Assessing the element of surprise of record-breaking flood events

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.