'Alarming' surge in rainfall burst intensity raising Sydney flood risks

Rainfall intensity during short rapid bursts has increased 40% in the past two decades in the Sydney region, raising risks from flash flooding as outdated infrastructure is overwhelmed, climate researchers say.

Latest research, outlined in an article on The Conversation website today, uses Bureau of Meteorology radar maps produced by overlapping stations at Newcastle, Terrey Hills and Wollongong to examine changes in short rainfall extremes that have been difficult to study in the past.

“Our research has found an alarming increase of at least 40% in the rate at which rain falls in the most intense rapid rain bursts in Sydney over the past two decades,” the researchers say. “This rapid increase in peak rainfall intensity has never been reported elsewhere, but may be happening in other parts of the world.”

The 20% per decade increase in the intensity of rapid bursts, which last less than an hour and often as little as ten minutes, was found despite little if any change in daily or even hourly rainfall over the same region.

The Conversation article is authored by University of Melbourne Research Fellow Hooman Ayat and UNSW Climate Change Research Centre professors Jason Evans and Steven Sherwood.

The researchers say the findings, published in the latest issue of the journal Science, have major implications for Sydney’s preparedness for flash flooding, with more intense downpours likely to overwhelm stormwater systems designed for past conditions.

“For our old structures, they were definitely not built to handle these kind of extremes, and that means if they do fail then the damage is likely to be a lot worse than you’d expect, because in 10 minutes you could totally flood a house,” Professor Evans says in a video.

The findings are consistent with expectations from a warming climate but further study is needed into drivers of the change, the researchers say.

“We looked at things like El Nino and other modes of variability in the climate system and it’s not related to those so it leaves climate change as one possibility,” Professor Sherwood said.

“We do think that in a warmer climate the atmosphere is becoming more unstable and that means that you do expect bigger bursts of rain in a really short timescale.”

The rapid rainfall burst events are typically highly localised and can be part of a larger storm or form independently.

“An intensification of 40% in only two decades means we must re-evaluate existing flood control systems and standards. We also need to explore whether it is happening elsewhere or unique to Sydney,” the article says.

The article is available here.