Vegetation In Open Channel Research Articles

Predicting Bulk Average Velocity with Rigid Vegetation in Open Channels Using Tree-Based Machine Learning: A Novel Approach Using Explainable Artificial Intelligence.

Predicting the bulk-average velocity (UB) in open channels with rigid vegetation is complicated due to the non-linear nature of the parameters. Despite their higher accuracy, existing regression models fail to highlight the feature importance or causality of the respective predictions. Therefore, we propose a method to predict UB and the friction factor in the surface layer (fS) using tree-based machine learning (ML) models (decision tree, extra tree, and XGBoost). Further, Shapley Additive exPlanation (SHAP) was used to interpret the ML predictions. The comparison emphasized that the XGBoost model is superior in predicting UB (R = 0.984) and fS (R = 0.92) relative to the existing regression models. SHAP revealed the underlying reasoning behind predictions, the dependence of predictions, and feature importance. Interestingly, SHAP adheres to what is generally observed in complex flow behavior, thus, improving trust in predictions.

Analytical Solution of Suspended Sediment Concentration Profile: Relevance of Dispersive Flow Term in Vegetated Channels

AbstractSimulation of suspended sediment concentration (SSC) has great significance in predicting the sediment transport rate, vegetation growth, and the river ecosystem in channels. The present study focuses on investigating the vertical SSC profile in the vegetated open channel flows. To this end, a model of the dispersive flux is proposed in which the dispersive coefficient is expressed as partitioned linear profile above or below the half height of vegetation. The double‐averaging method, that is, time‐spatial average, is applied to investigate the vertical SSC profile in the vegetated open channel flows. The analytical solution of SSC in both submerged and emergent vegetated open channel flows is obtained by solving the vertical double‐averaging sediment advection‐diffusion equation. The morphological coefficient, a key factor of dispersive coefficient, is obtained by fitting the existing experimental data. The analytically predicted SSC agrees well with the experimental measurements, indicating that the proposed model can be used to accurately predict the SSC in the vegetated open channel flows. Results show that the dispersive term can be ignored in the region without vegetation, while the dispersive term has significant effect on the vertical SSC profile within the region of vegetation. The present study demonstrates that the dispersive coefficient is closely related to vegetation density, vegetation structure, and stem Reynolds number but has little relation with flow depth. With a few exceptions, the absolute value of the dispersive coefficient decreases with the increase of vegetation density and increases with the increase of stem Reynolds number in the submerged vegetated open channels.

Approaching model of Manning’s coefficient due to an Effect of density and height of vegetation in open channel

The presence of vegetation growing in the channel can cause increasing the Manning’s coefficient. The magnitude of Manning’s coefficient is depended on the characteristic and type of vegetation. The purpose of the research is to study and model the effect of density and height of vegetation on Manning’s coefficient in open channel. The study was conducted in open channel with 15.5 m length, 0.5 m width and 1.0 m height. At the centre part of the channel is planted with Elephant grass (Pennisetum Purpureum). The effect of different density and height of vegetation were run with fixed discharge to measure the velocity and water profiles in order to obtain Manning’s coefficients. The effect of those variables on the developed model was evaluated using statistical analysis of variance and paired t-test with significance level of 0.05. The results showed that Manning’s coefficient is increasing as increasing density and height of vegetation. If compared to un-vegetated channel, Manning’s coefficient increase in between 4.92 - 54.07%. Based on statistical analysis, it is found that Fcalculation = 367.776 > Fcritic = 19.452, tcalculation = 12.298 and 22.464 > tcritic = 2.571 and R2 = 0,967, showing that the density and height of vegetation significantly influence on the Manning’s coefficient with strong correlation variables in the model.

Investigation of flow resistance exerted by rigid emergent vegetation in open channel

The issue of the resistance to flow in open channels with vegetation has been considered by several researchers mainly experimentally, but the case of rigid emergent vegetation with linear stem arrangement is scarcely investigated. In the present work, the results are presented of an experimental investigation related to the case of rigid emergent vegetation that has been modeled by placing small rods on the bottom of a laboratory flume in aligned configuration. Tests have been executed by varying the flow rate, the bottom slope and the number and the diameter of the rods, by directly measuring the drag force exerted by the flow on a given number of rods, and the water-level profiles. A new expression has been devised for the drag coefficient as a function of the vegetation density, weakly dependent on the stem Reynolds number that allows the use of the former also in large-scale cases. The experimentally measured forces exerted by the flow on the rods have been also compared with the results obtained by applying the momentum equation in integral form to given control volumes, exhibiting a general agreement, but also showing that the use of this technique for the evaluation of the drag coefficients can give rise to not negligible errors. One of the experimental tests has been numerically simulated with the RANS technique (Reynolds-Averaged Navier–Stokes equations), and it is found that the results, mainly in terms of water-level profiles, confirm the ability of such a numerical technique in investigating this complex category of flow cases.

Flow over Flexible Vegetated Bed: Evaluation of Analytical Models

The development of vegetation in the river bed and in the banks can affect the hydrodynamic conditions and the flow behavior of a watercourse. This can increase the risk of flooding and sediment transport. Therefore, it is important to develop analytical approaches to predict the resistance caused by vegetation and model its effect on the flow. This is the objective of this work which investigates the ability of different analytical models to predict the vertical velocity profile as well as the resistance induced by flexible submerged vegetation in open channels. Then it is possible to select the appropriate model that will be applied in the real case of rivers. The model validation is determined after a comparison between the data measured in the different experiments carried out and those from literature. For dense vegetation, the role of the Reynolds number is emphasized in particular with a model using the Darcy-Brinkman equation in the canopy. With a simple permeability, this model is relevant to estimate friction. However, for larger Reynolds number, models based on the fully turbulent flow assumption provide better results.

Experimental Study on Flow Resistance Over Rigid Vegetated Channel

Vegetation is an important part of the ecological channel, and flow structure over vegetation in ecological channel is rather complex. The vegetation resistance to flow is affected by many factors, and there is still no general calculation method to express it. Thus, it is necessary to explore the mechanism of flow resistance over vegetation in open channel. In this paper, the rigid cylindrical sticks arranged in the open channel were used to simulate the stems of non-submerged vegetation in order to investigate the effects of vegetation on the flow. Through theoretical analysis and experimental verification, it shows the form drag caused by vegetation is closely related to the vortex volume created by vegetation. In addition, the total resistance of vegetated channel can be determined by two methods based on vegetation density and flow resistance partitioning, respectively, and both of them are verified by the experimental results. What's more, for rigid vegetation, the drag coefficient is closely related to vegetation density, spacing between vegetation patches and the Reynolds number. The empirical formula among them was established through the experimental results.

Determination of Emergent Vegetation Effects on Manning’s Coefficient of Gradually Varied Flow

Based on mechanical derivation, a new formula showing the effects of emergent vegetation on Manning’s coefficient was established for complex gradually varied flow affected by rigid unsubmerged vegetation in open channels. The new formula indicates that the Manning’s coefficient is a quadratic sum of its vegetal and boundary components with coefficients related to vegetation parameters. The variation characteristics of the total Manning’s coefficient and its vegetal component calculated by the new formula were investigated by a series of laboratory experiments under different vegetation coverage area ratios. It was found that the variations of both the total Manning’s coefficient and its vegetal component followed a linear decreasing trend along the vegetation section. Moreover, linear and power empirical formulas were constructed for the average total Manning’s coefficient and its vegetal component related to the vegetation coverage area ratio, respectively. The findings of the research may expand the theoretical connotations of the Manning’s coefficient for open channel hydraulics and demonstrate practical significance to engineering applications of the Manning’s coefficient for gradually varied flow in vegetated channels.

The effect of density and height of vegetation in open channel on the Manning’s coefficient

One of an important parameter to study water flow in channel is Manning’s coefficient (n). This coefficient is produced not only from material forming the channel, but also due to presence of vegetation in channel. The aim of research is to study the effect of vegetation density and height on Coefficient of Manning in open channel. The study was conducted in open channel with 15.5 m long, 0.5 m wide and 1.0 m high. At the center of 3 m long of channel is planted with Sweet flag (Acorus calamus). The impact of various densities and heights of sweet flag were examined using fixed discharge to measure velocity and slope energy. Coefficient of Manning was obtained using Manning’s equation. Statistical analysis of variance with significance level of 0.05 was adopted to evaluate the effect of variables. The result showed that increasing density and height of Sweet flag make decreasing velocity and increasing Coefficient of Manning to be 0,043-0,064, or 1.40-2.10 times compared to un-vegetated channel (no=0.031). Based on statistical analysis, it is found that Fcalc=15.646 and 16.928 for vegetation density and height respectively, which is larger than FStat=2,603. It can be concluded that density and height of vegetation significantly influence on Coefficient of Manning.

Turbulence Scale Characteristics in a Patchy Vegetated Open Channel

Turbulence Scale Characteristics in a Patchy Vegetated Open Channel

Determination of Flow Resistance Coefficient for Vegetation in Open Channel: Laboratory study

This study focused on determination of flow resistances coefficient for grass in an open channel. Laboratory works were conducted to examine the effects of varying of roughness elements on the flume to determine flow resistance coefficient and also to determine the optimum flow resistance with five different flow rate, Q. Laboratory study with two type of vegetation which are Cow Grass and Pearl Grass were implementing to the bed of a flume. The roughness coefficient, n value is determine using Manning’s equation while Soil Conservation Services (SCS) method was used to determine the surface resistance. From the experiment, the flow resistance coefficient for Cow Grass in range 0.0008 - 0.0039 while Pearl Grass value for the flow resistance coefficient are in between 0.0013 - 0.0054. As a conclusion the vegetation roughness value in open channel are depends on density, distribution type of vegetation used and physical characteristic of the vegetation itself

A Well-balanced Finite Volume Scheme for Shallow Water Equations with Porosity: Application to Modelling Flow through Rigid Vegetation

Strong interactions exist between flow dynamics and vegetation in open-channel. Depth-averaged shallow water equations can be used for such a study. However, explicit representation of vegetation can lead to very high resolution of the mesh since the vegetation is often modelled as vertical cylinders. Our work aims to study the ability of a single porosity-based shallow water model for these applications. More attention on flux and source terms discretizations are required in order to archive the well-balancing and shock capturing properties. We present a new Godunov-type finite volume scheme based on a simple-wave approximation and compare it with some other methods in the literature. A first application with experimental data was performed.

Sediment incipient motion on movable bed in rigid vegetation environment

Vegetation in open channels has a significant influence on the characteristics of bed load movement. To date, studies of incipient motion of sediment in open channels with submerged vegetation have been restricted by limitations in statistical approaches and measuring equipment. Researchers have also neglected incipient motion of sediment in the presence of vegetation when deriving semi-empirical equations for the rates of bed load transport, and have generally considered it the same as that without vegetation, which is clearly inconsistent with, and does not represent, the actual conditions. In this study, we therefore studied the vegetation and sandy bed in a natural river to determine the characteristics of incipient motion of sediment within rigid vegetation on a movable bed. From the spatially averaged velocity, we then described the incipient motion of sediment under the water-vegetation interface. We found that there were three phases of incipient motion of sediment, namely static, partially in motion, and entirely in motion. We defined the incipient motion of sediment from the conditions for the third process. We derived a semi-empirical equation for the sediment incipient velocity in the presence of submerged vegetation from our experimental results, and found that, of the factors explored, the vegetation factor Fv had most influence on the incipient motion velocity of sediment with vegetation. For a given vegetation density, the average depth of the scour holes around the vegetation elements were linearly related with the spatially averaged velocity of the flow under the water-vegetation interface. These results demonstrate that our method gave reasonable estimates of the spatially averaged velocity for incipient motion of sediment.

Probability Distribution of Turbulent Velocity Fluctuations in a Patchy Vegetated Open Channel

The probability density function (PDF) for the stream wise velocity fluctuations was obtained for various cross sections in a patchy vegetated open channel. The pdf obtained for all the cross sections appeared to be qualitatively Gaussian, hence adequate to predict the extreme values arising from coherent structures of the patchy vegetated open channel for two experimental conditions. Relative to gravel bed, it is shown that the vegetation stem density produces asymmetric velocity fluctuations over the vegetated bed due to more distortion of large scale flow structures. At the boundary region however, the pdf is negatively skewed in experiment two relative to experiment one. This asymmetry is attributed to the enhanced lateral transport of momentum at the boundary region.

Phenomenological Features of Turbulent Hydrodynamics in Sparsely Vegetated Open Channel Flow

Phenomenological Features of Turbulent Hydrodynamics in Sparsely Vegetated Open Channel Flow

Incipient motion of sediment in presence of submerged flexible vegetation

The presence of submerged vegetation on river beds can change the water flow structure and alter the state of sediment motion. In this study, the incipient motion of sediment in the presence of submerged flexible vegetation in open channels was investigated in a laboratory experiment. The vegetation was simulated with flexible rubber cylinders arranged in parallel arrays. The effect of the vegetation density, water depth, and sediment grain size on the incipient motion was investigated. The experimental results indicate that the incipient motion velocity of sediment increases as the vegetation density decreases and the water depth and sediment grain size increase. With flexible plants, the incipient motion velocity of sediment is lower than it is without vegetation, and is larger than it is with rigid vegetation. A general incipient motion velocity equation was derived, which can be applied to both flexible and rigid vegetation conditions.

Erratum for “Velocity Profiles in Vegetated Open-Channel Flows: Combined Effects of Multiple Mechanisms” by Nina Nikora, Vladimir Nikora, and Tom O’Donoghue

Erratum for “Velocity Profiles in Vegetated Open-Channel Flows: Combined Effects of Multiple Mechanisms” by Nina Nikora, Vladimir Nikora, and Tom O’Donoghue

Velocity Profiles in Vegetated Open-Channel Flows: Combined Effects of Multiple Mechanisms

AbstractVertical profile of longitudinal velocity in vegetated channels reflects complex mechanics of flow-vegetation interactions and determines the bulk flow velocity and flow rate. Most available models of velocity profiles in vegetated channels are based on a single physical concept that underpins theoretical considerations and data interpretation. However, measured velocity profiles suggest that the use of a single concept is not sufficient to cover all possible scenarios of flow-vegetation interactions. As a result, a number of models in which different concepts are applied to different flow regions have been recently developed. Within this framework, the overall velocity profile is represented with a set of linked segments. Although such segment-based models have improved velocity profile description, there is a need for more robust approaches and better analytical formulations. This paper proposes a new approach where a vertical velocity profile in vegetated channels is modelled as a linear superp...

Study on the effects of vegetation density in reducing bed shear stress on the downstream slope of earthen embankment

River bank, coastal erosion and embankment failures are endemic and recurrent natural hazards, causing loss of lands and livelihoods along major rivers and coastlines. Overflow is the most damaging factor for earthen embankments on the downstream (d/s) slope due to the erosion of surface material which is caused by the high bed shear stress and supercritical flow over the entire d/s slope. Vegetation planted on d/s slope may have a considerable effect in reducing bed shear stress which makes erosion. However,the influence of vegetation for flow resistance as well as bed shear stress control has been studied a little. Therefore, the aim of this research is to introduce the use of natural resources like vetiver grass for controlling bed shear stress which actually causes embankment erosion starts from the high flow region that is, in supercritical flow state. A small-scale experiment is performed with four different densities of vegetal cover considering different discharges on d/s slope. Vegetative barrier affect the mean flow rate by induced drag force against hydraulic forces. Hydraulic characteristics and mechanism of vegetation in open channel during overflow are studied through flume experiment. Results demonstrated that vegetation markedly reduced the effective bed shear stresses which are the influencing factor for causing erosion on the d/s slope, toe and bed of embankment. Results also revealed a significant reduction of damage due to erosion and risk of failure from overtopping flow can be achieved by providing dense vegetation cover on the d/s slope of earthen embankment. Key words: Embankment, downstream slope, drag force, bed shear stress, vegetation density.

A depth-averaged 2D shallow water model for breaking and non-breaking long waves affected by rigid vegetation

This paper presents a depth-averaged two-dimensional shallow water model for simulating long waves in vegetated water bodies under breaking and non-breaking conditions. The effects of rigid vegetation are modelled in the form of drag and inertia forces as sink terms in the momentum equations. The drag coefficient is treated as a calibrated bulk constant and also determined using two empirical formulas as functions of stem Reynolds number, Froude number, and vegetation volume fraction. The governing equations are solved using an explicit finite-volume method based on rectangular mesh with the Harten, Lax, and van Leer approximate Riemann solver with second-order piecewise linear reconstruction for the streamwise convection fluxes, a second-order upwind scheme for the lateral convection fluxes, and a stable centred difference scheme for the water surface gradient terms. The model was tested using five laboratory experiments, including steady flow in a flume with alternate vegetation zones, solitary wave in a vegetated flatbed flume, long-wave runup on a partially-vegetated sloping beach, the dam-break wave overtopping an obstacle, and breaking the solitary wave on a sloping beach. The computed water levels, flow velocities, wave heights, and runups are in generally good agreement with experimental observations. The model was then applied to assess the hydrodynamic effectiveness and limitations of vegetation in coastal and river protection. It is shown that vegetation along the coastal shoreline has a positive benefit in reducing wave runup on sloping beaches, whereas vegetation in open channels causes conflicting impacts: reducing inundation in the downstream areas, but increasing flood risk in a certain distance upstream.

Velocity distribution for open channel flows with suspended vegetation

Suspended vegetation in open channels retards flow, which, as a result the vertical distribution of streamwise velocities, will deviate from the classic logarithmic law. To clearly understand the hydraulic properties of turbulent flow in an open channel with suspended vegetation, we analyzed hydraulic mechanisms by vertically dividing the flow region into three parts extending from the channel bed to the water surface: a basal non-vegetated layer, an internal vegetation layer and an upper vegetation layer. For each layer, the streamwise velocity was obtained by mathematically solving the momentum equations. Mixing length theory was adopted to determine the shear stress in the internal and non-vegetation layers, and some parameters were determined using published data from Plew’s detailed laboratory measurements. The good agreement between the analytically and experimentally predicted vertical distributions of streamwise velocity indicated that the three-layer model is both applicable and reliable.