The response of salt intrusion to changes in river discharge and tidal mixing during the dry season in the Modaomen Estuary, China

The increase of salt intrusion in recent years in the Modaomen Estuary, one of the estuaries of the Pearl River Delta in China, has threatened the freshwater supply in the surrounding regions, especially the cities of Zhongshan, Zhuhai in Guangdong Province and Macau. A numerical modeling system using nested grids was developed to investigate the salt transport mechanisms and the response of salt intrusion to changes in river discharge and tidal mixing. The steady shear transport induced by estuarine circulation reaches maximum and minimum, respectively, during neap and spring tides, while the tidal oscillatory transport shows an opposite pattern. The net transport is landward during neap tides and seaward during spring tides. The salt intrusion length responding to constant river discharges generally follows a power law of −0.49. The dependence of salt intrusion on tidal velocity is less than that predicted by theoretical models for exchange flow dominated estuaries. The response of salt intrusion to change in tidal velocity depends largely on river discharge. When river flow increases, the impact of tidal velocity increases and the phase lag of response time decreases. The asymmetries of salt intrusion responding to increasing and decreasing river discharge (tidal velocity) are observed in the estuary.