Supplied: FRNSW
)What does it mean when a natural disaster is described as a "one-in-50-years" or "one-in-100-years" event? Well, not what most people think and that has to change, according to a leading risk assessment expert.
Thomas Mortlock, from consultancy Risk Frontiers, said "100-year floods" don't just happen once every 100 years.
In fact, 100-year floods have happened twice in a fortnight in the same place.
"In fact, it's only about 63 per cent."
Confused? You're not alone.
Supplied: National Library of Australia
)A flood of confusion
As Sydney's Warragamba dam overflowed last week, NSW Premier Gladys Berejiklian described the floods as a one-in-50-year event, and the Mid North Coast floods as a one-in-100-year event.
Her statements unleashed a torrent of comments on social media, some pointing out their region had suffered multiple 100-year events in recent years, others seeing the frequency of 100-year events as proof that global warming was making extreme weather worse.
So what is a one-in-50-year flood, for instance, and are they becoming more common?
"What Gladys Berejiklian is talking about is an event that has a one-in-50 chance of being exceeded in any one year," said associate Professor James Ball, of the University of Technology Sydney's School of Civil and Environmental Engineering.
Similarly, a one-in-100-year event has a one-in-100, or 1 per cent chance of happening in any single year in a single location, said Dr Ball.
He said engineers used computer models that factored in previous floods to calculate points on maps where the flood risk reached the 100-year, or 1 per cent mark.
A flood that matches those points is a 100-year flood.
Town planners use these points in deciding where houses can and cannot be built.
"The 1 per cent, or the one-in-100-year mark, is usually the critical level," Dr Ball said.
'A false sense of security'
While this might make sense to engineers, the 100-year term can be misleading for the public, said Mr Mortlock.
"It gives a false sense of security really, that if you experience a flood to that magnitude, then you're OK, because you've got another 100 years to wait until that event is likely to occur again. And that's certainly not the case," he said.
In fact, he said, climate drivers like La Niña increased the chance of multiple extreme weather events clustering.
"La Niña certainly does load the dice in favour of floods," Mr Mortlock said.
"Brisbane had two 100-year floods within a fortnight in 1893," said environmental historian Margaret Cook from the University of the Sunshine Coast.
In 1893, La Niña helped break a drought that had gripped the then colony of Queensland. Brisbane received over a metre of rain in February alone, triggering a series of floods.
They remain Brisbane's second and third biggest recorded floods.
The devastating Brisbane flood of 2011 was also in a La Niña year.
Incredibly, that was only Brisbane's seventh biggest flood.
Mr Mortlock said homeowners would better understand risk if they were told they had, for example, a 1 per cent chance of flooding each year, rather than being told they were in a 100-year flood zone.
"So instead of talking about the one-in-100-year flood, let's talk about the 1 per cent flood," he said.
"I think that really does add immediacy to the discourse when we're communicating flood risk."
'More we don't know'
So is the kind of rainfall that causes 100-year flood events becoming more common and is this evidence of climate change?
Probably not, according to Dr Andrew King, from the ARC Centre of Excellence for Climate Extremes at the University of Melbourne.
He said the perceived frequency of 100-year events in Australia was probably due to the size of the continent.
"If you're looking across all of Australia, it's quite likely that you'll get some extreme events somewhere," he said.
And what about the impact of climate change on flooding?
He said that was partly related to the amount of water the lower atmosphere could hold increasing by about 7 per cent for every degree Celsius of warming.
And the Australian climate has warmed by 1.4 degrees since 1910.
In other words, more water in warmer air leads to more rain and more floods.
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)But Dr King said there were no trends in "multi-day heavy rain events" like last week's across NSW.
"Even in other parts of Australia, there aren't really strong trends," he said.
Dr King says storms like those NSW just experienced require more than increased moisture-holding capacity.
They need very particular weather patterns and for those patterns to stay in place for a while.
"We don't really know how weather patterns are changing," he said.
"It doesn't appear that they're changing a huge amount, but it's quite hard to tell.
"Our computer models probably don't do a great job of accurately simulating these kind of weather patterns. So it's really hard to tell how climate change is affecting them."
Supplied: Bureau of Meteorology
)Dr King said this did not minimise the fact that climate change was a huge problem.
"We know that temperature extremes are seeing really big changes," he said.
"But for the multi-day rainfall extremes, which are really important in Australia, we don't really have a good handle on how climate change is affecting them."
And Dr King cautions that just because science doesn't completely understand the connection between flood frequency and climate change, it doesn't prove climate change isn't impacting flooding in Australia.
"There's more we don't know than we do know," he said.
Supplied: State Library of New South Wales
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