Lunch Talk Series by Earth, Ocean and Atmospheric Sciences (EOAS) Thrust, HKUST (GZ) - Diahaline Exchange Flow as a Method to Analyse Estuarine Dynamics/Salinity Mixing and Diahaline Exchange Flow in a Large Multi-Outlet Estuary with Islands  

1) Diahaline Exchange Flow as a Method to Analyse Estuarine Dynamics (Speaker: Hans Burchard)

Estuaries are characterised by inflow of riverine freshwater from land and salty water from the ocean, two water masses that mix and form outflowing brackish water. These mixing processes can be quantified as decay of salinity variance in the estuary. Starting from the Knudsen relations, a simple bulk mixing estimate has been found. Due to the large range of salinity continuously occurring in estuaries, mixing and exchange flow analysis with respect to salinity coordinates does largely simplify the results. The universal law of estuarine mixing that has recently been derived by this author prescribes that the mixing per salinity class equals twice the freshwater runoff times the salinity of the salinity class. This law motivates to identify the hotspots of diahaline mixing in estuaries and to describe underlying hydrodynamic processes.

2) Salinity Mixing and Diahaline Exchange Flow in a Large Multi-Outlet Estuary with Islands (Speaker: Xiangyu Li)

The relationship between the salinity mixing, the diffusive salt transport, and the diahaline exchange flow is examined using salinity coordinates. The diahaline inflow and outflow volume transports are defined in this study as the integral of positive and negative values of the diahaline velocity. A numerical model of the Pearl River Estuary (PRE) shows that this diahaline exchange flow is analogous to the classical concept of estuarine exchange flow with inflow in the bottom layers and outflow at the surface. The inflow and outflow magnitudes increase with salinity, while the net transport equals the freshwater discharge Qr after sufficiently long temporal averaging. In summer, intensified salinity mixing mainly occurs in the surface layers and around the islands. The patchy distribution of intensified diahaline velocity suggests that the water exchange through an isohaline surface can be highly variable in space. In winter, the zones of intensification of salinity mixing occur mainly in deep channels. Apart from the impact of freshwater transport from rivers, the transient mixing is also controlled by an unsteadiness term due to estuarine storage of salt and water volume. In the PRE, the salinity mixing and exchange flow show substantial spring–neap variation, while the universal law of estuarine mixing m = 2SQr (with m being the sum of physical and numerical mixing per salinity class S) holds over a longer averaging period (spring–neap cycle). The correlation between the patterns of surface mixing, the vorticity, and the salinity gradients indicates a substantial influence of islands on estuarine mixing in the PRE.