What is Flooding?

Flooding is defined as relatively high water levels which overtop natural or artificial banks in any part of a stream, river, estuary, lake or dam or any local overland drainage before it enters a watercourse.

Flooding can also be due to coastal inundation resulting from storm related elevated sea levels or large waves (including tsunami).

Types of Flooding

Flash Flooding

Flash flooding usually occurs in the headwaters and results in rapid rises in water over a short period.

Especially in places where the land is steep and close to the headwaters of watercourses or streams, flooding can happen within a few hours or less of heavy rainfall and is called flash flooding.

  

Although a warning of the chance of heavy rain might be possible, the arrival time and depth of floodwater cannot normally be predicted. Flood depth can be many metres above the ground level but flooding does not last more than a few hours.

 

When flash flooding is likely, leaving low-lying homes and businesses well before flash flooding begins (evacuation) is the best action to take, but only if it is safe to do so . If you are trapped by rising floodwater, seek refuge in the highest part of a sturdy building. Stay there and call '000' (triple zero) if you need rescue. 

Riverine Flooding

Riverine flooding occurs when rivers burst their banks, inundating surrounding low lying land. In places where land is less steep and you are farther away from the headwaters, flooding may rise relatively slowly (compared to flash flooding) over many hours or days.

The expected arrival time and the depth of floodwater can often be predicted so a flood warning can be issued. The depth of floodwater in these locations can be great and in some coastal rivers water can rise 10-20 metres above their normal level.

Staying inside a house, even one which you think is high enough, may be very dangerous. If you are warned to evacuate it is always best to move to a safe location before floodwater arrives.

In places where the land is flat and a long way from the headwaters of rivers, such as inland NSW, the floods may take many days, weeks, or even months to arrive. The depth will not be great but huge areas can be flooded and people and animals can be isolated for many weeks.

Other Types of Flooding

Other types of flooding include those caused by tsunamis and dam and pipeline failures.

It does not have to rain to cause flooding. Floods can result from a number of other causes. These include:

Tsunami

Tsunamis such as the Indian Ocean Tsunami of 2004 can flood significant areas of low lying coastal areas with sea water. Flooding affects not only coastal areas but may extend along estuaries and coastal lakes and rivers some distance from the ocean. Flooding caused by tsunamis may carry very large amounts of debris. 

Dam Failure

Although dam failures are rare, their effects can be significant. In New South Wales dam safety is monitored and some dams have early warning systems for communities that lie downstream of the dam and close to the dam. Should dam failure occur, significant downstream flooding with potentially swift flowing water and high amounts of debris can occur. Dam safety is the responsibility of the Dam Safety Committee NSW

Pipeline Failure

Sometimes pipelines carrying water can rupture causing localised flooding. The effects can range from minor inconvenience to significant releases of water that can erode roads and flood yards and houses.

Such an event occured in Ryde in western Sydney in Juy 2005 causing the flooding of 19 properties and the release of an estimated 100 million litres of water.

Storm Surge

Sea levels can be elevated above the highest high tide of the year due to the action of tropical and mid-latitude cyclones.

Under tropical and mid-latitude cyclones locally low pressure can cause sea levels to rise as there is less air pressing down on the sea. This is a particular problem with tropical cyclones where atmospheric pressures can be very low. Information on storm surge including safety information is available from the Bureau of Meteorology Storm Surge page

Persistent very strong onshore winds can result in the sea being 'piled up' against the shore locally raising sea levels and potentially flooding low lying coastal areas.