The coastal macro-vortices dynamics in Hong Kong waters and its impact on water quality

Large-scale macro-vortices could be formed in the lee or around natural obstacles such as islands and headlands during the interaction between geographical features and coastal circulations induced by tides and winds. Tidal and wind-induced vortices play a fundamental role in transport processes of sediments and nutrients and the effects are then reflected on water quality of estuaries and coastal areas. In the present study, the coastal hydrodynamics of the Pearl River Estuary and Hong Kong waters was analyzed in detail using a high-resolution model based on the Finite-Volume Coastal Ocean Model (FVCOM), forced by tides and seasonal winds. The results showed that the local dynamics was strongly affected by the continuous shedding of macro-vortices. A classification of these coherent structures was suggested based on their typical evolution to distinguish between tidal vortices and wake vortices using four non-dimensional parameters. The spatial distribution, the seasonality and the probability density functions of their geometrical characteristics were presented and discussed in detail. The results suggested that tidal currents played a dominant role in most of the Pearl River Estuary and the Hong Kong waters, generating for the entire year a high and constant number of macro-vortices that occupied most of the north channels of Hong Kong. Finally, we linked the vortices distribution and their typical features with several important water quality parameters, suggesting possible explanations of the observed trends.