Total Exchange Flow and Mixing in a Tidally Driven Estuary

The exchange flow in estuaries is typically characterized by an inflow of salty water near the bottom and a surface outflow of brackish water, resulting from the mixing of river and ocean water. The exchange flow has been found to be a major driver of residence times and water quality conditions in estuarine systems and has also been found to influence biogeochemical processes such as hypoxia, nutrient fluxes, and the transport of contaminants. 
This study focuses on the spatial and temporal variability of exchange flow in a highly turbid and tidally-driven estuary located on the southwest coast of France, The Gironde. Stratification in the Gironde varies from well-mixed to partially stratified during low to high river flow conditions, respectively. Using validated numerical simulations, the exchange flow was calculated using the Total Exchange Flow (TEF) methodology under various river regimes. A mixing ratio was also quantified to assess the relative contribution of shear production to the salinity variance dissipation. Results show that the exchange flow in the Gironde is characterized by up to four salinity layers at the mouth reducing to two layers upstream. In addition, TEF outflow transport (Qout) increases slightly with river discharge, while the most substantial variability in Qout is due to along-channel variability in bathymetry and estuary width. The mixing ratio indicates that salinity variance dissipation was more influenced by the shear production than buoyancy near both the mouth and head of the estuary during spring tide. Towards the middle of the estuary, the ratio shows a weak contribution from the shear production whereas the mixing of salinity was high. During neap tide, the contribution of shear production to the salinity variance was elevated only downstream and mixing of salinity dominated. The next steps of this work will be to assess the impact of sediments on TEF and to compare the magnitude of the exchange flow in the Gironde to other systems globally.