The mixing characteristics in the marine environment of the Euboean Gulf are studied. The Estuarine and Lake CΟmputer Model three-dimensional hydrodynamic model has been used, to simulate numerically the effects of the strong tide conditions, the atmospheric forcing, and the oceanographic conditions. Water age was calculated in all computational cells and its renewal was examined with the “pure” water of the open sea both on the surface layers, where the effect of tide and wind was pronounced, as well as on the deeper layers and bottom. It was investigated if in surface layers the tide and the wind restore the water of the study area, thus preventing its renewal. In the remote area, the mixing and dilution of the pollutants contained in the treated municipal waste of five installed diffusers in this complex hydrodynamic field, generated by the aforementioned loads, is simulated. It was found that even a slight stratification density has the effect of restricting the vertical mixing and entrapment of dirt in the deeper layers. The combined impact of municipal waste effluents on selected areas of high economic, tourist and environmental value in the Euboean Gulf was assessed. It was found out, that the Diffuser 2 has a great effect in all the five selected areas, because it has greater waste water discharge and because of the tide, who produce a velocity field and transfer the pollutants in longer distances. It was also investigated indicatively in an important area, whether the presence of tide contributes to better mixing and dispersing of pollutants and to better water quality or, on the contrary, as it restores remote pollutants that were released earlier.
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