More frequent flooding? Changes in flood frequency in Switzerland since 1850

SummarySevere flood events have occurred in many Swiss catchments in the last decade. Have flood frequencies changed over the last 150 years in Switzerland? And is the high frequency observed recently a nationwide phenomenon? To answer these questions, we analysed streamflow data from 83 stations with a record length of up to 105 years, complemented with data from historical floods dating back to 1850. Multiple trend analysis of the annual flood series showed only few negative trends. The number of stations with positive trends was especially high, when the period of 2001–2007 was included into the analysis. The temporal and spatial distribution of flood events with return periods larger than 10 years, and the large scale flood events of the last 150 years were analysed as well. Periods rich in floods alternated with periods poor in floods, showing large regional differences especially between northern and southern Switzerland. The second half of the 19th century was rich in floods, both in northern as well as in southern Switzerland. In southern Switzerland and the northern Grisons, flood frequency was high again between 1940 and 1960, a period poor in floods in northern Switzerland. Here flood frequencies increased again only after 1968. The recent increase in flood frequency and discharge has been most pronounced along the central and western northern flank of the Alps. Our data suggest that, since 1900, periods with many floods in northern Switzerland have corresponded to periods with few floods in southern Switzerland and vice versa. The differences also suggest that changes in large scale atmospheric circulation might be responsible for the fluctuations in flood frequency. The fluctuations in flood frequency should be considered in defining design floods for flood protection measures.