Floods occur when water covers land that is normally dry. They can be caused by prolonged or very heavy rainfall, severe storms or tropical cyclones, and the monsoonal (wet season) rains in the tropics. Other causes include snow-melt and dam failure.
In the coastal regions of Australia, floods can also be caused by the sea. Seawater flooding can happen during storm surges, king tides and tsunamis.
People who live in low-lying areas, or close to rivers, creeks or major stormwater drains are usually most at risk from flooding. Heavy rains and storms in one area can cause flooding in another. When the water level of local rivers and streams rises it can cause them to overflow, sometimes spilling out over thousands of kilometres.
Flooding can be caused by a range of situations that include:
- Seawater flooding – coastal areas may be flooded when a cyclone or severe storm causes a surge of sea water.
- Tidal flooding – floods that are caused by high tides that coincide with higher than normal river levels.
- Run-off from rivers and dams – flooding can be caused when river systems need to carry more water than usual following a snow-melt or when dams start to overflow.
- Urban drainage – flash flooding is a serious problem in some cities when the drainage systems fail. People can be caught in stormwater drains, trapped in their cars or even swept off the roads by water.
Is it always bad news?
Floods can have both positive and negative impacts. Sometimes floods can bring welcome relief for people, livestock and areas experiencing drought conditions.
Flooding can also be a natural way for wetland areas, swamps and native waterways to survive. Underground aquifers and soils with high salt levels also benefit from floods.
Types of floods
There are three common types of floods that affect Australia:
Inland rivers in the vast, flat areas of Western Australia, central/western New South Wales and Queensland often flood. These floods may take days to build-up, but can last for weeks or even months.
The damage caused by slow-onset floods includes:
- major loss of livestock
- rural towns being cut-off and isolated
- crop damage
- damage to roads and railways.
Rapid-onset flooding occurs more quickly than slow-onset flooding. Rapid-onset floods can be much more damaging and pose a greater risk to loss of life and property because there is generally less time to take preventative action from a faster, more dangerous flow of water. This type of flooding can affect most of our major towns and cities.
Flash flooding results from relatively short, intense bursts of rainfall, often during thunderstorms. It can occur in almost all parts of Australia and poses the greatest threat to life. People are often swept away after entering floodwaters on foot or in vehicles. These floods can result in significant property damage and major social disruption. They are a serious problem in urban areas where drainage systems are often unable to cope with large amounts of water in a short time.