Flash flooding risks increase as storm peak downpours intensify, research shows

  • Thirty-year weather records from 79 locations across Australia reveal peak downpours during storms are intensifying, leading to greater flash flood risks in cities.
  • Findings true across all Australian climate zones, suggesting global implications.
  • Results published in the journal Nature Geoscience.

Sydney, Australia, Tuesday 9 June: Patterns of peak rainfall during storms will intensify as the climate changes and temperatures warm, leading to increased flash flood risks in Australia’s urban catchments, new UNSW Australia research suggests.

Civil engineers from the UNSW Water Research Centre have analysed close to 40,000 storms across Australia spanning 30 years and have found warming temperatures are dramatically disrupting rainfall patterns, even within storm events.

Essentially, the most intense downpours are getting more extreme at warmer temperatures, dumping larger volumes of water over less time, while the least intense periods of precipitation are getting weaker. If this trend continues with future warming, the risk of flooding due to short-term extreme bursts of rainfall could increase even if the overall volume of rain during storms remains the same. The findings were published today in the journal Nature Geoscience.

“These more intense patterns are leading to more destructive storms, which can significantly influence the severity of flood flows,” says lead author and PhD candidate Conrad Wasko, from the UNSW School of Civil and Environmental Engineering. “The climate zones we studied in Australia are representative of most global climates, so it’s very likely these same trends will be observed around the world.” 

Previous studies have looked at rainfall volumes over the total duration of storms, but this latest UNSW study is the first to look at temporal rainfall patterns within storms. Australian Bureau of Meteorology data from 79 locations across the country were used instead of computer simulations.

“Our results were consistent across all the climate zones in Australia, regardless of season or storm type, without exception,” said co-author and UNSW engineer, Professor Ashish Sharma. “This was an unexpected finding, and it supports our hypothesis that increasing temperatures are changing rainfall patterns.”  

“It means that most people in Australia can expect to see intensification in the magnitude of flash flooding in smaller catchments, particularly in urban or residential areas,” says Professor Sharma.

The paper quantifies this increase for a range of Australian cities representing different climates and finds that for a five degree Celsius temperature rise flood peaks could increase by an average of 5% to 20% for a typically sized catchment.

The increase in flood peaks are estimated to be about 12% for Sydney, 10% for Perth, 19% for Hobart and 45% in Darwin, which has a tropical climate and a markedly different storm temporal pattern that the other cities. While these increases are for a medium sized typical catchment, and represent the 1 in 100 year flood extremes, the fact that they represent the change only through intensification of the within-storm rainfall (and not an increase in its volume, which is supposed to go up at 7% per oC because that much more moisture is held in a warmer atmosphere) is a cause of concern.

These results highlights the need for local councils to think about redesigning stormwater and road infrastructure, and updating guidelines about where it’s safe to build homes, says Sharma.

The team analysed the 500 largest storms by rainfall volume from each of the 79 locations. These storms were divided into five equal time-periods to determine when rainfall was heaviest.

Wasko says the next step is to incorporate the findings into a model that can predict rainfall in future climates. “That’s the big caveat,” he says. “Is our present-day climate indicative of potential changes to future climates? Or will we begin to see entirely new weather systems emerging as climates change?” 

The full article is available at dx.doi.org/10.1038/ngeo2456.

Media Contact: Myles Gough, UNSW Media Office, 0491162717, myles.gough@unsw.edu.au

Professor Ashish Sharma, a.sharma@unsw.edu.au | Conrad Wasko, conrad.wasko@unsw.edu.au