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SUMMARYA new two-layer, two-dimensional mathematical model employing a finite difference method based on numerically generated boundary-fitted orthogonal co-ordinates and a grid 'block' technique for unsteady boundary problems is developed which can be used to simulate flows with density stratification in a natural water-body with complicated topography. In the model the turbulent exchange across the interface is treated empirically and a timesplitting finite difference method with two fractional steps is employed to solve the governing equations. The model is calibrated and verified by comparing the computational results with data measured in Tolo Harbour, Hong Kong. The simulation results mimic the field measurements very closely. The computation shows that the model reproduces the two-layer, two-dimensional tidal flow with density stratification in Tolo Harbour very well. The computed velocity hodographs show that the tidal circulations at various positions in each layer have different patterns and that the features of the patterns are independent of the tidal type except for their scales. The computed Lagrangian pathlines show that the tidal excursion is dependent on the tidal type, especially in the inner harbour and side-coves.

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Carbon to chlorophyll-a ratio (CCHL) is formulated based on the assumption that adaptive changes in carbon to chlorophyll occur so as to maximize the specific growth rate for ambient conditions, including solar radiation and water temperature. With the dynamic CCHL, an unsteady two-layered, two-dimensional eutrophication numerical model for density stratified coastal waters has been developed. Saturated light intensity (IS) is determined as weighted average of the light intensity for previous three days to incorporate light acclimation by phytoplankton. The bottom water anoxic condition during summer in Tolo Harbour, Hong Kong is successfully reproduced by the present method. Otherwise, the simulation with a constant CCHL gave a wrong result.

Based on a numerically-generated boundary-fitted orthogonal curvilinear coordinate system, a 2-layered, 2-d mathematical model for unsteady flows with complicated boundaries is presented. It is employed to evaluate the change of flow patterns such as water surface level, velocity vector, Lagrangian pathline and friction factor in sinegenerated meandering compound channel in terms of water depth ratio between in main channel and in flood plain. It is concluded that the over-bank flow is impacted severely by the meandering main channel flow when the flow depth on flood plain is shallow, and is almost independent of that when hu/hi is greater than 0.9, except the super-elevation of water surface due to the meandering in-bank flow. The bulk friction factor (f or f78C, ) changes with hu/hi very interestingly.