Study And Application Of Three-Dimensional Numerical Modeling Of Bed Deformation In Curved Channel And Salinity Transport In Estuary

River meandering pattern is the normal feature of natural rivers, and straight rivers are the exception while meandering rivers are the rule. The secondary flow with the main flow direction produces the well known three-dimensional helical flow patterns that can significantly influence the spreding and mixing of suspended sediment particles, bedload sediment transport behavior and the bathymetry evolution. Accurate representation of the features of secondary flow is very challenging in rivers simulaitons because the bed shear stress and sediment transport in alluvial rivers are influenced to a large degree by the strength and redistribution of momentum by the secondary flow. The transversal slope which balance the force induced by the secondary flow against the slope is the key point to make the bathymetry reach equilibrium state, meanwhile, it will also force the bedload particles to deviate from the direction of flow drag force, in addition, the value of critical of incipient bed shear stress must be modified to reflect the influence of the local bed surface slope. These two phenomena must be taken into account in rivers numerical models which try to predict the bedload transport rate.The hydrodynamic and transport processes in estuaries are very complex and highly dynamic due to the mixture of salt and fresh water as well as the synchronous impacts of runoff, tides, winds and waves. Complexities can also arise because of the increasing engineering projects like as estuarial and shorline regulations, dredging of navigation channels and land reclamations and so on, all of these acts will result in the original coastlines and landforms were artificially transformed. It poses more and higher challenges to the numerical models being considered the impacts of multiple composite factors, fitted the changes of irregular shorelines, adapted to the complex landform features and included density stratification transport problems.With regard to the problems with significant three-dimensioanl characteristics in flow, mass transport and bathymetry evolution, the three-dimensional numerical models about detailed flow field and mass transport have been enjoyed widespread applications and are becoming an important research tools in solving hydraulic engineering problems, with the rapid development of computer hardware technology and growing sophistication of numerical model theories. In this paper, based on fundamental theories of conservative laws of mass and momentum, sediment transport, numerical discretization, algebraic equations etc, the research focuses on flow, sediment transport and bed deformation in channel bends as well as tidal current and salinity transport process in estuary district, and it takes the computer programming technology as research tools. A fully3D non-hydrostatic numerical model of hydrodynamics, mass transport and bed deformation was developed and applied to the studies about bed deformation in Chibakou waterway of Yangtze River as well as tidal current and salinity transport process in Oujiang River Estuary at Wenzhou, respectively.The horizontal unstructured triangular grid and vertical boundary-fitted coordinate system, which can fitted excellently the irregular shorelines and complicated landform features in natural rivers, coastlines and islands were employed in the numerical model. Based on the staggered grid arrangement, the governing equations and turbulence model were discretized by using finite volume method along with Perot’s scheme and one-order upwind scheme. The layer-integrated method and explicit two steps projection method were employed to solve the Reynolds-averaged Navier-Stokes equations, while, the discretized Poisson equation for pressure is symmetric and positive definite, and thus it can be solved effectively by the preconditioned conjugate gradient method. With regard to the strong flow with significant three-dimensioanl characteristics, However, In cases such as short wave motion, abruptly changing local flow field (e.g., local scour hole around bridge pier and spur dike) or bed topography (e.g., tidal creek, artificial navigation channel and submerged jetty), as well as flows with strong density stratification. The fully3D non-hydrostatic model has a much widespread applicability, because that it contains the non-hydrostatic term in which the vertical acceleration was considered, moreover, the zero pressure boundary condition at the free surface can be approximated very accurately.In the mass transport model, the salinity and suspended sediment transport were generally simulated using an advection-diffusion equation while additional settling velocity term was included in suspended sediment transport, and the bedload transport is computed with empirical or semi-empirical bedload transport rate foumulas. With respect to the local value changes of the critical bed shear stress due to bed deformation and the deviation of bedload sediment transport due to the influence of transverse bedslope, the Van Rijn correction method was used that deduces the critical bed shear stress for particles on bed slopes and three different methods (e.g., Struiksma, Ikeda and Engelund) were taken into account by introducing an extra gravitational term to modify the direction of the bedload flux, In addition, a coupled or semi-coupled bed evolution model about uniform sediment particles at equilibrium conditions was presented and it has the ability to predict the unsteady temporal variation of bed deformation process before the equilibrium condition was reached.Lastly, the sediment transport and bed deformation model was verified through a series of classic open channel bends (e.g.,193°,140°,180°and S-shaped flumes), and then it was applied to study the river bed deformation in Chibakou waterway of Yangtze River. Besides, the salinity transport model was also verified by a wave over submerged bar experiment and a lock exchange experiment, respectively, and then the influence on ambient flow pattern and redistribution of tidal prism as well as salinity distribution after a blocking project were reproduced in Oujiang River Estuary at Wenzhou. It indicates that the model is capable of solving practical engineering problems.