Keywords: three-dimensional modelling, far-field, Glafkos stream, hydrodynamic circulation, MIKE-3-FM, Patras Gulf, sewage plume.

The spread of the sewage plume, discharged into the Gulf of Patras by the treatment plant of the city of Patras, has been studied using three-dimensional numerical simulations. The worst-case dilution of the treated sewage is examined in the near shore region up to the swimming zone for the winter-time regime, during which, according to field measurements, the receiving waters are homogeneous. The hydrodynamic circulation in the Gulf of Patras is simulated using the hydrodynamic module (HD) of the MIKE 3 FM code. The advection-diffusion of the sewage effluent is simulated using the transport module (TR), which uses the simulated flow field as the input. Both the cases of a conservative and non-conservative pollutant species are examined [1]. During the winter season, the most adverse condition is that of no wind and, as expected, it is under these conditions that the highest concentrations of conservative pollutants are observed near the coast. For all the cases examined, however, the simulations show that the effected dilution is greater than 200 even for conservative pollutants. It is therefore confirmed that the advection-diffusion field arising from the hydrodynamic circulation in the Gulf of Patras ensures adequate dilution of the sewage plume near the coastal zone, where the bathing areas are located.

Further, the interaction of the sewage plume with the Glafkos stream, discharging at the south-eastern region of the Gulf (i.e. to the east of the location where the treated sewage is discharged), is studied. Specifically, the most adverse condition of the wet season (which occurs during the winter regime), i.e. that of no wind is re-examined taking into account the stream discharge for a flood event. Simulations confirm that the stream flow discharging into the Gulf is deflected to the right by Coriolis forces, and its influence can be traced in a zone near the coastal region, where bathing areas are located. It follows that the effect of stream's plume is restricted only to the right of the axis of the stream discharge, affecting only the south-eastern beaches of the Gulf, leaving unaffected the south-western coasts. At this time lack of measurements make it impossible to assess whether the Glafkos' waters advect any pollution into the Gulf. Since, the stream's discharge is deflected towards the east, it can be concluded that the only cause of pollution in the west of the sewage plume arises from the sewage plume itself.

1

M.E.W.S.P., "The Gulf of Patras: technical details", Technical Report, 1-6, 2009. (in Greek) http://www.deyap.gr/environment/clean_sea.html

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