Asymmetric Compound Channel Research Articles

Effect of vegetation on flow hydraulics in compound open channels with non-prismatic floodplains

Abstract The present paper aims to evaluate the effect of emergent rigid vegetation density on the flow's turbulence structure and hydraulic parameters at the non-prismatic floodplains. The experiments were performed using the physical model of the asymmetric non-prismatic compound channel. The results show that the velocity distribution in the vegetation flow is more influenced by the drag force caused by the vegetation than by the bed shear stress and does not follow the law of logarithmic velocity distribution throughout the non-prismatic section. The intense velocity gradient at the interface of the main channel and the floodplain leads to the development of strong secondary currents, increased Reynolds shear stresses, apparent shear stresses and momentum exchange in this region. Vegetation also decreases mean kinetic energy in the floodplain and increases it in the main channel. The mean turbulence exchange coefficient for the non-prismatic compound channels without vegetation was 0.23 and for the divergent and convergent compound channels was 0.035 and 0.020, respectively. The comparison of the local drag coefficient results shows that the fluctuations of this parameter are greater in the divergent section than in the convergent section due to the strong secondary currents in the interface.

Combined effect of a mobile bed and floodplain edge vegetation on compound channel conveyance

This technical note presents results from flume experiments conducted in a straight prismatic asymmetric compound channel with two different floodplain roughnesses and rigid emergent vegetation along the floodplain edge in combination with a mobile and fixed main channel bed, respectively. It is found that the contribution of floodplain roughness and vegetation along the floodplain edge to the bulk friction factor is considerably reduced when studied in combination with a mobile main channel bed featuring bedforms. This in turn means that the effect of vegetation along the floodplain edge on the conveyance capacity and bulk friction factor may be overestimated if it is studied solely with fixed bed conditions. The presented results reveal the need to improve our understanding of the complex interplay between mobile beds and vegetation along the floodplain edge and its combined effect on compound channel conveyance capacity and hydraulics.

Flow structures in asymmetric compound channels with emergent vegetation on divergent floodplain

Flow structures in asymmetric compound channels with emergent vegetation on divergent floodplain

Linear-scale models for discharge estimation: asymmetric compound open channel flows

In this study, three recent methods of discharge estimation are evaluated, namely, the interacting divided channel method, the modified divided channel method and the momentum transfer divided channel method, which include momentum exchange on the interface for prediction of discharge. These methods are compared intensively using 22 experimental flume data sets with six river data sets taken from the literature comprising 204 test runs in total. These data sets are categorised as asymmetric compound channels with a wide range of width ratio (B/b), aspect ratio (b/h) and bed slope ([Formula: see text]), where [Formula: see text] is the total width of channel at the bankfull condition, [Formula: see text] is the main channel bed width and [Formula: see text] is the bankfull main channel height. All these three methods are capable of predicting both overall and zonal discharge for small- to large-scale channels, including natural rivers. However, the interacting divided channel method is the least erroneous, with an average percentage error of 5.83%. Further investigations should include in-depth discussion on zonal discharge prediction, particularly for floodplain discharge, which has been an area of concern and interest in the areas of the water environment and sustainable design of rivers.

Experimental Study of Smooth Asymmetric Compound Channels Flow: An Investigation of the Interaction of Flow Using Scaling Argument for Prediction of Overall Discharge

A simple model for the apparent shear stress on the vertical interface between the floodplain and main channel in asymmetric smooth compound channels is proposed using experimental data obtained in this study. The turbulent structure, including Reynolds shear stress in asymmetric compound channel flows, is investigated for three different flow depths. The lateral distribution of the apparent shear stress obtained shows that the total apparent shear stress has a negative peak near the junction edge in the main channel. Furthermore, the intensity of the advection terms and the Reynold shear stress near the interface are investigated as the function of the bankfull height and floodplain width. The momentum transport due to Reynolds stress and secondary current between main channel and floodplain is finally modeled as depth ratio using scaling argument. The validation of the current model on three datasets shows an accurate prediction of overall discharge for the asymmetric smooth compound channels.

Numerical Modelling of Flow Characteristics in an Asymmetric Trapezoidal Compound Channel with Vegetation Patches

Vegetation is an important component of rivers, present naturally alongside the banks. In this research study, three-dimensional computational fluid dynamics (CFD) code ANSYS FLUENT was used to solve the Reynolds Averaged Navier-Stokes equations and to simulate flow features in an asymmetric vegetated compound channel. Three-dimensional flow characteristics and turbulence properties were captured well by the Reynolds stress turbulence model (RSM). A total of five flow cases were considered in this work. Among these, four cases were of circular vegetation patches present on the floodplain edge and one was of non-vegetated asymmetric trapezoidal compound channel. Impact of vegetation patch density, patch center to center (c/c) spacing and flow blockage area has been investigated on various flow features and compared with flow characteristics in a channel without vegetation. Flow features include mean and depth-averaged velocities, secondary flows, Reynolds shear and normal stresses, turbulence intensities and turbulent kinetic energy. The results show that the presence of vegetation patches along the floodplain imparted flow resistance which shifted the zone of higher velocities towards outer wall of main channel and floodplain. Increased velocities in the main channel up to 24.8% was observed for larger flow blockage (aD, where a is the density of the vegetation patch and D is the diameter of patch) and smaller normalized c/c spacing (L/D, where L is the c/c spacing between two patches and D is the diameter of patch) of patches. Large transverse shear stresses existed around the vegetated zone due to the gradient of velocities and vortices in the main channel and floodplain. Floodplain, especially vegetated zone acquires less Reynolds stresses and turbulence making it favorable for deposition of sediments and growth of ecological species.

Zonal and Overall Discharge Prediction Using Momentum Exchange in Smooth and Rough Asymmetric Compound Channel Flows

AbstractThe apparent shear stress acting on the vertical interface between floodplain(s) and main channel has been shown to be very influential for the estimation of the zonal and overall discharge...

Estimation of apparent shear stress of asymmetric compound channels using neuro-fuzzy inference system

Discharge calculations and conveyance estimations in rivers are of prime importance during high flow events. Conventional methods using single global resistance parameters are incapable of assessing discharge with high precision; thus incorporating apparent shear into these methods not only improves the results but also addresses the momentum transfer phenomena over sub-sections. Apparent shear stress modelling for asymmetric channels with different geometric and roughness parameters is found to be monotonic and tedious in linear regression. To improve its accuracy and efficiency, one of the modelling tools is to apply adaptive network-based fuzzy inference system (ANFIS). In this paper, apparent shear stress is trained, modeled and tested for a wide range of asymmetric channels including various experimental and field data. It is found that asymmetrical channels have large positive shear force acting at the vertical imaginary interface between the floodplain and main channel. The positive apparent shear force indicates that the slower floodplain flow retards the faster flow in the main channel. Therefore, considering correct shear stress in the discharge estimation will result in higher accuracy. The application of ANFIS in modelling the output variable of apparent shear stress is done using five geometrical and hydraulic parameters of varying range from small-scale experiments to real field measurements as the input variables. The coefficient of determination (R2) for the modeled apparent shear is 0.96, which is exceptionally high as comparison to past models. Furthermore, comparison of discharge calculation using modeled apparent shear stress and other conventional methods has shown that the results obtained using vertical interface shear stress have higher accuracy.

A Computational Study of Interaction of Main Channel and Floodplain: Open Channel Flows

The assumption of steady uniform flow permits the computation of the velocity isoline, secondary current and turbulent statistics in open channel flows. However, it becomes important to choose appropriate turbulence models to capture the length scale of turbulence near the interfacial zone of compound channels. This paper not only focusses on capturing the longitudinal vortex and primary mean velocity but also extrapolates the results of numerical analysis to understand the interaction between the main channel and floodplain in asymmetric compound channels. The results of computational fluid dynamics simulation showed that the velocity isoline bulging near the bed of the floodplain and sidewall at the junction, due to high-momentum transport by secondary current, can be captured with Reynolds stress model. Furthermore, by applying the three different cases of channels with varying geometrical aspects, the maximum velocity simulated showed similar results to the experiments where the structure of primary mean velocity is seen to be influenced by momentum transport due to the secondary current.

Apparent Shear Stress and Its Coefficient in Asymmetric Compound Channels Using Gene Expression and Neural Network

AbstractFlow interaction over the interface between main channel and floodplains affects the overall discharge capacity and discharge distribution in compound open channels. Many investigators have...

Flow distribution in a compound channel using an artificial neural network

In compound channels, much of the hydraulic resistance may be assigned by channel and floodplain geometry and bed roughness. Understanding the distribution of flow in the subsections of a compound river channel is a tedious work due to complex momentum transfer at the junction. The zonal discharges are mostly dependent upon the hydraulic resistance of the corresponding subsections. Hence, different experimentations in laboratory channels are required to be investigated by analyzing the dependence of momentum transfer on individual flow capacities of the subsections. So, experiments are performed in non-homogeneous roughness beds of asymmetric compound channels to examine the flow behavior. Total 272 numbers of experimental data sets comprising wide ranges of width ratio, relative flow depth, aspect ratio and roughness ratio with the present experimental data series are used for both training and validation of the model. Previous models can provide good results only for specific ranges of independent parameters whereas the back propagation of artificial neural network (BPNN) model is capable of performing well for the global ranges of independent parameters. This is because BPNN is able to perform the nonlinear mapping between the dependent and independent variables during the training. The efficacy of the models is verified with the standard statistical error analysis for the data sets. The BPNN model is found to perform well as compared to other researcher’s models.

Discharge prediction in asymmetric compound channels

Due to the exchange of momentum at the junction between the main channel and floodplain, the prediction of stage discharge relationship in a compound channel is not an easy task as in simple channel. Despite the availability of 2D and 3D flow models, 1D flow models are rather preferred for its fewer data requirements, easy access and shorter processing time. As a substitute, a simple 1D methodology is proposed here which includes effects due to lateral momentum transfer at junction by introducing a momentum transfer coefficient between the adjacent compartments. The model estimates the individual flow carried by the two compartments of an asymmetric compound channel considering the percentage boundary shear force distribution as an input parameters. The boundary shear force carried by floodplain is parameterized in terms of the channel dimensions, flow depth and roughness characteristics. The resulting average flow velocities in sub-sections and momentum transfer coefficient at junction are determined from a set of physics based equations. Utilizing the outputs, the quantification of momentum exchange in terms of apparent shear force is achieved by the difference between main channel and floodplain flow velocities along with this momentum transfer coefficient. Experiments are undertaken to investigate the asymmetric geometry in flow distribution of a compound channel. The performance of the method in calculating overall discharges competes well with previous investigators models, seven traditional methods and Conveyance estimation system (CES). The new model and its corollary are tested for their validity against the experimental channels and also found to predict the discharge well in real world application.

Evaluating Methods for Discharge Prediction of Straight Asymmetric Compound Channels

Evaluating Methods for Discharge Prediction of Straight Asymmetric Compound Channels

TRANSPORTATION OF BED LOAD AND ITS FORMATION FOR INBANK FLOW: A PHYSICAL MODELLING APPROACH

Floods hit various states in Malaysia and caused damage to properties, infrastructures, human suffering and even loss of lives. Frequent flood incidents and soil erosion are reported which lead to sedimentation problems in the drainage and river systems due to urbanisation. It is important to understand the sedimentation process and the behaviour of bed forms in the water course for post-flood events. However, it is risky and dangerous to conduct field study during occurrence of flooding. Laboratory study has been chosen as an alternative to fulfill the objectives. Experimental investigations on the bed load transport and bed forms in rectangular open channels have been undertaken. Small and large scale physical models are used in the laboratory. The effects of flow on mobile sandy bed channel and bed load transportation are studied by using a modified flumes with an asymmetric straight compound channel. However, the findings on rate of transport and bed formation for non-flooding cases are presented in this paper. It is found that the bed formation profiles for both channels are different due to scale and flume characteristics. The bed forms, erosion and deposition processes are significantly influenced by the water velocity in the channel. The bed forms observed for large flume are repeating ripples and dunes; meanwhile variable of ripples are observed in small flume.

Depth-averaged simulation of flows in asymmetric compound channels with smooth and rough narrow floodplains

ABSTRACTDepth-averaged hydrodynamic models are predominantly used in numerical simulations of compound channel flows. One of the most popular methods for the depth-averaged simulation is Shiono and Knight method (SKM). This method accounts for the effects of bed friction, lateral turbulence and secondary flows, via three key parameters f, λ and Γ, respectively. The conventional expressions that are developed to calibrate these parameters are generally based on experiments in compound channels with wide floodplains. In this study, the application of SKM to an asymmetric compound channel with a narrow floodplain is examined in terms of the calibration requirements. Two sets of experiments that have smooth and rough floodplains are conducted and then simulated by SKM. In smooth floodplain cases, the results reveal that SKM model with the conventional calibration expressions of f, λ and Г is reasonably capable of predicting the distributions of depth-averaged velocity and boundary shear stress in the main channel. However, in the floodplain region, the expressions recommended for calibrating Г need to be modified to improve the predicted results in that region. In cases of the rough floodplain, the results indicate that only the values of λ in the main channel need to be changed from its conventional values to improve the predictions.

Prediction of apparent shear stress in an asymmetric compound channel

There is always an exchange of momentum between the deep main channel and its shallow floodplains of a compound channel section. When rivers have single side floodplains, they are known as asymmetric compound channels. Momentum transfer phenomenon has the effect of increasing floodplain shear and decreasing the main channel shear. In symmetrical compound channels, the momentum transfer occurs from both sides of the main channel to the floodplains whereas, for an asymmetrical compound channel, there is a stronger interaction takes place as compared to the symmetrical case. This causes more non-uniform boundary shear stress distribution as compared to the symmetrical cases. Experiments are conducted to measure the boundary shear stress distribution of an asymmetric compound channel that helps to quantify the momentum transfer in terms of apparent shear stress at the interface. Analysis of the apparent shear is useful in deciding the choice of appropriate interface planes for stage–discharge relationship of a compound channel. In the present work, an improved expression to predict the apparent shear stress, especially in an asymmetrical compound channel is developed. The strength of the new expression to predict apparent shear stress is also found to be better than the models of other investigators.

Application of the Shiono and Knight Method in asymmetric compound channels with different side slopes of the internal wall

The Shiono and Knight Method (SKM) is widely used to predict the lateral distribution of depth-averaged velocity and boundary shear stress for flows in compound channels. Three calibrating coefficients need to be estimated for applying the SKM, namely eddy viscosity coefficient (λ), friction factor (f) and secondary flow coefficient (k). There are several tested methods which can satisfactorily be used to estimate λ, f. However, the calibration of secondary flow coefficients k to account for secondary flow effects correctly is still problematic. In this paper, the calibration of secondary flow coefficients is established by employing two approaches to estimate correct values of k for simulating asymmetric compound channel with different side slopes of the internal wall. The first approach is based on Abril and Knight (2004) who suggest fixed values for main channel and floodplain regions. In the second approach, the equations developed by Devi and Khatua (2017) that relate the variation of the secondary flow coefficients with the relative depth (β) and width ratio (α) are used. The results indicate that the calibration method developed by Devi and Khatua (2017) is a better choice for calibrating the secondary flow coefficients than using the first approach which assumes a fixed value of k for different flow depths. The results also indicate that the boundary condition based on the shear force continuity can successfully be used for simulating rectangular compound channels, while the continuity of depth-averaged velocity and its gradient is accepted boundary condition in simulations of trapezoidal compound channels. However, the SKM performance for predicting the boundary shear stress over the shear layer region may not be improved by only imposing the suitable calibrated values of secondary flow coefficients. This is because difficulties of modelling the complex interaction that develops between the flows in the main channel and on the floodplain in this region.

Depth-Averaged Velocity and Boundary Shear Stress Prediction in Asymmetric Compound Channels

When a river overflows, either side of its flood plain during flood is named as asymmetric compound channel; however, if two adjacent flood plains are inundated, it is called as symmetric compound channel. Due to the complex flow mechanism at the junction between main channel and flood plain, modeling of depth-averaged velocity and boundary shear stress in an asymmetric compound channels becomes an exigent task. The analytical solution of the Shiono and Knight Method accounts three governing parameters, i.e., lateral shear, bed friction and secondary flows for prediction of these flow variables. In order to investigate the influence of these governing coefficients on flow parameters, experiments were employed in asymmetric compound channels. Variation of boundary shear stress and depth-averaged velocity across the channel was investigated. Some experimental data sets from large flood channel facility and from measurements taken by other researchers have also been used in this study. The variation of three calibrating coefficients, i.e., eddy viscosity coefficient (\(\lambda \)), friction factor (f) and secondary flow coefficient (k) against the geometric and hydraulic parameters, has been studied. Expressions for secondary flow coefficients in terms of hydraulic and flow parameters have been procured for different panels which are required to get the distribution of boundary shear and depth-averaged velocity. The performance of the models has been successfully proved for experimental channels and natural river data sets.

Discharge assessment in an asymmetric compound channel by zero shear interface method

Rivers are often flanked by either side floodplain and appear as an asymmetrical compound section during flood. In an asymmetrical compound section, the apparent shear stress at different interfaces varies greatly due to stronger interaction at the junction between main river section and its adjoining floodplain. In a symmetrical compound channel, the momentum transfers uniformly from main channel to both its floodplains. But in case of a compound channel with asymmetrical floodplain, there is a stronger flow interaction at the interface region. In this study, a new approach to estimate the discharge in asymmetric compound channels by adopting zero shear interfaces at the junction is described. Some approaches for predicting flow using different area methods in asymmetric channels are reported. On the basis of multi linear regression analysis, an improved area method has been proposed. The performance of proposed model is successfully validated with own experimental data and the flood channel facilities of UK data. The advantage of the current area correction model is compared well with previous methods. The applicability of the new approach in realistic situations for predicting the discharge has been precisely achieved.

Accuracy of Numerical Simulation in Asymmetric Compound Channels

In the recent years, compound channels received more attention in hydraulic engineering for their role in estimating and calculating the hydraulic parameters of natural rivers. Usually, for determination of the hydraulic parameters of compound channels, physical models are used which have cost and time assumption. According to the wide usage of numerical modeling in hydraulic engineering, this paper aims to evaluate the accuracy of numerical models in compound channels by simulating the hydraulic parameters in nine different types of compound channels and to compare them with experimental data. These simulations can show the interaction between velocity and vorticity and other hydraulic parameters by using contours and graphs along the channel length which help to have more and better understanding about their changes. Numerical simulations were performed using the renormalization-group turbulence model and volume-of-fluid free surface model for determining the level of fluid. Values of convergence ratio and the grid convergence index were calculated for evaluating the extrapolated values from numerical modeling and the sensitivity of the model solution to the numerical discretization, respectively, which indicates a proper validation of grid spacing and refinement selection for optimizing the calculation process. The comparison between numerical and experimental results shows a good agreement. The extracted numerical results show that by changing the floodplain width and depth, the water surface level changes 4–20% and 5–34%, respectively. Moreover, the numerical results show an increment of 20 and 145% in Froude and Weber numbers in floodplains, respectively, because of increment of velocity in floodplain.