Flood

Floods in Australia usually occur over a smaller area than droughts but still cause damage to ecosystems and property. Floods can occasionally affect large areas of Australia as waters move down rivers into the interior of the continent. After the initial devastation, floods can be beneficial to natural ecosystems. The effects of floods can be felt for many years and can even result in changes within the ecosystem.

Floods

Droughts in Australia are often broken with heavy rains and flooding and this flooding can have devastating results. Most areas of Australia experience occasional flooding. This could be in the form of localised flash flooding from intense thunderstorms or it could be more widespread as heavy rain falls over the catchments of river systems. When this happens, normally quiet-flowing streams can spill out over thousands of square kilometres of surrounding country.

Floods may cause loss of lives, the death of any livestock that cannot move to higher ground and damage to homes, businesses, roads, equipment and ecosystems. It may also take some time before things can be fixed and this lost production can add considerably to the costs of the flood. Many animals that survive the flood are often threatened by death from starvation as it takes time for the vegetation to return. Overall, flooding is Australia's costliest form of natural disaster, with losses estimated at over $400 million a year.

See Image 1

Tropical flooding

In northern Australia, floods are usually caused by tropical cyclones in summer or early autumn which can produce as much as 1000 millimetres of rain in a few days. The official 24 hour rainfall record of 907 millimetres was set on 3 February 1893 at Crohamhurst, northwest of Brisbane, causing devastating floods in Brisbane. In January 1979, tropical cyclone 'Peter' dumped 1947 millimetres in 48 hours at Bellenden Ker in North Queensland.

Flooding outside the tropics

Outside the tropics, coastal areas of eastern Australia mostly receive flooding rains from a weather system known as an 'east coast low' that develops occasionally over the adjacent Tasman Sea. In other places, flooding occurs mostly in winter and spring as fronts move through. Flooding over inland areas is usually associated with tropical systems that move south. Some inland floods, such as those of Lake Eyre, may be a result of rain falling hundreds of kilometres away. The flood moves slowly downstream and takes months to move down into the interior. In 1973-1974, the normally dry Lake Eyre was flooded to a depth of ten metres.

See Image 2

Flooding and La Niña/El Niño

The term 'La Niña' is applied to climatic conditions that are the opposite of the better known El Niño. La Nina occurs when the temperatures of the central and eastern Pacific Ocean are cooler than normal and when there is increased cloud cover over Australia.

In Australia (particularly eastern Australia), La Niña events are associated with an increased likelihood of wetter conditions. The La Niña years of 1916, 1917, 1950, 1954-1956 and 1973-1975 were accompanied by widespread and severe flooding. Flooding, however, can occur in other years and often accompanies the breakdown of El Niño in late summer or autumn.

Positive effects of flood

Although floods cause widespread damage to property, livestock and crops, they do provide some positive ecological effects to the natural ecosystems. Floodwater soaks into the soil and fills up ground and underground water supplies. This water can then provide a long-term water supply to both native plants and agricultural crops. Floods also wash out excess salt and chemicals from the soil and the rivers and help to clear away debris and other pollution. The water deposits fertile soil on river flats and provides an environment that enables fish, birds and other organisms to breed and increase in numbers. If the flood reaches the desert, the plants can regenerate and start to bloom.

Adapting to the flood

Some ecosystems have adapted to regular floods and need floodwater to function. The River Red gums that grow along the Murray River are well adapted to the floods that used to occur frequently. The floods dispersed the seeds and deposited fertile soil around their roots. The damming of the Murray and the increased use of weirs and irrigation has meant that the river flow has been reduced and that floods no longer occur.

See Image 3

Other organisms, particularly those that live in the desert, floodplains or surrounding areas, are also well adapted to floods. The life cycle of plants and animals within these areas is geared toward high productivity in short time-spans. There is a large population increase after a flood and a substantial decrease in population when the area is dry for extended periods.

Floods, droughts and bushfires affect Australia's ecosystems. Other effects on the ecosystems are the various interactions that take place within them. These interactions will be explored in the next topic.