The inundation of land that is normally dry through the overflowing of a body of water as a result of unusual and rapid accumulation of runoff of surface waters. Floods are estimated to be the most costly natural disaster in Australia, causing millions of dollars of damage to buildings and critical infrastructure.

Floods are often caused by excessive heavy rainfall but may also be caused by factors such as cyclonic storm surges, high tides, tsunamis, dam failure and more.

There are three common types of floods that affect Australia: slow-onset floods which are from inland rivers and may take days to build up; rapid-onset floods which occur quickly; and flash floods which result from relatively short, intense bursts of rainfall, often from thunderstorms.. 

Image: Flash flooding in Toowoomba during 2011 Queensland Floods, Tim Swinson (Wikimedia Commons)