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The occurrence of record‐breaking flood events continues to cause damage and disruption despite significant investments in flood defences, suggesting that these events are in some sense surprising. This study develops a new statistical test to help assess if a flood event can be considered surprising or not. The test statistic is derived from annual maximum series (AMS) of extreme events, and Monte Carlo simulations were used to derive critical values for a range of significance levels based on a Generalised Logistic distribution. The method is tested on a national data set of AMS of peak flow from the United Kingdom, and is found to correctly identify recent large events that have been identified elsewhere as causing a significant change in UK flood management policy. No temporal trend in the frequency or magnitude of surprising events was identified, and no link could be established between the occurrences of surprising events and large‐scale drivers. Finally, the implications of the findings for future research examining the most extreme flood events are discussed. [ABSTRACT FROM AUTHOR]

In November 2009, record-breaking rainfall resulted in severe, damaging flooding in Cumbria, in the north-west of England. This paper presents an analysis of the river flows and lake levels experienced during the event. Comparison with previous maxima shows the exceptional nature of this event, with new maximum flows being established at 17 river flow gauging stations, particularly on catchments influenced by lakes. The return periods of the flood peaks are estimated using the latest Flood Estimation Handbook statistical procedures. Results demonstrate that the event has had a considerable impact on estimates of flood frequency and associated uncertainty. Analysis of lake levels suggests that their record high levels reduced their attenuating effect, significantly affecting the timing and magnitude of downstream peaks. The peak flow estimate of 700 m3 s"1 at Workington, the lowest station on the Derwent, was examined in the context of upstream inputs and was found to be plausible. The results of this study have important implications for the future development of flood frequency estimation methods for the UK. It is recommended that further research is undertaken on the role of abnormally elevated lake levels and that flood frequency estimation procedures in lake-influenced catchments are reviewed. [ABSTRACT FROM AUTHOR]

The depth–duration–frequency (DDF) model developed as part of the Flood Estimation Handbook (FEH) enables estimates of design rainfall to be made at any location in the UK. This study presents a shortcut method for estimating seasonal design rainfall for a summer and winter season, respectively, using a simple seasonal correction factor in combination with the FEH DDF estimate of design rainfall. The seasonal correction factors were derived for the summer and winter season for a range of durations. The observed correction factors were linked to standard average annual rainfall available from the FEH CD-ROM, thereby enabling the use of this method at any location in the UK. The results obtained in this study are shown to correspond well to the summary statistics on seasonal rainfall presented in the Flood Studies Report. [ABSTRACT FROM AUTHOR]