In cities, flooding and rainfall extremes to rise as climate changes

May 9, 2016, University of New South Wales
Emergency responders discuss options for rescuing a woman trapped on the roof of her car in flood waters during the 2010-2011 Queensland floods in Australia. Credit: Timothy Swinson/Flickr

Cities face harsher, more concentrated rainfall as climate change not only intensifies storms, but draws them into narrower bands of more intense downpours, UNSW engineers have found. This has major implications for existing stormwater infrastructure, particularly in large cities, which face higher risks of flash flooding.

In the latest issue of Geophysical Research Letters, doctoral student Conrad Wasko and Professor Ashish Sharma of School of Civil and Environmental Engineering at the University of New South Wales show the first evidence of storm intensification triggering more destructive storm patterns.

"As warming proceeds, storms are shrinking in space and in time," said Wasko, lead author of the paper. "They are becoming more concentrated over a smaller area, and the rainfall is coming down more plentifully and with more intensity over a shorter period of time. When the storm shrinks to that extent, you have a huge amount of rain coming down over a smaller area."

Wasko and Sharma, working with collaborators at the University of Adelaide, analysed data from 1,300 rain gauges and 1,700 temperature stations across Australia to see how air temperature affected the intensity and spatial organisation of storms.

They found that atmospheric moisture was more concentrated near the storm's centre in warm storms than in cooler ones, resulting in more intense peak rainfalls in those areas. The storms were clearly shrinking in space, irrespective of the amount of rain that fell.

Although the data is sourced from Australia, this has global implications, said Sharma. "Australia is a continent that spans almost all the climate zones in the world - Mediterranean, tropical, temperate, subtropical - everything except the Arctic or Antarctic. So the results hold a lot of value - we are finding the pattern repeating itself over and over, happening around Australia and around the world.

"Look at the incidents of flooding in Mumbai or in Bangkok last year - you see urban streets full of water," he added. "You see it now in Jakarta and Rome and many parts of Canada. That's because the stormwater infrastructure cannot handle the rain, and part of the reason there's more rain is the increase in global temperatures."

Most urban centres have older stormwater infrastructure designed to handle rainfall patterns of the past, which are no longer sufficient. "The increase is especially noticeable in urban centres, where there is less soil, unlike rural areas, to act as a dampener," said Sharma. "So there is often nowhere else for the water to go, and the drainage capacity is overwhelmed. So the incidence of flooding is going to rise as temperatures go higher."

Wasko, lead author of the paper, said scientists have long suspected that the intensity of rainfall would be boosted by climate change, as the warming air raises the carrying capacity of moisture. But while extreme rainfall has been rising, little was known about the mechanisms causing it. The latest study shows that storms are changing in spatial terms.

It follows a study by the same authors in Nature Geoscience in June 2015 showing that storms were also changing their 'temporal pattern' - that is, getting shorter in time, thereby intensifying. When it comes to flash flooding, the amount of rain that falls over a period of time is much more important than the total volume of rainfall that a given storm delivers. This study was the first to show that was disrupting the temporal rainfall patterns within storms themselves.

If both spatial and temporal changes in storms continue, as they are likely to do as the world warms, there will be more destructive flooding across the world's major urban centres.

In their Nature Geoscience paper, the duo calculated that floods in some parts of Australia would likely increase by 40%, especially in warmer places like Darwin. "If you add the spatial pattern from this latest paper, you will probably increase this 40% number to maybe 60%," said Sharma.

Earlier this year, a pivotal framework for infrastructure maintained by the Institution of Engineers, the Australian Rainfall and Runoff national guidelines, were updated for the first time since 1987, a process that took three years. It's now clear, said Sharma, that these will need to be adjusted, as the safety and sustainability of Australian infrastructure adapts to a warming climate.

And there are still unknowns to contend with, he added.

"When we say that the storms are shrinking in space and shrinking in time, and we say floods will increase, we are making an assumption that the volume of water coming down is not changing," said Sharma. "That assumption is very conservative, because you would expect the air to hold more moisture. If you factor in that in as well, there'll be even more , and more floods."

Explore further: Flash flood risks increase as storm peak downpours intensify

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Display comments: newest first

1.2 / 5 (17) May 10, 2016
Freeman Dyson says a warmer world has always been a kinder world for humans. Why does disagree with Professor Dyson?

It has been colder (4th and 5th Century C.E.; Little Ice Age) and warmer (Viking Era) than now in historical times, well before the Industrial Revolution. The hockey stick is a contrived falsehood.
3.9 / 5 (19) May 10, 2016
Oh, it's the fan boyz trying to get their hearthrob in bed with cut and paste duty! That's why his name is "shittest".
4 / 5 (20) May 10, 2016
Is there urban slang for when a young guy goes after an ol' coot? There must be. Not really up on these things. Whatever that is, it seems to be the only explanation short of that he's a parrot. Pretty sad, really. Dyson has told his misquoting suitors to bugger off, and not with him. Yet they still persist. You can tell the lot. Always "Professor Dyson". You notice that they never do that with anyone else. Has he ever taught for more than a year anywhere?

Proves royally the difference between jerks and virtuous individuals. Where's Feynman? Too virtuous, died prematurely. He would have called Dyson out as a coot straight-away, and Dyson would have said, "Yes, sir!" Instead we've got idiots like this troll grovelling after a 92 year old man.
May 10, 2016
This comment has been removed by a moderator.
4.7 / 5 (12) May 10, 2016
Oh, it's the fan boyz trying to get their hearthrob in bed with cut and paste duty! That's why his name is "shittest".

lol, it's one of antigorillascience's old socks, shooty the potty miss, he's getting paid to friskly stay wake to be the first to comment and make his stupidity known, very consistently i must say :D
4.7 / 5 (12) May 10, 2016
Where is the science and data in this article? Instead it reads like a fortuneteller with phrases like this:

"you will probably increase this to maybe 60%..."

"they are likely to do this as the world warms...."

"the duo calculated that floods in some parts of Australia would likely increase by 40%...."

"And there are still unknowns to contend with...."

Very poor.

Not as poor as you are in lacking of any credibility wrt to your airhead lookout on the subject, any subject here, bartvart another sock of antigoracle aka antisciencegorilla, can never back up any statements he say.... here monkey monkey ;)
5 / 5 (7) May 10, 2016
As from Dr. Alley's talk at AGU yield losses due to higher temperatures and too many hot days in a row are already solid metrics that hotter is bad for crops globally, denialistas claim this golden brick road to hotter strewn with riches ...

Reality is we hit 3.05-ppm last year alone, long-term average is 1-ppm/1000-years over the Pleistocene big jump up after the last ice-age was about 1-ppm/180-years we're a tad faster, eh?

At 410-ppm off the Washington coast right now we're committed to 25m/82ft of sea-level rise only question is how fast, are we doing a damn thing to slow things down, we hit 3mm/year last year?

At the current rate we hit 800-ppm in less than 130-years, all of Antarctica is then committed to being melted it starts to really go fast after 600-ppm and by 750-ppm you kill all the coral reefs of any meaning to humanity as a bonus.

The denialista trolls are really getting heavy on these comments lately as if this is social media not science people.
5 / 5 (9) May 10, 2016
1m by 2100, 4-6 degrees, ocean sea life collapse and I predict civilisation collapse by 2050 (at the latest). At that point, CO2 emissions will collapse but I still think we'll see virtual human extinction around 2100.
1 / 5 (8) May 10, 2016
1 / 5 (9) May 11, 2016
1m by 2100, 4-6 degrees, ocean sea life collapse and I predict civilisation collapse by 2050 (at the latest). At that point, CO2 emissions will collapse but I still think we'll see virtual human extinction around 2100.

Wow!! I must offer you my pity. It must be a heavy burden, hauling around all that stupid and yet you don't seem to tire.
1 / 5 (8) May 11, 2016
It couldn't be that these cities, through population increase and paving of the land surface, have rapidly outgrown their aging infrastructure.
The desperation of the AGW Cult to manufacture their non-existent doom and gloom, is pitiful.
1 / 5 (7) May 11, 2016
In their Nature Geoscience paper, the duo calculated that floods in some parts of Australia would likely increase by 40%, especially in warmer places like Darwin.

Oh the me.. "science" of the AGW Cult. Never mind that Darwin has been cooling for over a 100 years.

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