CHARACTERISTICS AND DRIVING MECHANISMS OF MIXING AND STRATIFICATION IN SMALL ESTUARIES USING THE POTENTIAL ENERGY ANOMALY EQUATION AND ITS DERIVATIVE

This study aims to analyze the different mechanisms driving the mixing and stratification of the water column in small estuaries with large intertidal zones by using numerical modeling to calculate the potential energy anomaly and its derivative. Numerical results show that the Suances Estuary presents an ellipse of mixing and stratification variability driven by, firstly, the river flow (seasonal cycle – monthly time scale), secondly, the tidal phase (ebb-flood cycle – diurnal time scale) and, lastly, the tidal magnitude (spring-neap cycle – fortnightly time scale). Furthermore, these results explicitly highlight how the driving mechanisms can vary for the same estuary geometry at different locations due to diurnal, fortnightly and seasonal changes in forcing. In the Suances estuary, it was found that mixing is intensified in locations with bathymetric variability along-estuary. Most of the time, the stratification zone is located between km 4 and 8 of the estuary, varying according to the flood-ebb cycle. At the fortnightly time scale, the stratification is enhaced during neap to spring transition, moving the stratification zone landward. However, when a high runoff occurs, the tide plays a secondary role, and the fluvial contribution flushes away all the estuarine water from the main channel to the mouth, centering the stratification in this area. The influence of the river flow is extremely important in the estuary, since the higher the flow, the higher the displacement of the stratification zone to the outside of the estuary.