Salinity mixing in a tidal multi-branched estuary with huge and variable runoff

Abstract

The study of estuarine hydrodynamics within an isohaline framework has been well established in recent years. However, most studies of estuarine mixing and exchange flow using salinity coordinates have been conducted for idealized estuaries or realistic estuaries with simple bathymetry and constant runoff. In this study, a realistic numerical model is applied to the Changjiang River estuary, a complex multi-branched estuary with huge and variable runoff. The destruction of salinity variance is used to quantify salinity mixing, in which numerical mixing accounts for about one third of the total mixing. The results of total mixing per salinity class mS indicate that the universal law of estuarine mixing applies to realistic simulations under huge and variable runoff when averaged over two spring-neap cycles (28 days). During the flood season, volume integrated salinity mixing is approximately twice that of the dry season due to the double runoff. Salinity mixing at different channels displays distinct characteristics in both horizontal and vertical distributions. The patchy distribution of effective vertical diahaline velocity implies that the volume flux across an isohaline surface can exhibit significant spatial variability. The most vigorous salinity mixing occurs in the North Passage, where about 58 % of the freshwater enters the sea. We found that the estuarine mixing within different branches is linearly correlated with the corresponding water transport. Dikes fringing the navigational channel on both sides downstream of the North Passage significantly increase the length of saltwater intrusion in the North Passage, alter the distribution of horizontal and vertical flow velocities, and further cause significant changes in local mixing, especially near the mouth of the dikes.