| Abstract: | The Odra river flood of July through August 1997 transported a large additional volume of water into the Szczecin Lagoon area for a period of about one month. The dispersion of this water in the Szczecin Lagoon and Pomeranian Bight was simulated using the operational hydrodynamic model of the North Sea and Baltic Sea operated by the Federal Maritime and Hydrographic Agency of Germany (BSH). The model system receives as input meteorological forecast fields from the EUROPA model of the German Weather Service. As a result of the model simulation, the temporal development of the river plume can be described as follows: First the eastern part of Szczecin Lagoon, the Zalew Wielki, filled with flood water displacing θnormalρ Odra river water from that area. After about a week, Odra river flood water started to flow into the Pomeranian Bight. Its dispersion within Szczecin Lagoon was by no means uniform. The Kleines Haff, the western part of the Lagoon, was not much affected at first. When large labelled water masses had already left the Zalew Wielki area through the Swina river, at most only about half the water volume in Kleines Haff had been replaced by Odra flood water. In the Pomeranian Bight, the concentration was higher at the coast of Usedom – at least initially – than at the coast of Wolin. After 30 August 1997, northwesterly winds caused undiluted Baltic water to flow from the northern to the southern part of the Pomeranian Bight, pushing the water body marked, or distinguished, by Odra water eastward along the coast of Wolin. At the same time, outflow began from Kleines Haff through the Peenestrom into the Greifswalder Bodden. Due to light winds, and hence limited vertical mixing in summer, the proportion of freshwater in Baltic surface water reached about 50% in the southern Pomeranian Bight. Near Rügen, it fell below 10%. Within 2 months of stronger wind caused major shifts of the water bodies concerned. The scale considerations and model simulations discussed in this paper allowed qualitative estimates to be made in the course of the flood event, which were later confirmed by measurements, presented at a HELCOM (Helsinki Commission, Baltic Marine Environment Protection Commission) Scientific Workshop in January 1998. |
