Changes in flood frequencies in Switzerland since 1500

摘要

In northern Switzerland, an accumulation of large flood events has occurredsince the 1970s, preceded by a prolonged period with few floods(Schmocker-Fackel and Naef, 2010). How have Swiss flood frequencies changedover the past 500 years? And how does the recent increase in floodfrequencies compare with other periods in this half millennium? We collectedhistorical flood data for 14 Swiss catchments dating back to 1500 AC. Allcatchments experienced marked fluctuations in flood frequencies, and we wereable to identify four periods of frequent flooding in northern Switzerland,lasting between 30 and 100 years (1560–1590, 1740–1790, 1820–1940and since 1970). The current period of increased flood frequencies has notyet exceeded those observed in the past. We tested whether the floodfrequency fluctuation could be explained with generalised climatic indiceslike solar activity or the NAO. The first three periods of low floodfrequency in Switzerland correspond to periods of low solar activity.However, after 1810 no relationship between solar activity and floodfrequency was found, nor could a relationship be established betweenreconstructed NAO indices or reconstructed Swiss temperatures. We foundre-occurring spatial patterns of flood frequencies on a European scale, withthe Swiss periods of frequent flooding often in phase with those in theCzech Republic, Italy and Spain and less often with those in Germany. Thepattern of flooding in northern Switzerland and the Czech Republic seem tobe rather similar, although the individual flood events do not match. Thiscomparison of flooding patterns in different European countries suggeststhat changes in large scale atmospheric circulation are responsible for theflood frequency fluctuations.