Probability of flooding

• Understanding the occurrence of flood events
• Why are no two floods the same?

Understanding the occurrence of flood events

Although very large floods are unlikely to happen, it is important to be aware they can and do occur. A key point to remember is that despite the widely used term, a one in a hundred-year flood, there is no such thing as a flood that only occurs once every one hundred years or once every 50 years and so on.

Unfortunately, these terms have caused confusion, are misleading and impact the understanding of the likelihood of a flood occurring. Some people incorrectly believe that if they experience a one in a hundred year flood, they will not experience another that big for another 99 years.   

The correct way to describe the likelihood or chance of a flood occurring is Annual Exceedance Probability (AEP). AEP is the likelihood of a flood of a given size occurring in any one year. It can be expressed as a fraction or a percentage. Council's Moreton Bay flood viewer shows the AEP for the region.

While we do not know when the next flood will occur, we can estimate the likelihood (also referred to as chance or probability) of a certain size flood occurring.

A great example to explain this is if you throw a dice you have a one in six chance of rolling the number five. A one in six chance is the same as a 17 percent chance. Similarly, we can say that a certain sized flood has a one in a hundred chance of occurring or being exceeded each year. A one in a hundred chance is the same as a one percent chance. This flood is then called the one percent AEP flood, or the one in a hundred AEP flood.

If you roll a five and pick up the dice to roll again. You still have a one in six chance of rolling a five again. Go ahead and try it. It is not impossible to get a five twice in a row. It is the same with floods. If you have a one in a hundred AEP flood, you still have a one in a hundred chance of that flood or a bigger one, happening again. It is common for a region to experience multiple large and rare floods only several years apart or even in the same year.

Another way to think about the occurrence of flood events is to consider the chance of it occurring throughout a mortgage or over a lifetime. For example, the one percent AEP flood has a one percent chance of occurring in any given year, however, during the span of a 30-year mortgage, a home in the one percent AEP (100-year) floodplain has a 26 percent (that’s about a one in four chance) of being flooded at least once during those 30 years. The value of 26 percent is based on probability theory that accounts for each of the 30 years having a one percent chance of flooding.

Flood Probability

This table provides probabilities of experiencing a given sized flood once or more in a lifetime, 80 years, as well as once or more over a 30-year mortgage. 

The Moreton Bay region has recently experienced two very large rare one in 1000 AEP flood events in Saltwater Creek in Deception Bay in May 2015 and February 2022.

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Why are no two floods the same?

If you have lived in the area for a long time you may have experienced or seen different flood events throughout the region and noticed they were considerably different. This is because the size and behaviour of a flood in a particular location will vary depending on many things such as:

• how dry or wet the soil is before it rains
• where the rain falls within the catchment
• how intense the rain is, that is how much the rain falls, and for how long
• if the landform has changed since the last flood for example reduced vegetation from recent flooding, bushfire or land use.

This is why the historically significant flood events throughout the region for example those which occurred in 2011 and 2022 are different. 

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