maanantai 9. syyskuuta 2013

Eliisa Lotsari: Flooding in Finland

During last few years, it has been reported great floods in Finland and elsewhere in the world. For example in 2012 autumn, heavy and prolonged rains caused flooding in Finland, and in 2013 there were widespread floods in Europe due to the rainy spring. In addition to the prolonged rain periods, flooding can be caused in Finland also by spring snow melt, frazil ice and ice dams, but also they may relate to exceptionally high sea water levels. Presently, Finland can be divided broadly into three main watershed areas. The coastal watersheds are rather small, and they experience rain and snow melt induced flooding. The wide lake area watersheds experience more long lasting floods and Northern rivers experience mainly spring floods due to the snow melt.

According to the Intergovernmental Panel on Climate Change (IPCC) the climate change may also effect on the hydrological cycle, and thus also on flooding. The climatic change has been forecast to be greater in Northern latitudes than the global average. Especially the extreme events have been forecast to intensify.

Lately, there has been increasing amounts of studies related to the climate change and its effects on flooding on global, regional and national scale. In my PhD entitled “Fluvial processes and their future magnitudes: combined field observation and simulation approaches”, it was shown that due to the climatic warming, the discharges may change greatly in Finland. However, the many hydrological scenarios had great temporal and spatial differences between river reaches. Particularly great seasonal changes are to be expected, since discharges may increase in autumns and winters. Also the importance of the spring flooding as the channel modifying event is expected to diminish from the present.

The great discharge events, for example the once having a 100 year recurrence interval, are forecast to diminish on average by the end of the 21st century. Particularly diminishing flood events are expected in Northern Finland. On the other hand, the great discharges are expected to increase in the lake area and their outlet rivers. One of these areas is Kokemäenjoki River at Pori city area, which is already presently one of the greatest flood risk areas in Finland. However, on average, the spring discharge peaks are forecast to diminish. Important is that the autumn and winter discharges may increase in Northern Finland, in the coastal area, and in the lake area. In addition, the spring snow-melt discharge peak is forecast to occur two-three weeks earlier in the end of the 21st century than presently. Even though the flood inundation extent can be forecast to have same direction in the change as the discharges, the change in flooding extent is not linear to the discharge changes due to the local, particularly topographical, effects. In addition, the sea level and its changes may locally have greater effect on flood inundation than the changes in discharges.

According to the EU’s Floods Directive (2007/60/EC), also the climate change impacts are to be taken into account in flood risk assessments. The 21 greatest flood risk areas have been defined for Finland already in the end of 2011 by The Ministry of Agriculture and Forestry. The city of Pori has been defined as the greatest flood hazard and risk site, since there is a great amount of population living in the flood risk area. The flood hazard and risk maps will be created from these areas by the end of 2013 and the flood risk management plans are to be made by the end of 2015. However, despite the already great amount of studies related to flooding, further research on these issues is still needed on global, regional and local scale along with continuously enhancing future hydrological and flood simulation approaches.



Lotsari, E. 2012. Fluvial processes and their future magnitudes: combined field observation and simulation approaches. Doctoral thesis (article-based). Department of Geography and Geology; Geography Division. Annales Universitatis Turkuensis A II 270.

Ministry of the Environment.

Veijalainen, N., Lotsari, E., Alho, P., Vehviläinen, B. and Käyhkö, J. 2010: National scale assessment of climate change impacts on flooding in Finland. Journal of Hydrology 391: 333–350.