“While they’re closely related, they are not the same thing and misunderstanding the difference can lead to confusion about flood risk, approvals, and design requirements.”
If you’ve ever read a flood report or planning document, you’ve probably seen the terms ARI and AEP used, sometimes interchangeably. While they’re closely related, they are not the same thing and misunderstanding the difference can lead to confusion about flood risk, approvals, and design requirements.
ARI stands for Average Recurrence Interval. This describes the long-term average time between events of a certain size. For example, a 100-year ARI flood means that, on average, a flood of that magnitude occurs once every 100 years. Importantly, this does not mean it happens regularly every 100 years, nor does it mean that once it occurs you’re “safe” for another century. It’s a statistical average based on long-term rainfall or flood records.
AEP stands for Annual Exceedance Probability. This approaches the same concept from a probability perspective. It describes the chance of an event occurring in any given year. A 1% AEP flood has a 1% chance of being exceeded in any year. That chance resets every year, regardless of what happened the year before.
This is where confusion often arises. A 100-year ARI event and a 1% AEP event are often treated as equivalent, but they’re simply two different ways of expressing the same likelihood. The AEP terminology tends to be more intuitive because it focuses on probability rather than time. Saying “there’s a 1% chance this flood will occur this year” is clearer than saying “this is a 100-year flood.”
From a planning and engineering perspective, AEP is now the preferred terminology in most Australian guidelines and council policies. It provides a clearer understanding of risk and avoids the false sense of security that ARI language can create. That said, ARI is still widely used in practice, particularly in rainfall data and older flood studies, so engineers are fluent in both.
So which one matters more? In practical terms, neither is “better,” but AEP communicates risk more clearly, especially to non-technical audiences. When councils refer to a 1% AEP flood level, they’re defining a design benchmark that balances safety, risk, and practicality – not predicting when the next flood will occur.
Understanding the difference between ARI and AEP helps property owners, developers, and decision-makers make more informed choices. Flooding isn’t about certainty or timelines—it’s about probability and risk. And that’s exactly what these terms are designed to describe.
