Citizens flee as Brisbane braces itself for worst floods in a century

(AFP/GETTY IMAGES)

Residents of Australia's third-largest city were warned yesterday to prepare for the worst floods in more than a century, as raging waters that have killed at least 11 people in inland Queensland towns surged towards the state capital, Brisbane.

The abrupt escalation of the flood crisis came after a freak storm sent a wall of water crashing through Toowoomba, 80 miles west of Brisbane, and neighbouring towns on Monday. Last night 77 people were still missing in the Lockyer Valley area, with more torrential downpours hampering the search and rescue operation.

Australians were still struggling to come to grips with the sudden tragedy in the Lockyer Valley when yet more dire warnings were issued – this time for Brisbane, which had been expected to escape relatively lightly. Yesterday forecasters said the Brisbane river was poised to break its banks in spectacular fashion, reaching a peak on Thursday and swamping 20,000 homes and businesses.

The city centre emptied almost instantly, with shops closing and office workers fleeing by bus, train and car. Thousands of people in low-lying suburbs were urged to leave their homes, with the Brisbane Lord Mayor, Campbell Newman, describing the situation as "very serious". Supermarkets reported panic buying, and military helicopters were on standby.

In the Lockyer Valley, the 11 confirmed fatalities include at least five children. The state premier, Anna Bligh, said the death toll would rise "potentially quite dramatically", and she revealed that whole families were among the missing. "Mother Nature has delivered something terrible in the last 48 hours, but there's more to go," she said.

Queenslanders have been battling floods for weeks, with river after swollen river bursting its banks, but until Monday the waters were rising slowly, giving people time to evacuate. What happened in the Toowoomba area – where terrified people clung to trees and telegraph poles as the water rushed through – has changed the whole tenor of the crisis.

Although there had been warnings of flash floods, the Lockyer Valley was completely unprepared for the "inland tsunami" that tossed cars around like corks and lifted houses off their foundations. Ms Bligh said the "incredibly intense" deluge that preceded the lethal events – an estimated 150mm of rain fell in half an hour – could not have been predicted.

Brisbane, a city of two million, has at least had more notice of the natural catastrophe heading its way. But disaster management officials fear its impact will be massive. "We are in uncharted territory," said Mr Newman.

Brisbane's last major flood was in 1974, after which the Wivenhoe Dam was built to protect the city. But the dam is already well over capacity after weeks of heavy rain, and when the torrent of water from Toowoomba reaches the coast, dam managers will have to release it into the Brisbane river.

The result, it is feared, will be floods of a magnitude not seen since 1893, with 15,000 people expected to be affected. Mr Newman said: "Today is very significant, tomorrow is bad, and Thursday is going to be devastating."

In 1974, thousands of homes were flooded and 14 people died when the Brisbane river flooded. Yesterday it was already lapping at boardwalks and waterside buildings after breaching its banks in numerous places. Boats, and even a ferry pontoon, were ripped from their moorings.

A total of 18 people have died in Queensland's worst floods for at least half a century; the latest casualties include a mother and two children whose car was swept away.

In the Toowoomba area, 40 people were pulled to safety from rooftops. Sixty residents who sought refuge in a primary school in the town of Grantham remained huddled there without power yesterday, waiting to be rescued.

The Prime Minister, Julia Gillard, said she was "absolutely shocked" by television footage of the flash flooding. "The power of nature can still be a truly frightening power, and we've seen that on display in this country," she said.

Three-quarters of Queensland has now been declared a disaster zone, with the cost of the floods estimated at A$6bn (£3.8bn).

Q&A: So where is all the water coming from?

Is such heavy rainfall out of the ordinary in Australia?

It is quite normal for summer to be Australia's wettest season, when, per month, it receives almost three times more rain on average than London. However, with half of Brisbane's entire average monthly rainfall falling during just half an hour on Sunday, the scale of the deluge it is experiencing now is clear.

Why is the flooding happening now?

The underlying reason for this heavy rain is a weather system called La Niña. This natural cycle, which is currently approaching its strongest since records began in the 19th century, affects weather worldwide by causing sea temperatures in the eastern Pacific to drop while those in the west rise. Although the difference may be only a single degree Celsius and occurs off the coast of Peru – thousands of miles east of Australia – it can have a profound effect on the weather in Australia. Warmer seas lead to more rain, while strong winds drive tropical weather usually bound for the Americas towards Australia. Indeed, Argentina is also counting the cost of an opposite consequence of La Niña: drought. This weather system is the opposite of the better known El Niño, and can last for between three and six years.

Why is Brisbane suffering so badly?

The city is situated on the Pacific coast just a few miles from Australia's westernmost point, and Queensland's state capital has borne the brunt of the storms coming off the ocean. It has just had its wettest month on record. Added to this, the city not only sprawls around the Brisbane River and a number of smaller creeks that are all bursting their banks, but is also built on a low-lying flood plain at the end of the Lockyer Valley, which has been collecting water from the surrounding area and funnelling it into Brisbane. A so-called "king tide" means sea levels are simultaneously at their highest, and with the rain continuing to fall, the city is under siege from the water on four fronts. Brisbane could still have been spared the current floods had it not already experienced such a wet spring. This meant that the ground, which has been drought-ridden for the past few years, is now fully saturated and incapable of storing more water.

Rob Hastings