Flexible Vegetation Research Articles

Space evolution of turbulent structures over a flexible vegetation area

En un cauce natural o artificial es usual la presencia de vegetación en el fondo y en la pared. Los efectos que produce la presencia de ésta es una alteración en el campo de flujo. La magnitud de las alteraciones en el campo del flujo depende de la extensión, altura, densidad y flexibilidad de la vegetación. Para analizar a detalle las estructuras turbulentas, en este trabajo se realizó un estudio experimental en laboratorio en un canal de sección rectangular con un tramo de vegetación sumergida, flexible y artificial. La región de medición se ubicó aguas arriba de la zona de vegetación hasta el final de esta misma, además se determinó la asimetría que puede inducir la presencia de la pared del canal. Las mediciones se realizaron con equipo de velocimetría acústica de efecto Doppler (ADV). Los resultados obtenidos identifican el desarrollo de la capa de mezcla a partir de los patrones de velocidad media y los esfuerzos de Reynolds. En el caso de los perfiles de velocidad media sobre la capa de vegetación fue posible identificar un punto de inflexión donde se identificaron inestabilidades tipo Kelvin-Helmholtz. Con el fin de analizar los procesos en la capa de mezcla se propone un modelo fenomenológico basado en el análisis de la curvatura de las velocidades promediadas. Además, se determinó la aportación en el transporte de momento de las estructuras turbulentas con un análisis de cuadrantes con las velocidades fluctuantes.

The Impact of Plant Oscillation on Dispersion in Emergent Aquatic Canopies

AbstractFlexible canopies bend and oscillate in both the in‐line and cross‐flow directions due to periodic forcing associated with vortex shedding. The resultant plant motion impacts the vegetation wake structure and, thus, the rate of lateral dispersion in these environments. Despite significant improvements in our understanding of dispersion in rigid canopies, a reliable framework to predict mixing in oscillating canopies is still lacking. This research demonstrates how plant oscillation can profoundly impact rates of lateral mixing in steady flows. The lateral dispersion coefficients were evaluated experimentally, using photographs of injected dye plumes within two types of emergent flexible model vegetation. Results revealed a significant increase in the rates of lateral dispersion (by up to 45%) due to the plant oscillation. A predictive model, based on a redefined vegetation density that incorporates the impact of plant oscillation, was developed that can accurately predict dispersion in emergent canopies. A quantitative prediction of dispersion in oscillating flexible vegetation is a significant step toward a more accurate description of material transport in aquatic environments.

Effect of Rigid Vegetation Arrangement on the Mixed Layer of Curved Channel Flow

Curved channels and aquatic vegetation are commonly present in the riverine environment. In this study, the effects of vegetation density and distribution on the hydrodynamic characteristics of a mixed layer developed over a 180-degree curved channel were investigated through flume experiments. Wooden sticks were used to simulate rigid vegetation distributed along the half side of the channel, and a 200 Hz acoustic Doppler velocimeter (ADV) was employed to measure the three-dimensional instantaneous velocity at five selected cross sections along the curved channel. Experimental results show that the vegetation covering the half of the channel significantly affects the hydrodynamic structure of the curved channel flow, and the unequal vegetation resistance induces the K-H instability at the vegetation and non-vegetation interface, resulting in a standard hyperbolic tangent function of streamwise velocity distribution along the lateral direction. The influence of curve position on turbulence kinetic energy is far greater than that of vegetation density and vegetation distribution. The peak value of turbulent kinetic energy is comprehensively affected by vegetation density and distribution, and the peak position of turbulent kinetic energy at the interface is changed by different vegetation distribution. The combined effect of the curve and the partly covered vegetation increases the mixing between the water bodies, enhancing turbulent kinetic energy, and vegetation along the concave bank plays a more significant role. For turbulent bursting, the inward and outward interactions are mainly bursting events in the vegetation area, while ejections and sweeps are dominant in the non-vegetation area. However, the critical vegetation condition to initiate large-scale coherent structure (LSS) in the mixed layer and the influence of flexible vegetation need to be further studied in the future.

Flow Structure and Short-Term Riverbed Evolution in Curved Flumes

River bending is the major effect responsible for bed topography and bank changes. In this study, fluid velocity (measured by a three-dimensional Doppler advanced point current meter) and bed topographical data have been collected in 40 sections of an experimental model. The whole flume was composed of an organic glass bend, upstream and downstream water tanks, two transition straight sections, a circulation pump, and a connection pipeline. Each section has been found to be characterized by a primary circulation and a small reverse circulation, with some sections even presenting three more or more circulation structures. The minimum circulation intensity has been detected in proximity to the top of the curved channel, while a region with small longitudinal velocity has been observed near the concave bank of each bend, corresponding to the flat bed formed after a short period of scouring. The maximum sediment deposition and scour depth in the presence of a uniform distribution of living flexible vegetation within 10 cm of the flume wall have been found to be smaller than those observed in the tests conducted without vegetation.

Verification of Methods for Determining Flow Resistance Coefficients for Floodplains with Flexible Vegetation

In terms of the hydraulic effect of plant flexibility, of particular note is the calculation formula that was proposed by Kouwen, which combines the roughness of the riverbed with the plant community parameter MEJ (including the modulus of elasticity). Kouwen’s method was developed on the basis of laboratory experiments with low vegetation (grasses). According to the authors of this work, the method can also be used to evaluate the resistance of medium vegetation (shrubs) deforming under the influence of water flow. The main objective of the presented research was to verify the application of Kouwen’s method in order to calculate the flow resistance coefficient λ for quasi-regular formed plant obstructions (e.g., basket willow plantations). In a water laboratory, a comprehensive study of the biomechanical and hydraulic properties was carried out for flexible shrubs in floodplains. The results of the hydraulic measurements were compared with the results of the calculations that were made by four various methods using the Chezy-Manning, Garbrecht/Pasche, Lindner/Kaiser, and Kouwen formulas. For all of the flows through the vegetated zone that was tested, the best results were obtained when using the Kouwen calculation procedure and the worst were found for the Lindner formula, which did not include information on the plant flexibility.

Derivation of new resistance principle on flow-induced morphological response of flexible vegetation.

Water flow under vegetated environments is a noteworthy research topic in environmental hydraulics and restoration ecology, and this research is particularly important for maintaining water transport and streambed stability in water ecosystems. The calculation of the resistancecoefficient in vegetated water flow is the core of this research. But there are still problems such as complex expressions and low simulation accuracyin this research field. To solve this scientific problem, this research, based on the theoretical study of environmental hydraulics and genetic algorithm, selected three basic parameters of vegetation submergence, resistance length and curvaturedegree, and successfully constructed the formula for calculating the resistance coefficient for flexible vegetated flow by usinga wide range of data sets. New quantitative relationship between the drag coefficient and the relative roughness of flexible vegetation was establishedin this study. The formula ofdrag coefficients for flexible vegetation conditions has a more concise form and can besuccessfully applied to both flexible andrigid vegetation.Asflexible vegetation is deformed under the action of water flow, and the quantitative expressions of Vogel number and relative roughness are given quantitatively through the analysis of its physical properties. Overall, this study improves the basic theoretical study of vegetated flow in environmental fluid dynamics and provides scientific theoretical support for vegetationrestoration.

Flow and turbulence in unevenly obstructed channels with rigid and flexible vegetation

The pattern of vegetation along the cross-section of macrophyte-dominated shallow waters is generally uneven, which affects water velocity and turbulence. This study examined the velocity and turbulence in the open channel with an uneven transverse distribution of vegetation in laboratory flume experiments. Two vegetation patterns were tested: emergent vegetation which covered part of the channel, and a symmetrical combination of submerged and emergent vegetation canopies along the lateral direction of the flume. The flow was measured using a three-dimensional Acoustic Doppler Velocimeter. The velocity and turbulence characteristics were analyzed under three vegetation densities and five discharge scenarios. Results showed that the longitudinal mean velocity changed with vegetation density and flow discharge when vegetation was unevenly distributed in a lateral direction. The strong variation in shear stress at the emergent vegetation–open water intersections and submerged–emergent vegetation intersections resulted in large-scale vortices at the interfaces. The formation processes of stem-scale turbulence and shear-scale turbulence under different vegetation scenarios were discussed. A turbulent kinetic energy model within partly obstructed vegetation canopies was established, which helped to identify the development of horizontal and vertical coherent vertices.

Fluid–structure interaction in a flexible vegetation canopy in an open channel

Submerged flexible vegetation occupies the core of aquatic ecosystem research. Hydrodynamics of submerged flexible vegetation and its interaction with the flow in an open channel are of great significance in studying the mass and momentum transports in the flow. In this study, a numerical model for highly flexible vegetation based on large eddy simulation and the immersed boundary method was used to simulate the flow–vegetation interaction. It is recognised that alternate vortices with opposite sense rotations appear at the flow–vegetation interface. These vortices prompt the vegetation canopy to have wave-like coherent waving motion, commonly called the monami phenomena. The spatial scale and the spreading velocity in the streamwise direction of these vortices determine the wavelength, frequency and amplitude of the vegetation coherent waving motion. In this study, the fast Fourier transform method was applied to analyse the factors affecting the characteristics of the vegetation coherent waving motion. It is revealed that, as the flow velocity increases, the wavelength of the coherent waving motion decreases, while the frequency and amplitude increase. Besides, as the vegetation spacing increases, the wavelength and amplitude of the coherent waving motion increase, but the frequency decreases. Furthermore, an increase in the relative density of vegetation magnifies the amplitude of coherent waving motion without affecting the wavelength and frequency.

The role of riparian vegetation flexibility in a bio‐hydro‐morphodynamic simulation

AbstractBio‐hydro‐morphodynamic simulation is an essential tool for analyzing fluvial processes under the influence of riparian vegetation, and has developed over the past decade. However, most bio‐hydro‐morphodynamic models have neglected the flexibility of vegetation. The flow‐induced deformation of flexible vegetation changes the resistance to water flow. Therefore, flexibility affects water flow surrounding vegetation patches, and alters bed shear stress below and surrounding patches. To investigate the effects of vegetation flexibility in a fluvial, bio‐hydro‐morphodynamic simulation, we incorporated a model for predicting flexible vegetation reconfiguration in a bio‐hydro‐morphodynamic model based on Delft3D. We studied effects by considering and neglecting vegetation flexibility within a gravel bed river with alternate bars. The vegetation used in the simulation was Phragmites japonica, a widely existing reedy grass in riparian environments. Our results indicate that vegetation flexibility can impact river hydrodynamics and morphology development processes. When accounting for vegetation flexibility, water depth decreases, averaged Shields stress within a vegetation patch increases, and channel width with sediment transport increases. Erosion and deposition within the channel are additionally reduced. The significance of vegetation flexibility depends on multiple factors (i.e., the stem density growth rate, rigidity, and the river bed material). Given a fast stem density growth rate or a large rigidity, differences between flexible and rigid vegetation cases could be neglected. The difference between flexible and rigid vegetation was maximized when the bed gravel size fell within a certain range. Our results suggest that the impact of vegetation flexibility on river morphology depends on the mobility of river bed material. A very active or inactive bed reduced the importance of vegetation flexibility on river morphology.

The Deformation and Shear Vortex Width of Flexible Vegetation Roots in an Artificial Floating Bed Channel

As an ecological measure to deal with river water quality problems, artificial floating islands have been widely used all over the world, but the research on root deformation and shear vortex width under the action of artificial floating islands is rare. In this paper, the relationship between the deformation of vegetation roots and parameters of vegetation roots under different hydrodynamic conditions is experimentally studied. The results show that the Cauchy number (Ca) value gradually increases with the increase of velocity, and that the smaller the diameter is, the greater the Ca value is. The value of the buoyancy number (B) will increase with the increase of root length and will decrease with the increase of root diameter. The corresponding deformation formula of flexible root systems under hydrodynamic conditions is obtained, and has high simulation accuracy. Based on theoretical analysis and machine learning, a formula for the shear vortex width of flexible vegetation is established, δ=0.361+0.0738lelαCdf. The research results can provide a theoretical basis for hydrodynamic and solute transport in artificial floating island channels.

A Semicoupled Shallow-Water Model for Vertical Velocity Distribution in an Open Channel with Submerged Flexible Vegetation

A Semicoupled Shallow-Water Model for Vertical Velocity Distribution in an Open Channel with Submerged Flexible Vegetation

Modelling wave attenuation through submerged vegetation canopies using a subgrid canopy flow model

Coastal canopies formed by aquatic vegetation can significantly modify nearshore wave processes, which has motivated numerous laboratory and field studies investigating how wave attenuation occurs for a wide range of canopies as a function of wave conditions and canopy properties. To predict wave dissipation by canopies, numerous formulations have been derived to describe canopy drag through use of empirical drag coefficients. In many cases, drag coefficients are used as a tuning parameter to account for complex processes that govern how wave-driven flows interact with individual plants within canopies. This approach, however, can greatly limit the predictive capabilities of both empirical and numerical models. In this study, we extend a non-hydrostatic (phase-resolving) depth-integrated wave model (XBeach non-hydrostatic) with an efficient and robust subgrid canopy flow model to account for the important properties of submerged canopy flows that govern canopy drag forces and wave dissipation. A combination of wave heights, velocities and direct canopy force measurements, as well as data obtained from literature, were used to validate the model, showing the model has good skill in capturing the governing flow dynamics for both rigid and flexible vegetation canopies. By incorporating the canopy flow dynamics, the extended model allows for application of well-established drag coefficients for individual canopy elements to accurately predict wave attenuation over submerged vegetation canopies, rather than relying on empirical formulations or calibration. With the current extension, the model remains computationally efficient, allowing for use on coastal (∼km) scales. • A canopy flow subgrid model was implemented in the open-source XBeach model. • The model was verified using observed wave heights, velocities and canopy forces. • It can accurately predict wave evolution over rigid and flexible submerged canopies. • The model is robust and computationally efficient for use in large scale applications. • Not accounting for canopy flow can lead to substantial errors in wave predictions.

Influence of submerged flexible vegetation on turbulence in an open-channel flow

AbstractIn this study, we present a three-dimensional numerical model for the interaction of flow with submerged flexible vegetation, based on a large-eddy simulation and the immersed boundary method. The model innovatively realises the interaction between the flow and highly flexible vegetation with clustered leaves. Besides being a three-dimensional model of motion with full degrees of freedom, this study improves the consideration of the motion of the vegetation in all directions, and in addition the energy and momentum transfer in the spanwise direction. Furthermore, we perform a flume experiment for the flow with submerged flexible vegetation, the results of which are used to validate the simulation effects of the numerical model. It is found that the numerical model can effectively simulate the velocity profiles and the movement of vegetation induced by the flow. Using the model to analyse the flow–vegetation interaction, we find that the movement of vegetation is closely related to the flow velocity. As the flow velocity increases, both the offset angle and the vegetation swaying amplitude increase. Compared to vertical rigid vegetation, the tilting of flexible vegetation does not significantly change the velocity difference and the magnitude of the turbulent kinetic energy between the inside and the outside of the vegetation canopy, but it does weaken the disturbance to flow, thus reducing the resistance to flow. However, the swaying of vegetation dose significantly increase the velocity difference between the inside and the outside of the canopy. It forms Kelvin–Helmholtz hairpin vortices intensifying the turbulence production, and enhancing the disturbance and resistance to flow.

Numerical investigation of the wave interaction with flexible vegetation: model setup and validation for a single stem study case

Aquatic flexible vegetation plays a very important role in ecosystem, and has been widely used in river or coastal bank revetment. Flexible vegetation contributes to wave attenuation and soil retention. In this study, a fluid-structure bidirectional coupled numerical model (FSC model) was developed based on the codes in-house software HydroFlow@ to study the interaction between water flow and flexible vegetation. The water wave was simulated using the non-hydrostatic numerical model. Based on the nonlinear theory of elastic thin rod, a dynamic numerical model of flexible vegetation (FV model) was developed using the Finite Element Method (FEM) to simulate the large bending deformation and finite extension of a thin rod. A domain extension method was used to transfer the contact force between waves and the vegetation stem in the coupling process. The FSC model was validated using available experimental results focusing on a single stem dynamic simulation coupling with the free surface open channel flow simulation. The numerical results were in good agreements with the experiments. Relative errors of maximum deflection were less than 10%. Asymmetrical bending during a wave period were captured well compared with the measurements.

Numerical investigation of the dynamics of flexible vegetations in turbulent open-channel flows

Numerical investigation of the dynamics of flexible vegetations in turbulent open-channel flows

Hydraulic Jump on a Partially Vegetated Bed

AbstractHydraulic jump may form when a supercritical flow runs over a vegetated bottom and change the downstream flow hydraulics and fluid properties after air‐water mixing and energy dissipation at the flow transition. Unlike the abundant literature on hydraulic jump characteristics on engineered or natural sediment rough beds, few studies of vegetation roughness effects have been reported. New experiments were conducted by creating hydraulic jumps with identical inflow Froude numbers on a smooth bed, fully and partly grated rough beds, and partially vegetated grate‐rough beds covered with artificial plants of different densities. The free‐surface dynamics and air‐water flow properties were investigated with primary focus on the interaction between the flow and flexible vegetation barrier/roughness, in addition to the effects of Froude number and inflow roughness conditions. The characteristic jump and jump roller lengths were found shortened as the jump roller was deflected over the vegetation layer. The flow structure modification was observed with a steepening of the roller free‐surface profile and enhanced surface wave propagation, especially when the flow experienced an invert roughness discontinuity at the toe. An increasing bubble detection frequency was obtained in nonvegetated channels when the bed was roughened, but additional vegetation on the rough invert caused reduction in the bubble frequency and cross‐sectional mean air concentration by inducing roller deflection and air bubble detrainment. The results indicated that the effects of perturbing artificial flexible vegetation with relatively large plant height could be different from uniformly distributed rigid bed roughness elements and complicated by the time‐varying flow‐vegetation interaction.

Living on the edge: How traits of ecosystem engineers drive bio-physical interactions at coastal wetland edges

Salt marshes and mangrove forests provide critical ecosystem services such as reduced sediment erosion and increased hydrodynamic buffering. Sediment transport and hydrodynamics can be influenced by specific functional traits of the plants (for example, flexibility vs. rigidity) and community traits (for example, spatial density). While there is a growing body of literature on plant trait and hydrodynamic interactions, direct comparative studies of sediment transport and scour development in and around intertidal wetland edges are scarce. In this study we systematically compared the effects of plant traits on sediment budgets around the lateral edges of intertidal wetlands under controlled hydrodynamic and sedimentary conditions using full scale vegetation mimics with contrasting flexibilities and densities. Experiments were carried out in a large-scale flume, using two spatial densities each of flexible and rigid vegetation mimics. We measured unconsolidated sedimentary bed-level changes in experimental runs using waves only, currents only, and waves combined with currents. Both mimic types dampened the energy of the incoming flow, highlighting the role of rigid and flexible aquatic vegetation in providing coastal protection. The rigid vegetation mimics’ lateral edge experienced larger velocities, more energetic turbulence, and local scour around individual stems. Scour around stems could influence the lateral expansion of the rigid vegetation ecosystem by reducing sediment stability and thus decreasing seedling establishment success. The flexible plant mimics produced lower turbulence at their leading edge, which resulted in sediment being deposited over a shorter distance into the patch than in the rigid mimics. Decreased vegetation density caused reduced sediment erosion at the leading edge and less sediment accumulation within the vegetation patches for both the rigid and flexible mimics. The hydrodynamic and sedimentary processes identified for both ecosystems are linked to different feedbacks. A positive feedback was identified in which vegetation attenuates hydrodynamic energy allowing sediment accumulation within the patch. A negative feedback was identified where large velocities caused flow divergence and erosion outside of the vegetation, and would therefore compromise its lateral expansion. High densities of rigid vegetation enhance this negative feedback. Lower density flexible vegetation, however, combined with less energetic hydrodynamic conditions facilitate the expansion of vegetation patches as they cause less flow divergence and therefore less erosion. The strong flow divergence observed in the rigid vegetation cases highlight their importance for buffering hydrodynamics but at the cost of increased erosion within the front end of patches and along their lateral edges.

Space evolution of turbulent structures over a flexible vegetation area

Space evolution of turbulent structures over a flexible vegetation area

Modeling wave attenuation by submerged flexible vegetation with XBeach phase-averaged model

Understanding and modeling wave attenuation by flexible vegetation have emerged as essential issues in coastal and ocean engineering. Given that the existing XBeach phase-averaged wave model can only simulate wave attenuation by rigid vegetation, and previous studies of wave energy dissipation by vegetation are mainly restricted to linear wave theory. This study aims to extend the XBeach phase-averaged wave model to simulate wave attenuation by both homogeneous and heterogeneous submerged flexible vegetation, and determine wave attenuation simulation deviation based on different wave theories. Hence, the existing flexible vegetation dynamic model is incorporated into the XBeach phase-averaged wave model. The reliability of the coupled model in simulating wave attenuation by both homogeneous and heterogeneous submerged flexible vegetation is revealed. Simulation results also demonstrate that the computed wave attenuation rate based on Stokes second-order wave theory is larger than that based on linear wave theory within the parameters used in this investigation. Meanwhile, wave attenuation simulation deviation between these two wave theories enlarges as the Ursell number increases within the parameters used in this investigation. This study can broaden the application of the XBeach model and lay the technical foundation for future engineering practice of nature-based coastal defence.

A Methodology for Studying the Hydroelastic Response of Submerged Flexible Vegetation

AbstractMost fluvial vegetation canopies embody some degree of structural flexibility, resulting in hydroelastic phenomena linking plant deformation and fluid flow. Quantification of such coupled fluid‐structure motion is critical in investigating key fluvial processes. The present work presents an experimental protocol that enables such quantification. High‐speed particle image velocimetry data of unidirectional flow surrounding idealized, submerged canopy elements, consisting of polyurethane rubber models, together with the corresponding solid displacement field, were simultaneously obtained by combining fluorescent imaging and refractive index matching. The operating principle of the approach employed herein involves seeding the two phases with different tracers, facilitating independent interrogation of the dynamics of each. Time‐resolved data allowed for observation of the dynamic link between a deformable object and the surrounding flow field.