This paper has expressed the probability of a flood event using the Average Recurrence Interval (ARI) rather than the Average Exceedance Probability (AEP). However it should be noted that:
“Use of the terms "recurrence interval" and "return period" has been criticised as leading to confusion in the minds of some decision makers and members of public. Although the terms are simple superficially, they are sometimes misinterpreted as implying that the associated magnitude is only exceeded at regular intervals, and that they are referring to the elapsed time to the next exceedance." (IEA, 1987).
Conversion of ARI to AEP can be conducted using the table below.
AEP (%) AEP ARI
99.75 1.002 0.17 98.17 1.02 0.25 95.02 1.05 0.33 86.47 1.16 0.50 63.21 1.58 1.00 50.00 2 1.44 39.35 2.54 2.00 20.00 5 4.48 18.13 5.52 5.00 10.00 10 9.49 5.00 20 20 2.00 50 50 1.00 100 100
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