The flooding event of Odra river: Measurements and calculations of the impact in the Odra estuary

Wolfgang Rosenthal
GKSS Forschungszentrum Geesthacht GmbH, Max-Planck-Straße, 21502 Geesthacht, Germany.
Tel.: +49-4152-87-1516, Fax: +49-4152-87-1565,
E-mail: wolfgang.rosenthal@gkss.de


The origin of the flood were extraordinary high precipitation rates in the mountains at the border between the Republic of Tchekia and Poland. The respective catchment area lies mainly in Poland with small extensions into the Republic of Tchekia and into Germany.

The flood caused casualties and severe damage along the banks of the river. In the lower Odra river large areas of cultivated land were flooded after the breaking of dikes. However in the area before and below the city of Szczecin the flood level was only moderate or even small due to rapid run-off into the Szeczecin lagoon and from there into the Baltic Sea and due to the opening of polders to increase the effective run-off cross-section of the river. Nevertheless the situation in the lagoon was exceptional because of the large water masses, that carried increased amount of dissolved and suspended material containing nutrients and other potentially harmful substances towards the Pommeranian bight. The hydrodynamic conditions and the quantitative behaviour of the water masses in the Szczecin Lagoon will be described in this presentation.

When the extend of the damages in the upper Odra river became known in July 1997 there was a danger that the flood wave will cause devastation also in the area of the German-Polish border and in the area of the Odra estuary in Szczecin and downwards in the Szczecin lagoon. The Szczecin lagoon receives the fresh water mainly from the Odra river and has three exits to the Baltic Sea . It covers an area of about 685 km2 and has an average depth of 3.8m. The Polish-German border crosses the lagoon approximately in North-South direction between the western part (Kleines Haff) and the eastern part (Zalew Wielki). Before the flood arrived in Szczecin it was possible two install two piles and a floating pontoon with sensors in the Kleines Haff and to get the permission for field measurements by ship cruises in the Zalew Wielki. The organisation of the field experiments was shared between the Maritime Research Institute in Szczecin and the GKSS research centre in Geesthacht. The measurements have been done in close co-operation with University Greifswald and Energiewerke Nord in Greifswald/Lubmin.

History of the flood event with emphasis on the estuary of the Odra river
There is a typical meteorological situation developing frequently in the area of the Odra origin and along the river catchment in June/July. A slowly moving depression brings large amounts of precipitation which is causing damage to the dikes and subsequent flooding of cultivated areas along the river. Because it occurs around St. Johannis day, the flooding is known by the name St.Johns flood in the river region. In summer 1997 such an event brought extraordinary large amounts of run-off water. In the first phase of extreme precipitation from 3. to 9. of July the total amount of precipitation was about 6 km3 in the catchment area and in the second phase between 17. and 21. July again about 4 km3. Due to large overflooded areas the two maxima of the water level merged to only one longlasting period of high water in the middle part of the river A major dambreak south of Frankfurt/Oder (Ziltendorfer Niederung) is visible in the water level measurements downward of Frankfurt/Oder but this new double peak of the water has not been caused by the original two precipitation time periods.

The differences of the water level in the Szczecin lagoon were only around 40 cm during the extreme outflow event. The water started falling on the 21 of July due to the drop of the water level in the Baltic . The average salinity of the lagoon did not change in the central part before the 24. July where a station near to the centre of the lagoon shows a strong drop in conductivity. The arrival of low salinity water in the Swina straits indicates that after 5 days the water of the Zalew Szczecinski, (that is the eastern part of the lagoon) is washed out to the Pommeranian bay. In the Kleines Haff (the western part of the lagoon) the low salinity front needed about 11 days to cross the lagoon from west to east and to exit into the Baltic. So the main transports went on in the eastern part of the lagoon and through the Swina straits.

A not yet completely answered question is the quantitative water transport in the different places of measurement along the lower part of Odra river and through the Szczecin lagoon and the straits to the Baltic Sea. We shall describe the respective status of research.

In the lagoon itself current patterns form that are important for the flushing and retention capabilities. The current patterns were estimated by measuring with automatic stations the drift of water bodies and compare that with model results that were achieved with different assumptions on border values.

Conclusion and lesson from the event
For the benefit of the people of the lower Odra region there is a necessity of combined exercises for emergency situations and the improved co-ordination of counter measures between respective governmental authorities on the German and on the Polish side of the river.