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[EN] In this study, we report large eddy simulations of turbulent flow in a periodic compound meandering channel for three different depth conditions: one in-bank and two overbank conditions. The flow configuration corresponds to the experiments of Shiono and Muto (1998). The predicted mean streamwise velocities, mean secondary motions, velocity fluctuations, turbulent kinetic energy as well as mean flood flow angle to meandering channel are in good agreement with the experimental measurements. We have analyzed the flow structure as a function of the inundation level, with particular emphasis on the development of the secondary motions due to the interaction between the main channel and the floodplain flow. Bed shear stresses have been also estimated in the simulations. Floodplain flow has a significant impact on the flow structure leading to significantly different bed shear stress patterns within the main meandering channel. The implications of these results for natural compound meandering channels are also discussed.

A mathematical model is used to predict velocity and bed shear stress in a compound channel flow. Model parameters such as non-dimensional eddy viscosity and secondary flow term in the mathematical model based on Shiono and Knight Method (SKM) are reconsidered using the UK-FCF data. The turbulent shear stresses obtained by three turbulence models are compared with those in the FCF data and the results are no significant differences between them. The concept of the secondary flow in the SKM is redundant through the data analysis as a result a new one is introduced in representing the shear stress in the momentum equation. The shear stress is now expressed by three mechanisms of momentum transfer between the main channel and the floodplain. The analytical solutions to the SKM are used to predict velocity and bed shear stress with the new concept of the shear stress. An effect of various parameters in the SKM to the distributions of velocity and bed shear stress is also investigated.

Overbank flow in a meandering channel displays a complex coherent flow structure resulting from the interplay between the floodplain and main channel flows. Understanding and modeling such coherent flow structure are of great importance for addressing river engineering and management issues where the design solutions are required to retain natural channel features and to maintain a balance between environmental, ecological and amenity issues. In recent years, numerical models have increasingly been used for river flow modeling in order to understand their flow behavior. The main aim of this paper is to predict and investigate the overbank flow behavior in the physical model of meandering channels using three-dimensional (3D) numerical model. The performance of the model is first assessed by comparing predicted results with the limited experimental data. The experiment data were obtained from a 1:5 scale physical model of the River Blackwater and a flume study in Loughborough University. The predicted distributions of the streamwise velocity, secondary vectors, turbulent parameters and bed shear stress are used to investigate the coherent flow structures along the channel, in both the horizontal and inclined floodplain cases and main channel mobile sand bed case. The results show that the three-dimensional model predicts the coherent flow structures reasonably well.

Studies were carried out to understand the effect of floodplain roughness on flow structures, sediment transport rates and bedforms in a mobile meandering channel with overbank flows. Three floodplain roughnesses were examined in this study. Flow structures and bedforms were measured using a three component laser Doppler anemometer (LDA) system and digital photogrammetry, respectively. Comparisons of flow structures and bedforms between different floodplain rougheness are made. Considerable changes in the flow structure and bed form were observed. In particular, multiple secondary flow cells along the meandering channel occur at deeper water depths as the floodplain roughness increases. These cells also cause a series of wavy bedforms along the meandering channel. This paper is separated into two parts. Part I concentrates, in detail, on the flow structure with bedforms in the meandering channel for overbank flow as to change of floodplain roughness. The sister paper, Part II, concentrates on bed form formation during flood, sediment transport rates and flow resistance in the main channel and floodplain.

This paper compares results derived from a physical model representing the River Blackwater, to numerical simulation using both a two-dimensional (2D) and three-dimensional (3D) model. The 2D model solves the shallow water depth-averaged continuity and Navier-Stokes equations with the depth-averaged form of the k −ε turbulence model for free surface flow. The 3D model solves the three-dimensional Reynolds-averaged continuity and Navier-Stokes equations with the renormalization group (RNG) k −ε turbulence model for steady-state flow. The model results are evaluated against the experimental data obtained from a 1:5 scale physical model of the River Blackwater, located at the UK Flood Channel Facility. The predicted distributions of the streamwise velocity, secondary vectors and bed shear stress are used to investigate the flow processes along the channel. The 2D and 3D model results show that the three-dimensional model predicts the flow fields much better than the two-dimensional model.

This paper presents the hydrodynamic behaviour of the two-stage River Blackwater model with horizontal and inclined floodplains using two-dimensional (2D) and threedimensional (3D) numerical models. The 2D model solves the shallow water depth-averaged continuity and Navier-Stokes equations with the depth-averaged form of the k −ε turbulence model for free surface flow. The 3D model solves the three-dimensional Reynoldsaveraged continuity and Navier-Stokes equations with the renormalization group (RNG) k −ε turbulence model for steady-state flow. The performance of the models is assessed by comparing predicted results with the experimental data obtained from a 1:5 scale physical model of the River Blackwater, located at the UK Flood Channel Facility. The predicted model results are used to investigate the flow processes along the channel in both the horizontal and inclined floodplain cases.

This paper presents a number of complex issues associated with the methodology and preliminary analysis of velocity and discharge measurements using various techniques applied to the River Blackwater, UK. An ADCP deployment procedure was adapted in order to take into account the difficulties of using such equipment in small rivers. The velocity profile in the middle of main channel has been measured using both ADCP and ADV, and averaged over a time scale of 800 s and 300 s respectively, to obtain a stable value. A discrepancy of approximately 6.8% was observed in the mean streamwise velocity between both measurements. A good agreement between surface velocity measurements using PIV and ADCP in cross-sections located in straight and meandering reaches of the river was observed. Finally, several numerical models have been benchmarked against a physical scale model of the river.

Current state of the art computational fluid dynamics (CFD) and in particular, river flow modelling, require accurate estimation of the “free surface” in order to accurately predict the three dimensional flow field along a river. Such models are increasingly important for management of river basins and mitigation of river floods as the incidence of extreme rainfall events increases, causing widespread flooding, disruption and loss of life. To increase the accuracy of such flow models, it has become necessary to calibrate model outputs using field data, demanding improved measurement of the flow field along rivers.A substantial and funded research project being conducted at Loughborough University is developing image based river measurement methods using a combination of Particle Image Velocimetry (PIV) and close range photogrammetry (CRP). This paper will report on the use of digital close range photogrammetry to measure the dynamic topographic water surface exhibited by real and flooding rivers.A pair of Nikon D80 (10 Mega-pixel) digital cameras have been purchased, each equipped with a variables zoom lens (f: 18-70mm). The two cameras have been synchronized using two cables connected via a single relay operated switch, tests demonstrating that the accuracy of synchronization is better than 100th of a second. The cameras are mounted on two standard camera tripods, providing convergent and stereoscopic coverage of the river reach, which is between 10 and 20m distant. In initial tests, conventional photogrammetric control was provided using temporary targeted points, coordinated using a Reflectorless Total Station. Subsequent work is being conducted on a semi-engineered river at Farnborough, in the UK, where fixed targets have been permanently installed and coordinated to be in position necessary for the two year duration of the project.Imagery is being processed using the Leica Photogrammetry System (LPS); commercial software which provides the ability to automatica

Experiments were conducted in meandering channels with non-mobile and mobile beds to measure flow rates, velocities, turbulent kinetic energies, bedforms and sediment transport rates for overbank flows. The behaviour of bedform in meandering channels with overbank flows was observed using digital photogrammetry, with velocity measurements taken with a Laser Doppler Anemometer. The bedform structure and velocity distributions along the meandering channel were obtained for bank full flow and three overbank flow depths. Important interactions between the flow structure and bedform were observed along the meandering channel. The sediment transport rates collected during the experiment showed three phases; an increase in the sediment transport rate up to the bankfull level, a small decrease as the flow goes overbank up to a relative depth ratio of 0.3 and then an increase again for higher flow depths. The regions of higher turbulent kinetic energy were identified. The total energy losses due to friction, secondary flow and interfacial turbulence in the lower layer flow of the main channel were compared in both the non-mobile and mobile bed cases.

The paper presents a quasi two-dimensional (2D) model calculating depth-averaged velocity and bed shear stress in a straight compound channel with a vegetated floodplain. The model numerically solves the depth-averaged Navier–Stokes equation for the streamwise motion of flow (quasi 2D). Reduction in volume of water due to vegetation is modelled as porosity. Drag force due to vegetation is modelled as an additional momentum sink term in the Navier–Stokes equation. Estimation methods for model parameters—drag coefficient, shading factor, porosity, friction factor, eddy viscosity and advection term—are discussed. The predictive capability of the model is assessed against experimental data with regard to distributions of depth-averaged velocity, bed shear stress, transverse shear stress and stage-discharge. The results show that the quasi 2D model reproduces a reasonable simulation of the flow field.