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The simplest explanation for the Brisbane River flooding in 2011 is that there was simply too much rainfall in the catchment area.

In early 2011, Australia (Brisbane included) had been coming out of an El Niño weather pattern for some time, which means that many parts of the region had seen gradually increasing rainfall. The opposing weather pattern, La Nina was setting in. The rainfall recorded in September 2010 made it Australia's wettest month overall in 110 years.

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 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 the main reason why the Brisbane River flooded.

Subsequent inquiries have pointed the finger at senior engineers acting too late to release water from Wivenhoe. Had the water begun to be released earlier, it is believed the floods in Brisbane would not have been so bad.

It should be noted that the excessive rainfall had also caused the ranges to the west of Brisbane and Ipswich to collect an unusually large amount of water, leading to higher water levels in the Bremer River, a tributary of the Brisbane River. This compounded the problem of the Brisbane River flood levels.

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What is the history of Brisbane's floods?

Four major floods have occurred in Brisbane.The first was on 14 January 1841. This was the biggest flood of the Brisbane River since records began with the river being approximately 8.6 metres above the high tide mark. There was limited destruction to property as the town was still very young and undeveloped.The next major flood began on 5 February 1893. These floods came within 7cm of the 1841 levels.The worst floods in terms of death and loss of property occurred in January 1974 when tropical Cyclone Wanda developed into a rain depression that dumped hundreds of millimetres of rain on an already saturated southeast Queensland. This flood reached a height of 5.5 m. Since the construction of Wivenhoe Dam on the Brisbane River within a few years of this flood, Brisbane was believed to be flood-proof.The most recent major floods to hit Brisbane occurred on 11 January 2011. On this date, the Brisbane River again broke its banks. The flood was the result of a La Nina weather pattern, together with the state being saturated by the rainfall resulting from a cyclone system that hit in the north on Christmas Day 2010. Wivenhoe Dam, which was designed to prevent any further floods, exceeded 160% capacity, requiring the release of a massive amount of water, comparable to the volume of Sydney harbour.


Where did the Brisbane flood start?

There were a number of factors that contributed to the origin of the Brisbane flood. Ultimately, it could be said that the Brisbane flood started at Wivenhoe Dam, largely as a result of a torrent that started further west. 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 filled the quarry and rushed 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. It was this release of massive volumes of water in a short period of time which started the flood in Brisbane.


When did the Brisbane floods occur?

Four major floods have occurred in Brisbane.The first was on 14 January 1841. This was the biggest flood of the Brisbane River since records began with the river being approximately 8.6 metres above the high tide mark. There was limited destruction to property as the town was still very young and undeveloped.The next major flood began on 5 February 1893. These floods came within 7cm of the 1841 levels.The worst floods in terms of death and loss of property occurred in January 1974 when tropical Cyclone Wanda developed into a rain depression that dumped hundreds of millimetres of rain on an already saturated southeast Queensland. This flood reached a height of 5.5 m. Since the construction of Wivenhoe Dam on the Brisbane River within a few years of this flood, Brisbane was believed to be flood-proof.The most recent floods occurred inBbrisbane on 11 January 2011,when the Brisbane River again broke its banks. The flood was the result of a La Nina weather pattern, together with the state being saturated by the rainfall resulting from a cyclone system that hit in the north on Christmas Day 2010.Wivenhoe Dam, which was designed to prevent any further floods, exceeded 160% capacity, requiring the release of a massive amount of water, comparable to the volume of Sydney harbour.