19 hours ago
Flood maps don't account for the real risk of climate change, Brisbane engineer warns
Flood maps around Australia drastically underestimate the impact climate change will have on rainfall, according to an accomplished civil engineer.
The Australian Rainfall and Runoff (ARR) guidelines' chapter on climate change factors was updated last year, laying out how storms will increase in intensity and frequency depending on how quickly the world warms.
This is because the hotter the atmosphere, the more moisture it can hold, which means more rain.
Hot air also holds more energy that needs to be dispersed, which will lead to more storms.
Brisbane civil and environmental engineer Alan Hoban said what used to be considered a worst case scenario for how global warming will impact rainfall is now looking more likely.
"Prior to the update of the guideline people made some general estimates about what sort of changes in rainfall intensity you would need to account for, for climate change," he said.
"People often just increase rainfall by 10 per cent, sometimes 20 per cent."
But Mr Hoban said without a rapid turn to renewable energy, rainfall intensity would increase by 20 to 30 per cent in large storm events and by 50 to 60 per cent in shorter storms that often cause flash flooding.
While the overall rainfall numbers were worrying, it's the shorter, more intense storms that concerned Mr Hoban.
"As we see these rain events become more and more intense, particularly the short duration storms, we're going to see more of that flash flooding coming through our environment," he said.
It's not welcome news for Hossein Khoshgou, who works at a billiards hall on a flood prone street in East Brisbane.
"Each time we see on the forecast that rainfall higher than 8 millimetres is coming up we use the flood barriers and prepare ourselves," he said.
Mr Khoshgou said the business had flooded "more than five times" since opening two years ago, and had to close for "at least a month each time".
The business flooded twice in December last year due to short intense storms and again during the deluge brought on by Ex-Tropical Cyclone Alfred.
"I think the drainage in this suburb is not working good to collect all the rainfall," he said.
He said the business could not be insured for flooding.
Dr Margaret Cook, whose book River with a City Problem is considered the most complete history of Brisbane's flooding, said development was also impacting flash flooding.
Many storm drains were designed when most suburban blocks held one house with a backyard.
"Now you've got bigger houses or lots of houses or a unit block taking up that whole block, so you've lost all that green space," Dr Cook said.
She said that resulted in a lot more water entering the drains as well as more people using the water system.
"Every hard surface we build changes how floods will react," she said.
In Brisbane, the council said it had spent "up to" $85 million dollars each year enhancing, renewing, and maintaining Brisbane's drainage system" since 2021.
With the changing climate and increased development, Mr Hoban said even a small increase in flood levels could "significantly increase the area that is flooding".
He offered a flood study on the area around Breakfast Creek in Brisbane's inner north as an example.
"The study was released last year and it only accounts for an 8.5 per cent increase in rainfall intensity whereas under this new approach you should be doing 20 or 30 per cent," Mr Hoban said.
Even with the 8.5 per cent increase, the area predicted to flood in a 1 in 100 year rainfall event is much larger.
Many Brisbane suburbs have had two floods in the past 15 years — 2011 and 2022 — that exceeded that marker.
In an area included in the new Breakfast Creek study, residents spoke about higher insurance premiums and increased risk to their homes from a nearby storm drain that had overflowed.
More than one said they were considering selling up before too long.
Mr Hoban urged councils to update their flood maps in line with the updated guidelines despite the "political challenge" it can present.
"If a local government flood map increases the flood level it can have really big implications for the community," he said.
"It might devalue properties or limit the possibility of properties to develop.
"But similarly if they don't update those flood maps [people] aren't making informed decisions about where they should be buying property."
In a statement, Brisbane City Council said they were "undertaking a rolling program of flood studies" to update flood mapping.
"The ARR relevant at the time is incorporated into our modelling," the council said.
Mr Hoban said the responsibility to limit these damages fell with those making decisions that impact climate change.
"What becomes really important here are the energy choices we make for the 21st century," he said.
Dr Cook said cities with flooding issues must heed the warning and view it as an opportunity for more resilient building and planning.
