18.104.22.168 Extreme river floods
The most comprehensive available global study examined worldwide information on annual extreme daily flows from 195 rivers, principally in North America and Europe, and did not find any consistent trends, with the number of rivers showing statistically significant increases in annual extreme flows being approximately balanced by the number showing a decrease (Kundzewicz, 2004) (see Section 22.214.171.124). However, in terms of the most extreme flows, when data were pooled across all the rivers surveyed in Europe, a rising trend was found in the decade of the maximum observed daily flow, with four times as many rivers showing the decade of highest flow in the 1990s rather than in the 1960s.
Again, with a focus only on the most extreme flows, a pooled study examined great floods with return periods estimated as greater than 100 years on very large rivers (with catchments greater than 200,000 km2) in Asia, North America, Latin America, Europe and Africa (Milly et al., 2002). From the pooled record of all the rivers, the observed trend in the population of 100-year flood events, at a 95% confidence interval averaged across all basins, has been positive since the Mississippi floods in 1993 and can be detected intermittently since 1972. Analysis of available long-term river flow records shows that since 1989 more than half of Scotland’s largest rivers (notably those draining from the west) have recorded their highest flows (Werrity et al., 2002). Of sixteen rivers surveyed, with a median record of thirty-nine years, eight had their maximum flow during the period 1989 to 1997, a period of high NAO index (based on the pressure difference between Iceland and the Azores) values consistent with storm tracks bringing high levels of precipitation to the northern UK.