Why did the Queensland floods occur?

Answer 1

The main reason why the 2010-2011 floods happened in Queensland was, quite simply, because of the unusual amount of rain.

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.

Australia had been coming out of an El Niño for some time, which means that many parts of Australia had seen gradually increasing rainfall. The rainfall recorded in September 2010 made it Australia's wettest month overall in 110 years. Queensland already experienced flooding in early 2009 (at the same time that southern Australia had the terrible Black Saturday bushfires), and higher than normal rainfall in 2010, increasing towards the end of the year. By then, the ground was simply too saturated to hold any more water. Add to that the effects of the cyclone which crossed the north Queensland coast at Christmas time, bringing excessive rainfall to north and central Queensland, and the ground was waterlogged. (A similar pattern led to the Brisbane floods of 1974.)

Radar images show how large the cyclone system was, even though it was only a category one cyclone, the lowest grade. Low wind speeds do not necessarily mean low rainfall, and in this case the cyclone brought large amounts of rainfall right along the coast. Some of the water runoff from the north flowed down through the inland river system; some of the rainfall fell into the catchment areas of coastal rivers further south. The rivers broke their banks more easily, and there was nowhere for the excess water to go.

Rain continued to fall heavily throughout early January. On 10 January 2011, Toowoomba, a city which sits at an elevation of 700m at the top of the Great Dividing Range, received 150 mm of rain within a 40 minute period. The ground could not hold any more water, and the waters from the escarpment at the top of the range rushed in a torrent towards the city. Stormwater drains and creeks coming through the two natural valleys in the city were already overflowing, unable to cope with the sudden torrent of rain. The water from the escarpment at the top of the range then rushed down the mountainside, creating a wall of water, a 7m high inland "tsunami", that went through the city and down the range. This wall of water rushed through Lockyer Valley to the catchment areas of the main dam that protects the city of Brisbane from flooding, sending its capacity to 190%. The gates had to be opened, sending the equivalent of two Sydney Harbour's worth of water into the Brisbane River each day. This is why Brisbane and Ipswich flooded. Subsequent inquiries have pointed the finger at senior engineers acting too late to release water from Wivenhoe.