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Floods occur when water covers land that is usually dry. Floods are fairly commonplace in Australia, costing Australians $300-$400 million in damage every year. Floods can be deadly, but deaths due to flood are preventable. Most flood deaths occur when people underestimate the depth and current of flood waters and try to cross on foot, by car or by swimming. Flooding, unlike some other natural disasters, is usually fairly localised. Sometimes, however, many floods can take place over a large area. See image 1

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. (Refer Chapter 4) 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 Niņo Southern Oscillation (ENSO.) (Refer Topic 2, Chapter 1) During La Niņa years, heavy rains fall in Australia. Floods can, therefore, be more common in La Niņa 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. (Refer Chapter 4) 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.

Types of floods

The most common type of flood in Australia happens when a river overflows its natural banks. There are two main types of such floods, slow-onset floods and rapid-onset floods. Slow-onset floods usually occur on inland rivers such as those found in central and western New South Wales, central and western Queensland and parts of Western Australia. As their name suggests, these floods take at least a week to develop and can persist for months. As heavy rain falls, the river is unable to accommodate the extra water. This causes the river to overflow its banks. Slow-onset floods can result in damage to crops, livestock, rail lines, roads and property.

Rapid-onset floods occur more quickly, but they can be more catastrophic since there is less warning than with slow-onset floods. Rapid-onset floods occur on rivers found in coastal areas and the mountain headwaters of major rivers. Since these rivers drain more quickly than slow-moving inland rivers, flooding happens more quickly, over the course of a couple of days.

Flash floods occur when extremely heavy precipitation due to intense storms is more than local drainage systems, either natural or man-made, can accommodate. These floods occur with little or no warning, and as a result, often cause loss of life. Flash floods are an increasing problem in cities, which have inefficient drainage. See image 2

Flood protection

Floods can be prevented through flood mitigation projects. These include building dams upriver of flood-prone areas and building levees, or walls, around riverbanks to prevent water from reaching inhabited areas. Unfortunately, these defences can fail in extreme circumstances. See image 3

The best defence against floods is early detection. The Bureau of Meteorology monitors river levels and rainfall and issues flood warnings as necessary. Some rivers have electronic data collection systems that transmit river levels to meteorologists. Some floods, such as flash floods, are not easy to predict, however.

A severe flood: Great Floods of 1990

In 1990, unusually high rains caused flooding across parts of New South Wales, Queensland and South Australia. In these floods, six people were killed along with a million farm animals. In April and May, heavy rainfall caused by tropical cyclones and other extreme weather systems caused two rivers to overflow their banks. One river was the Bogan River in central New South Wales, where the town of Nyngan was flooded when its levee broke. Over 2000 people were evacuated from Nyngan, most by helicopter. The damage was estimated at $50 million with a majority of the damaged property uninsurable.

At the same time, the Warrego River in Queensland overflowed its banks. This caused a flood in the town of Charleville. The entire town, with a population of more than 3000, was evacuated. A ';tent city' for residents and relief workers was put up near the airport, which was located on higher ground and was not as affected by the flooding.