Drivers risk death when driving into flood water: new study

Andrew Gissing
Fran Molloy
28 November 2018
Faculty of Science and Engineering


As torrential rain and flash flooding hit NSW on Wednesday, drivers were warned again it is never safe to enter flood waters - as Macquarie research has shown, a slight error of judgement can be the difference between life and death.

At least a dozen drivers were rescued from floodwaters across Sydney on Wednesday, leading police to issue repeated warnings to commuters to stay off the roads as torrential rain lashed areas across News South Wales.

Deadly risk: Flood waters are the nation’s second-highest natural disaster killer after heatwaves.

Roads were flooded, train services suspended, and thousands of homes and businesses left without power after a month's worth of rain fell in two hours.

The NSW State Emergency Service says it made 12 flood rescues by 10am - including in Marrickville, West Pymble, West Ryde, Silverwater and Macquarie Park - and urged drivers not to enter floodwater, warning that a "major cause of death during floods is by people entering floodwater".

It follows extreme weather in Tasmania in May, which saw police issued repeated warnings reminding residents to avoid driving into floodwaters, as heavy rains and wild storms saw cars swept along Hobart streets, many roads closed and hundreds of calls for help lodged with emergency services.

Despite this, at least two people had to be rescued after they ignored warnings and tried to drive through flood waters in New Town, Hobart, and one boy was winched to safety by helicopter after riding his pushbike into a danger zone.

Stormy Sydney: Flood water on the road at Northbridge in the suburb of Castlecrag on Wednesday. Credit: Supplied/Richard Denyer.

Research by the Bushfire and Natural Hazards Cooperative Research Centre found that most of the 178 flood-related deaths in Australia since 2000 have resulted from motorists driving into flood waters.

Australians just can’t help themselves. Even though floodwaters are the nation’s second-highest natural disaster killer – after heatwaves – we keep driving into them, and new research suggests that’s because many of us think we know how to work out when it’s safe.

Drivers think they can judge safe floods

Macquarie research fellow Andrew Gissing says that driving through floodwater is an entrenched social behaviour - and many people make a deliberate choice to do it.

“The safety of floodwater can often be very difficult to judge - in particular the speed and depth of the water and what might be underneath,” Gissing says. “Making only a slight error of judgement can be the difference between life and death.”

His team conducted research following West Australian floods in 2017, funded by the Department of Fire and Emergency Services. Using focus groups and surveys of residents, they examined what determines people’s decision-making around entering flood waters. About 84 percent of those interviewed had experienced flooding on the road while driving.

The research, to be presented at the upcoming Floodplain Management Australia conference, found that while nearly everyone agreed that entering floodwater is dangerous – most people also believed they could assess some circumstances where it’s safe.

“For example, 26 percent believed that it was somewhat or completely safe to drive through knee-deep still flood waters,” he said.

“The majority of drivers who had entered flood waters had done so on multiple occasions and would do so again if faced with the same circumstances."

Many of these drivers also didn’t have good knowledge of the limitations of their vehicles in floods, he adds.

“When uncertain, some drivers claimed to take further actions to reduce the uncertainty, such as walking into the flood water to assess water depths and velocity before deciding to enter, or watching another vehicle drive across.”

There were three key things that drivers thought they could judge: the level of danger involved in the flood water, the safety of their vehicle, and their own skill as a driver.

“It can be very hard to judge the depth and the speed of flood waters, which can rise up very quickly,” he said.

Research shows that cars can sink in deep water in just a few minutes, and many deaths from floods involve passengers trapped in submerged vehicles.

Natural hazards research shows that cars can be swept quickly from a roadway into deep water nearby, and safety measures that governments can implement include installing roadside barriers, depth indicators, good lighting and prioritising road closures in known danger zones.

“Despite prominent warning signs and road closures, though, people persist in driving into floods,” Gissing said.

Getting the message across

Education campaigns and warnings from emergency services generally agree on the message to never enter flood waters. But this message may be ineffective for many drivers – because even though they acknowledge the dangers, many who hear the message have escaped harm when they’ve previously driven into flood waters.

“Also, given the differing definitions as to what flood waters are, the message is open to individual interpretation,” Gissing said.

He suggests that better messaging could question whether people can accurately interpret flood conditions and whether it’s safe to cross floodwaters, despite previous experience.

“We need to highlight how much uncertainty there is when you assess flood waters and how experience from previous floods won’t translate each time because they are all different.

“Misjudging flood water depths by just centimetres can be deadly.”

Andrew Gissing is an emergency risk management and resilience expert and Adjunct Fellow in the Department of Environmental Sciences.


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