Floods

Floods in Australia are often associated with tropical cyclones in northern areas and slow-moving low-pressure cells further south. Even the vast tracts of desert that cover much of our interior are subject to occasional floods.

How are floods categorised?

For the purpose of issuing the appropriate warnings, flood events in Australia are categorised into Minor, Moderate or Major Floods.

Minor flooding:

Causes inconvenience. Low-lying areas next to watercourses are inundated which may require the removal of stock and equipment. Minor roads may be closed and low-level bridges submerged.

Moderate flooding:

In addition to the above, the evacuation of some houses may be required. Main traffic routes may be covered. The area of inundation is substantial in rural areas requiring the removal of stock.

Major flooding:

In addition to the above, extensive rural areas and/or urban areas are inundated. Properties and towns are likely to be isolated and major traffic routes likely to be closed. Evacuation of people from flood affected areas may be required.

Are there different types of floods?

Floods can generally be identified as either river floods or flash floods based on how they form and the rainfall intensity responsible for them.

Flash floods:

Brief but intense flash flooding sometimes accompanies slow-moving thunderstorms, which can dump huge quantities of rain across a small area in a short time. Put simply if the capacity of a system is unable to handle the amount of rain falling in it a flood will result. This immediate flooding in response to high intensity rainfall is called flash flooding.

Some factors may increase the chance of flash flooding aside from rainfall intensity. For example, extremely heavy rainfall across a city where water tends to run off rather than soak in and heavy rain across an already saturated catchment.

River Floods:

River flooding occurs when intense or prolonged rainfall causes greater inflow into a river system than it can handle and water overtops the banks, spreading across the surrounding area.