23/07/2026
New Flood Plan: What You Need to Know about the 0.5 AEP
There is a lot of discussion recently in town about flood risk, with terms such as "0.5% AEP" and "1 in 200-year flood" appearing in the Ensign, the news, council reports, LIMs and engineering assessments. So what do they actually mean?
A 0.5% AEP (Annual Exceedance Probability) simply means there is a 0.5% chance of a flood of that size, or larger, occurring in any given year.
It does not mean the flood will only happen once every 200 years. The probability resets each year, so two such floods could occur only a few years apart, or even in consecutive years.
Those who are a little older may recall that the now-abandoned township of Kelso experienced two major floods in just 15 months, in 1978 and 1980. Both were regarded as "100-year floods" at the time, and the second ultimately led to the decision to relocate the township. It is a good reminder that a "1 in 100-year flood" describes the annual probability of an event occurring, not that it can only happen once every 100 years.
Understanding this terminology is important when buying property, subdividing land, or considering future development, as it is one of the tools councils, engineers and insurers use to assess flood risk.
