Understanding flood terminology

Flood terminology expressed throughout draft Feasibility Study will include:

AEP (%) – Annual Exceedance Probability

ARI (1-in-x) – Average Recurrence Intervals

EY – Exceedances per Year

These terms describe how likely a given flood event is to occur each year.. You may heard terms such as a ‘1-in-100 year’ flood before, for example. This would be expressed in the above measures as: 1% AEP, a 100-year ARI, or an 0.01 EY.

They are also expressed in terms of rainfall, and you may hear or see the term ‘rainfall event’. This term is interchangeable with ‘flood event’.

There are two elements to these terms:

Probability

A 1-in-100-year flood describes a probability for that flood event; meaning, there is a 1% likelihood of such a flood occurring in any given year. It is the likelihood that this kind of flood event might occur. This probability was calculated using historical flood records and modelling of potential future floods in a location.

There is no guarantee that a 1-in-100-year flood will only occur once every hundred years – two such floods may well occur within months of each other. However, statistically it is more likely that such floods are significantly less frequent than, say, 1-in-1-year floods, which are statistically 100 times more likely to occur.

Intensity

The lower frequency (‘1-in-100-year’) suggests higher intensity – greater discharge and flooding depth. The more 'frequent' flooding - the kind that can occur in an area with regular rainfall events - often has less discharge and floods to lower depths above the ground surface.

So a 1-in-1 year flood event would have lower flood depths than a 1-in-2-year flood event, and significantly lower flood depths than a 1-in-100 year flood.

Flooding events of 0.5EY or more are considered frequent or very frequent.

With the Parkway winding through a floodplain, and close to Middle Creek, heavier rainfall events mean there is a higher probability of overbank flooding spreading to parts of the Parkway, even with more frequent flood events. The draft Feasibility Study is looking at options to make the Parkway less prone to flooding during these events.

Probability, not frequency

Frequency measures in this report – including ARI (Average Recurrence Interval), expressed in months of years, EY (Exceedances per year), or AEP (Average Exceedance Probability), are measures of probability or likelihood. They don’t predict the actual timing of a flood event.

For example, 1-in-3 month floods are not spaced 3 months apart; but there is a high likelihood they may occur four times within a year.

Sources: Australian Bureau of Meteorology