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What is a flood?

In simple terms, flooding occurs when an area that is typically dry becomes inundated with water. It's important to note that no two floods are identical, as they can differ based on factors such as the amount, duration, and distribution of rainfall.

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What causes floods?

Australia experiences three primary types of flooding:

1. Riverine Floods: Riverine floods are among the most common types of flooding in Australia. They typically result from heavy rainfall and the land's inability to absorb the excess water. When the land becomes saturated, the surplus water flows into river systems, causing them to exceed their capacity. This overflow leads to water spilling into adjacent low-lying areas.

2. Flash Floods: Flash floods can occur in almost any location and are characterized by short but intense bursts of rainfall. They pose a significant danger as they can develop rapidly, particularly in urban areas where drainage systems may struggle to cope with the sudden influx of water. Due to their swift onset, flash floods can be challenging to predict and warn for.

3. Coastal Floods: Coastal floods result from specific conditions such as low-pressure systems or strong onshore winds, which elevate sea levels above their usual levels. This phenomenon creates a storm surge that can lead to flooding in low-lying areas near tidal waterways and coastal areas.

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What is 1% AEP (Annual Exceedance Probability)?

An 'Annual Exceedance Probability' (AEP) represents the likelihood that a flood of a specific magnitude or greater will happen within a one-year period. For instance, a 1% Annual Exceedance Probability (AEP) Flood indicates a 1-in-100 chance that a flood of that size or larger could occur in any given year. It's important to note that the term "1% AEP," also known as the "1-in-100 year flood," does not imply that if a flood occurs in one year, it won't happen for the next 99 years. Likewise, it does not guarantee that if there has been no flooding for 99 years, a flood will occur in the following year. As an example, certain regions in Australia have experienced two 1-in-100 year floods within a single year.

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What is PMF?

The Probable Maximum Flood (PMF) is the most significant flood event that could theoretically take place at a specific location. It typically relies on a theoretical estimate of the maximum possible rainfall, known as probable maximum precipitation, and is of much greater magnitude than a 1% Annual Exceedance Probability (AEP) flood. The areas that would be submerged by a PMF are commonly referred to as Flood Prone Land or the Floodplain. Local Councils and Emergency Services may use the PMF flood information for disaster planning and emergency response, as well as for the regulation of sensitive and high-risk development projects.