A mathematical model for estimating the pollution exchange coefficient of small tidal embayments

摘要

A novel analytical tidal prism model has been developed for predicting the pollution flushing characteristics of small, well-mixed tidal embayments. The model relates the water quality response of the basin to the external forcing effects of the tide, initial pollutant loading and freshwater inflow. In general, the pollution flushing depends not only upon the geometry of the basin and the tidal range, but also on the proportion of effluent water which leaves the basin on an ebb tide, mixes with the surrounding coastal water and partially returns on subsequent flood tides. This effect has been taken into account in the mathematical model by the inclusion of a 'return flow' factor. The efficiency of the tidal flushing can be expressed in terms of a pollution exchange coefficient which measures the proportion of water exchanged with the sea each tidal cycle. The mathematical formulation has been verified by comparing predictions against experimental pollution data from a 1:400 scale laboratory harbour. The results indicate that the proposed mathematical model offers a viable and robust method for determining the pollution exchange coefficient of well-mixed tidal basins.