Annual Exceedance Probability (AEP) refers to the probability of a flood event occurring in any year. The probability is expressed as a percentage. For example, a large flood which may be calculated to have a 1% chance to occur in any one year, is described as 1%AEP.
The Queensland State Planning Policy, defines AEP as, "the likelihood of occurrence of a flood of a given size or larger in any one year; usually as a percentage. For example, if a peak flood discharge of 500 cubic metres per second has an AEP of 5%, it means that there is a 55 risk (i.e. a probability of 0.05 or a likelihood of 1 in 20) of a peak discharge of 500 cubic metres per second or larger occurring in any one year. The AEP of a flood event gives no indication of when a flood of that size will occur next."