Compound Channel Flow Research Articles

Overbank Flow in Symmetrically Narrowing Floodplains

Experimental data are presented for flow in compound channels with symmetrically narrowing floodplains. In such a geometry, the flow behavior presents similarities with the more complex flow in a meandering compound channel, yet without the curvature effects, because of mass transfers between the floodplains and the main channel, and secondary currents induced in the main channel. An estimation of the momentum transfer generated by the mass transfer is found significant compared to the frictional losses. It mainly depends on the geometrical parameters and is practically independent of the friction slope. Free-surface profile computations are performed with the exchange discharge model (EDM) to incorporate the effects of the momentum transfer in terms of an additional head loss. Agreement was found between measured and computed water surfaces, thus validating the EDM approach.

STUDY ON THE EFFCTS OF VANE WORKS REDUCING SCOUR DEPTH IN A CURVILINEAR COMPOUND CHANNEL

It is important to evaluate the effect of vane works not only in the simple uniform bend but also in the reach of various forms in river. In this study we investigate the effects of vane works reducing scour depth in the compound channel whose plan form changes longitudinally. In this paper, at first we describe the idea of the design of vane works based on the recent topics of the flow and bed topography characteristics in a compound meandering channel, and show vane works fulfill their functions through flood from vane's effects in simple and compound channel flows. Bed variation in a channel with vane works is reproduced by using 3D-flow model considering drag and lift forces exerting on vanes.

Three-Dimensional Numerical Simulation of Compound Channel Flows

A three-dimensional numerical study is presented for the calculation of turbulent flow in compound channels. The flow simulations are performed by solving the three-dimensional Reynolds-averaged co...

Application and validation of numerical codes in the prediction of compound channel flows

This paper attempts to compare the accuracy of a two-dimensional (2D) depth-averaged code (TELEMAC-2D) and a three-dimensional (3D) code (SSIIM) in the numerical simulation of free surface flows. Assessment was conducted using benchmark data collected from a meandering compound channel at the UK EPSRC Flood Channel Facility. For both codes a standard k–ε turbulence model was employed to simulate the flow field. Assessment of both codes was based on the prediction of depth-averaged velocity traverses around a meander bend, flow features inferred by velocity vectors in the horizontal plane, and water surface elevations for both low and high relative flow depths. In addition, predictions with the 3D code of velocities parallel and perpendicular to the main channel direction are compared with those observed in the experimental programme. Both codes predict the lateral distribution of depth-averaged velocity in the main channel with reasonable accuracy at low relative flow depth, with the 3D code predicting better than the 2D code (maximum percentage errors in depth-averaged velocity are 16% and 22% respectively). At a higher flow depth no extra accuracy in the prediction of depth-averaged velocity is to be gained from the 3D model than from the 2D model, although the 3D model can predict the correct location and direction of secondary currents. For all flow conditions better accuracy was achieved for both codes at the apex section where the mean flow direction is more aligned with the local main channel direction. An accuracy of within 20% can be expected for predictions of depth-averaged velocity magnitude from a 3D model using a standard isotropic representation of turbulence.

Application and validation of numerical codes in the prediction of compound channel flows

This paper attempts to compare the accuracy of a two-dimensional (2D) depth-averaged code (TELEMAC-2D) and a three-dimensional (3D) code (SSIIM) in the numerical simulation of free surface flows. Assessment was conducted using benchmark data collected from a meandering compound channel at the UK EPSRC Flood Channel Facility. For both codes a standard k–ε turbulence model was employed to simulate the flow field. Assessment of both codes was based on the prediction of depth-averaged velocity traverses around a meander bend, flow features inferred by velocity vectors in the horizontal plane, and water surface elevations for both low and high relative flow depths. In addition, predictions with the 3D code of velocities parallel and perpendicular to the main channel direction are compared with those observed in the experimental programme. Both codes predict the lateral distribution of depth-averaged velocity in the main channel with reasonable accuracy at low relative flow depth, with the 3D code predicting better than the 2D code (maximum percentage errors in depth-averaged velocity are 16% and 22% respectively). At a higher flow depth no extra accuracy in the prediction of depth-averaged velocity is to be gained from the 3D model than from the 2D model, although the 3D model can predict the correct location and direction of secondary currents. For all flow conditions better accuracy was achieved for both codes at the apex section where the mean flow direction is more aligned with the local main channel direction. An accuracy of within 20% can be expected for predictions of depth-averaged velocity magnitude from a 3D model using a standard isotropic representation of turbulence.

Application and validation of numerical codes in the prediction of compound channel flows

This paper attempts to compare the accuracy of a two-dimensional (2D) depth-averaged code (TELEMAC-2D) and a three-dimensional (3D) code (SSIIM) in the numerical simulation of free surface flows. Assessment was conducted using benchmark data collected from a meandering compound channel at the UK EPSRC Flood Channel Facility. For both codes a standard k–ε turbulence model was employed to simulate the flow field. Assessment of both codes was based on the prediction of depth-averaged velocity traverses around a meander bend, flow features inferred by velocity vectors in the horizontal plane, and water surface elevations for both low and high relative flow depths. In addition, predictions with the 3D code of velocities parallel and perpendicular to the main channel direction are compared with those observed in the experimental programme. Both codes predict the lateral distribution of depth-averaged velocity in the main channel with reasonable accuracy at low relative flow depth, with the 3D code predicting better than the 2D code (maximum percentage errors in depth-averaged velocity are 16% and 22% respectively). At a higher flow depth no extra accuracy in the prediction of depth-averaged velocity is to be gained from the 3D model than from the 2D model, although the 3D model can predict the correct location and direction of secondary currents. For all flow conditions better accuracy was achieved for both codes at the apex section where the mean flow direction is more aligned with the local main channel direction. An accuracy of within 20% can be expected for predictions of depth-averaged velocity magnitude from a 3D model using a standard isotropic representation of turbulence.

Application of a 3D numerical model to a river with vegetated floodplains

This paper describes the application of a three-dimensional computational fluid dynamics code to a large-scale river scheme. One of the primary aims of this paper is to provide a tool, which utilises a roughness closure derived from physical processes, requiring minimal calibration. Accordingly, a roughness closure based on the traditional drag-force approach is implemented. Unlike other implementations of this approach, the drag force is only introduced in the momentum equations and not into the turbulence closure. This ensures that the coefficients of the turbulence closure model (in this case the k−ε scheme) do not require recalibration for each application. An existing vegetated floodplain is used as a reference site and parameters characterising the dimensions of riparian vegetation and its distribution are quantified. A 100 year flood event on a considerable reach length (3500 m) of the lower River Rhine in South-West Germany is then simulated. Mean floodplain velocities are measured using dilution gauging techniques and these are compared with the computed values. Given information such as plant distribution and geometric properties of the various plant populations, the proposed tool can predict floodplain velocities, water elevation and hydrodynamic features indicative of vegetated compound channel flow.

FLOW AND BED TOPOGRAPHY IN A COMPOUND CHANNEL CONSISTING OF DIFFERENT PLAN SHAPES

In the previous studies on compound channel flows, experimental channels of simplified plan shape, straight or uniform sinuosity have been used. However, rivers generally consist of reaches of different plan shapes continuously. Therefore, it is important to clarify as a whole effect of channel shape and hydraulic condition on flow and bed formation. In this paper, effects of plan shape and relative depth on sediment discharge were examined And characteristics of flow and bed topography were investigated in a compound channel consisting of different plan shape (straight, curved and meandering). As the results, sediment discharge in compound straight and meandering channel showed a different manner with relative depth variation. And flow and bed topography were affected mainly by the upstream channel shape. Especially, at transition section from straight to meandering maximum scouring occurred due to non-equilibrium of sediment discharge with large relative depth.

Muskingum Method, EXTRAN and ONE-D for Routing Unsteady Flows in Open Channels

In the present study the numerical modelling aspects of unsteady flow in simple and compound channels are examined through the application to a laboratory data set of the simple hydrologic routing method Muskingum and the unsteady flow dynamic routing models EXTRAN and ONE-D. Model simulations were compared and evaluated by applying various statistical measures to determine the degree of goodness-of-fit between simulated and observed hydrographs. In terms of overall performance, the Muskingum method proved to be a simple and reliable method avoiding complicated mathematical and numerical computations for the cases considered.

Critical depth prediction in straight compound channels

Critical depth is the depth at which the flow in an open conduit undergoes a transition from subcritical to supercritical regimes. The prediction of critical depth in a prismatic channel can be achieved to a high degree of accuracy. This is not the case for two-stage channels (main channel with flanking floodplain), also commonly known as compound channels. Traditionally, critical depth is predicted based on minimum specific energy, and this paper investigates the validity of this and several other methods of critical depth prediction for compound channels. It has been found that flow in compound channels does not undergo a sharp critical flow transition as occurs in prismatic channels. Instead, the change between fully subcritical and fully supercritical occurs over a depth range, whereby different regions in the cross-section maybe either sub- or supercritical. This has been termed a ‘mixed-flow’ region. It has been found that the point of minimum specific energy does not coincide with either the beginning or the end of this mixed flow transition, rendering such predictions ineffectual. A new method for determining critical depth in a compound channel is presented and compared with other prediction techniques. Methods of incorporating the effects of the momentum interaction have been used to improve the prediction of the velocity profile within the channel and locate the mixed flow transition zone.

Critical depth prediction in straight compound channels

Critical depth is the depth at which the flow in an open conduit undergoes a transition from subcritical to supercritical regimes. The prediction of critical depth in a prismatic channel can be achieved to a high degree of accuracy. This is not the case for two-stage channels (main channel with flanking floodplain), also commonly known as compound channels. Traditionally, critical depth is predicted based on minimum specific energy, and this paper investigates the validity of this and several other methods of critical depth prediction for compound channels. It has been found that flow in compound channels does not undergo a sharp critical flow transition as occurs in prismatic channels. Instead, the change between fully subcritical and fully supercritical occurs over a depth range, whereby different regions in the cross-section maybe either sub- or supercritical. This has been termed a ‘mixed-flow’ region. It has been found that the point of minimum specific energy does not coincide with either the beginning or the end of this mixed flow transition, rendering such predictions ineffectual. A new method for determining critical depth in a compound channel is presented and compared with other prediction techniques. Methods of incorporating the effects of the momentum interaction have been used to improve the prediction of the velocity profile within the channel and locate the mixed flow transition zone.

Critical depth prediction in straight compound channels

Critical depth is the depth at which the flow in an open conduit undergoes a transition from subcritical to supercritical regimes. The prediction of critical depth in a prismatic channel can be achieved to a high degree of accuracy. This is not the case for two-stage channels (main channel with flanking floodplain), also commonly known as compound channels. Traditionally, critical depth is predicted based on minimum specific energy, and this paper investigates the validity of this and several other methods of critical depth prediction for compound channels. It has been found that flow in compound channels does not undergo a sharp critical flow transition as occurs in prismatic channels. Instead, the change between fully subcritical and fully supercritical occurs over a depth range, whereby different regions in the cross-section maybe either sub- or supercritical. This has been termed a ‘mixed-flow’ region. It has been found that the point of minimum specific energy does not coincide with either the beginning or the end of this mixed flow transition, rendering such predictions ineffectual. A new method for determining critical depth in a compound channel is presented and compared with other prediction techniques. Methods of incorporating the effects of the momentum interaction have been used to improve the prediction of the velocity profile within the channel and locate the mixed flow transition zone.

Turbulent flow across a natural compound channel

The measurements and primary analysis of turbulent flow across a compound channel (River Severn, England) are presented. The velocity was measured using a three‐dimensional acoustic Doppler velocimeter in combination with a directional current meter. The statistical flow structure is examined against existing analytical formulations derived for single channel flows based on laboratory studies. The existence of a vertically two‐layer structure around the interface between the main channel and the floodplain is demonstrated, indicating (1) a vertical shear‐dominated flow zone near the bed; and (2) away from the bed a transverse shear‐dominated flow zone with enhanced turbulent mixing. The temporal spectra clearly reveal the occurrence of anisotropic turbulence both in the main channel and over the floodplain. The present findings necessitate the resolution of both transverse and vertical structures for advanced modeling of compound channel flows. The measured data can be used to assess the performance of mathematical river models.

Gravitational and Shear Instabilities in Compound and Composite Channels

Linear analysis of gravitational instabilities in the presence of a shear layer and shear instabilities in the presence of a free surface is performed. This study is relevant to shallow mixing layers, such as flow in compound and composite channels and inflows at channel junctions. The variations of the channel bed, velocity profile, Froude number, and friction coefficients with the transverse (lateral) coordinate are considered. It is found that there is a threshold Froude number above which the flow is unstable with respect to gravity waves and below which the flow is unstable with respect to shear waves for a certain range of the bed friction number. For values of Froude number larger than the threshold value, the influence of the shear layer and channel walls on the characteristics of the gravitational instability is strong when the channel and the shear layer are of comparable width. This influence reduces as the channel becomes wider and disappears in the limit when the channel width becomes infinite. When the Froude number is below the threshold value, free surface deformation in the form of gravitational waves exerts a strong stabilizing influence on the shear instability. In particular, the value of the critical bed friction number decreases when either the Froude number of the fast stream (main channel) or the slow stream (flood plain) increases. That is, shallow mixing layers become more stable as the Froude number increases. Comparisons of the linear stability calculations with experimental data show reasonable agreement.

Comparison of turbulence models for stage-discharge rating curve prediction in reach-scale compound channel flows using two-dimensional finite element methods

This paper attempts to assess the accuracy of constant eddy viscosity, Elder and k–ϵ turbulence models in the numerical simulation of reach-scale compound channel flows using two-dimensional (2D) finite element methods. Assessment was conducted using benchmark stage-discharge data collected from straight and meandering compound channel configurations at the UK Engineering and Physical Science Research Council (EPSRC) Flood Channel Facility. For mesh resolutions and topologies used in reach-scale studies, all models were found to be adequate predictors (<5% error in predicted flow depth) of the stage-discharge relationship at moderate overbank flows (Figs. 1 and 2) . However, at inbank and low overbank flows the Elder and k–ϵ turbulence models can reproduce stage-discharge points with much greater accuracy than the constant eddy viscosity model. Hence, for an unsteady simulation where low flows are relevant a constant eddy viscosity turbulence closure may prove problematic. In terms of computed lateral distributions of depth-averaged velocity for both channel configurations, at higher depths (Relative depth=0.666) all turbulence models predict the velocity with greater accuracy than at a lower depth (Relative depth=0.333). At this latter depth, all turbulence models predict the depth-averaged longitudinal velocity distribution with poor accuracy (>20% error). Also, sensitivity of the turbulence parameter calibration with respect to the predicted flow depth showed that the constant eddy viscosity model's performance can be highly dependent on the choice of turbulence parameter value.

The influence of planform on flow resistance in mobile bed compound channels

A large-scale test facility, the United Kingdom Flood Channel Facility, has been used to compare observations of a range of overbank flows in straight compound channels and meandering compound channels with similar geometries, floodplain roughnesses and mobile main channel bed material. For straight and meandering channels with smooth floodplains, the single channel Manning's n values were similar, but single channel Manning's n values for the meandering channel with roughened floodplains are approximately 50% greater than those for the straight channel with similarly roughened floodplains. At higher relative depths, Yr > 0·25, the floodplain Manning's n values become similar for both the straight and meandering channels indicating that most of the overbank flow for the meandering channel is in the valley direction at these depths. The meandering main channel with roughened floodplains was found to have the largest mobile bed features.

The influence of planform on flow resistance in mobile bed compound channels

A large-scale test facility, the United Kingdom Flood Channel Facility, has been used to compare observations of a range of overbank flows in straight compound channels and meandering compound channels with similar geometries, floodplain roughnesses and mobile main channel bed material. For straight and meandering channels with smooth floodplains, the single channel Manning's n values were similar, but single channel Manning's n values for the meandering channel with roughened floodplains are approximately 50% greater than those for the straight channel with similarly roughened floodplains. At higher relative depths, Yr &gt; 0·25, the floodplain Manning's n values become similar for both the straight and meandering channels indicating that most of the overbank flow for the meandering channel is in the valley direction at these depths. The meandering main channel with roughened floodplains was found to have the largest mobile bed features.

NUMERICAL SIMULATION OF MEANDERING COMPOUND CHANNEL FLOW WITH STEEP GRADIENT BED USING CIP METHOD

It is important for river planning and improvement to predict flows in steep gradient slope river. In this study, weapply 2-D shallow water equation model to the experiment of meandering compound channel flow with steep gradient bed. We use CIP method, which can simulate the unsteady and rapid flow without the numerical viscosity.We examine the characteristics and application of this model. The result of simulation can describe water level and velocity, but cannot describe the detail of change about water level. We also estimate bed shear stress and vortex from this simulation.

COMPARISON BETWEEN EXPERIMENT AND COMPUTED FLOOD FLOW CHARACTERISTICS IN A COMPOUND MEANDERING CHANNEL

Extensive development in river flood plains should be achieved in parallel with minimization of potential inundation risk of over-bank flooding. Flood risks of property damage and loss of life can be eliminated through accurate flow predictions and by proper application of mitigating measures such as flow regulation structures. This paper presents a comparison between experimental results and a 2-dimensional unsteady flow numerical model simulations of flood propagation in a compound meandering channel. The model is based on a finite volume discretization on a staggered grid with upwind scheme in flux is presented, it handles drying and wetting process for a flood plain, complex geometry and discontinuities, which are the main requirements for modeling compound channel flows. The main objective of this research is to advance the existing knowledge on flood flow phenomenon in compound channels through comparative studies of unsteady flow experiments and numerical simulations. Comparison between measured data and model simulations show generally good agreement.

Modelling suspended sediment deposition on a fluvial floodplain using a two-dimensional dynamic finite element model

In this paper we outline a new numerical model for predicting floodplain sediment deposition resulting from out-of-bank flow in reach-scale natural compound channels. Simulation of this problem requires models capable of dealing with the hydraulic and sediment transport effects of a dynamically moving inundation front, as well as a complex set of flow processes including momentum exchange between main channel and floodplain, spillage of water across meander loops and the impact of complex topography. Whilst the treatment of dynamic moving boundary problems is difficult, but attainable in finite element hydraulic codes, the necessity to include dry areas within the model generates a number of problems that numerical solvers for fluvial sediment transport have, to date, failed to overcome. Accordingly, we develop a two-dimensional finite element approach that specifically accounts for sediment transport in domains undergoing wetting and drying. The hypothesis that at the reach-scale a two-dimensional depth averaged representation of flow and suspended sediment is able to reproduce observed deposition patterns is then tested against average annual rates determined using 137caesium analysis of floodplain sediments. Using reasonable parameterisation and no calibration of the sediment transport component, the developed model is able to replicate an encouraging amount of the observed spatial variability in this data set. Whilst further testing of both the hydraulic and sediment transport components of the model is undoubtedly required, the results provide an initial assessment of reach scale process dominance for floodplain systems.