Computational Study Of The Impacts Of Protection Measures For Central Bars On Waterway Evolution

Central bars commonly exist in the middle and downstream reaches of the Yangtze River, which play an important role in stabilizing the navigational channels. When a central bar is stable, it provides a stable planform for the navigation; however, if a central bar features considerable erosion or deposition, the navigational condition may deteriorate due to the variation of the planform of the navigational channel.Although there have been a number of investigations on the characteristics of sediment transport and water flow around central bars, its understanding remains premature. Existing experimental studies can be summarized as two categories. One is for practical application purposes, by means of reproducing a small-scale version of a natural river central bar. The second category is for research purposes, conducted in rectangular flumes aiming to unravel the effects of the protection measures for central bars on the flow, sediment transport and morphological evolution.Existing mathematical models are mostly decoupled or partially coupled, based on simplified equations, with a poor representation of the natural river, the central bar and the protection measures. Moreover, three-dimensional modelling studies of flow, sediment transport and bed evolution arount central bars are rarely reported. There is therefore an urgent need to establishing a fully coupled model, so as to fully unravel the water and sediment transport characteristics around central bars after implementation of different protection measures.This dissertation presents a new two-dimensional coupled model for sediment-laden flow under the framework of shallow water theory. The governing equations are based on the complete shallow water hydrodynamic equations, fully accounting for not only the water flow evolution, but also the induced sediment transport and bed morphological evolution, as well as their effects on the flow evolution. The governing equations are numerically solved using the second-order Godunov-type scheme along with HLLC (Harten, Lax and van Leer with Contact wave restored) approximate Riemann solver. Unstructured triangular grid suitable for complex boundary shapes is used to represent the computational domain. The DFB method independent of the discretisation scheme is used for the bed slope source term, making the present model well-balanced. The friction source terms are treated in a fully implicit way to alleviate numerical instabilities. The model is systematically validated and calibrated for benchmark cases widely used in the world, as well as a channel flume experiments conducted at Chongqing Jiaotong University. It is shown that the computed water level and flow velocity agree well with the experimental data. By calibrating a modification coefficient in the ZF empirical formula, satisfactory agreements between the computed and measured bed deformation are obtained. These indicate that the present model can resolve the sediment-laden flow aroung river central bars reasonably accurately.Protection place for the head of central bars can be different. The2D coupled model is applied to investigate the effects of the different protection location at the flume scale. The following observations are obtained from computational results. First, the effects of the flexible mattress beach protection are mainly focused on the project area as well as its upstream and downstream areas; the higher the contral water depth of the central bar, the less it affects the flow velocity. Second, the effects of the implementation of fish-bone type dividing dike is qualitatively similar to that of the flexible mattress beach protection in terms of the variation of navigational depth, yet the former has a more obvious than the latter. Both measures results in obvious deposition in the head of central bar and different extent of scour on the two sides of the central bar.The CFD software FLUENT is applied to resolve for the three-dimensional flow structure over protected cerntral bars. Its numerical results are compared with those from the2D coupled model. Reynolds stress model is used for turbulent closure. The volume of fluid (VOF) method is used to capture the free surface. Central difference method is applied to diffusion terms, and second-order upwind difference method for the advection terms. The SIMPLE algorithm is used for the governing equations. Hexahedron mesh is used to represent the computational domain, which results in a minimum volume of20mm×10mm×8mm. Numerical comparison indicates that the computed flow velocity and water level from the FLUENT agree with those by the present2D model and the experimental data rather well. After the implementation of the fish-bone protection, obvious turbulent flow regime is observed at the downstream of the central bar. The zone with maximum velocity transits between the left and right side of the central bar with a certain period. The maximum shear stress is located around the head of the fish-bone protection. Due to the fish-bone protection, bed shear stress at the two side of the central bar is enhanced appreciably. The minimum bed shear stress is located at the protection area of the fish-bone protection. The distribution of bed shear stress along the two sides of the central bar shows a higher correlation with the distribution of flow velocity. The present2D coupled model is applied to investigate a field scale central bar at the Shashi reach in the middle part of the Yangtze River, where a project of fish-bone protection was implemented. With and without the fish-bone protection, the differences in water level, flow velocity and bed deformation are numerically studied. It is shown that the implementation of the fish-bone protection makes water level in the upstream reach of the central bar increase, and that in the downstream reach of the central bar slightly decrease. This effect is more obvious at high flow discharge than a mediate flow discharge. Irrespective of the magnitude of flow discharge, flow velocity at the protection area of the fish-bone dike decreases with the appearance of appreciable deposition, whereas at other places flow velocity increases with some extent of bed scour.In summay, the effects of central bar protection projects on navigational flow and sediment evolution are systematically investigated by numerical modeling under both laboratory and field scales. This research has potential applications for practical projects.