What causes major floods in Australia?

Answer 1

Australia is a very flat country. Running down the eastern coast is the Great Dividing Range, so there are the usual coastal plains there. To the west of the range are vast expanses of flat country, and when there is a lot of rainfall (which happens infrequently), the rivers quickly swell and break their banks, sending massive floods across these great plains.

Australia spends many years in drought, often caused by the El Niño effect. When an El Niño occurs, changes in sea surface temperatures cause a shift in air pressure which, in turn, can result in climatic anomalies, such as severe droughts in Australia. These years are punctuated by years of normal rainfall, but occasionally, Australia gets the opposite to an El Niño, which is La Niña. This means that weather conditions, etc, are in reverse to those seen during El Niño, and Australia experiences far more rain than usual. When this happens, many parts of Australia will see gradually increasing rainfall over the period of several months. The ground becomes too saturated to hold any more water. This often occurs during the spring and summer season, which is also cyclone season. Add to the already super-saturated ground the effects of a cyclone which becomes a rain depression once it crossed the coast, and the scenario is one of excessive rainfall leading to the ground becoming waterlogged. (This was the pattern that led to both the Brisbane floods of 1974 and 2011).