CFD modelling of meandering channel during floods

The three-dimensional Reynolds-averaged Navier–Stokes (RANS) and continuity equations are solved using a standard computational fluid dynamics (CFD) solver to predict flow in a compound meandering channel. High-quality experimental data from the UK Flood Channel Facility (FCF) are used to validate the computational results. The flow velocities, free-surface elevation, bed shear stress and turbulent kinetic energy are predicted reasonably well. The measured and predicted flows are analysed qualitatively and quantitatively to improve further understanding of mean flow, turbulence and secondary flow structures in a compound meandering channel. The streamwise component of the mean vorticity equation is used to quantify the behaviour of secondary flow circulations in terms of their generation, development and decay along the meandering channel. The turbulent kinetic energy equation is used to understand energy expenditure mechanisms of secondary flow circulations. The numerical results show that one of the shear stresses significantly contributes towards the generation of the streamwise vortex and the production rate of turbulent kinetic energy

Water surface and velocity measurement-river and flume

Understanding the flow of water in natural watercourses has become increasingly important as climate change increases the incidence of extreme rainfall events which cause flooding. Vegetation in rivers and streams reduce water conveyance and natural vegetation plays a critical role in flood events which needs to be understood more fully. A funded project at Loughborough University is therefore examining the influence of vegetation upon water flow, requiring measurement of both the 3-D water surface and flow velocities. Experimental work therefore requires the measurement of water surface morphology and velocity (i.e. speed and direction) in a controlled laboratory environment using a flume but also needs to be adaptable to work in a real river. Measuring the 3D topographic characteristics and velocity field of a flowing water surface is difficult and the purpose of this paper is to describe recent experimental work to achieve this. After reviewing past work in this area, the use of close range digital photogrammetry for capturing both the 3D water surface and surface velocity is described. The selected approach uses either two or three synchronised digital SLR cameras in combination with PhotoModeler for data processing, a commercial close range photogrammetric package. One critical aspect is the selection and distribution of appropriate floating marker points, which are critical if automated and appropriate measurement methods are to be used. Two distinct targeting approaches are available: either large and distinct specific floating markers or some fine material capable of providing appropriate texture. Initial work described in this paper uses specific marker points, which also provide the potential measuring surface velocity. The paper demonstrates that a high degree of measurement and marking automation is possible in a flume environment, where lighting influences can be highly controlled. When applied to a real river it is apparent that only lower degrees of automation are practicable. The study has demonstrated that although some automation is possible for point measurement, point matching needs to be manually guided in a natural environment where lighting cannot be controlled

Camera calibration for water-biota research: the projected area of vegetation

Imaging systems have an indisputable role in revealing vegetation posture under diverse flow conditions, image sequences being generated with off the shelf digital cameras. Such sensors are cheap but introduce a range of distortion effects, a trait only marginally tackled in hydraulic studies focusing on water-vegetation dependencies. This paper aims to bridge this gap by presenting a simple calibration method to remove both camera lens distortion and refractive effects of water. The effectiveness of the method is illustrated using the variable projected area, computed for both simple and complex shaped objects. Results demonstrate the significance of correcting images using a combined lens distortion and refraction model, prior to determining projected areas and further data analysis. Use of this technique is expected to increase data reliability for future work on vegetated channels

Modeling of vegetated rivers for inbank and overbank flows

Model parameters such as friction factor and eddy viscosity in the Shiono & Knight method (SKM) are considered through experimental data obtained from a vegetated open channel. The experiment was conducted in a rectangular open channel with cylindrical rods as vegetation. Velocity, Reynolds stresses and boundary shear stress were measured with Acoustic Doppler Velocimetry (ADV) and a Preston tube re-spectively. Both friction factor and eddy viscosity were calculated using the measured data and found to be not constant in the shear layer generated by rods. The analytical solutions of SKM to predict velocity and boundary shear stress currently in use were based on the constant assumption of these parameters. In this pa-per a new analytical solution was derived by taking into a variation of these parameters account and was also verified with the experimental data. This solution was also applied to flow in compound channel with vegeta-tion. The new solution gives a good prediction of the lateral distribution of depth-averaged velocity and boundary shear stress in vegetated channels, and it predicts the boundary shear stress better than that of the original solution without considering the secondary flow term in particular

Flow resistance of one-line emergent vegetation along the floodplain edge of a compound open channel

Experiments have been conducted in straight compound channels with and without one-line emergent vegetation along the floodplain edge, in which stream-wise velocities and boundary shear stresses have been measured. The experimental results show that the velocity distribution in the vegetation case is considerably different from that in the no vegetation case and the boundary shear stress is also significantly reduced by the additional flow resistance caused by the vegetation at a similar relative water depth. The apparent shear stress distribution which has been calculated with the boundary shear stress and weight component in the vegetation case is totally different from that in the no-vegetation case. New formulae for friction factors for the with and without vegetation cases are developed using vegetation density and flow parameters. The drag force caused by the vegetation is obtained for two different vegetation density cases and the magnitude of its effect on total flow resistance is then investigated. The force balance method is used to predict discharge and this is compared with the discharge predicted by the new formula. A further analysis of the selection of vegetation spacing is carried out, determining its effect on stage-discharge

Flow characteristics in meandering channels with non-mobile and mobile beds for overbank flows

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 application of LS-PIV to a small irregular river for inbank and overbank flows

This paper examines the feasibility of applying a mobile, large scale particle image velocimetry (LS-PIV) system to a 300 m reach of a small river in order to estimate the discharge. Detailed velocity measurements at a number of locations were carried out using an acoustic Doppler current profiler (ADCP) and acoustic Doppler velocimetry (ADV) for inbank, bankfull and overbank flows. The lateral distributions of the velocity index k (i.e., the ratio of the depth-averaged velocity to the surface velocity) were found to be influenced by the secondary currents, channel vegetation and flow conditions. An attempt is made to quantify the relationship between secondary flow and the velocity index. Appropriate conclusions and advice relating to the practical use of a LS-PIV system as applied to a small river are given