On the Dispersion and Diffusion Near Estuaries and Around Islands

A series of diffusion experiments using surface drifters were performed in the coastal waters of south-western Taiwan near a small estuary and around an island. Two stages or regimes of drifter dispersion can be identified for all experiments: a coherent regime in the beginning stage when the drifters remained clustered with little dispersion occurred, and a rapid-disperse regime in the latter stage when the separation of drifters grew to the order of a few kilometres. Our results show that near estuaries the extent of the dispersive influence is limited to the tidal excursion distance, and the dispersion coefficient is about 12 15 m2 s−1. On the other hand, the dispersion coefficient downstream of an island in a strong flow ( 1•5 m s−1) can reach a much larger value (45 m2 s−1), which can be attributed to the enhanced mixing and transport processes due to island wakes. Theoretical scaling parameters for coastal water flows are also presented to predict the formation of island wakes. After applying three different methods to exclude the effects of shear flow, the turbulent diffusivities for our experiments amount to 0•2 5 m2 s−1. Analysis of the differential kinematic properties indicates that a close relationship exists among the dispersion, diffusion and the effect of shear flow.