A team from the University of Adelaide's school of civil, environmental and mining engineering has unveiled findings that shed light on the issue of flood risk in the latest issue of Nature Climate Change.

By analyzing data from rainfall gauges across the greater metropolitan area of Sydney, the researchers found that the potential for flood risk can both increase and decrease in the same geographic area.

Flash flood is caused by short but heavy rain events, and is usually localized. Source: www.srh.noaa.govRivers can create two different types of floods: one is caused by heavy rain sustained over long periods of time that might affect large catchments over a larger geographic area; the other is flash flooding that is caused by short but heavy rain events, and is usually localized.

"One of the common assertions of the climate change discussion is that 'flood risk will increase'. And on balance, yes that's probably correct, but we've found the issue is much more complex than such a blanket statement," says corresponding author and senior lecturer Seth Westra.

"At the global scale we're increasingly confident that flood risk will change, because a warming atmosphere means more heavy rain. 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," he says.

The data that the team has been using to draw its conclusions were collected every five minutes from 1966 to 2012.Results also show a distinct seasonal variation, they say. In summer, extreme rainfall increased strongly, while in the remaining seasons, the changes were smaller and sometimes extreme rainfall even decreased.

The research shows that short but intense rainfall events increased, while longer sustained heavy rainfall events tended to decline. The team says this complicated implications for flood risk, since floods in small catchments are usually caused by short rainfall events, whereas floods in large catchments require longer periods of heavy rainfall.

“Each region will have its own unique features that determine the flood risk from both short and long-term rainfall events,” Westra says."What we are showing is that historical changes to rainfall patterns are much more complicated than is commonly appreciated." This means there is a lot more nuance in how flood disasters might change as a result of climate change, which he says has not been part of the commentary on flood risk.