Researchers from the University of Adelaide have discovered much greater complexity in assessing the potential for flood risk than previously realised.

A team from the School of Civil, Environmental and Mining Engineering analysed rainfall data from greater metropolitan Sydney that was collected every five minutes from 1966-2012.

They say historical changes to rainfall patterns are more complicated than previously thought, which makes understanding the impact of climate change less certain.

The research shows risk levels can both increase and decrease in the same geographic area, which challenges the common assertion that climate change means greater flood risk.

Flood hydrology researcher Seth Westra says on balance that assertion is probably still right, but regional factors also play a role.

“At the global scale we’re increasingly confident that flood risk will change, because a warming atmosphere means more heavy rain,” Dr Westra said.

“However, for any individual location, the changes to flood risk will depend on each region’s rainfall patterns. Under certain circumstances, the flood risk may actually decrease.”

The researchers identify two major types of river flood: one caused by heavy rain over long periods that might affect a wide geographic area; the other caused by short but extremely heavy rain spells, usually localised, and often called flash flooding.

“There’s a lot more nuance in how flood disasters might change as a result of climate change, which hasn’t been part of the commentary on flood risk until now,” Dr Westra said.

The findings are published in the journal Nature Climate Change.