CHARACTERISTICS AND DRIVING MECHANISMS OF MIXING AND STRATIFICATION IN SUANCES ESTUARY USING THE POTENTIAL ENERGY ANOMALY BUDGET EQUATION

Jagoba Lupiola ^1,2* Javier F Bárcena ¹ Javier García-Alba ¹ Andrés García ¹

• ¹ Environmental Hydraulics Institute (IHCantabria), Santander, Spain
• ² IntelliAlert Technologies S.L, Zamudio, Spain

This study first applies a method to analyze the different mechanisms driving the mixing and stratification of the water column in Suances estuary, a small estuary with large intertidal zones, by using numerical modeling to calculate the potential energy anomaly (ϕ) and its derivative (𝜙 𝑡 ). Numerical results show that Suances estuary presents an ellipse of mixing and stratification variability driven by, firstly, the river flow (seasonal cyclemonthly time scale), secondly, the tidal phase (ebb-flood cyclediurnal time scale) and, lastly, the tidal magnitude (spring-neap cyclefortnightly 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. The predominant driving mechanisms in Suances estuary are straining (S) tending to stratify the estuary, river and tide advection (A) tending to mix it and non-linear straining (N), caused by contributions from intertidal areas that favor mixing or stratification according to the tidal cycles. The rest of the mechanisms can be discarded as being practically insignificant in the total contribution. Most of the time, the stratification zone is located between km 4 and 8 of the estuary, an area where the straining term predominates, since this is where the two water masses of different density converge, varying according to the flood-ebb cycle. Finally, in the upper and lower parts of the estuary, advection terms predominate, as these are areas where the flow velocity acquires a major role in displacing mixed or stratified fronts.