Environmental monitoring and hydrodynamic studies were carried out to understand the forcing mechanisms that are responsible for the circulation patterns of one of the several small coastal lagoons of the Sardinia, in the center of Mediterranean, Italy. Tide and wind are the main causes of circulation flow in these small and shallow semi-enclosed basins. The semidiurnal tidal wave reflects from the closed end without amplitude reduction, and produces a free-surface oscillation (seiche) with a higher frequency. The wind has a local effect on these oscillations and generates a secondary circulation. Our study also evaluated the accuracy of the description of the seiche and wind effects in the formulation of a plane 2D model of lagoon hydrodynamics. Data were collected monthly on the horizontal and vertical distribution of currents, salinity and temperature of the water in the lagoon during the year of monitoring. The average temperature of the lagoon was 3-4 °C higher than the air temperature, with well defined horizontal gradients, except in the wet fall season, when the water and air temperatures were the same. During the wet season the horizontal salinity gradients were negative from the sea channel towards the inside the lagoon (about 0.01 per meter), and the vertical gradients were positive towards the bottom (about 4 per meter). During the dry season the salinity of the whole lagoon was a little lower than that of the sea, without stratification. The density-induced axial velocity was negligible. 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 internal seiche is one of the mechanisms responsible for driving residual currents. While the employed 2D-horizontal model was accurate in the modeling of tide-generated flow, the numerical predictions of wind-induced circulation patterns were not as satisfactory, due to the poor temporal and spatial resolution of the wind data. The depth-averaged model accurately simulated the observed seiche oscillation within the lagoon, and reproduced the amplification of the seiche oscillations when the wind was stronger, but the 5 minute time step of our monitoring was too large to quantify accurately the relationship between residual seiche-induced circulation and wind stress.
|Titolo:||Tidal and Wind Dynamics in a Small Coastal Lagoon in the Mediterranean Sea|
|Parole Chiave:||environmental monitoring; lagoon modeling; seiche; shallow water; tide dynamics; wind-driven flow|
|Settore Scientifico Disciplinare:||Settore AGR/08 - Idraulica Agraria e Sistemazioni Idraulico-Forestali|
|Data di pubblicazione:||2007|
|Appare nelle tipologie:||01 - Articolo su periodico|