Australia Weather News

A decade ago, as Brisbane's record floods receded, many residents were left shocked and awed that such a devastating inundation could happen to a modern city.

But while there is no denying Queensland's rainfall events of late 2010 and early 2011 were exceptional, Brisbane had flooded before — and it will flood again.

It is a history we can ill afford to forget as we continue to live and build on the flood plain.

Floods of the 1800s

The most astounding thing about floods in Brisbane is that they continue to take us by surprise.

Even the official Commission of Inquiry set up in the wake of the 2011 floods stated:

"The disastrous floods which struck south-east Queensland in the week of January 10, 2011 were unprecedented, in many places completely unexpected, and struck at so many points at once that no government could be expected to have the capacity to respond seamlessly and immediately everywhere, and in all ways needed."

Yet there have been many examples of floods in Brisbane before 2011.

Peaks in 1841, 1893 and 1974 all eclipsed 2011's mark on the Brisbane gauge, and Indigenous knowledge and the soil record shows floods occurred in the catchment well before European settlement.

The first flood since European settlement was also the biggest, when the Brisbane city gauge reached 8.43 metres in 1841.

The only time this level has been challenged since was in 1893 when an ex-tropical cyclone, followed by yet more rain, brought about two major flood peaks just centimetres short of the 1841 level, with a third smaller peak in between.

The triple flood in just over a fortnight destroyed both the Victoria and Indooroopilly bridges, and newspaper reports at the time estimated 500 homes were washed down the river.

Margaret Cook, an environmental historian, tells the tale of Eleanor Bourne, who lived in West End and was on holiday at Redcliffe in February 1893.

"The first time she knew there was a flood happening in Brisbane was when she found her gate washed up on the beach," Dr Cook said.

"That would be quite a shocking way to find that your house is flooded."

Dr Cook is the author ofA River With A City Problem,a history of flood and the management thereof in the Brisbane River.

"I chose that title because I wanted to make it obvious that, in my opinion and scientifically, the river came first.

"The river was here and British settlers arrived and settled on a flood plain, and then the problem begins."

Debate about how to protect Brisbane and control the flooding date back almost as far.

Dam construction

In a piece of history that will surprise few, there were many calls for reform, reports commissioned, blanching at costs and delays before there was a decisive action to manage the flood risk.

According to Dr Cook, Somerset Dam was first mooted after the 1893 disaster, but little was done until the 1930s when Brisbane was faced with a severe drought.

"That really gave us some momentum to build a dam which would provide water supply and also hold back the floods," she said.

It also doubled as an employment project, so in 1959 Somerset became the first dual-purpose dam in the Southern Hemisphere.

In a twist on the common narrative, the construction of Wivenhoe Dam was also largely driven by the need for water.

As far back as the 1960s it was clear Somerset Dam would be unable to meet Brisbane's rapidly growing water needs by the 1980s.

The 1974 floods certainly provided incentive to speed up the process of a new dam.

Dam and be damned

Wivenhoe and Somerset dams provide Brisbane with drinking water and reduce the flood risk — but they have drawbacks:

Before their construction we used to get floods quite regularly, making us more aware of flood, but now we are far more unconscious of the fact Brisbane does flood, Dr Cook said.

Humans have an extraordinary capacity to forget or ignore inconvenient past tragedy in favour of hoping for the best.

Brisbane relies on the water held in the dams and they can run low, as anyone who lived through the four-minute showers of the millennium drought will be able to testify.

But if the dams are filled to the brim to maximise our water stores for the next drought, it leaves no capacity to absorb or delay the next flood.

A key limitation of the dams is that they only capture water from the northern section of the Brisbane River catchment.

The dams have no control over run-off from the Lockyer Valley, Bremer River or local catchments downstream from Wivenhoe.

So where exactly the water falls has a big impact on how high the flood is in Brisbane.

"That's the problem of flood modelling, isn't it? That it's all based on ifs, buts and maybes," Dr Cook said.

None of the water that devastated Grantham in 2011 or which caused the Bremer River to reach 19.4 metres in Ipswich could have been stopped by the Wivenhoe or Somerset dams.

Modern floods

Cyclone Wanda in 1974 was a relatively weak cyclone — but that did not prevent it from dumping huge amounts of rain on the Brisbane River catchment as an ex-tropical cyclone, resulting in the city's biggest flood since 1893.

In contrast, there was no cyclone in 2011, just huge amounts of rain over several months resulting in saturated catchments up and down the Queensland coast.

According to Andrew Watkins, head of climate monitoring at the Bureau of Meteorology, 1973-74 and 2010-11 were the two strongest La Niñas on record, indicating climatic conditions primed to bring rain.

In 2011, by the time Brisbane reached its flood peak on January 13, more than 20 Queensland towns had experienced a flood peak with significant effects, according to the BOM, some multiple times.

The most devastating impacts were witnessed in Toowoomba and Grantham where a wall of water swept away cars, homes and lives.

In Brisbane, the city gauge at the end of Edward Street reached 4.46 metres; in Ipswich, the Bremer River reached 19.4 metres.

It becomes more difficult to compare floods post dam completion, but according to the BOM's analysis of known floods in the Brisbane and Bremer river basin:

"Inflows into Somerset and Wivenhoe dams were nearly double those of 1974 and can be compared to the 1893 flood event."

Modelling presented to the Commission of Inquiry showed that without the dams, the peak of the 2011 flood would have reached approximately six metres at the Port Office gauge.

That is higher than the 1974 flood peak but over two metres less than in 1841 and 1893.

Flood maps for the 1974 and 2011 floods are available on the Brisbane City Council website.

Some sought to damn management

Many in Brisbane believed Wivenhoe Dam was supposed to prevent the city from flooding again, so as the waters abated, it did not take long for the muddy finger-pointing to start.

With much debate in the public sphere, the Commission of Inquiry covered at length how and if the dam operators used the dam operation manual and its "W" strategies.

As part of the inquiry, the commission hired independent expert hydrologist Mark Babister.

"Mr Babister concluded, in light of the information available at the time, that, allowing for the limits of the strategies in the Wivenhoe manual, the flood engineers achieved close to the best possible flood mitigation result for the January 2011 flood event," the inquiry's final report stated.

The chapter on the management of the operation of the Wivenhoe and Somerset dams concluded:

"There is, it is obvious, plenty of scope for argument about whether adherence to the manual strategies would have made a difference to the way in which the flood engineers actually operated the dam; but the possibility certainly exists that they would have responded more quickly to the developing conditions of January 9 had their mindset been one of applying strategy W3.

"Ascertaining the practical result of acting more quickly also is subject to the uncertainties inherent in the modelling; but again, the possibility exists of at least some improvement in the flooding outcome for Brisbane and Ipswich."

The Queensland Government, Seqwater and Sunwater were all found liable in a class action in 2019 for the negligent operation of the dams that worsened the flooding.

The dam operators are currently appealing this finding.

The decade since

Since 2011 residences in Grantham have been moved to higher ground, but areas in Brisbane continue to be developed despite the fact they fall within the 1974 and 2011 flood zones.

Architect James Davidson, whose practice JDA specialises in climate change mitigation and flooding, is not a believer in clearing out the flood plain per se.

"What I am really interested in is more appropriate forms of development," he said.

"We can design our way around a lot of the issues."

He said there had been a general recognition since 2011 that there was a problem to be dealt with in Brisbane.

An example is the Flood Resilient Homes Program, in which Brisbane City Council is providing funding and expertise to make flood-prone properties better able to handle rising water.

New regulations also require a habitable floor above the flood level.

When looking at property in Brisbane, it is important to do your research; maps of various flood types and likelihoods are available on the Brisbane City Council website.

Bearing in mind the record on which these maps are based is short, every event is different and multiple one-in-100-year events can happen in your lifetime.

Living in the flood zone is the reality for many — it is about being aware of and managing the risks.

"It will happen again. It's just a matter of when," Dr Cook said.

This year

This year is also a La Niña, but so far it has been weaker and rainfall has been significantly less than in the lead up to 2011, according to the BOM's Dr Watkins.

"Having said that, we do have to remember February and March is the peak of the tropical cyclone season," he said.

A cyclone or ex-tropical cyclone coming through would certainly wet up the catchment.

"While we have a La Niña, we always have to be cautious in Queensland, but having said that, the risk would not be as high this year as it was in 2011."