Dynamics of saltwater intrusion in the Modaomen Waterway of the Pearl River Estuary

Observations indicate an abnormal characteristic of saltwater intrusion in the upper Modaomen Waterway of the Pearl River Estuary, i. e., the maximum salinity occurs during the neap tide or the coming moderate tide instead of during the spring tide. To explore the associated dynamic mechanisms, a high resolution three-dimensional numerical model was set up based on the Finite Volume Coastal Ocean Model (FVCOM), which covered the entire river network, the Pearl River Estuary, and the adjacent sea. Numerical experiments illustrated that the upper Modaomen Waterway is significantly influenced by the saltwater intrusion from the Hongwan Waterway, a narrow and shallow channel connecting the Modaomen Waterway to the sea. Specific topography, spring-neap tidal variation, local wind stress, and their interaction drive an up-estuary residual current in the Hongwan Waterway, which is much stronger during the neap tide than during the spring tide. As a result, more saltwater in the Hongwan Waterway is spilled over into the Modaomen Waterway during the neap tide or the coming moderate tide. This is the inherent dynamic mechanism why the saltwater intrusion in the upper Modaomen Waterway reaches its maximum during the neap tide or the coming moderate tide. Besides, we also found that the winter prevailing wind can pronouncedly enhance the saltwater intrusion in the Modaomen Waterway.