What is a flood?

A flood is an overflow of water beyond the normal limits of a watercourse. Flooding occurs when water extends over what is usually dry land. This can happen when it escapes from a natural watercourse, such as a lake, river or creek. It can also happen when water is released from a reservoir, canal or dam.

There are two types of flooding, riverine and flash.

Riverine flooding is where rivers break their banks and water covers the surrounding land. It’s mostly caused by heavy rainfall, but can also be caused by king tides, storm surge, snowmelt and dam releases.

In inland parts of Queensland, New South Wales and Western Australia, riverine flooding can affect thousands of square kilometres for weeks or even months at a time.

Flash flooding occurs within six hours of rain falling. It can happen after a short burst of heavy rain, such as from a thunderstorm.

Flash floods can be a serious problem in urban areas if drainage systems can’t cope and tend to affect a localised area.

What causes floods?

Heavy rainfall

The most common cause of riverine and flash flooding is heavy rainfall.

Rivers are formed over thousands of years. Every river is different and forms in response to:

• the usual amount of local rainfall and runoff
• the local topography, vegetation and soil types.

These features stay relatively constant through time, with only rainfall varying.

Rivers have a maximum capacity to carry water. More rainfall than usual, creates more runoff than usual. This runoff can’t be carried by the river channel, so it spills out onto the land. When the time between rainfall and flooding is longer than six hours, this is a riverine flood.

Riverine floods can happen shortly after rainfall, in localised areas. River flood peaks are often reached within days of rain falling. But given many Australian rivers are long, floods can also occur weeks or months after rain, sometimes hundreds of kilometres away. For example, flood water in the upper Murray River, Australia’s longest at over 2,500 km, may take months to reach the river mouth in South Australia.

King tide

The term ‘king tide’ is widely used to describe an exceptionally high tide. These tides are a natural and predictable part of the tidal cycle. The time of year they occur varies by location and between years. They can have very noticeable effects where the ocean meets the land at beaches, estuaries, harbours and other coastal locations.

King tides can increase the impact and extent of riverine flooding. Imagine a town near the coast that lies beside a coastal river. If the river is in flood, parts of that town may be inundated. If the flood occurs at the same time as a high king tide, floodwater will have less opportunity to drain to the sea. More of that town is likely to be flooded and to a higher level.

Storm surge

A storm surge is a rise above the normal seawater level along a shore caused by a storm. The storm often brings strong onshore winds and typically results from reduced atmospheric pressure. Storm surges often accompany a tropical cyclone. Away from the tropics, storm surge can occur due to an intense low pressure system.

Coastal flooding is likely during a storm surge. And if storm surge combines with a riverine flood the area and extent of flooding can increase.

Snowmelt

Snow can be thought of as a reservoir of water, waiting for enough warmth to run down the hill. When conditions warm rapidly, snow can melt quickly. When this happens it can release volumes of water too great for the downstream river channels, causing riverine flooding.

Dam releases

Dams can hold vast amounts of water, but all dams have a capacity. If a dam is close to capacity, dam operators may release water and sometimes this can cause flooding. We work closely with dam operators in times of flood to ensure they have the latest weather information. Operators use this information to manage their dams in a way that minimises impacts on communities downstream.

How dangerous is floodwater?

People don’t think of floodwater as dangerous, compared to raging fires, or the howling gales of a tropical cyclone. Yet flooding is a serious threat – it’s Australia’s second-most deadly natural disaster (Haynes 2017). The danger is often underestimated, with devastating consequences. People have lost their lives when entering floodwaters on foot or in their car. Sadly, most flood injuries and fatalities are preventable.

Most people are shocked to learn how dangerous it is to drive into floodwater. A small car weighing about 1 tonne can be moved by floodwater which is only 15 cm deep, flowing at a rate of 1 m per second. In 60 cm of floodwater a small car will completely float away. This is because of the car’s buoyancy from airtight doors and inflated tyres.