News coverage of floods is inevitably accompanied by footage of stranded motorists who have attempted to drive through floodwaters, despite warnings from emergency services. The recent floods that hit the Australian east coast were no different, but a new study out of the University of New South Wales (UNSW) may give drivers pause before risking their lives in the next downpour. Researchers have found that vehicles – even burly four-wheel drives – can be swept away in even remarkably shallow water.

In an attempt to discover just how easy it is for motorists to be swept away in floodwaters, engineers at the UNSW Water Research Laboratory used a specially configured test tank to replicate conditions encountered by motorists in flash floods. In what they claim is a world first, the team used actual cars rather than the vehicle miniatures that have been used in previous experiments – and the results surprised them.

According to the researchers a small car like a Toyota Yaris, which weighs just over one tonne (1.1 tons), can be moved in just 15 cm (6 in) of water flowing at a rate of 3.6 km/h (2.2 mph). Ramp the water level up to 60 cm (24 in), and the little Yaris will float away.

And those who think their big four-wheel drive is better qualified to ford every stream, think again. A Nissan Patrol weighing in at 2.5 tonnes (2.76 tons), with an official wading depth of 70 cm (27.6 in), can be made unstable in just 45 cm (17.7 in) of flowing water. Once those floodwaters swelled to 95 cm (37.4 in), it was completely floating, and could be pushed with almost no effort.

If you're wondering how two tonnes of expensive metal can be turned into a floating bath toy, some of the blame can be attributed to the airtightness of modern vehicles. Although being so well sealed helps make climate control systems more efficient, it also makes the vehicle more likely to float when confronted with floodwaters.

"People don't realize that even slow-moving water packs a powerful punch," says Principal Engineer Grantley Smith. "Water is heavy: each cubic meter (35 cu-ft) weighs about 1,000 kg (2,204 lb). If a house is exposed to floodwaters two meters (6.6 ft) deep and 20 meters (66 ft) wide – travelling at a steady 1 metre/second – the force is equivalent to being hit by a 40-tonne (44-ton) semitrailer every 15 seconds."

The tests were funded by UNSW, as well as the NSW State Emergency Service and the NSW Office of Environment and Heritage.

A timelapse video of the experiment is below.

View gallery - 2 images