Subscribe now for unlimited access.
$0/
(min cost $0)
or signup to continue reading
Emergency crews expect to be on standby until at least Monday as another east coast low is forecast to bring heavy rain and possible flooding to parts of New South Wales, including the Illawarra.
The second winter storm in as many weeks comes as new research indicates there is a significant increase in the frequency of major floods hitting Australia's eastern seaboard.
As of late Friday, Bureau of Meteorology models were yet to settle on the timing and location of the east coast low although it is not expected to be as extensive or long-lasting in its impacts as the massive storm that struck two weeks ago.
Widespread rainfall, though, is expected to bring heavy falls to parts of the coast and the western slopes.
"It is difficult to accurately predict rainfall totals while still gauging how the system will develop, but at the moment all of the guidance is pointing to the coastal areas north of Port Macquarie and south of Sydney receiving their heaviest rainfall on Saturday night and Sunday," said Jane Golding, acting NSW regional director of the bureau.
The NSW State Emergency Service was planning to "keep a large watching brief over NSW," Becky Gollings, an SES spokeswoman, said. "Everyone's on alert.
"For now, though, the focus will be on the mid-north coast down to the Victorian border, especially the Illawarra region around Wollongong,’’ Ms Gollings said.
Authorities will also be watching for how much rain falls over Sydney's catchment region, with major dams close to full.
The bureau issued a flood watch on Friday afternoon, identifying a greater than 70 per cent chance of flooding in nine river valleys in NSW from Sunday onwards.
Most of the nine are expected to have minor flooding, although the Nepean-Hawkesbury is among three catchments that may get moderate flooding.
"It's highly dependent on where the low forms," Mohammed Nabi, a bureau forecaster said, adding Sunday will be Sydney's wettest of the coming days with 20-40 mm expected. Saturday is likely to start with a morning shower before conditions clear.
The frequency of major flood events along Australia's eastern seaboard is increasing, with climate change one of the possible factors, senior Bureau of Meteorology researchers say.
For now, weather models are yet to settle on the area of most rain, although the north-east corner of the state all the way to Victoria will see widespread falls, Mr Nabi said.
"It's highly dependent on where the low forms," Mr Nabi said.
With the eastern seaboard still largely sodden after record daily rainfall about two weeks ago, emergency services are standing by for possible flooding wherever the east coast low develops.
Flood risks will be elevated if the dams overflow at the same time that nearby rivers are swollen. On present forecasts, Sydney's catchments can expect about 50 mm of rain from the event, Mr Nabi said.
Coastal regions will endure strong winds and dangerous surf conditions but the risk of a storm surge is less of a concern than two weeks ago, he said. Even so, some beaches remain vulnerable to further erosion.
New research from Scott Power and Jeff Callaghan, of the bureau, indicates that major flood events are already on the increase based on their study of a 1500-kilometre stretch of eastern Australia from Brisbane down to Bega since 1860.
Major floods were defined as those events which caused extensive flooding within 50 kilometres of the coast, or inundation that extended 20 kilometres along the coast, with at least two catchment areas involved.
The frequency of such events has roughly doubled to two a year over the past 150 years, with about half the increase since the end of the 19th century.
"There is a statistically significant increasing trend in major flood frequency over the full period," the authors wrote in their paper.
The range was also widespread, with "the overwhelming majority of sites in the study region [showing] increasing trends", including all but one of the sites closest to the coast.
The majority of the sites also revealed that the largest amount of daily rain received each year was increasing.
They attributed the trend to natural climate variability and "possibly" from human-induced climate change.
Cars can be carried away by even shallow water, study show
With another east coast low predicted for Sydney this weekend, researchers at the University of New South Wales have demonstrated that cars can be swept away easier than people in flooded water.
On Friday, a team from the university's Water Research Laboratory dropped cars into a large tank at Manly Vale in Sydney's north to show how much force it would take for them to be washed from the road.
Principal engineer Grantley Smith, who led the research, said he was surprised at "just how little water it took to make even a large vehicle unstable".
"They became vulnerable to moving floodwaters once the depth reached the floor of the vehicle," Mr Smith said. "Even in low water depths and slow flow speeds, floodwaters had a powerful enough force to make them float away."
In world-first tests using actual cars instead of miniature models, the researchers found a small car like a Toyota Yaris – which weighs 1.05 tonnes – was moved by water just 15 centimetres deep that had a flow speed of 3.6 km/h. It was carried away in 60 centimetres of water.
A larger Nissan Patrol four-wheel drive, at 2.5 tonnes, was moved by 45 centimetres of water and began floating in 95 centimetres of water, able to be pushed along by just a finger.
The cars were moved so easily partly because even shallow water can be deceptively strong, and partly because modern cars are so air-tight that instead of taking on water they get pushed along by it.
"People don't realise that even slow-moving water packs a powerful punch," Mr Smith said. "Water is heavy: each cubic metre weighs about 1000 kilograms."
Three motorists died in storms in early June when their cars were swept away by floodwaters.
One man died in Leppington in Sydney's south-west when his ute was washed from a causeway. Another was fatally swept away in the NSW Southern Highlands. And a third man was killed after he was caught in floodwaters near the Cotter Dam in Canberra.
Greg Newton, the Acting Commissioner for the State Emergency Service (SES), who helped to fund the experiment, said more than 80 cars were rescued when they became stranded in water during that storm.
"People need to re-think their actions and not drive into floodwater, because by doing this they are not only placing their lives at risk, but the lives of our volunteers who have to go out and rescue them," he said.
"Entering floodwater is the number one cause of death and injury in flood, so everyone should stay out and stay alive."
On Friday afternoon, the Bureau of Meteorology released a fresh warning for a trough off the northern NSW coast which was expected to intensify into an east coast low as it moved south on Sunday and Monday.
"There is still uncertainty surrounding the exact timing and location of the East Coast Low," the Bureau said in a statement.
Minor to moderate flooding was also expected for waterways including the Nepean Hawkesbury River, Richmond and Wilsons River and the Orara River in northern NSW.
Georgina Mitchell, smh.com.au