ANNUAL EXCEEDANCE PROBABILITY

Annual Exceedance Probability is the
probability, expressed as a percentage, that a flood of a given magnitude will be equalled or exceeded in any one year. The term has replaced terms such as ‘1-in-100-year-flood’ which are now seen as outdated and unhelpful because of the perception that the phrase reflects chronological forecasting rather than statistical probability. Semenov and Bengtsson (2002) predict changes to rainfall distributions in the Auckland region of New Zealand due to climate change. In a worst case scenario for Auckland, a flood that is rated with an Annual Exceedance Probability of 0.02 (a 1-in-50-year-flood) under the current climate will have an Annual Exceedance Probability of less than 0.10 (1-in-10-year flood) by 2100.
Mark