Major floods follow a pattern in eastern Australia

Major flood no random event study says

A new study has identified a 40 year flood cycle in eastern Australia. File photo AAP / Dave Hunt.

A new study has identified a 40 year flood cycle in eastern Australia. File photo AAP / Dave Hunt.


New study exposes myth of random one-in-one-hundred-year floods


Australia is subject to so many elemental forces that events like floods are viewed as just one among a long list of unpredictable natural disasters we have to cope with.

But these assumptions are set to be overturned by new research, which draws on historic grassroots research and cutting edge climatology to show that floods, in the eastern states at least, are far from the random events we once assumed.

Floods across Australia’s eastern seaboard have occurred in 40 year cycles, with 35 low-risk years punctuated five years of high flood risk, according to University of Newcastle Associate Professor Anthony Kiem.

“We found that those 5 year periods which make up 16 per cent of our study period, accounting for 80pc of major floods which occurred in that time,” Mr Kiem said.

Designating a flood a one-in-100-year event when it occurred within the five year risk zone (an event with a 1 per cent chance of occurring in any given year) is risky and misleading, he said.

That is shown by clusters of big floods in South East Queensland which correspond to the 40 year rule, with sever events in the five year window in 1974 and then 37 years later in 2011, and gain in 2013.

The study drew on 120 years of streamflow data from the major catchments in SE Qld. Within this dataset, the likelihood of large floods per year was on average 4.9 times higher within the five-year flood-prone periods.

Mr Kiem said the pattern was mirrored where the weather is dominated by common influences.

“These flood cycles apply for all of eastern Australia - anywhere that is impacted by El Nino and the Southern Oscillation,” he said.

“The impact varies, with the strength and timing of the cycle changing depending on local factors, but the fundamental cyclical finding is true, there is a disproportionate clustering of floods in these five year periods.”

The 40 year flood cycle is thought to be influenced by a complicated relationship between ENSO and the Interdecadal Pacific Oscillation.

This is an ocean-weather pattern centered on the mid-latitudes of the Pacific Ocean which cycles between positive and negative phases, where warm and cool surface water swaps between the eastern and western sides of the Ocean.

But climate change is disrupting dominant weather patterns, loading more variables into the 40 year flood cycle.

Droughts are expected to become more severe and when it does rain, it is likely to be in more intense bursts.

“We have seen a long-form pattern in the historical records, and we can assume there will be some sort of future cycle, but the question is, will we get stuck in just one part of it?, There is still a lot uncertainty around all of this,” Mr Kiem said.

The study was published in the Australasian Journal of Water Resources and co-authored by Greg McMahon, a Brisbane-based independent consultant on flood risks.

Mr McMahon brings considerable experience to the topic.

He was part of Qld’s ‘roadshow’ in the 1980s, a government investigation into the big floods of the ‘70s that had engineers traversing floodplains and scouring data for preemptive warning signs.

Its findings zeroed in on a 40 year cycle and predicted the major floods around 2013.

Several years ago Mr McMahon heard a conference presentation by Mr Kiem on weather patterns and flood cycles and told him afterwards about the roadshow, and how the patterns it identified in SE Qld matched his predictions.

“It’s always good science when multiple lines of evidence match up, and it's given us a high degree of confidence here,” Mr Kiem said.

But despite the strong evidence, decision makers in Australia have underestimated the real risks posed in these flood prone periods, because they have been lulled into a false sense of security in low-risk years.

“Areas designed to deal with a certain chance of flooding may find the likelihood of these events have been significantly underestimated, which could have serious consequences,” Mr Kiem said.

The story Major floods follow a pattern in eastern Australia first appeared on Farm Online.


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