Currents and transport of sewage water in the coastal area of Vaasa, Northern Baltic Sea - a study with current measurements and modelling

The sewage water of the city of Vaasa, located at the Gulf of Bothnia, pollutes the surrounding coastal areas and beaches near the city. The sewage outlet is planned to be moved to some other place in order to decrease the harmful effects in the most valuable areas close to the city. The currents and transport processes in the area were examined in order to find the most suitable outlet location, which would minimize the effects of wastewater. An extensive set of current measurements gives an idea of typical current patterns in different conditions. The currents were calculated with a 3-dimensional nested hydrodynamic model, which was validated with the current measurements. The calibrated transport model was used to describe the effects of sewage water, when the outlet is located in four different places. The main sewage water impact area varies to a great extent depending on the outlet location. Also the outlet depth affects strongly the wastewater mixing. Comparison between the present situation and one of the main alternative outlet locations are discussed in this paper. Model simulations and analyzed current measurements describe the comparative effects of different sewage water outlet locations. The results are of great practical value when the location alternatives are examined. Background Municipal wastewaters are typically discharged in the vicinity of or just at the coastline, thus affecting strongly the quality of the coastal waters. One solution to improve the situation, in addition to reduce the wastewater load, is to remove the outlet location to an offshore position. The improved mixing conditions may lead to clear reduction of the negative effects in the coastal area. This paper deals with a study on the impacts of the municipal wastewaters discharged from the town of Vaasa to the Gulf of Bothnia in the Baltic Sea. In the study, the impact of discharging the wastewaters in various, alternative locations was investigated. The alternative outlet locations were selected within a distance of a few kilometers from the coastline, still inside the Vaasa archipelago. The archipelago is composed of a great number of islands with complicated configuration. In this area the water depth is typically less than 10 metres. Methods Reliable information of the currents and mixing processes are of major importance when assessing the impacts and cost-benefit relation of relocating the wastewater outlet. This is why special attention was paid to measuring the flow pattern in the Vaasa archipelago. Nine measurement locations were selected (figure 1). Measurements were made close to the surface at each site and close to the bottom at the deepest sites. The deepest sites were located on the side of the shipping channel (sites 2, 4 and 8), considered also as potential area of the new outlet. Aanderaa recording devices were used for wind and current measurements with a sampling interval of 10 minutes. The measurement period lasted about two months. With the absence of tide, wind is the predominant current inducing factor in the Finnish coastal waters.