all runs, the currents evolved as expected through inertial, drag-dominated and viscous regimes. The positions at which transitions between the regimes occurred were measured and analysed. In the fully-vegetated runs, both transition positions varied linearly with aH for aH < 0.8, and were constant when aH > 0.8. We argue that the variation at lower values of aH is caused by non-canopy drag forces becoming non-negligible compared to the canopy drag. An equation is derived that models, as a function of a and H, the size a vegetation patch needs to be for its effect on turbidity currents to be the same as that of a continuous canopy. The sediment depositional flux rate for fine particles from the currents within the vegetation was greater than that for coarse particles, by a factor of 1.57. This suggests that bed sediment deposited within canopy patches by turbidity currents will be on average finer than that in gaps between patches, as has been found previously for currents and waves. Thus, this effect will contribute to the development of inter-tidal and shallow sub-tidal landscapes characterized by patches of dense vegetation and fine sediments, surrounded by bare regions with coarser sediments. Our results imply that the distances over which the phenomena we document occur in typical inter-tidal and shallow sub-tidal contexts are of the same order of magnitude as sizes of patches of saltmarsh plants and seagrasses. This indicates that the reported patch length effects are highly relevant to understanding eco-hydrological interactions in these contexts.

Canopy modeling of aquatic vegetation: A geometric optical approach (AVGO)

Aquatic vegetation is an important component in the ecosystems of wetlands and coastal waters, for example it can take up harmful substances and pollutants from the water. Constructing a remote sensing model of aquatic vegetation can deepen our understanding of the spectral characteristics and directional reflection characteristics of the aquatic vegetation canopy under various environmental conditions. However, the existing aquatic vegetation models are limited to continuously distributed canopies. This paper proposes an aquatic-vegetation- geometric-optical (AVGO) model at canopy scale for discrete aquatic vegetation. After dividing the reflectance of water background into specular components and diffuse components, the reflectances of all reflecting surfaces are classified into different reflectance components based on the differences in their materials and on whether the reflectors are illuminated by direct sunlight. Then the scene bidirectional reflectance factor (BRF) is regarded as an area-weighted sum of different reflectance components. Detailed verifications were conducted to ensure that the simulation model operates as intended. The performance of the AVGO model was qualitatively validated with the WCRM model and showed the physical plausibility of the model prediction. Quantitative validations against an artificially designed experiment were consistent with the bidirectional reflectance and the nadir-viewed spectral reflectance. This model can be used for canopy modeling of discrete emergent or submerged aquatic vegetation. Potential applications include retrieval of biophysical or biochemical parameters and aquatic vegetation abundance based on hyperspectral data.

Predicting Current-Induced Drag in Emergent and Submerged Aquatic Vegetation Canopies

Canopies formed by aquatic vegetation, such as mangroves, seagrass and kelp, play a crucial role in altering the local hydrodynamics in rivers, estuaries and coastal regions, and thereby influence a range of morphodynamic and biophysical processes. Prediction of the influence of canopies on these hydrodynamic processes requires a fundamental understanding of canopy drag, which varies significantly with both flow conditions and canopy properties (such as density and submergence). Although our knowledge on canopy drag has increased significantly in recent decades, a conclusive, physics-based description for canopy drag that can be applied to both emergent and submerged canopies is currently lacking. Here, we extend a new theoretical canopy drag model (that employs the velocity between canopy elements as the reference velocity) to submerged aquatic canopies. The model is validated for the first time with direct measurements of drag forces exerted by canopies across broad ranges of flow conditions and canopy density and submergence. The skill and broader applicability of the model are further assessed using a comprehensive set of existing experimental data, covering a broad range of natural conditions (including flexible vegetation). The resulting model provides a simple tool to estimate canopy drag forces, which govern hydraulic resistance, sediment transport and biophysical processes within aquatic ecosystems.

Sediment deposition from turbidity currents in simulated aquatic vegetation canopies

AbstractA laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders. The cylinders were intended primarily to simulate aquatic vegetation canopies, but could equally be taken to represent other arrays of obstacles, for example forests or offshore wind turbines. The turbidity currents were generated by mixing naturally sourced, poly‐disperse sediment into a reservoir of water at concentrations from 1·0 to 10·0 g l−1, which was then released into the experimental section of the flume by removing a lock gate. For each initial sediment concentration, runs with obstacle arrays with solid plant fractions of 1·0% and 2·5%, and control cases with no obstacles, were carried out. The progress of the current along the flume was characterized by the array drag term, CDaxc (where CD is the array drag coefficient, at the frontal area of cylinders per unit volume, and xc is the position of the leading edge of the current along the flume). The downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume. Analysis of these deposits divided them into fine (2·2 to 6·2 μm) and coarse (6·2 to 104 μm) fractions. At the beginning of their development, the gravity currents proceeded in an inertia‐dominated regime until CDaxc = 5. For CDaxc &gt; 5, the current transitioned into a drag‐dominated regime. For both fine and coarse sediment fractions, the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime, remained approximately constant at the early drag‐dominated regime, and then rose and peaked at the end of the drag‐dominated stage. This implies that, when passing through arrays of obstacles, the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles.

Does the canopy mixing layer model apply to highly flexible aquatic vegetation? Insights from numerical modelling

Vegetation is a characteristic feature of shallow aquatic flows such as rivers, lakes and coastal waters. Flow through and above aquatic vegetation canopies is commonly described using a canopy mixing layer analogy which provides a canonical framework for assessing key hydraulic characteristics such as velocity profiles, large-scale coherent turbulent structures and mixing and transport processes for solutes and sediments. This theory is well developed for the case of semi-rigid terrestrial vegetation and has more recently been applied to the case of aquatic vegetation. However, aquatic vegetation often displays key differences in morphology and biomechanics to terrestrial vegetation due to the different environment it inhabits. Here we investigate the effect of plant morphology and biomechanical properties on flow–vegetation interactions through the application of a coupled LES-biomechanical model. We present results from two simulations of aquatic vegetated flows: one assuming a semi-rigid canopy and the other a highly flexible canopy and provide a comparison of the associated flow regimes. Our results show that while both cases display canopy mixing layers, there are also clear differences in the shear layer characteristics and turbulent processes between the two, suggesting that the semi-rigid approximation may not provide a complete representation of flow–vegetation interactions.

Canopy modeling of aquatic vegetation: A radiative transfer approach

Aquatic vegetation in shallow waters can absorb nitrogen and phosphorus from eutrophic waters, and effectively control water bloom. However, our knowledge of the interaction between solar radiation and aquatic vegetation is limited. This restricts the protection of aquatic vegetation. This paper proposes a radiative transfer approach for homogeneous aquatic vegetation canopy, based on the theories for terrestrial vegetation and Case II water. The effects of rough water surface and bottom reflection are also taken into consideration. Validations against an independent Monte Carlo model and field experiment data exhibit well consistencies on the bidirectional reflectance of homogeneous aquatic vegetation canopy. As expected, the results for sparse canopy degenerate. This model can be used for canopy modeling of emergent, floating or submerged aquatic vegetation. Potential applications include retrieval of biophysical or biochemical parameters based on hyperspectral data and physical process.

Progress in the extension of a second-moment closure for turbulent environmental flows

An advanced second-moment closure for the double-averaged turbulence equations of porous medium and vegetation flows is proposed. It treats three kinds of second moments which appear in the double-averaged momentum equation. They are the dispersive covariance, the volume averaged (total) Reynolds stress and the micro-scale Reynolds stress. The two-component-limit pressure–strain correlation model is applied to model the total Reynolds stress equation whilst a novel scale-similarity non-linear k–ε two-equation eddy viscosity model is employed for the micro-scale turbulence. For the dispersive covariance, an algebraic relation is applied. Model validation in several fully developed homogeneous porous medium flows, porous channel flows and aquatic vegetation canopy flows is performed with satisfactory agreement with the data.

Destabilising effects of plant flexibility in air and aquatic vegetation canopy flows

With an analytical model coupling a mixing layer flow with an oscillating vegetation canopy through a drag force, we show that for both the cases of wind over a crop field and a water stream over aquatic plants, the dynamics of the plants is responsible for increasing the growth rate of the coupled instability in the lock-in range through two mechanisms. Because the flow and the vegetation canopy move in phase and thus minimise their interactions, the drag dissipation is decreased. The correlation between the two components of the perturbation flow velocity in the mixing layer is increased and makes the perturbation flow more efficient at extracting energy from the mean flow.

Turburence structure and coherent motion in vegetated canopy open-channel flows

A lot of aquatic plants are observed in actual rivers and they have significant effects on hydrodynamic characteristics such as the velocity distributions, turbulence and coherent structure, as well as mass and momentum exchanges between the vegetated and non-vegetated zones. Aquatic vegetation canopies are responsible for water quality, nutrient and particulate removal, and can reduce turbidity and sediment transport significantly. In the present study, we investigated turbulence structures and coherent motion in vegetated canopy open-channel flows, on the basis of non-intrusive LDA and PIV measurements as well as LES calculations. Section 2 deals with the theoretical consideration on submerged canopy open-channel flows, in which the importance of double-averaging techniques for both time-average and space-average was highlighted. Section 3 describes experimental techniques. Section 4 deals with mean flow structure within and over the canopy, in which the dispersive properties were highlighted. Section 5 considers turbulence structure, in which Reynolds stress, turbulence intensities and turbulent kinetic energy (TKE) budget were discussed. Further, quadrant conditional analysis and two-point space-time correlation revealed the importance of coherent structures near the vegetation edge. Section 6 examines the effects of submergence depth on coherent structure and compares them with terrestrial canopy flows. The whole flow region was divided into three sub-zones, i.e., the emergent zone, the mixing-layer zone, and the log-law zone. In particular, coherent eddies such as sweeps and ejections were highlighted on the instantaneous contour plane of Reynolds stress and vorticity. Finally, Section 7 deals with some LES simulations, which predicted large-scale coherent eddies reasonably in aquatic canopy flows.