Tidal, seiche and wind dynamics in a small lagoon in the Mediterranean Sea

Environmental monitoring and hydrodynamic studies were carried out to understand the forcing mechanisms that are responsible for the circulation patterns of the several small coastal lagoons of Sardinia, in the center of the Mediterranean Sea, Italy. The water level observed in the Calich lagoon showed a free-surface oscillation with a semidiurnal period (tide) and a second order oscillation with higher frequency and lower amplitude (seiche). The wind has a local effect on these oscillations and generates a secondary circulation. Numerical analysis confirmed that the dynamics of the lagoon is mainly controlled by a balance between accelerations, barotropic pressure gradients, and bed and wind stresses, and highlighted that the surface seiche is one of the mechanisms responsible for driving residual currents. The employed 2D-horizontal model of lagoon hydrodynamics, forced with tides only at the basin boundary, reproduced the natural mode oscillation period of 0.5 h of the basin. Besides, our study evaluated the accuracy of the simulation of the seiche and wind effects. The plane 2D model, forced with the measured wind stress above the basin surface, simulated stronger seiches by strength and variability in the wind field, in agreement with the measured seiche and wind data. The 5-min time step of our monitoring was, however, too large to quantify accurately the relationship between seiche-induced residual circulation and wind stress. Simulation results indicate that the wind variations at the time scale of minutes could affect the strength of the seiche oscillation. Nevertheless, this variability had insignificant effect on the frequency, suggesting that the oscillatory response is predominantly linear. This analysis showed that tidal, atmospheric and seiche forcing mechanisms are responsible for the circulation patterns of these small Mediterranean wetland areas, generating water level oscillations each one with specific frequency, but with amplitudes of the same order.