Vegetation Drag Research Articles

Interrill erosion and roughness parameters of vegetation in rangelands

In recent decades, the micro-region of Paraiba state, Brazil, has been undergoing the process of replacing its native coverage by crops, mainly pastures for grazing. This land use change has led to the detachment of soil due to the water erosion. Thus, this research had the merit of analyzing the hydraulic roughness parameters generated by pasture emergent vegetation in field condition under interrill erosion. This study was conducted in a randomized block with 4 treatments and 5 replicates, totaling 20 experimental plots. The treatments comprised different slopes (15%, 25%, 35% and 45%) in an Ultisol. The mean flow velocity and the rate of infiltration in the pasture varied significantly with the increase in slope, contributing to increasing erosion rates until 35% slope. For the slope equal to 45%, the largest soil loss was not observed, chiefly because the crop covers offset the effects generated by the increase in the slope and also due to the remotion of surface horizon (A horizon) which was removed by previous water erosion, which now acted directly on the B horizon. The vegetation drag coefficient (CD′) of the pasture exhibited an increase when occurred a reduction of turbulent flow between plants. In the condition of increasing in flow depth (h) there was a greater energy flux between the mass of water and the plant structure of Brachiaria decumbens that resulted in the decrease of the vegetation resistance force (Fdrag).

Large eddy simulation of flow and mass exchange in an embayment with or without vegetation

This paper presents a 3D large eddy simulation model to investigate the flow phenomena in an embayment with or without vegetation in a straight channel. This model takes into account the existence of vegetation by adding the vegetation drag force to the momentum equations. The equations of mass, momentum were implemented in σ-coordinate, and the operator splitting method is used based on finite difference schemes. A fluctuating velocity generated by synthetic eddy method is imposed at the inlet boundary. The model is first validated by two cases: flow in an embayment without vegetation in a straight channel, and flow in an open channel with submerged vegetation. The comparisons show a good agreement with the measurements. The model is then applied to simulate flow patterns and mass exchange in the embayment with vegetation and/or varying the embayment aspect ratios. Lastly, the model is applied to the field.

Three-dimensional hydrodynamic modeling of coastal flood mitigation by wetlands

The mitigation of storm tides by coastal wetlands is investigated by enhancing a well-established three-dimensional hydrodynamic model to include vegetation effects on mean flow and turbulence quantities. The influence of vegetation upon the mean flow characteristics is modeled through the stem-induced form drag and inertia force incorporated into the momentum equations. Vegetative source and sink terms are added to the transport equations of the Mellor–Yamada turbulence closure model in order to properly simulate the vertical mixing process that takes place within vegetated water columns. The model adopts two well-established empirical formulas to determine the vegetation drag coefficient. Although these formulas have been established under laboratory conditions that differ from those tested in the present study, quantitative and qualitative comparisons reveal the satisfactory performance of the enhanced model to simulate complex flow–vegetation interactions. Model validation against laboratory experiments show that the vegetation-induced drag force added to the momentum equations is the main contributor to the flow-vegetation interactions. The original model that lumps the resistance due to vegetation with the bottom roughness coefficient produces locally incorrect vertical distributions of mean flow velocity and turbulence quantities. The enhanced model is applied to simulate the impacts of intertidal salt marshes of Jamaica Bay, NY, on the storm tide produced by Hurricane Irene. The model results indicate that the salt marshes played only a minimal role in mitigating the peak water elevations induced by Irene. On the other hand, the presence of the marshes causes higher velocities in non-vegetated areas such as deep channels, and lower velocities in vegetated areas, and thus redistributes energy around the bay, with likely feedbacks on water quality, marsh stability, and the response to sea level rise.

The importance of accurately representing submerged vegetation morphology in the numerical prediction of complex river flow

AbstractThis paper reports a novel method for the incorporation of complex plant morphologies into a computational fluid dynamics (CFD) model, allowing the numerical prediction of flows around individual plants. The morphological complexity, which comprises the vertical and lateral distribution of individual branches and leaves is captured through terrestrial laser scanning (TLS) and is maintained in the numerical prediction of flow fields. This is achieved where the post‐processed, voxelized plant representation is incorporated into a CFD scheme through a mass flux scaling algorithm (MFSA). Flow around Prunus laurocerasus has been modelled under foliated and defoliated states following the removal of leaves. The complex plant morphologies are shown to produce spatially heterogeneous downstream velocity fields, with velocity profiles that deviate significantly from the idealized inflected shape. Rapid transition between the high velocity free stream zone and the zone of reduced velocity in the plant wake indicate shearing of flow, with the point of reattachment extending up to seven plant lengths downstream. The presence of leaves significantly modifies the flow field response, with development of a second, more pronounced wake structure around the dense foliage. This approach provides a full flow numerical description of the pressure field, enabling the vegetative drag force to be quantified. For the example given here, drag force is an order of magnitude greater for the foliated state. The methodology outlined here demonstrates the importance of accurately representing complex plant morphology in hydraulic models, and allows drag forces and coefficients to be calculated for specific plant species. Copyright © 2015 John Wiley & Sons, Ltd.

Linear stability analysis for monami in a submerged seagrass bed

The onset of monami – the synchronous waving of seagrass beds driven by a steady flow – is modelled as a linear instability of the flow. Unlike previous works, our model considers the drag exerted by the grass in establishing the steady flow profile, and in damping out perturbations to it. We find two distinct modes of instability, which we label modes 1 and 2. Mode 1 is closely related to Kelvin–Helmholtz instability modified by vegetation drag, whereas mode 2 is unrelated to Kelvin–Helmholtz instability and arises from an interaction between the flow in the vegetated and unvegetated layers. The vegetation damping, according to our model, leads to a finite threshold flow for both of these modes. Experimental observations for the onset and frequency of waving compare well with model predictions for the instability onset criteria and the imaginary part of the complex growth rate respectively, but experiments lie in a parameter regime where the two modes can not be distinguished.

A new formulation for vegetation-induced damping under combined waves and currents

Based on energy conservation a new analytical formulation for the evaluation of wave damping under the combined effect of waves and both following and opposing currents is presented. The formulation obtained for regular and random waves allows the derivation of analytical expressions for the vegetation drag coefficient as a function of wave damping parameters. These parameters are calibrated using a unique experimental set obtained in a large-scale wave basin considering the interaction of waves and currents with real vegetation representative of salt marshes, namely Spartina anglica and Puccinellia maritime. Comparisons show the quality of the analytical formulation under different hydrodynamic conditions, vegetation species and various Reynolds numbers formulated in terms of plant characteristics such as the deflected plant length accounting for the flow-induced bending of the vegetation. The new formulation can be useful to be implemented in phase averaged and phase resolving numerical models of wave propagation.

Representing plants as rigid cylinders in experiments and models

Simulating the morphological adaptation of water systems often requires including the effects of plants on water and sediment dynamics. Physical and numerical models need representing vegetation in a schematic easily-quantifiable way despite the variety of sizes, shapes and flexibility of real plants. Common approaches represent plants as rigid cylinders, but the ability of these schematizations to reproduce the effects of vegetation on morphodynamic processes has never been analyzed systematically. This work focuses on the consequences of representing plants as rigid cylinders in laboratory tests and numerical simulations. New experiments show that the flow resistance decreases for increasing element Reynolds numbers for both plants and rigid cylinders. Cylinders on river banks can qualitatively reproduce vegetation effects on channel width and bank-related processes. A comparative review of numerical simulations shows that Baptist's method that sums the contribution of bed shear stress and vegetation drag, underestimates bed erosion within sparse vegetation in real rivers and overestimates the mean flow velocity in laboratory experiments. This is due to assuming uniform flow among plants and to an overestimation of the role of the submergence ratio.

Single-Layer Model for Average Flow Velocity with Submerged Rigid Cylinders

AbstractA single-layer velocity model is proposed in this study to predict the cross-sectional average velocity for open channel flows with submerged rigid cylinders. This model is preferable to previous two-layer models in that it does not involve evaluations of vegetation drag coefficient and hydrodynamic height of roughness elements. In the development of the single-layer model, the concept of the hydraulic radius is applied to determine the length scale of the entire cross-section affected by the submerged vegetation. The resulting friction factor scales largely with the ratio of stem height to flow depth with exponent 3/2, and also decreases slightly with an increasing Reynolds number. Comparisons of the single-layer model with experimental data show that its performance is comparable to the previous two-layer models, and the model is also applicable to open channel flows subject to submerged flexible vegetation. However, the proposed model cannot be extended to two extreme conditions including flows...

An improved Cauchy number approach for predicting the drag and reconfiguration of flexible vegetation

An improved model to describe the drag and reconfiguration of flexible riparian vegetation is proposed. The key improvement over previous models is the use of a refined ‘vegetative’ Cauchy number to explicitly determine the magnitude and rate of the vegetation's reconfiguration. After being derived from dimensional consideration, the model is applied to two experimental data sets. The first contains high-resolution drag force and physical property measurements for twenty-one foliated and defoliated full-scale trees, including specimens of Alnus glutinosa, Populus nigra and Salix alba. The second data set is independent and of a different scale, consisting of drag force and physical property measurements for natural and artificial branches of willow and poplar, under partially and fully submerged flow conditions. Good agreement between the measured and predicted drag forces is observed for both data sets, especially when compared to a more typical ‘rigid’ approximation, where the effects of reconfiguration are neglected.

Prediction of Velocity Distribution in Straight Channel with Rigid Vegetation

A laboratory study to explore the effect of vegetation in terms of rigid cylindrical roughness on the hydraulics of flow in an open channel is presented. The study consists of an extensive set of flume experiments for flows with unsubmerged rigid cylindrical stems of various concentrations arranged in regular staggered configurations. The study will be helpful for finding roughness and local velocity in a channel with rigid vegetation. The effect of emergent rigid vegetation on the prediction of the effective vegetal drag coefficient, Cd for various flow depth combinations has been explored. Vegetal drag coefficient Cd is found to vary with the non-dimensional hydraulic, geometric and surface parameters such as vegetation densities, relative depth ratio of water depth h to plant height hs, Reynolds's number Re and Froude's number Fr etc. The suitable dependencies of the vegetal drag for the non-dimensional hydraulic and geometric parameters are presented. Analysis and comparisons with flexible vegetation has also been done with the present experimental work on rigid vegetation. Parameters affecting the velocity drag coefficients for both the cases are discussed, compared and results are summarized and presented

Hydraulic roughness due to submerged, emergent and flexible natural vegetation in a semiarid alluvial channel

In the semiarid area of Brazil, vegetation in natural streams generates hydraulic resistance and becomes critically important in the design of soil conservation practices. However, most researchers have studied vegetation roughness under laboratory conditions. The present work has been studied under the rare phenomenon of flow conditions and roughness due to Beach Morning Glory (Ipomoea pes-caprae) in field conditions of submerged and emergent flexible vegetation on the Jacu River, an intermittent stream in the semiarid region of Brazil. The hydraulic roughness of the Jacu River was characterized by the vegetal drag coefficient (CD′) of 5.93 m−1 under submerged condition and 2.702 m−1 underemergent condition, which is sensitive to the amount of turbulence, especially when I. pes-caprae is submerged. Two zones of the velocity profile were witnessed under submerged and emergent conditions: a shear zone and a fast free flow zone. In the shear zone turbulent mixing and a linear increase in velocity were observed at a submergence ratio between 0.6 and 1, followed by the velocity-defect law. The fast free flow zone occurred on the top of vegetation where there was a decreasing shear dispersion and the velocity followed the logarithmic law for a submergence ratio greater than unity.

Numerical investigation of wave attenuation by vegetation using a 3D RANS model

Vegetation has been recognized as an important natural shoreline protection against storm surges and waves. Understanding of wave–vegetation interaction is essential for assessing the ability of vegetation patches, such as wetlands, to mitigate storm damages. In this study the wave attenuation by vegetation is investigated numerically using a 3-D model which solves the Reynolds-Averaged Navier–Stokes equations (RANS) by means of a finite-volume method based on collocated hexahedron mesh. A mixing length model is used for turbulence closure of the RANS equations. The water surface boundary is tracked using the Volume-of-Fluid (VOF) method with the Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM) to solve the VOF advection equation. The presence of vegetation is taken into account by adding the vegetation drag and inertia forces to the momentum equations. The model is validated by several laboratory experiments of short wave propagation through vegetation over flat and sloping beds. The comparisons show good agreement between the measured data and calculated results, but the swaying motion of flexible vegetation which is neglected in this study can influence the accuracy of the wave height predictions. The model is then applied to one of the validation tests with different vegetation properties, revealing that the wave height attenuation by vegetation depends not only on the wave conditions, but also the vegetation characteristics such as vegetation height and density.

Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

AbstractThe presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed‐shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed‐shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.

Wave attenuation in mangroves: A quantitative approach to field observations

Coastal mangroves, dwelling at the interface between land and sea, provide an important contribution to reducing risk from coastal hazards by attenuating incident waves and by trapping and stabilizing sediments. This paper focusses on relations between vegetation densities, wave attenuation rates, sediment characteristics and sedimentation rates in mangroves. These processes were studied along two cross-shore transects through mangroves fringing estuaries in the southern Andaman region of Thailand. Volumetric vegetation densities in these mangroves were ranging up to 32‰, depending on the water depth. Generalized total wave attenuation rates increased from 0.002m−1 in the sparsely vegetated forest fringes with Avicennia and Sonneratia species, up to 0.012m−1 in the dense Rhizophora vegetation in the back of the forests. The total wave attenuation rates integrate effects of shoaling and energy losses due to various bio-physical interactions within the mangrove ecosystem. Wave attenuation in the mangroves is presumably dominated by energy losses due to vegetation drag, since wave attenuation due to bottom friction and viscous dissipation on the bare mudflats is significantly lower than those inside the mangrove vegetation.Additionally, wave attenuation in the mangroves was found to facilitate enhanced net sediment deposition and a gradual fining of the bed material. These findings corroborate the coastal defence function of mangroves by quantifying their contribution to wave attenuation and sediment trapping. The explicit linking of these properties to vegetation composition and structure facilitates modelling studies investigating the mechanisms determining the coastal defence capacities of mangroves.

Wind effect on diurnal thermally driven flow in vegetated nearshore of a lake

In this study, a highly idealized model is developed to discuss the interplay of diurnal heating/cooling induced buoyancy and wind stress on thermally driven flow over a vegetated slope. Since the model is linear, the horizontal velocity can be broken into buoyancy-driven and surface wind-driven parts. Due to the presence of rooted vegetation, the circulation strength even under the surface wind condition is still significantly reduced, and the transient (adjustment) stage for the initial conditions is shorter than that without vegetation because of the reduced inertia. The flow in shallows is dominated by a viscosity/buoyancy balance as the case without wind, while the effect of wind stress is limited to the upper layer in deep water. In the lower layer of deep regions, vegetative drag is prevailing except the near bottom regions where viscosity dominates. Under the unidirectional wind condition, a critical dimensionless shear stress $$\Gamma _{cri} $$ to stop the induced flow can be found and is a function of the horizontal location $$x$$ . For the periodic wind condition, if the two forcing mechanisms work in concert ( $$\theta =0$$ ), the circulation magnitude can be increased. For the case where buoyancy and wind shear stress act against each other ( $$\theta =\frac{1}{2}$$ ), the circulation strength is reduced and its structure becomes more complex. However, the flow magnitudes near the bottom for $$\theta =0$$ and $$\theta =\frac{1}{2}$$ are comparable because surface wind almost has no influence.

A 3-D phase-averaged model for shallow-water flow with waves in vegetated water

A 3-D shallow-water flow model has been developed to simulate the flow in coastal vegetated waters with short waves. The model adopts the 3-D phase-averaged shallow-water flow equations with radiation stresses induced by short waves. It solves the governing equations using an implicit finite volume method based on quadtree rectangular mesh in the horizontal plane and stretching mesh in the vertical direction. The flow model is coupled with a spectral wave deformation model called CMS-Wave. The wave model solves the spectral wave-action balance equation and provides wave characteristics to the flow model. The model considers the effects of vegetation on currents and waves by including the drag and inertia forces of vegetation in the momentum equations and the wave energy loss due to vegetation resistance in the wave-action balance equation. The model has been tested using several sets of laboratory experiments, including steady flows in a straight channel with submerged vegetation and in a compound channel with vegetated floodplain and random waves through a vegetated channel and on a vegetated beach slope. The calculated water levels, current velocities, and wave heights are in general good agreement with the measured data.

Determining drag coefficients and their application in modelling of turbulent flow with submerged vegetation

Vegetation is a key aspect of water resources and ecology in natural rivers, floodplains and irrigation channels. The hydraulic resistance of the water flow is greatly changed when submerged vegetation is present. Three kinds of drag coefficients, i.e., the drag coefficient for an isolated cylinder, the bulk drag coefficient of an array of cylinders and the vertically distributed or local drag coefficient, have been commonly used as parameters to represent the vegetation drag force. In this paper, a comprehensive experimental study of submerged stems in an open channel flow is presented. Empirical formulae for the three drag coefficients were obtained based on our experimental results and on data from previous studies. A two-layer model was developed to solve the mean momentum equation, which was used to evaluate the vertical mean velocity profile with each of the drag coefficients. By comparing the velocity distribution model predictions and the measurement results, we found that the model with the drag coefficient for an isolated cylinder and the local drag coefficient was good fit. In addition, the model with the bulk drag coefficient gave much larger velocity values than measurements, but it could be improved by adding the bed friction effect and making choice of the depth-averaged velocity within the canopy layer.

Laboratory study on wave dissipation by vegetation in combined current–wave flow

Coastal wetlands such as salt marshes and mangroves provide valuable ecosystem services including coastal protection. Many studies have assessed the influence of plant traits and wave conditions on vegetation-induced wave dissipation, whereas the effect of tidal currents is often ignored. To our knowledge, only two studies investigated wave dissipation by vegetation with the presence of following currents (current velocity is in the same direction as wave propagation) (Li and Yan, 2007; Paul et al., 2012). However, based on independent experiments, they have drawn contradictive conclusions whether steady currents increase or decrease wave attenuation. We show in this paper that this inconsistency may be caused by a difference in ratio of imposed current velocity to amplitude of the horizontal wave orbital velocity. We found that following currents can either increase or decrease wave dissipation depending on the velocity ratio, which explains the seeming inconsistency in the two previous studies. Wave dissipation in plant canopies is closely related to vegetation drag coefficients. We apply a new approach to obtain the drag coefficients. This new method eliminates the potential errors that are often introduced by the commonly used method. More importantly, it is capable of obtaining the vegetation drag coefficient in combined current–wave flows, which is not possible for the commonly used calibration method. Based on laboratory data, we propose an empirical relation between drag coefficient and Reynolds number, which can be useful for numerical modeling. The characteristics of drag coefficient variation and in-canopy velocity dynamics are incorporated into an analytical model to help understand the effect of following currents on vegetation-induced wave dissipation.

The role of rooted emergent vegetation on periodically thermal-driven flow over a sloping bottom

Thermal-driven flow is generated due to topographic or vegetation-shading effects. Asymptotic solutions by assuming a small bottom slope are derived to discuss effects of rooted emergent vegetation and interactions between emergent vegetation and sloping topography on thermal-driven flow during diurnal heating and cooling cycles. The results show that the zero-order horizontal velocity is significantly reduced by vegetative drag, and the time lag between the change of horizontal velocity and the reversal of pressure gradient is also shortened. The solutions reveal that the viscous effect is dominant in very shallow water, and the drag force becomes important as the water depth increases. The inertial term is only important at the very beginning stage of flow initiation. Different vegetation distributions can significantly change the temperature fields that then affect patterns of thermal-driven circulation and exchange flowrates. For the case of tall vegetation growth in shallow water, and when the deep water side is open, the effects of vegetation shading may interfere with the topographic effects and dramatically alter the flow patterns. The blockage of solar radiation due to vegetation shading can determine the patterns and magnitude of thermal-driven flow. By means of the derived asymptotic horizontal velocity, exchange flow rates can be estimated, which are in good agreement with previous studies. The limitation and valid ranges of asymptotic solutions are finally discussed.

Relative Significance of Microtopography and Vegetation as Controls on Surface Water Flow on a Low-Gradient Floodplain

Surface water flow controls water velocities, water depths, and residence times, and influences sediment and nutrient transport and other ecological processes in shallow aquatic systems. Flow through wetlands is substantially influenced by drag on vegetation stems but is also affected by microtopography. Our goal was to use microtopography data directly in a widely used wetland model while retaining the advantages of the model’s one-dimensional structure. The base simulation with no explicit treatment of microtopography only performed well for a period of high water when vegetation dominated flow resistance. Extended simulations using microtopography can improve the fit to low-water conditions substantially. The best fit simulation had a flow conductance parameter that decreased in value by 70 % during dry season such that mcrotopographic features blocked 40 % of the cross sectional width for flow. Modeled surface water became ponded and flow ceased when 85 % of the cross sectional width became blocked by microtopographic features. We conclude that vegetation drag dominates wetland flow resistance at higher water levels and microtopography dominates at low water levels with the threshold delineated by the top of microtopographic features. Our results support the practicality of predicting flow on floodplains using relatively easily measured physical and biological variables.