Eutrophication Model for a Coastal Bay in Hong Kong

Flow and transport in a natural water body commonly interact with density stratification and in some cases the stratification may be characterized as a two-layered system. A rigorous, two-layered, two-dimensional (2D) finite difference numerical model for eutrophication dynamics in coastal waters, based on the numerically generated, boundary-fitted, orthogonal curvilinear grid system as well as a grid “block” technique, is proposed here. The model simulates the transport and transformation of up to nine water quality constituents associated with eutrophication. The structure of the model is based on a generally accepted framework with the exception of the interaction between the two layers via vertical advection and turbulent diffusion. Some kinetic coefficients are calibrated with field data specifically for the scenario in Tolo Harbour, Hong Kong. The pollution sources are unsteady and hourly solar radiation is imposed. Sediment oxygen demand (SOD) and nutrient releases from sediment are incorporated in the model based on the relevant in-situ sampling analysis. The hydrodynamic variables are predicted simultaneously with a hydrodynamic model previously developed. The computed results show that the present model successfully reproduces the stratification tendency in all the water quality constituents, showing an obvious bottom water anoxic condition during the summer, which is consistent with the density stratification and the unsteady layer-averaged 2D eutrophication processes in Tolo Harbour.