Μελέτη της υδροδυναμικής μεταφοράς και διάχυσης ρυπαντικών φορτιών στον Ευβοϊκό υπό παλιρροιακές συνθήκες και παράκτιες περιβαλλοντικές επιπτώσεις
Authors: Τσιρογιάννης Ευάγγελος 
Published: September 2021

In this doctoral thesis, the hydrodynamic characteristics of the Euboean water column, as they are influenced by the prevailing meteorological and tidal conditions in the area, and the influence of these conditions on the mechanisms of Euboean water circulation were investigated. The research was carried out using a numerical model based on fluid computational methods. (Computational Fluid Dynamics). These methods are widely considered in the science of fluid engineering, solving equations that describe the flow of fluid. Hydrodynamic circulation in the marine environment, characterized by the effect of intense tide, atmospheric charge and bathymetry peculiarity, is a complex phenomenon. The physical and hydrodynamic characteristics of this flow are absolutely crucial for the vertical mixing of the marine masses and thus the mixing of their physicochemical parameters, such as nutrients and oxygen. The diffusion and hydrodynamic transport of passive pollutants, the renewal of water and the overall environmental status of the water body are also affected. Tidal currents in the Euboean Gulf are probably one of the oldest historically studied tidal phenomena. Eratosthenes, Pytheas, Poseidon, but also Stravon and Seneca were some of the ancients who studied this phenomenon. The Euboean Gulf has been the subject of a remarkable number of investigations since the early 20th century. Aeginitis made a thorough review of the literature and measurements regarding tide in the Euboean. Sterneck simulated the propagation of tidal waves along the axis of the northern and southern Euboean Gulf, assuming that the Strait of Euripos is closed and was able to describe the main features of the tide. The first hydrodynamic model for the study area was developed by Livieratos, while Tsiblis applied two models, one low resolution for the propagation of tidal waves in the north and south Euboean and one high resolution focused on the Strait of Euripos.Despite the fact that the tidal phenomenon in the Euboean has been studied since ancient time, even today there are no systematic measurements and not enough relevant research. Tidal stations as well as satellite elevation data have been used to systematically monitor sea level change on the coasts of southern Europe. There are a total of 17 tidal stations located on the shores of the Mediterranean Sea and the Atlantic Ocean, but not in the Euboean Gulf. In the wider region the tide dynamics in a system of interconnected seas of the Mediterranean, the Marmara Sea and the Black Sea have been investigated by Ferrarin et. al., 2018, and the numerical results described the main tidal characteristics of this marine system, but without specific information on the Euboean Gulf. The contribution of tidal currents has been also examined by Poulos et. al., 2001 on the resuspension, erosion and deposition of suspended sediments in the marine area of Avlida, located within the research area of the present work. However, there is a significant lack of knowledge in the literature on the dynamic characteristics of the intense tide present in the Euboean Gulf, and its impact on the interconnected water system of the northern and southern Euboean Gulf. There is a lack of knowledge about the hydraulics of water exchange due to tidal and other loads between the two bays connected through a narrow and shallow channel, as well as the mode of transport and dispersion of pollutants. In the present PhD thesis the numerical hydrodynamic and environmental simulation of the Euboean is performed using the three-dimensional hydrodynamic model ELCOM and its associated environmental model CAEDYM. It includes the study of the hydrodynamic and biochemical characteristics of the Euboean water column. The hydrodynamic transport and diffusion of municipal waste was investigated, with both inert and biodegradable contaminant loads containing and disposing in the marine area under investigation, which were also simulated.Briefly, the effects of tides, atmospheric loads and oceanographic conditions on mixing and transport of pollutants were simulated. It was investigated the renewal of the waters of the Euboean Gulf with the "clean" water of the open sea by calculating the age of the water in all cells. The renewal was 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 effect of density stratification on limiting vertical mixing and entrapment of pollutants in the deeper layers was examined. Furthermore, the combined impact of the municipal waste effluent on selected areas of high economic, tourist and environmental value in the Euboean Gulf was assessed. Initially, the hydrodynamic circulation in the Euboean Gulf region was simulated computationally by applying the three-dimensional hydrodynamic model ELCOM. This model includes the effect of intense tide, which characterizes the study area, the Coriolis force due to the rotation of the earth, and the effect of climatic factors such as: total solar radiation, atmospheric pressure, relative humidity, precipitation altitude, and wind speed and direction. To the best of our knowledge, there is no comparable research for the area under consideration. The purpose of this thesis is to investigate the effect of tidal flow on the physical and hydrodynamic characteristics of the examined marine area such as salinity, water temperature, density stratification, water circulation, flow velocities etc. These characteristics are considered absolutely crucial for the vertical mixing of the marine masses and therefore for the mixing of their physico-chemical parameters, such as nutrients and oxygen and also for the renewal of the waters of the Euboean Gulf. The ultimate goal of this thesis is to investigate the diffusion and hydrodynamic transport of passive and real pollutants in this area, under the influence of the above parameters and conditions. The ELCOM model has been used successfully in many relevant surveys around the world, such as the northern Aegean Sea, the northern Adriatic Sea near the research area of this dissertation, and the Persian Gulf, where the tide was even simulated. The study area 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. These two bays are connected through the Euripos River, a narrow and shallow 40 m x 40 m x 10 m canal. The main feature of this marine area, as already mentioned, is the presence of intense tides. ELCOM, used for numerical simulation of the region, uses hydrodynamic and thermodynamic models to simulate the time behavior of stratified flows, under the influence of environmental actions. The numerical method applied by ELCOM solves the non-constant Navier-Stokes equations for uncompressed flow, ignoring the non-hydrostatic pressure conditions. For the horizontal turbulence, the Reynolds-averaged Navier-Stokes equations are solved, and the turbulence closure is performed using a turbulence coefficient, while for the vertical turbulence a vertical mixing layer model is applied. The transfer of passive and active scalar sizes (eg tracers, salinity and temperature) is also carried out using the ULTIMATE QUICKEST semi-selective conservation method.The area of the Euboean Gulf, selected for the numerical simulation of the present study, is bounded on the west by Central Greece-Attica, east of Euboea, and the northern and southern Aegean, which constitute its "open" border conditions. simulation space. A variable dimensional rectangular computational grid was formed, constructed of rectangular computation cells measuring 40 m x 100 m up to 400 m x 400 m. The gradual change of the size of the computational cells was necessary, due to the restrictive width of 40 meters on the Euripos on the one hand, and the avoidance of excessive number of cells on the other, in the case of the use of fixed cells of 40 mx 40 m, which made it impossible to perform the computation code. A total of 20 layers were defined in the vertical direction of the computing space. The first 8 layers of the surface were of variable thickness from 2.5 m to 5.0 m with a gradual increase, while the remaining 13 deeper layers to the bottom had a constant thickness of 5.0 m. This vertical discretization, by selecting a vertical grid as dense as possible, was made to: (a) To simulate as accurately as possible the hydrodynamic circulation of the 10 m depth strait of Euripos, which was absolutely crucial for the communication between the northern and southern Euboean Gulf. (b) Better capture the results in deep coastal areas of particular interest; and c) To achieve the greatest possible accuracy in the vertical distribution of velocities (in particular of tides), density stratification and tracers concentrations in depth. The maximum simulation depth reached -86.0 m in the southern part of southern Euboean and the computational network consisted of a total of 225,351 active water cells. At the bottom, a turbulent benthic boundary condition was used, by applying a constant coefficient of friction. The open boundary conditions were applied to the northern and southern boundaries of the computational space, which allowed passive water flow or outflow from each cell according to the flow requirements. Based on observations and measurements reported in the literature, boundary conditions of variable sea level were imposed at the northern and southern boundaries of the computational space in order to simulate the tide.The literature search did not provide systematic measurements of temperature and salinity for the northern and southern borders of the Euboean Gulf with the open Aegean Sea, so that they can be used as boundary conditions in the present study. All that is available is temperature and salinity forecasts at these locations from the records of the valid COPERNICUS marine environment monitoring service, and these forecasts provide the initial and limiting conditions for these parameters. The time step between the recycles was 1.0 min and the simulation was performed for one year (2016), which was considered as a typical year. Meteorological data were introduced every 10 minutes, namely wind speed and direction, atmospheric pressure, air temperature, relative humidity, rainfall amount and total solar radiation. These data were measured by meteorological stations of the National Meteorological Service, located in the cities of Lamia and Aliarto, adjacent to the study area. The movement of water from the open marginal conditions of the northern and southern Aegean was simulated using two conservative tracers. These two indices, with a dimensional concentration at the open boundaries equal to the unit, are intended to monitor the source of water in each cell of the computational space and their mixing with Euboean water during simulation development. The simulation was completed using the CAEDYM environmental model, which works with the three-dimensional hydrodynamic ELCOM model to calculate the transfer and mixing of environmental characteristics. CAEDYM calculates all the abiotic and biotic elements of an aquatic ecosystem, such as oxygen, nutrients, elements of carbon (C), nitrogen (N) and phosphorus (P). It also calculates multiple functional phytoplankton groups, such as zooplankton and marine organisms, benthic communities, and more. It solves a system of differential equations, which include a set of processes of carbon (C), nitrogen (N), phosphorus (P) and dissolved oxygen (DO) cycles, several orders of magnitude of suspended solids, as well as phytoplankton dynamics. The collaboration between ELCOM and CAEDYM models is an integrated arithmetic system, consisting of a set of arithmetic models (hydrodynamic, thermodynamic, biogeochemical) capable of accurately rendering the function and behavior of an aquatic body, which also has an ecosystem, such as is that of the Euboean. This combination of the above numerical models is considered to be the strongest point of their superiority over the other models in this category. Also, the ability to simulate urban waste water available to the Euboean by underwater conduits using numerical traces makes the ELCOM-CAEDYM system an important and most suitable tool for monitoring hydrodynamic transport and diffusion. the waste and the pollutants contained therein. This monitoring takes into account the prevailing meteorological loads as well as the effect of the high tide, which results in the appearance of a high velocity field at the Strait of Euripos, with maximum values approaching 2.5 m / s. This kind of simulation in the study area has not been done in the literature to date. In particular, in this doctoral thesis the following were done:Simulation of the computational space for a typical year with the ELCOM hydrodynamic model and for a shorter time with the CAEDYM environment with their reliability test. Calibration of the ELCOM hydrodynamic model based on salinity and temperature forecasts and based on field-measured water velocities. In the environmental model CAEDYM the calibration was based on predictions of dissolved oxygen, chlorophyll- (a) and nutrient concentrations in the Euboean water column. The results were analyzed and evaluated and the way in which tidal but atmospheric loading and sinking affect hydrodynamic circulation as well as water mass mixing was investigated in detail. The results of this simulation for mean salinity and mean temperature were compared with COPERNICUS projections for all months of the annual simulation, at specified locations and for various depths, and were considered satisfactory. The validity of the results was further confirmed by the predicted maximum flow velocity in the Strait of Euripos, which was equal to the measured value in the natural field, and the depth of the tide effect was also investigated. The analysis revealed velocity fields both at the surface and at different depths at each time point. It was found that the distribution of velocity in depth at the Strait of Euripos is approximately constant. It was found that the distribution of water density with depth is mainly influenced by temperature change and less by salinity change. At various times (especially during warmer months) the formation of depth-stratification was controlled, potentially preventing the vertical mixing of the marine masses and consequently the mixing of their physicochemical parameters, such as nutrients and oxygen.It was investigated, the renewal of the waters of the Euboean Gulf with "pure" open sea water by calculating the age of the water in all cells. Waters were re-examined both on the surface layers, with the effect of tide and wind, as well as on the deeper layers, but also on the bottom. The temporal evolution of water renewal in the surface, intermediate and bottom layers was also investigated. The mixing, transport and dilution of pollutants in the distant area were also simulated within the complex hydrodynamic field created by the aforementioned charges. These pollutants are contained in treated municipal waste from six simulated offshore pipelines operating in the wider area that have been simulated. The effect of density stratification on limiting vertical mixing and entrapment of pollutants in the deeper layers was also investigated. At the same time, the combined impact of the effluent from the above- mentioned pipelines, in selected areas of high economic, tourist and environmental value, on the Euboean Gulf was assessed. 6) The study and investigation of tidal charge, as well as other atmospheric parameters of the Euboean Gulf, the evolution of its hydrodynamic and biochemical characteristics, as well as the dynamics of transport and diffusion of urban-industrial wastewater available in it this PhD thesis, contributes research and scientific to: It provides a detailed presentation and assessment of the hydrodynamic and biochemical characteristics of the Euboean water column in specific locations and in coastal areas. 7) For the first time, the use of appropriate computational indicators is used to assess the impact of municipal waste that ends up in the Euboean coastal areas under the prevailing tidal and meteorological conditions. 8) Areas identified, which due to slower than other renewables, tend to be more "vulnerable" to pollution, than urban waste water, which is available in the wider Euboean area, but also from pollutants of different origins , which are not considered in this PhD thesis. 9) Emphasizes the need to correlate the tidal effect on the variation of the dilution and diffusion of urban pollutants available in the wider Euboean region with the dimensioning of the wastewater treatment plants from which they originate, with a view to optimization the economics of construction, design parameters, future new installations or extensions of existing ones. 10) There is also a need for systematic measurements of the change in the level of the free surface of the sea, on the northern and southern borders of the Euboean with the open sea of the Aegean. The need for up-to-date and systematic measurements of physicochemical parameters and other pollutants in the area in order to exploit these data in future simulations with similar computational models is also highlighted.Finally, from the overall results of this PhD thesis, it is evident that the numerical research methodology proposed, validated and evaluated for the case of the Euboean can be the basis but also an integrated and reliable research tool for other similar hydrodynamic systems, which will be subject to different effects, with application to other areas.