Causes of floods

Floods are most often caused by high levels of precipitation. In Australia, this is mostly caused by unusually heavy rainfall, although in other parts of the world, floods can be started by melting snow. In northern Australia, flooding is often caused by tropical cyclones and other large tropical storms. In south-eastern Australia, floods are usually caused by low pressure systems that develop in the late winter and spring, although some strong tropical systems can affect the area as well. Flooding of this sort can be affected by the El Niño Southern Oscillation (ENSO.) During La Niña years, heavy rains fall in Australia. Floods can, therefore, be more common in La Niña years.

Another less common cause of flooding is when seawater comes up onto dry land. This can happen for a number of reasons. Low pressure systems, such as tropical cyclones, can cause a storm surge to inundate the land.Tsunamis, or giant waves created by earthquakes, volcanic eruptions or meteor collisions, can also inundate the land with seawater.

Another rare cause of flooding is dam failure. Dams have been built on some rivers to generate hydroelectric power and reduce the severity of floods downstream. If these dams fail, due to poor engineering or an earthquake, however, flooding will occur downstream.