Analysis of the stratification in the Guadiana estuary

The construction of new dams in the Guadiana River (Portugal/Spain) motivated the analysis of the hydrodynamics of its estuary under strong river flows. The main objectives were to determine under which combinations of river flow and tidal amplitudes stratification occurs, and to characterize the velocity and salinity structure under these conditions. High-resolution numerical simulations were performed using a three-dimensional baroclinic model (ELCIRC) with a level 2.5 Mellor and Yamada turbulence closure scheme. Field data collected in the scope of this study were used both to validate the model and to complement the analysis of the estuarine behavior. The data were adequately reproduced and results were consistent with theory. Model results also provided additional details on the spatial distribution of salinity. For instance, the model shows that stratification decreases near the tip of the salt wedge at the end of flood. Some empirical criteria to predict stratification conditions were shown to be inadequate for the Guadiana estuary. Both data and model results were used to verify the hypothesis that pelagic organisms able to migrate vertically during the tidal cycle can take advantage of the baroclinic circulation to remain in the estuary even at high river flows. Although residual velocities are directed seaward at all depths, it was shown that, under stratified conditions, the vertical phase lags in the velocity field are sufficient to allow an organism with vertical migration capabilities to move upstream.