Vegetation Drag Research Articles

Density‐driven exchange flow between open water and an aquatic canopy

Differences in water density can drive an exchange flow between the vegetated and open regions of surface water systems. A laboratory experiment has been conducted to investigate this exchange flow, using a random array of rigid, emergent cylinders to represent the canopy region. The flow pattern was captured using a CCD camera. The velocity of the current entering the canopy and the volume discharge both decrease with increasing vegetative drag and also decrease gradually over time. Theoretical predictions for velocity and discharge rate are developed and verified with experimental observations. Extensions to field conditions are also discussed.

Flow resistance models for flexible submerged vegetation

Summary This paper investigates the variation of hydraulic roughness parameters with flow depth for submerged flexible vegetation. In an experimental study flexible grass canopies are used to examine the impact of stem height, grass type and degree of submergence on the flow resistance properties. Results reveal that Manning’s n roughness coefficient increases with decreasing flow depth reaching an asymptotic constant at lower levels of vegetation submergence. Data suggest that the value of the constant is dependent on the vegetation height and the vegetation properties. Currently the n - UR method underpredicts the Manning’s n and the data from this study extends the USDA method for lower bed slopes and for smaller UR values. The vegetal drag-Reynolds number model is extended for submerged flexible grass linings of differing heights and types. Similar values of the vegetative characteristic number k are evaluated as from previous studies yet for contrasting vegetation types. The vegetal drag coefficient value was found to be strongly correlated to the vegetation height, and follows a power law relationship. The exponent value was found to be independent of the vegetation characteristics.

Modeling microphysical influences on optical turbulence in complex areas

An earlier paper showed that there is a growing need for increasingly accurate and reliable numerical models to predict optical turbulence conditions, especially in complex (nonuniform) signal propagation environments. Thus, we present a finite-difference computer model to demonstrate a viable approach for predicting the microphysical (microclimate) influences on optical turbulence intensity (Cn2) around the ARL A_LOT Facility and its surroundings (which consist of multiple building arrays and forests). Our multi-dimensional prototype model begins to address optical turbulence conditions along more complex lines-of-sight and begins to account for inhomogeneities in Cn2 brought about by horizontal changes in landscape, wind flow, temperature, and humidity. For now, the model physics represent advection, pressure gradient, eddy diffusion, and vegetation drag force processes. Simple mechanisms to predict the heat and moisture source terms have also been incorporated. Initial model results have been quite encouraging. The model code is computationally efficient and extremely flexible with regard to modifications and debugging. We anticipate that this kind of computational research will be an important vehicle for investigating Cn2 and related laser-optic propagation effects in complex areas.

Effects of aquatic vegetation on flow in the Nansi Lake and its flow velocity modeling

Aquatic vegetation has significant effects on flow in waters. In this article, four types of water areas were analyzed according to the field survey on water depth and vegetation in the Nansi Lake for the East Line Project of Water Transfer from South to North in China (WTSNC). The depth-averaged 2-D hydrodynamic models with and without consideration of the effects of aquatic vegetation on flow were established to simulate flow fields in vegetated and non-vegetated zones in the Nansi Lake. With the established models, flow fields were predicted under the conditions of water transfer from south to north. The results indicate that when the drag force term exerted by aquatic vegetation is considered, the computed velocities agree well with the measured data, whereas as the drag term is taken into account, the computed velocities are obviously larger than the measured data in the vegetated zone and considerably smaller in the non-vegetated zone, and the error range between the two velocities is large if this problem is dealt with the method of increasing the roughness coefficient of the lake-bed to reflect the vegetation drag force. In addition, it is demonstrated that the emerged vegetation exerts larger effects on flow than submerged vegetation comparing the results in the emerged vegetation (reed) zone (EVZ) and the submerged vegetation zone (SVZ).

Prediction of velocity profiles and longitudinal dispersion in salt marsh vegetation

To predict the behavior of solutes and suspended particles in wetlands, it is necessary to estimate advection and longitudinal dispersion. To better understand these processes, measurements were taken of stem frontal area, velocity, vertical diffusion, and longitudinal dispersion in a Spartina alterniflora salt marsh in the Plum Island Estuary in Rowley, Massachusetts. Vegetation volumetric frontal area peaked at 0.067 6 0.007 cm 21 near 10 cm from the bed. If the velocity profile in a dense emergent marsh canopy depends on the local balance between pressure forcing and vegetation drag, the velocity will vary inversely with canopy drag (i.e., velocity is minimum where the frontal area is maximum). In fact, the minimum velocity was observed at 10 cm from the bed. The momentum balance therefore provides a way to predict the velocity profile structure from canopy morphology. The vertical diffusion coefficient also depends on canopy characteristics, such that the vertical diffusion coefficient normalized by the velocity and stem diameter had a constant value of 0.17 6 0.08 at this study site. The canopy morphology also controls the longitudinal dispersion, observed in this study to be 4 to 27 cm 2 s 21

Drag coefficients for modeling flow through emergent vegetation in the Florida Everglades

Hydraulic data collected in a flume fitted with pans of sawgrass were analyzed to determine the vertically averaged drag coefficient as a function of vegetation characteristics. The drag coefficient is required for modeling flow through emergent vegetation at low Reynolds numbers in the Florida Everglades. Parameters of the vegetation, such as the stem population per unit bed area and the average stem/leaf width, were measured for five fixed vegetation layers. The vertically averaged vegetation parameters for each experiment were then computed by weighted average over the submerged portion of the vegetation. Only laminar flow through emergent vegetation was considered, because this is the dominant flow regime of the inland Everglades. A functional form for the vegetation drag coefficient was determined by linear regression of the logarithmic transforms of measured resistance force and Reynolds number. The coefficients of the drag coefficient function were then determined for the Everglades, using extensive flow and vegetation measurements taken in the field. The Everglades data show that the stem spacing and the Reynolds number are important parameters for the determination of vegetation drag coefficient.

Numerical Solution of Fully Developed Flow with Vegetative Resistance

This paper presents a numerical solution of the Reynolds averaged Navier-Stokes and the near-wall \ik- (turbulent kinetic energy) and \Iω\N- (specific dissipation or dissipation per unit kinetic energy) transport equations, which are modified to include vegetative drag terms. For similar treatment of the model coefficients, the use of the near-wall \Ik-ω\N model produces similar results to previous models that employed the standard \Ik-e\N models with wall functions. The study shows that reasonable predictions of streamwise velocity and Reynolds stress profiles can be achieved by adopting universal values for all model coefficients, but the calculated energy gradient can have significant error. The study also indicates that predictions of streamwise turbulence intensity are significantly improved by adopting the universal values of \IC\Dfk\N\N = 0.05 and \IC\Dfω\N\N = 0.16 rather than the theoretically based values, \IC\Dfk\N\N = 1.0 and \IC\Dfω\N\N = \Iβ/αβ*C\Dfk\N\N.

Large eddy simulation of free surface turbulent flow in partly vegetated open channels

AbstractA large eddy simulation (LES) model has been developed to simulate the hydrodynamic behaviour of turbulent flow in an open channel with a domain of vegetation. Vegetation is considered as an internal source of resistant force and turbulence energy. The model is modified from the LES model of Li and Wang (International Journal for Numerical Methods in Fluids 2000; 34), and is distinctive in that the subgrid scale turbulence is parameterized by a k–l model. The length scale of turbulence l is proportional to the grid size and the turbulence energy k is obtained from the solution of the turbulence energy transport equation. An operator splitting method, which splits the solution procedure into advection, diffusion and pressure propagation steps, is employed so that different numerical schemes can be used for the solution of different physical processes. The model has been applied to simulate open channel flow with transverse shear produced by vegetation drag. Some organized large eddies were found in the interface between the vegetated and non‐vegetated regions and the organized structure clearly has a life cycle. At the interface the transverse velocity profile exhibits a steep gradient, which induces significant mass and momentum exchange, acts as a source of vorticity, and generates high Reynolds stresses. The logarithmic vertical velocity variation becomes uniform in the vegetated domain. The agreement between the numerical results and the experimental data (Tsujimoto and Kitamura, KHL Progressive Report '92, Hydrology Laboratory, Kanazawa University, Japan, 1992; 21) is satisfactory. The present k–l LES model is proven to be a useful tool for engineering applications, as it can simulate the dynamic development of large eddies and the associated intermittent turbulence. Copyright © 2002 John Wiley & Sons, Ltd.

Mixing layers and coherent structures in vegetated aquatic flows

To date, flow through submerged aquatic vegetation has largely been viewed as perturbed boundary layer flow, with vegetative drag treated as an extension of bed drag. However, recent studies of terrestrial canopies demonstrate that the flow structure within and just above an unconfined canopy more strongly resembles a mixing layer than a boundary layer. This paper presents laboratory measurements, obtained from a scaled seagrass model, that demonstrate the applicability of the mixing layer analogy to aquatic systems. Specifically, all vertical profiles of mean velocity contained an inflection point, which makes the flow susceptible to Kelvin‐Helmholtz instability. This instability leads to the generation of large, coherent vortices within the mixing layer (observed in the model at frequencies between 0.01 and 0.11 Hz), which dominate the vertical transport of momentum through the layer. The downstream advection of these vortices is shown to cause the progressive, coherent waving of aquatic vegetation, known as the monami. When the monami is present, the turbulent vertical transport of momentum is enhanced, with turbulent stresses penetrating an additional 30% of the plant height into the canopy.

Contents of Volume 257

Mangroves have been shown to protect shorelines against damage from the combined hydrodynamic forces of waves and tides, owing to the presence of roots (pneumatophores) and tree trunks that enhance vegetative drag. However, field measurements within these environments are limited. We present field observations of flows from the seaward coast of Cù Lao Dung Island (Sóc Trăng Province) in the Mekong Delta, Vietnam. Measurements were made in two different seasons along a transect that crosses from mudflats to mangrove forest. Flows are also explored using an idealised numerical model. Both the data and model capture the flow transitions from mudflat across the fringing region to the forest interior. We observe a rotation of the obliquely incident flows toward an orientation nearly perpendicular to the vegetated/unvegetated boundary. The momentum balances governing the large-scale flow are assessed and indicate the relative importance of friction, winds and depth-averaged pressure forces. In the forest, drag coefficients were 10–30 times greater than values usually observed for bottom friction, with particularly effective friction in the regions of dense pneumatophores at the fringe and when water depths were lower than the height of the pneumatophores. Pressure gradient balances suggest that the drag induced by bottom friction from pneumatophores was dominant relative to drag from the larger, but sparser, tree trunks.

Flow structure in depth‐limited, vegetated flow

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open‐channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.

NUMERICAL STUDY ON THE FLOOD-FLOW STAGE IN GRAVEL-BED RIVER WITH THE EXCESSIVE REVERINE TREES

Recently reverine trees in the course of gravel bed-river have extremely increased and enlarged their covered-area over bars and flood plain. Such a situation brings about problems of river management for the environmental aspects as well as the safety of rivers against flood. In order to clarify such problems, we should get the knowledge about the flow with vegetation-covered zones.In this study, the applicability of a 2D-numerical flow model including vegetation drag effect is verified for the upstream reach of the river Watarase which has the averaged bed slope of about 1/150 and has the characteristics of the segment-1 reach with the excessive reverine trees. Calculated results are compared with the results obtained by the observation after flood and predict an increase in water-level depending on the vegetation density and predict the concentration field of suspended fine sand.

Drag, turbulence, and diffusion in flow through emergent vegetation

Aquatic plants convert mean kinetic energy into turbulent kinetic energy at the scale of the plant stems and branches. This energy transfer, linked to wake generation, affects vegetative drag and turbulence intensity. Drawing on this physical link, a model is developed to describe the drag, turbulence and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics, and covers the natural range of vegetation density and stem Reynolds' numbers. The model is supported by laboratory and field observations. In addition, this work extends the cylinder‐based model for vegetative resistance by including the dependence of the drag coefficient, CD, on the stem population density, and introduces the importance of mechanical diffusion in vegetated flows.

Shallow-Water Turbulence Modeling and Horizontal Large-Eddy Computation of River Flow

By introducing the concept of “SDS (subdepth scale) turbulence” to model three-dimensional (3D) turbulence with length scales less than the water depth and treating it explicitly with a proper separate modeling, an SDS-2DH model has been developed to simulate the evolution of horizontal large-scale eddies in shallow water. Applying this model to river flows with transverse shear due to vegetation drag, the horizontal large-scale (HLS) eddies were found to dominate horizontal momentum mixing. The bottom friction and vegetation drag, acting as sinks of vorticity, play the key roles in the development of the horizontal large-scale eddies and in Reynolds stress generation. The SDS-2DH model can directly describe effects of flow geometry, such as vegetation layer width, on the large-scale eddy development and, hence, predicts turbulence-mixing better than the k-e model.

Comparison of methods for measuring woody riparian vegetation density

Flow resistance caused by vegetative drag depends on vegetation density (Vegd), the area of vegetation projected onto a plane perpendicular to the flow direction per unit volume of flow. However, research concerning the accuracy and efficiency of field measurement of Vegdfor hydraulic investigations is limited. Measurements were conducted at two willow stands with different Vegdusing the horizontal point frame (HPF), board, and camera methods. Results obtained with the HPF method were considered accurate and used as a standard with which to compare the performance of the board and camera methods. It was concluded that the board method: (1) was the most rapid of the three; (2) tends to underestimate Vegd; and (3) performed better in sparse willows than dense willows. The camera method: (1) required less time than the HPF; (2) tends to underestimate Vegd; and (3) performed better in dense willows than sparse willows.