Vegetation In Open Channel Flow Research Articles

An improved formula for bed-load rate in open channel flows with emergent vegetation

There is an urgent need to predict the bed-load transport rate in vegetated river ecosystems to support restoration efforts. In response, we have developed a novel model for estimating the effective shear stress acting on the riverbed. This model is based on the energy equation and considers the intrinsic relationship between energy loss in the mean flow and turbulence generated by vegetation in open channel flows with emergent vegetation. Using this bed shear stress model, we assessed the performance of the Meyer-Peter–Müller (MPM) formula in predicting the bed-load transport rate in vegetated flows by comparing it with collected literature experimental data. The results revealed that the MPM formula does not provide accurate predictions. It tends to overestimate the bed-load transport rate when the dimensionless effective shear stress is approximately less than one and underestimate them when the dimensionless effective shear stress is approximately greater than one. This suggests that vegetation enhances and decreases the sediment transport rate when the dimensionless effective shear stress is approximately larger or lower than one, respectively. Consequently, we modified the coefficients of the MPM formula using extensive experimental data, leading to the development of a novel predictive formula for the bed-load transport rate in vegetated flows. This new formula outperforms existing literature equations and is effective for predicting the bedload transport rate, even for umbrella-like vegetation.

Flow through layered vegetation in open channel flows: effect on velocity and discharge distribution

Flow through layered vegetation in open channel flows: effect on velocity and discharge distribution

Numerical investigation on the effect of flexible vegetation in open-channel flow incorporating FSI

ABSTRACT The understanding and studying of the vegetation effect on the natural water flows is an important objective in modern scientific research regarding river management and restoration, with high environmental impact. Vegetation in the open-channel flow significantly affects the velocity of flow. In the present study, influence of the flexible vegetation on the flow characteristics of open-channel flow was resolved numerically using computational fluid dynamics. For the numerical study of the open-channel flow with flexible vegetation, fluid–structure interaction (FSI) is used. The obtained results were then compared with the numerical results of rigid vegetation. The influence of different parameters such as depth of flow, flow velocity, slenderness ratio of vegetation and density of vegetation were studied. It was found that incorporation of FSI in the analysis is essential to accurately predict the flow characteristics. It was also found that the simulation using FSI is more critical in case of submerged vegetation in an open-channel flow for all flow velocities.

Estimation of drag coefficient of emergent and submerged vegetation patches with various densities and arrangements in open channel flow

ABSTRACT A large proportion of flow resistance can be due to vegetation in open-channel flows. Classic methods are unable to estimate flow resistance in the presence of vegetation. In this study, drag coefficient was estimated as a parameter to represent the flow resistance in emergent and submerged vegetation patches with different densities. Emergent vegetation was simulated by wooden cylinders 8 mm in diameter and 0.35 m high. Four emergent patches with different densities, arrangements, tandem, random, and circular, were examined. Also, submerged vegetation was simulated by nails 4 mm in diameter and 4.5 cm in height. The phenomenon of dip occurred throughout the dense patches because of secondary flow and the drag force generated by vegetation. Four methods were selected to estimate the drag coefficient and the balance approach-blockage factor method was used as the main method to estimate the patch drag coefficient. Results showed that velocity profiles were S-shaped and the drag coefficient of random vegetated patches was greater than the coefficient of tandem patches. The drag coefficient revealed a direct relation with vegetation density. In all patches, the initial rows had the highest contribution to flow resistance, so the drag coefficient decreased through vegetation patches.

Drag coefficient of in-line emergent vegetation in open channel flow

Along the banks of rivers, trees and bushes are often planted in a single line. In the case of trees, the trunks are simulated in hydraulic laboratories by a set of cylinders and the drag coefficient can be estimated with use of various different methodologies, including by direct measurement, using the momentum equation, equating turbulence intensity and drag force, numerical modeling, and genetic programming. However, for the sake of simplicity, many equations have been proposed in the scientific literature that allow its immediate estimation. Some of these equations are used in this work to verify their ability to reproduce experimental data obtained for in-line cylinders by Mulahasan and Stoesser (2017), who obtained the drag force by applying the momentum equation. Several statistical descriptors have been used for this purpose. We found that the equations derived from staggered and random arrangements generally overestimate by a large amount the CD values; instead, a few relationships and in particular one derived from a squared arrangement provide much better results.

A genetic programming-based model for drag coefficient of emergent vegetation in open channel flows

The estimation of drag exerted by vegetation is of great interest because of its importance in assessing the impact of vegetation on the hydrodynamic processes in aquatic environments. In the current research, genetic programming (GP), a machine learning (ML) technique based on natural selection, was adopted to search for a robust relationship between the bulk drag coefficient (Cd) for arrays of rigid circular cylinders representing emergent vegetation with blockage ratio (ψ), vegetation density (λ) and pore Reynolds number (Rep) based on published data. We utilize a data set covering a wide range of each parameter involved to cover all possible dependencies. A new predictor, which shares the same form with the Ergun-derived formula, was obtained without any pre-specified forms before searching. The dependence of the two parameters in Ergun equation on vegetation characteristics was also estimated by GP. This new Cd predictor for emergent vegetation with a relatively concise form exhibits a considerable improvement in terms of prediction ability relative to existing predictors.

A Review on Hydrodynamics of Free Surface Flows in Emergent Vegetated Channels

This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

Experimental and Numerical Study on Impact of Double Layer Vegetation in Open Channel Flows

AbstractThis paper studies the effects of different types and configurations of double layer vegetation on the flow of open channels. The vegetation is simulated through cylindrical dowels with a d...

Floating seed dispersal in open channel flow with emergent vegetation

AbstractIn this paper, we investigate the effects of stem density and flow velocity on interaction processes between two types of floating particles and rigid emergent vegetation in open channel flow. A new definition of the probability that a floating particle interacts with a stem is proposed and validated, and particle–stem interaction processes (including collision and capture mechanisms) are further explored, including a collision model proposed and validated to predict the probability of collision with a correction parameter. We also establish an appropriate length scale to analyse the particle–stem mechanism (for the vegetation characteristics and arrangement used in this study). A more accurate statistical model based on Gamma function is proposed to describe floating particle dispersal from unvegetated section to vegetated section, which is verified compared with exponential model, and two empirical formulas are proposed to apply the Gamma distribution to the particle dispersal in natural systems. We also propose a dimensionless parameter to evaluate the attraction capacity of the floating particle attached to the stem based on analysing forces acting on the particle. Phenomena observed in these experiments are presented for future analysis and research.

Resistance of Open-Channel Flow under the Effect of Bending Deformation of Submerged Flexible Vegetation

The resistance of an open-channel flow under the influence of bending deformation of submerged flexible vegetation canopy was experimentally studied in an indoor water flume. Altogether, 27 tests under the cross-combinations of three bulk flow velocities (V), three vegetation elastic moduli (E), and three vegetation stem thicknesses (b) were conducted. The total drag force experienced by the vegetation stem was accurately calculated based on the measured vertical profiles of flow velocity and the second order turbulent momentum in front of an individual plant, and this enabled the estimation of the overall drag coefficient (CD) for bulk flow. The results show that the bending deformation of vegetation helps to transfer momentum downward toward the channel bed, thus helping to reduce the overall resistance to flow. The Cauchy number (CY) is an important parameter representing the bending property of the vegetation in open-channel flow; it’s functionally related to the resistance of flow. For small deformation configurations, the bending angle (θ) and the relative bending displacement (δz/hp) are linearly dependent on CY and (CY)2, respectively. The CD and Manning’s n for the bulk flow are both power law functions of CY. The semiempirical formulas of CD and n proposed in this study are expected to apply to further experimental studies and river engineering.

Use of conventional flow resistance equations and a model for the Nikuradse roughness in vegetated flows at high submergence

Abstract This study examines the problem of flow resistance due to rigid vegetation in open channel flow. The reliability of the conventional flow resistance equations (i.e. Keulegan, Manning and Chézy-Bazin) for vegetated flows at high submergence, i.e. h/k >5, (where h = flow depth and k = vegetation height) is assessed. Several modern flow resistance equations based on a two-layer approach are examined, showing that they transform into the conventional equations at high submergences. To compare the conventional flow resistance equations at high submergences, an experimental methodology is proposed and applied to the experimental data reported in the literature and collected for this study. The results demonstrate the reliability of the Keulegan equation in predicting the flow resistance. Based on the obtained results, a model to evaluate the Nikuradse equivalent sand-grain roughness, kN, starting from the vegetation height and density, is proposed and tested.

Flow resistance of in-line vegetation in open channel flow

ABSTRACTOpen channel flow through a line of vertically oriented circular rods, designed to represent idealized bankline vegetation, is studied. Bankline vegetation is a common feature in rivers and streams and yet has not received as much attention as fully vegetated flows. Particular focus is placed on the effect of vegetation diameter on the flow resistance which is quantified via a drag coefficient. A large number of laboratory experiments of uniform flow through one-line of vegetation were carried out and drag coefficients were calculated via a momentum balance. Experimentally obtained drag coefficients are then compared to those calculated using a number of empirical expressions that have been proposed in the literature. Good agreement is generally observed. The results show that varying the diameter of the rigid emergent vegetation affects flow resistance, and that the drag coefficient of in-line vegetation exhibits limited sensitivity to the stem Reynolds number.

Rijit Gövdeli Bitkilerin Neden Olduğu Manning Katsayisinin Araştirilmasi

Investigation of Manning Coefficient Caused by Rigid Body Plants To study flows in man-made or natural channels, the identification of the roughness caused by vegetation is important. The roughness parameters used in several equations such as Manning, Chezy, and Darcy-Weisbach are the experimental coefficients that represent mostly sidewall characteristics. In open channel flows having natural or man-made vegetation to control the flow sections, the knowledge of flow velocities, flow depths and/or flow rates are crucial for planning and management studies. For such studies, the quantification of the roughness caused by the vegetation becomes vital. In this study, roughness caused by vegetation in open channel flows is investigated experimentally and a non-linear regression equation relating Manning roughness coefficient to flow and vegetation characteristics is proposed.

Calculation of Drag Coefficient for Arrays of Emergent Circular Cylinders with Pseudofluid Model

The emergent vegetation in open-channel flows is usually simulated using arrays of circular cylinders in laboratory experiments. Analysis of recent experimental data reveals that for a given Reynolds number, the drag coefficient of a cylinder in a dense array is larger than that of an isolated cylinder. A new approach is applied to parameterize the drag coefficient and Reynolds number for flows through arrays of emergent cylinders. The approach is developed based on the concept of pseudofluid, for which an analogy is made between the cylinder-induced drag in an open-channel flow and that induced by the cylinder settling in a stationary fluid. With the proposed parameterization, the experimental database is successfully reorganized in such a way that a generalized drag coefficient is related to a generalized Reynolds number by one single curve, which is valid for a wide range of solid fractions and Reynolds numbers. However, it should be mentioned that only rigid circular stems are considered in this study and their induced drag is assumed to be dominant in comparison with the channel bed resistance.

Representative roughness height of submerged vegetation

Roughness length scale is important in the evaluation of resistance caused by submerged vegetation in open channel flows. By transforming the concept of hydraulic radius, a representative roughness height is proposed in this study for quantifying effect of submerged vegetation on flow resistance in the surface layer. The proposed roughness height is characterized by its proportionality to both stem diameter and vegetation concentration and performs better than other length scales in collapsing resistance data collected under a wide range of vegetation conditions. An approach is then developed for estimate of the average flow velocity and thus resistance coefficients for both cases of rigid and flexible vegetation. Comparisons are also made between the present study and other four formulas available in the literature. This study also shows that all the formulas, if simplified for some simple conditions, can be unified in a general form.

Hydraulic Radius for Evaluating Resistance Induced by Simulated Emergent Vegetation in Open-Channel Flows

The resistance induced by simulated emergent vegetation in open-channel flows has been interpreted differently in the literature, largely attributable to inconsistent uses of velocity and length scales in the definition of friction factor or drag coefficient and Reynolds number. By drawing analogies between pipe flows and vegetated channel flows, this study proposes a new friction function with the Reynolds number that is redefined by using a vegetation-related hydraulic radius. The new relationship is useful for consolidating various experimental data across a wide range of vegetation density. The results clearly show a monotonic decrease of the drag coefficient with the new Reynolds number, which is qualitatively comparable to other drag coefficient relationships for nonvegetated flows. This study also proposes a procedure for correcting sidewall and bed effects in the evaluation of vegetation drag.

Hydraulic resistance of some selected vegetation in open channel Flows

AbstractVegetation in rivers has important roles in improving and restoring river environment. Other than it adds high aesthetic value to revetments, that it can be used as a levee protection in environmental friendly way. In open channel hydraulics, vegetation often causes changes in the flow resistance, usually resulting in the increase of flood stage. Both experimental and numerical researches have been conducted on flow resistance of vegetation in open channels, however, the researches were based mostly on the vegetation found in the region where the researches were conducted, and this restricts the generality of the results.In this study, three Korean natural vegetations, Zoysia matrella (Korean zoysia), Pennisetum alopecuroides (L.) Spreng. (Korean native vegetation) and Phragmites communis Trin. (Korean reed) were used in flume tests on the effect of vegetation in the channel on flow resistance. ‘n‐VR’ retardance curves were developed for each grass. All the grasses were tested under unmown conditions. Z. matrella was tested under fully submerged condition and other two were tested under both submerged and un‐submerged conditions.Resistance coefficient, expressed as Manning's n, converged to about 0.027 (total roughness) for Z. matrella as VR increased. Resistance coefficients for other plants were found affected by the states of the plants, that is, whether they were ‘green’ or ‘dormant’. Generally, resistance coefficients are higher when plants are ‘green’ than when they are ‘dormant’. This is because the resistance coefficient is influenced by the leaf elements of vegetation on the river flow in addition to the stem of vegetation. The interaction between the bending part of vegetation and the water surface can also increase the resistance coefficient. In terms of water depth, P. communis Trin were found more affected on the resistance coefficients compared to Z. matrella and P. alopecuroides (L.) Spreng. Copyright © 2008 John Wiley & Sons, Ltd.