More frequent floods to hit Australia: One-in-100-years 'completely wrong'

Governments and emergency services need to be better prepared for extreme floods by not only using available historical information but taking into account a changing climate.

That’s the view of Risk Frontiers general manager Andrew Gissing and CSIRO Climate Science Centre research director Dr Jaci Brown.

Yahoo News spoke with the experts from the CSIRO and Risk Frontiers – which specialises in natural hazards, climate risk and resilience – in the wake of the devastating floods that recently inundated large parts of NSW and Queensland.

Many public figures were labelling the disasters "one-in-100-year floods" and even "one-in-1000-year" or "one-in-3500-year" floods.

Mr Gissing said labelling the floods as such is problematic given that rainfall data for Australia only goes back around 200 years, and also because the assumption that major floods only happen in isolation is completely wrong.

“The 1867 flood in the Hawkesbury-Nepean was much bigger than this year’s floods. Even up at Gympie they had the second biggest flood on record but the biggest flood that happened in the 1800s was much bigger than the flood that occurred this year,” he said.

“The language being introduced using these numbers is unhelpful to the average person... It would be better to refer to historical floods or percentages. Whether it’s bigger or smaller than those historical floods for example.

“People often hear 'one-in-100-year flood' and think it shouldn’t happen again because it just happened. But when it just happened you have an even greater chance that you’ll get another flood in the next few months or year because the environment is just ready and primed for it.”

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Major floods need proper preparation

During the recent floods, NSW premier Dominic Perrottet claimed that they were “one-in-1000-year floods,” while deputy PM Barnaby Joyce went further, claiming floods of this magnitude only occurred “once in 3500 years”.

While governments and emergency services have copped criticism for under-preparing for the magnitude of the flooding, Gissing believes adequate data exists for authorities to account for the possibility of major flooding in urban communities.

“There are always uncertainties in any emergency response with storm and bushfire. It’s often prudent to strategically over-react in some instances,” he said.

“There’s mapping available of the Lismore flood plain and mapping for the probable maximum flood that we think is possible for the physics of the atmosphere et cetera. These events can be planned for in terms of mitigation, land use planning and emergency response. So just because a flood is very big doesn’t mean we shouldn’t be planning for another one.

“Ultimately when we’re talking about calling on enough resources it’s also about 'can we mitigate the risk in the first place?', so we’re not talking about pointing fingers and who is to blame, but celebrating that we’re able to reduce the risk.”

Gissing points to a voluntary land buy-back scheme in the Queensland town of Grantham as an example of good flood planning. Residents in areas impacted by flooding in 2011 were able to sell their homes to the council and rebuild on higher ground, resulting in fewer property losses when floods hit the region this year.

“What we want to see in future floods is more success stories like that,” Gissing said.

Weather records not reliable for future planning

Dr Brown goes even further, saying that the weather patterns of today can't be relied on when it comes to planning for coming years due to climate change.

“You can look back over time and understand if it’s a one-in-100-year event, but that’s looking at a climate that didn’t have climate change in it,” Dr Brown said.

“It’s not when climate change will happen, it’s happening now. We need to figure out what that means for climate in this decade, but also what that means for the next few decades. The weather your grandparents had is not the weather we have.”

The CSIRO State of the Climate 2020 report states that there has been an increase in total rainfall and the intensity of heavy rainfall incidence in Australia since the 1970s. The short duration or hourly duration of rainfall has increased by around 10 per cent or more in some regions, particularly in the north of the country, and these short-duration extreme events are associated with flash flooding.

There has also been a rise in the compounding of extreme events, which occurred recently with the low pressure system in Queensland meeting with the east coast low in NSW.

The 'new normal' in climate

Dr Brown says that major flooding occurring in the two years since the report was written shows we are dealing with a significantly changed weather system, which we need to plan for now.

“We need to be asking 'is this the new normal?' The question is not 'is this a one-in-100-year event?', it's 'in this new normal world, how often might this occur?'

“The next time we have a La Niña, and they happen every three to six years, we need to be thinking about what this La Niña will be like with global warming.

“We need to be prepared for these sorts of events.”

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