Nearshore current and pollutant transport in the island of Crete

abstract

The three dimensional hydrodynamic pollutant transport model IDOR3D was used to simulate the nearshore current structure and pollutant dispersion from various sources. These sources can be municipal or industrial discharges and if are not properly designed will eventually cause environmental problems in the coastal areas. The inflows from sewage treatment plants and creeks affect the current structure within a small distance from the source. Their influence was studied in a high resolution of 100 m model in map sections E1 to E4 (Fig. 1). For the circulation of the northern coasts of Crete were the major urban centers are located, a grid size of 2 km was used. Local meteorological stations provided the wind speed and direction. The bathymetric files and coastlines were obtained from hydrographic digital files and the Digital Bathymetric Model (DBM) produced the gridded data obtained via GIS. Additional information from the Digital Elevation Model (DEM) of Crete with the aid of GIS technology can provide information of point sources discharging at the exit of watersheds in the near shores of the island. Further watershed analysis (hydrological/hydraulic/ pollutant concentration analysis) for rain events provides the pollutographs of ephemeral creeks discharging to the coastal areas that are used as input to IDOR3D. By employing different receiving water conditions (isothermal and stratified) and pollutant source conditions, simulations were performed in the areas E3 to E4 of Fig. 1. The bay in map section E4 was used to simulate the fate of a pollutant discharged over a four-hour period from an ephemeral creek under sea isothermal conditions and a 5m/s NW wind. The environmental impact on the nearshore area and mitigation measures are discussed. The bay in the map region E3 was used to examine the behavior of a contaminant discharged continuously into the bay from a submerged outfall source at two different locations (a) at 10m deep, positioned 250m offshore and (b) at 27 m depth positioned 2 km offshore, with the purpose to analyze the movement of the contaminant with respect to the island and the relative impact to nearshore areas. Concentration time series for a continuous conservative pollutant released with a concentration of 1000ppm into Chania bay under sea isothermal conditions and a 5m/s NW wind for the case (b), are shown in Fig. 2. In the case (a) the contaminant will dispersed between the island and the mainland shallow passage which will potentially cause environmental problems in a highly touristic area. The dilution is less under stratified conditions due to the buoyant forces of the pollutant generated by temperature and density differences, which reduces the amount of mixing due to the contaminant moving quickly to the surface. Complete results from all the cases will be presented in the conference.