There have been few studies of salt wedge instability under the influence of tidal motion and of the behavior of interfaces and interface instability conditions when tidal waves propagate up the rivers. To clarify these phenomena, it is necessary to use a veltical two-dimensional or three-dimensional numerical model for analysis. In addition, the modeling of tidal wave effects at the upstream boundary is an important issue. Numerical analysis is carried out on the salt intrusion caused by the propagation of a tidal wave, with two-dimensional Reynolds and diffusion equations used as the governing equations under a boundary condition that allows tidal waves to pass through the upstream boundary. It was possible to explain the temporal variation of surface elevation as well as the vertical structure of flow and salinity during flood or ebb tides in the estuary. The location of isohalines changes periodically with variations in tide and depending upon the Froude number.
Estuaries are regions of water connected to the sea or ocean at one end and fed by sources of fresh water(rivers) at their landward extremities. In such regions, saline sea-water and fresh river-water meet each other. As the salt and fresh waters mix, the distribution of salinity in an estuary becames gradually varying function of space and time. Most of the mathematical models that have been developed to calculate the salinity distribution of estuaries are descriptive rather than predictive. A mathematical model offering predictive capability would need physical information on the spatial and temporal functions governing turbulent eddies and mass diffusivities as well as physically relevant boundary conditions of general applicability. Present knowledge of, for instance, the effect of stratification on vertical eddies and mass diffusivities is limited and it is for this reason that models tend to be descriptive rather than predictive.
