Search

The bar–pool morphology is a typical feature of meandering channels that strongly affects flow and transport patterns. In this study, the three-dimensional flow velocity was measured in detail at six cross-sections in two continuous reverse bends with pool–point bar topography. The streamwise velocity, cross-sectional distributions of circulation, net transversal flow, turbulent kinetic energy (TKE) and turbulent shear stress (TSS) were analysed. High values of the streamwise velocity, TKE and TSS were located in the point bar region (PBR), which were closer to the bed surface in the downstream bend. The circulations, including the main circulation (MC), upper remaining circulation (URC) and outer-bank circulation (OBC), were observed to interact with each other and affect the net transversal flow. The URC was found to exist in the PBR of the downstream bend, which restrained the tendency of the MC to move towards the inner bank. The OBC decreased the net transversal flux of fluid mass towards the inner bank or increased the flux towards the outer bank. The variations in the flow characteristics for different water depths were also analysed. The experimental data can help to validate numerical simulations and predict the evolution of meandering channels, which will provide a technical basis for river regulation. [ABSTRACT FROM AUTHOR]

The distribution of depth-averaged velocity is crucial to many engineering problems. This paper presents an analytical solution for the lateral depth-averaged velocity distribution in a 120° curved channel with asymmetric trapezoidal cross-sections in which the curved flow is not fully developed. The proposed analytical model was derived from the streamwise depth-integrated Reynolds-averaged Navier–Stokes equation, which considers the bed stress, Reynolds stress, secondary flow and local flow acceleration. In addition, practical boundary conditions are proposed to consider the trapezoidal cross-sections with linearly varying side beds and the flow separation at the inner bank near the bend entrance. The analytical solutions using the proposed model were found to be in good agreement with measured data. The main contributions of this study are the development of a conventional analytical model for sophisticated circumstances and the proposal of practical boundary conditions that consider flow separation. [ABSTRACT FROM AUTHOR]

Headcut erosion is a very common process occurring at the head of rills, crop furrows and ephemeral gullies. A numerical model of the headcut flow was built to study the wall shear stress characteristics downstream of the scour hole, which is very important in headcut erosion prediction models. The calculated nappe parameters are very close to experimental data and/or equation results. The calculated flow field, velocity centreline of the free jet part and the locations of the maximum velocity in the scour hole are also close to the experimental data. It was found that the upstream flow rate and downstream water level can affect both the maximum value and the distribution of the wall shear stress. Taking into account the affecting factors and referencing the form of the existing relations on the maximum wall shear stress caused by the impinging jet, the relation of the maximum wall shear stress occurring downstream of the headcut is obtained. The wall shear stress distribution can be described by exponential functions. The new prediction equation of the maximum wall shear stress can be used in existing headcut erosion prediction models for calculating equilibrium scour hole depth or scour hole deepening rate. [ABSTRACT FROM AUTHOR]

The energy gradient is a fundamental physical quantity of flow motion. To date, many studies have concentrated on uniform flow or fully developed curved flow, with the energy gradient approximated as the bed slope. This paper presents a new method to assess the lateral distribution of the apparent energy gradient in curved channels when curved flow is not fully developed. A dimensionless apparent energy gradient (K) is presented. An expression for K is explicitly formulated based on the Reynolds-averaged Navier–Stokes (RANS) equations and the lateral distribution of K is derived from measured velocity data. The method was applied in a 120° curved flume with asymmetric trapezoidal cross-sections. The results showed a bilinear distribution of K; that is, K remained nearly constant in the central region of the channel while, in the inner bank region, K increased linearly as the inner bank was approached. These results were derived for specific cases for curved channels with a side-slope, but the method for deriving K can be applied to other cases, thus providing a new way to study energy loss in curved channels to contribute to river management, such as estimation of discharge capacity and navigation improvement. [ABSTRACT FROM AUTHOR]

A series of experiments were conducted to reveal the effects of the sidewall shear stress distribution (SSSD) on homogeneous cohesive bank erosion processes in a U-shaped flume. The three-dimensional flow velocities were measured in detail and the turbulent kinetic energy method was employed to estimate the SSSD. The experimental results showed that the SSSD changed with the discharge and the point of the maximum shear stress varied within a range of 0·15–0·75 of the relative water depth. The variation of underwater bank topography was found to be consistent with the SSSD and the location of the maximum erosion point was not at the bank toe. The dominant failure mechanisms were observed to be tensile and toppling failures. Furthermore, a simplified bank erosion model (SBEM) was developed with consideration of the SSSD. Compared with the bank stability and toe erosion model, the SBEM can provide a more accurate simulation of bank erosion processes. This study is expected to enrich general understanding of cohesive bank erosion processes in curved channels, which will help to formulate effective strategies for river regulation. [ABSTRACT FROM AUTHOR]

After failure of a riverbank, a cohesive block of soil can temporarily lie at the toe of the bank until it is eroded away. To investigate the erosion of this collapsed block and its interaction with the river channel, a series of experiments was conducted under similar hydraulic conditions within a U-bend flume. The flow structure and channel topography were measured and analysed in detail. The presence of the collapsed block was found to cause a 1·1-1·6 times greater downstream bank retreat, while the counter-rotating circulation cell and smaller near-bank velocity protect the bank against erosion upstream of the block end. The scour percentage of the block head or end was about 1·5-2 times larger compared with that at the middle. Cohesive soil from the block fills the gap of non-cohesive riverbed material in the downstream convex bank and can result in a maximum bed deposition thickness increase of more than 300%. The results also provide a valuable experimental dataset and a theoretical basis for practical application. [ABSTRACT FROM AUTHOR]