Compound Open-channel Flows Research Articles

On asymmetric compound open channel flows with two distinct width floodplains: revisiting interfacial mixing layer and flow structures

On asymmetric compound open channel flows with two distinct width floodplains: revisiting interfacial mixing layer and flow structures

Large Eddy Simulation of Compound Open Channel Flows with Floodplain Vegetation

Floodplain vegetation is of great importance in velocity distribution and turbulent coherent structure within compound open channel flows. As the large eddy simulation (LES) technique can provide detailed instantaneous flow dynamics and coherent turbulent structure predictions, it is of great importance to perform LES simulations of compound open channel flows with floodplain vegetation. In the present study, a wall-modeled large eddy simulation (WMLES) method was employed to simulate the compound open channel flows with floodplain vegetation. The vegetation-induced resistance effect was modeled with the drag force method. The WMLES model, incorporating the drag force method, was verified against flume measurements and an analytical solution of vegetated open channel flows. Numerical simulations were conducted with a depth ratio of 0.5 and four different floodplain vegetation densities (frk = 0, 0.28 m−1, 1.13 m−1 and 2.26 m−1). The main flow velocity, secondary flow, bed shear stress and vortex coherent structure, based on the Q criterion, were obtained and analyzed. Based on the numerical results, the influences of floodplain vegetation density on the flow field and turbulent structure of compound open channel flows were summarized and discussed. Compared to the case without floodplain vegetation, the streamwise velocity in the main channel increased by 10.8%, 19.9% and 24.4% with the frk = 0.28 m−1, 1.13 m−1 and 2.26 m−1, respectively. The results also indicated that, when the floodplain vegetation density increased, the following occurred: the velocity increased in the main channel, while the velocity decreased in the floodplain; the transverse momentum exchange was enhanced; and the strip structures were more concentrated near the junction area of compound open channel flows.

Genetic algorithm-assisted data-driven model for boundary shear distribution and stage-discharge: Compound open channel flows

In this paper, a unified method has been proposed for the percentage shear force carried by floodplains using fewer non-dimensional parameters such as the floodplain's percentage area, the ratio of Manning’s roughness and the depth ratio. This new data-driven dynamic model for percentage shear force is obtained using a genetic algorithm (GA) program, a well-documented machine-learning software, which can examine the existing relationship among the variables and explore the influencing factors. GA facilitated a unified relationship between apparent shear force and the chosen parameters. The new proposed model is simple and accurate compared to the previous models, which were either complex or distinct for different configurations. The efficiency of the new model shows that the most predicted test case results are under the 5% error cap. The cohesive expression derived for smooth and roughened compound channels is the most significant advantage of the GA-based data-driven model, providing a simple and easy-use formula for engineers to apply for broad applications. Compared with the other available approaches, the model proposed provides the most accurate discharge prediction for various unique datasets.

Momentum Transfer–Equivalent States Assumption of the Apparent Shear Stress in Compound Open-Channel Flow

The use of apparent shear stress at the interface between the adjacent subregions of a cross section to represent the effect of momentum transfer is a common method for the one-dimensional calculation of compound open-channel flow. The apparent shear stress of the dividing lines is affected by the compound geometry and boundary conditions. The expressions of the apparent shear stress established with previous studies are very different. Empirical treatment was proposed in this study to simplify the expressions based on a momentum transfer–equivalent states assumption, in which (1) the apparent shear stress was assumed to be the difference of the momentum transfer from two sides of a dividing line, and (2) two assumed equivalent states were employed to define the equivalent value of the momentum transfer. The apparent shear stress could be calculated based on the deviation of the momentum transfer from its equivalent value. The new expressions were used to calculate the discharges and the boundary shear force. Comparisons between the calculated results based on different methods and the measured data showed that the proposed method improved the calculation of the subregion boundary shear force and the discharges in the compound cross sections.

Hydrodynamic performance and placement characterization of axial hydro-kinetic turbine in a compound open-channel

In this study, performance of an in-stream hydro-kinetic turbine and its impacts on a simplified river channel (known as a compound open-channel) flow has been numerically examined. To this end, a steady Reynolds-Averaged Navier-Stokes solver has been employed together with the moving reference method to simulate the turbine rotation as well as the Reynolds Stress Model (RSM) as the turbulence closure. The results showed that the presence of a hydro-kinetic turbine causes dramatic changes in the natural flow structure of the compound channel, which varies depending on the location of turbine installation. Comparison of turbine installation in various locations in the cross-section of the channel showed that minimum effects on the natural flow conditions have been occurred in the installation at the main-channel being far from the bed. It has also been observed that the bed shear stress remains virtually constant for different hydro-kinetic turbine locations. Values of the power coefficient indicate that the highest value of maximum power coefficient is obtained where the hydro-kinetic turbine is installed at the interaction zone of the compound open-channel. This observation implies that cellular secondary currents of the compound channel have positive effects on the turbine power. Finally, it is noted that the thrust coefficient variation with the installation location is insignificant compared to the variation of this coefficient with respect to the inlet velocity and the ratio of the blade flow speed to the flow speed.

Study on Conveyance Coefficient Influenced by Momentum Exchange Under Steady and Unsteady Flows in Compound Open Channels

Study on Conveyance Coefficient Influenced by Momentum Exchange Under Steady and Unsteady Flows in Compound Open Channels

Analysis and validation of mathematical models of secondary velocities along vertical and transverse directions in wide open-channel turbulent flows

Cellular secondary flows are inevitably present in turbulent flows through ducts, natural or artificial channels, and compound channels. Secondary currents significantly modify the characteristics of turbulent quantities, the pattern of primary flow velocity by causing dip-phenomenon. To understand the detailed mechanism and hidden cause, modelling of secondary flow velocities is crucial. In this study, proper mathematical models of secondary flow velocities along vertical and transverse directions are proposed for steady and uniform turbulent flow through wide open channels with equal smooth and rough bed strips. Starting from the continuity and the Reynolds averaged Navier–Stokes equations, governing equation for secondary velocity is derived first and then using appropriate boundary conditions (no-slip boundary conditions at channel bottom and free surface, and maximum vertical velocity in magnitude at the interface of two cellular secondary cells and at mid-depth of the channel. All these conditions are consistent with several experimental observations). A new model of the streamwise Reynolds shear stress is proposed for the entire cross-sectional plane and using it, the analytical solutions are obtained. Proposed models include the effects of viscosity of the fluid and the eddy viscosity model of turbulence. All suggested models are validated with existing experimental data in rectangular open-channel flows, compound open channel flows, and duct flows, and satisfactory results are obtained. Furthermore, models are also compared with existing empirical models from literature to show the effectiveness and superiority of proposed models. Apart from these, the obtained results from this study are used to investigate the effects of vertical and transverse secondary flow velocities on the settling velocity vector in a cross-sectional plane. Effective alternative models for the settling velocity vector are suggested. The model of settling velocity vector is also compared with the existing model. Finally, all results are justified from physical viewpoints.

Impact of depth ratio on flow structure and turbulence characteristics of compound open channel flows

Compound open channel flows appear in most natural rivers are of great importance in river management and flood control. In this study, large eddy simulations were carried out to simulate the compound open channel flows with four different depth ratios (hr = 0.10, 0.25, 0.50, and 0.75). The main flow velocity, secondary flow, Reynolds stress, and bed shear stress were obtained from numerical simulations. The depth-averaged streamwise momentum equation was used to quantify the lateral momentum exchange between the main channel and floodplain. The instantaneous coherent structures were presented by the Q criterion method. The impact of hr on flow structure and turbulence characteristics was analyzed. The results showed that with the increase of hr, the high velocity area in the main channel shifted to the floodplain, and the dip phenomenon became more obvious; the Reynolds stress largely contributed to the lateral momentum exchange within the flows near the side walls of floodplain; and the vortex structures were found to significantly increase in the floodplain region.

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.

Compound open-channel flows: effects of transverse currents on the flow structure

The authors wish to thank Fabien Thollet and Alexis Buffet for their technical support, and the anonymous referees for their valuable comments that have helped to improve the paper. This research work was funded by the French National Research Agency under grant N◦ANR-14-CE03-0010 (Flowres ANR project, https://flowres.irstea.fr/en/). Declaration of Interests. The authors report no conflict of interest.

Turbulent rectangular compound open channel flow study using multi-zonal approach

In this paper, an improved Shiono–Knight model (SKM) has been proposed to calculate the rectangular compound open channel flows by considering a multi-zonal (MZ) approach in modelling turbulence and secondary flows across lateral flow direction. This is an effort to represent natural flows with compound shape more closely. The proposed model improves the estimation of secondary flow by original SKM model to increase the accuracy of depth-averaged velocity profile solution formed within the transitional region between different sections (i.e. between main-channel and floodplain) of compound channel. This proposed MZ model works by sectioning intermediate zones between floodplain and main-channel for running computation in order to improve the modelling accuracy. The modelling results have been validated using the experimental data by national UK Flood Channel Facility. It has been proven to work reasonably well to model secondary flows within the investigated compound channel flow cases and hence produce better representation to their flow lateral velocity profile.

Flow structure at the downstream of a one-line riparian emergent tree along the floodplain edge in a compound open-channel flow

This study focuses on the effects of one-line emergent natural tree (Cupressus Macrocarpa) planted at the edge of the floodplain in a compound open-channel flow. The flow velocity and water level are measured and used to analyze the flow structure. The time averaged and depth-averaged streamwise velocity distributions with root mean square (rms) and time series of streamwise velocity distrbution are analyzed. The velocity distribution considerably changes along the compound channel. The streamwise velocity distribution fits with logarithmic distribution in the non-vegetated case, but for vegetated cases, the streamwise velocity distribution shows S shaped profile at the 1/3 part of floodplain (B f / 3) and main channel (B m / 3) close to the boundary between floodplain and main channel. Additionally, it is obtained that the presence of tree line increases turbulence intensity over the compound open-channel. Moreover, an oscillation period is obtained in the flow caused by tree line by analyzing time series of the streamwise velocity distribution. The oscillation is present everywhere in the floodplain and present at almost B m / 3 part of the main channel which is close to the junction between floodplain and main channel.

Flow structure in compound open-channel flows in the presence of transverse currents

The structure of free-surface flows is experimentally investigated in a laboratory flume with a compound cross-section consisting of a central main channel (MC) and two adjacent floodplains (FPs). The study focuses on the effects of transverse currents on: (i) mixing layers and quasi-two-dimensional coherent structures at the interfaces between MC and FPs; (ii) secondary currents developing across the channel; and (iii) large and very-large-scale motions that were recently observed in non-compound open channel flows. Transverse currents represent spanwise depth- and time-averaged flow from MC to FPs or vice versa. The study is based on one-point and two-point ADV measurements. Streamwise non-uniform flows are generated by imposing an imbalance in the discharge distribution between MC and FPs at the flume entrance, keeping the total flow rate the same for all scenarios. It is shown that even small transverse currents can be very effective in flow modification, as they can significantly displace the mixing layer, shear-layer turbulence, and coherent structures towards MC or FP, depending on the current direction. They can also alter the distribution and strength of the secondary currents. The interactions of quasi-two-dimensional coherent structures, very-large-scale motions, and secondary currents at different conditions are also part of this study.

CHRACTERISTICS OF SECONDARY FLOW AND COHERENT STRUCTURE OF COMPOUND OPEN CHANNEL FLOW ACCOMPANYING WITH CIRCULAR SIDE WALL OF MAIN CHANNEL

低水路側壁が円形の複断面開水路流れ（円形流れ）の流れ構造の特徴を，流速計測法と流れの可視化法を用いて検討し，低水路側壁が鉛直の流れ場（鉛直流れ）の特徴との差異を比較した．その結果，比較的水深が高い条件では，鉛直流れに形成される斜昇流と対を成す旋回流は，円形流れにおいても形成されるがそれは弱体化することが明らかとなった．低水深の流れ場では，鉛直流れに形成される扁平な旋回状の二次流れが円形流れでは消失することが明らかとなった．比較的高水深の条件において，鉛直流れでは高水敷先端部に大規模な縦渦構造が直線状に，時空間的に集中して形成されるが，円形流れの側壁上には縦渦構造の形成領域が蛇行しており，それが斜昇流と旋回流の弱体化に影響していることが推察された．

Effect of Floodplain Obstructions on the Discharge Conveyance Capacity of Compound Channels

Results of an experimental study into steady uniform flows in compound open channels with cylindrical obstructions designed to mimic emergent vegetation is presented. Two configurations—fully covered floodplain and one-line obstructions—are considered, and the hydraulic properties are compared to those of a smooth, unobstructed compound channel. Particular attention is given to the effect of obstruction (i.e., vegetation) density on the rating curve, drag coefficients, and spanwise profiles of streamwise velocity. Flow resistance is estimated using an established approach, and the results are in agreement with other experimental studies. It was shown that the obstruction configuration significantly influences the flow velocity in the main channel, and in the case of one-line obstructions the floodplain velocity is higher than for an unobstructed channel for a given flow rate. Spanwise velocity profiles exhibit markedly different characters in the one-line and fully covered configurations.

Porous media approach for RANS simulation of compound open-channel flows with submerged vegetated floodplains

The main goal of this study is the 3D numerical simulation of river flows with submerged vegetated floodplains. Since, vegetation layers are usually dense and present a large spatial heterogeneity they are here represented as a porous media. Standard semi-empirical relations drawn for porous beds packed with non-spherical particles are used to estimate the porous media parameters based on the averaged geometry of the vegetation elements. Thus, eliminating the uncertainty arising from a bulk drag coefficient approach and allowing the use of a coarser mesh. The free flow is described by Reynolds-averaged Navier–Stokes (RANS) equations, whereas the porous media flow is described by the volumetric-average of RANS equations. The volume-of-fluid method and an anisotropic explicit algebraic Reynolds stress model are used for free-surface and turbulence closure, respectively. The simulation approach is validated against results by other authors featuring vegetated flows in horizontal and rectangular open-channel. The computed results show that the time-averaged streamwise velocity and Reynolds shear stress vertical profiles are properly simulated. The validated approach was applied to simulate compound open-channel flows with submerged vegetated floodplains and compared with data obtained in an experimental facility. The results show that the proposed porous media approach is adequate to simulate flows with submerged vegetation on the floodplains.

Predictive modeling of discharge of flow in compound open channel using radial basis neural network

Predicting the flow discharge in open channel is the main parameters in the flood management. The concept of the compound open channel is the accurate approach for modeling the natural streams. Several ways as analytical approaches and artificial intelligence methods have been proposed for predicting the discharge in rivers in term of compound open channel concepts. In this paper the single channel method (SCM), coherence method (COHM), and divided channel method (DCM) as common analytical approaches were used to predict the discharge in the compound open channel and in follow to achieve more accuracy in flow discharge prediction the radian basis neural network (RBF) was developed. The performance of RBF was compared with other types of transfer function governed on neurons of neural network. The results showed that the DCM with horizontal separated boundary among the subsections with correlation of determination (R2 = 0.76) is accurate through the analytical approaches. Assessing the results of the MLP model showed that this model with (R2 = 0.95) is a bit more accurate than the RBF (R2 = 0.85) and analytical approaches.

Flood hydrograph estimation using an adjoint shallow-water model

This paper describes the estimation of the flood discharge hydrograph for a compound open channel floodplain using data assimilation. The hydrograph was produced iteratively using an adjoint shallow-water model with time-series data of observed water levels. Data assimilation was applied to a flood event that occurred in mid-September 1998 along a 20 km stretch of the lower Tone River in Japan. Both the estimated hydrograph and the flow field simulated using the hydrograph were verified by comparison with the results of aerial photographic analysis. Results show that the hydrograph based on the stage–discharge was overestimated, but it was reasonably corrected by the data assimilation technique used for this study. In addition, the resulting relation during the flood event exhibited a hysteresis loop characteristic, which is typical of compound open channel flows during flooding.

Φ-indices approach and multivariable regression analysis for prediction of discharge in asymmetric straight compound open channel flows

This study is related to flow measurement structures of asymmetric compound cross section mostly suggested for sediment laden-rivers, streams and wadis. Nine different models of asymmetrical rectangular cross sections were tested for a wide range of discharges. In each model, the stages and corresponding discharges were measured. From these measurements, the average discharges utilizing the Φ-indices Approach were determined. In addition, a multivariable regression model was derived with high accuracy for the prediction of discharge in asymmetric compound channels using five dimensionless parameters.The measured discharges were compared with the predicted ones obtained from the Φ-indices methods with vertical, horizontal and diagonal interface planes; FI-V, FI-D and FI-H, respectively, with the use of three well-known methods for the computation of the apparent shear stress acting on the interface perimeter, τa, and the multivariable regression model for discharge prediction. At lower relative depth values, almost all the tested methods predict discharge with acceptable accuracy. In addition, as the relative depth increases, the results demonstrate a suitable accuracy of the FI-V with the vertical division lines and the multivariable regression model method. The multivariable regression model presented has been identified as the best-fit model in 26 cases out of 29 through minimization of the mean squared errors.

Simulation of flow in compound open-channel using a discontinuous Galerkin finite-element method with Smagorinsky turbulence closure

The small-scale spatial variability of eddy viscosity which is characteristic for the turbulent shear stress in compound open-channel flows was studied and investigated in this paper. Different options including a constant value, zero-equation, one-equation, two-equation, and Smagorinsky turbulence models for parameterizing the eddy viscosity were developed in the framework of the discontinuous Galerkin finite-element SLIM model and applied for presenting the complex velocity profile in two different experimental data sets of laboratory flumes. A very good qualitative agreement was achieved between numerical results and measurement data for both velocity and flow depth of all experimental data sets in general. In addition, the calculation results showed that the turbulent Smagorinsky empiricism allowed a better presentation of non-uniform velocity in the floodplain and transition regions between plain and main channels than the others in all calculated cases. This empiricism predicted a very close variation of eddy viscosity in comparison with the results calculated by the depth-averaged Reynolds' stress and the lateral gradient of longitudinal velocity. The eddy viscosity varies significantly in the channel section; in particular the small values often occurred around the middle location of floodplains and the central location of the main channel while the large values appeared in the transition regions, presenting different minimum and maximum values of eddy viscosity in each flow region. The effects of eddy viscosity variation on lateral distribution of velocity profile were also investigated and discussed.