Near the mouth of an estuary, a dynamic interaction occurs between the flow and the density field. Density stratification and its effect on the flow field depends mainly on the level of turbulence. An intense turbulence mixing can cause a wellmixed situation, while the reverse can result in a strong stratified situation. Modelling of stratified flow in a tidal medium requires a vertical resolution of flow, density and turbulence fields, whereas a well-mixed homogeneous flow can be described by vertically averaged quantities. In these flow situations, turbulent transport processes have to be defined accurately for a predictive model. Within the limit of computational economy, the algebraic stress/flux model can be considered as the most appropriate choice as a turbulence closure for a predictive numerical model. In the present study, predictive numerical models have been developed in a 2D vertical plane for the stratified tidal flow and in a 2D horizontal plane for the well-mixed homogeneous tidal flow. Finite elements are used for the spatial discretization. Explicit coupling of the flow, density and turbulence fields has been achieved by using a three level semiimplicit time discretization. By using this time discretization, a finite element scheme has been developed. This scheme solves the governing equations sequentially and without iterations. Fourier analysis shows that the scheme is unconditionally stable. The scheme has the property of damping the shortest wavelengths selectively and spatial oscillations will not appear. For the advection-diffusion equation, a three level time scheme in finite elements has been found to be slightly more dissipative and dispersive than the Crank-Nicolson scheme. A tidal flume study performed at Delft Hydraulics under the E.C. Large Installation Plan (LIPl) has been used to verify the mixing length model experimentally. The 2D vertical plane finite element model has been applied to predict the flow, density and turbulence fields of the tidal flume experiments using various levels of turbulence closures. The mixing length model has been found to have a limited predictive ability. The predictive ability of the algebraic stress/flux model has been found superior than the predictive ability of the mixing length model. Both the numerical model and the experimental observations have been used to study some of the basic characteristics of turbulence and turbulent transport mechanisms in a stratified tidal flow. A depth averaged version of the algebraic stress/flux model has been developed by integrating the original stress/flux model vertically. The 2D depth averaged horizontal plane finite element model has been applied to predict the depth averaged flow, density and turbulence fields in model harbours of different geometries situated in a tidal regime . Six different cases have been studied with four different harbour geometries, of which, three experimental conditions were homogeneous and three experimental conditions were nonhomogeneous. Depth averaged momentum and mass transport have been described by the depth averaged algebraic stresslflux model. Model predictions are, in general, satisfactory within the limitations of model assumptions. The model has been applied to study the turbulent transport mechanisms and salt intrusion characteristics in different harbour geometries. |
