Mixing Characteristics Under Tide, Meteorological and Oceanographic Conditions in the Euboean Gulf Greece

Computational Water, Energy, and Environmental Engineering, 2019, 8, 99-123 https://www.scirp.org/journal/cweee ISSN Online: 2168-1570 ISSN Print: 2168-1562 Mixing Characteristics under Tide, Meteorological and Oceanographic Conditions in the Euboean Gulf Greece Evangelos Tsirogiannis, Panagiotis Angelidis, Nikolaos Kotsovinos Division of Hydraulics, Department of Civil Engineering, Democritus University of Thrace, Kimmeria Campus, Xanthi, Greece How to cite this paper: Tsirogiannis, E., Abstract Angelidis, P. and Kotsovinos, N. (2019) Mixing Characteristics under Tide, Meteo- The mixing characteristics in the marine environment of the Euboean Gulf rological and Oceanographic Conditions in are studied. The Estuarine and Lake CΟmputer Model three-dimensional the Euboean Gulf Greece. Computational hydrodynamic model has been used, to simulate numerically the effects of the Water, Energy, and Environmental Engi- neering, 8, 99-123. strong tide conditions, the atmospheric forcing, and the oceanographic con- https://doi.org/10.4236/cweee.2019.84007 ditions. 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 Received: September 27, 2019 Accepted: October 20, 2019 layers, where the effect of tide and wind was pronounced, as well as on the Published: October 23, 2019 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 Copyright © 2019 by author(s) and the remote area, the mixing and dilution of the pollutants contained in the Scientific Research Publishing Inc. This work is licensed under the Creative treated municipal waste of five installed diffusers in this complex hydrody- Commons Attribution International namic field, generated by the aforementioned loads, is simulated. It was License (CC BY 4.0). found that even a slight stratification density has the effect of restricting the http://creativecommons.org/licenses/by/4.0/ vertical mixing and entrapment of dirt in the deeper layers. The combined Open Access 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. Keywords Sewage Mixing, Water Age, Tide, Gulf of Evoikos, Greece DOI: 10.4236/cweee.2019.84007 Oct. 23, 2019 99 Computational Water, Energy, and Environmental Engineering E. Tsirogiannis et al. 1. Introduction The hydrodynamic circulation in coastal systems is often depending on various interacting processes such as tides, waves, and river inflows. Many researchers have spent their effort to model the effects of the tide [1]-[6]. Hydrodynamic processes significantly influence the mixing and transport of pollutants in coastal waters. Additionally, the tidal currents transport pollutants back and forth before sometime dispersing them into the sea [7]. In the Eastern Mediterranean the tidal currents are in general relatively weak and in most case have low speeds [8]. One of the earliest historically observed tidal phenomenon is in the Gulf of Evoikos (Greece). Eratosthenes, Pitheas, Po- sidonios [9], Stravon and Senekas [10] have studied this phenomenon in antiq- uity and other researchers from the early 20th century [9]-[14]. Although some research has been carried out in the wider area of southern Europe [15] [16] [17] [18] on sea level changes, but also within the Euboean Gulf [19] [20], there are still no systematic measurements and several available work today to describe and illustrate the various effects of tide on hydrodynamic circulation, as well as on diffusion and transport of pollutants [21]. The ELCOM (Estuarine and Lake CΟmputer Model) model, which was developed in the Center of Water Research in the Western Australia University, is a very good numerical model, for the hydrodynamic simulation of lakes, estu- aries and enclosed seas. This model has been used very well in various relevant studies around the world, such as in the North Aegean Sea [22] [23], in the Adriatic Sea [24], and in the Persian Gulf [25], a study in which also included a simulation of the tide. The water circulation in the North and South Euboean Gulf has been simulated in a recent work of the authors’ [21] with the use of the ELCOM model. As a result of the above mentioned, there is a lack of knowledge in literature for the tidal dynamics taking effect on the interconnected system of the North and South Gulfs of Evoikos. There is a lack of information on the tidal hydraulics and the tidal exchange of the two Gulfs, which are connected through a narrow and shallow channel. Only in a previous work of the authors’ [21] a computational simulation of the hydrodynamic circulation has been carried out in order to investigate the natural and hydrodynamic characteristics of this tidal flow, including, among others, the velocity field, the salinity, the water temperature, the density stratification, the water circulation, the flow rates, etc. But there is still a gap of knowledge for if and how these hydrodynamic characteristics influence the horizontal dispersion and the vertical mixing of the marine masses and, consequently, the mixing and dilution of various pollutants, as well as the water renewal in the Gulf of Evoikos. The present paper comes to fill this gap and give answers to these questions. In the present work, the effects of the tide conditions, the atmospheric forcing, and the oceanographic conditions in mixing and transport of pollutants are computationally simulated. It was investigated the renewal of waters of Euboean DOI: 10.4236/cweee.2019.84007 100 Computational Water, Energy, and Environmental Engineering E. Tsirogiannis et al. Gulf with the “pure” water of the open sea by calculating the age of the water in all computational cells. The renewal has been tested both on the surface layers, where the effect of tide and wind is stronger, as well as on the deeper layers and also on the bottom. The mixing, transport, and dilution of pollutants in the dis- tant region within the complex hydrodynamic field, generated by the aforementioned charges, were simulated. These pollutants are included in the treated municipal waste of five installed diffusers operating in the area. The effect of density stratification on limiting vertical mixing and entrapment of pollutants in the deeper layers was investigated. The combined impact of municipal waste effluents on selected areas of high economic, tourist and environmental value in the Euboean Gulf was assessed. 2. Materials and Methods The study area (Figure 1) consists of the Northern Euboean Gulf with a total surface area of 1060 Km2 and the Southern Euboean Gulf with an area of 900 Km2. Communication between these two atria is through Evripus Channel which substantially is a narrow and shallow 40 m × 40 m × 10 m canal, with the main characteristic of being strong tides [20]. It is typically reported that during the semi-period (T/2 = 6 hours) the maximum flow velocity in the Strait of Eu- ripus is about 2.5 m/s [26] [27]. The water circulation and pollutant mixing and transport in the North and South Euboean Gulf have been simulated using the ELCOM three-dimensional, finite difference hydrodynamic model, which has been developed for the simulation of lakes and enclosed seas. The model solves the system of the unsteady Reynolds-averaged Navier-Stokes (RANS) equations for incompressible flow and the scalar transport equations. It uses the Boussinesq approximation omit- ting the non-hydrostatic pressure terms. The change of the level of the free water surface is estimated by integrating vertically the continuity equation applied to the Reynolds-averaged kinematic boundary condition [28] [29]. The RANS equations are solved and the closing of the turbulent flow equation system in the horizontal direction is achieved using a turbulent viscosity coefficient, while, for the vertical direction, it is usinga vertical energy layer mixing model. The ELCOM model takes into account the meteorological load, such as the effect that the solar radiation, the wind, the rainfall, and other meteorological parameters have on the surface layer of the water. The model also can simulate the influence of the tide. The South Euboean Gulf and a part of the North Euboean Gulf chosen for the numerical simulation of the hydrodynamic circulation. A rectangular computational grid constructed with cells of variable dimensions, ranging between 40 m × 100 m and 400 m × 400 m. The width of the 40 m of the Evripus straits im- posed the lower limit of the cells’ dimensions. The gradual change of the computational cell dimensionsup to 400 m was necessary in order to avoid creating an excessive number of cells, which would have made the computational code impossible to execute. Because of these restrictions, the numerical simulation DOI: 10.4236/cweee.2019.84007 101 Computational Water, Energy, and Environmental Engineering E. Tsirogiannis et al. Figure 1. Map of the northern and the southern Gulf of Evoikos. included the entire southern Euboean Gulf and (1/3) of the northern Euboean Gulf. Otherwise, if the computational domain extended to include the whole of the northern Euboean Gulf, the number of computational cells increased dra- matically and the computational code was not possible to execute. The southern Euboean Gulf is of higher economic, tourist and environmental value compared to the northern Euboean Gulf.

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