Annual exceedance probability (AEP)
This is the probability of an event occurring in a year. It has been adopted by the Australian Bureau of Meteorology (BOM) and Australian Rainfall and Runoff (2019) as the preferred terminology for rainfall and flood magnitudes, superseding average recurrence intervals (ARIs).
A common standard for street drainage design will be an AEP of 20% or 10% as a minor storm, and 1% AEP as a major storm. The general threshold for flooding of properties is 1% AEP.
The BOM provides the following table showing the different terminologies:
Because it is difficult to apply AEPs for frequent events, the terminology of Exceedances per Year (EY) has been introduced. Note that 50% and 20% AEPs do not correspond directly to 2 and 5 year ARIs. Explanations are provided in the BOM site, at www.bom.gov.au/water/designRainfalls/ifd/ifd-faq.shtml#sec1q6.