Automotive

Water hazard – even big cars can be swept away in shallow floodwaters

Researchers at the University of NSW say just 15 cm of water can make a car unstable
Researchers at the University of NSW say just 15 cm of water can make a car unstable
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Even big four-wheel drives can be turned into giant floating toys 
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Even big four-wheel drives can be turned into giant floating toys 
Researchers at the University of NSW say just 15 cm of water can make a car unstable
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Researchers at the University of NSW say just 15 cm of water can make a car unstable

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.

Even big four-wheel drives can be turned into giant floating toys 
Even big four-wheel drives can be turned into giant floating toys 

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.

Source: University of New South Wales

Car flooding time-lapse - UNSW Water Research Laboratory

5 comments
Milton
Take the doors off next time. I had a wheeler that I would submerge on occasion (with no doors). The thing did great, but I went through a lot of alternators.
Mel Tisdale
It is not impossible to have an opening panel on the floorpan somewhere that is opened when sideways movement is detected via inertial navigation chips, or even a simple water depth measurement. It is much better to have a ruined interior trim than to be swept away.
tyme2par4
@Mel Tisdale: It's better to just not attempt to drive through flood waters... This study didn't even take into affect the movement of water. Yes, 24" of stagnant water can float a Yaris, but 12" of rushing water would easily wash it down stream. If the water is moving, don't ever try to cross it if you aren't 100% sure it's not more than a couple of inches deep.
Grunt
That's why old Land Rover Defenders are so good in floods. They let in so much water so quickly through all the various gaps that there is little chance of them actually floating. However, having said that, once flowing water reaches the sills, the exposed cross-sectional area increases rapidly and things can get problematical very quickly.
Bill Bennett
There is no way I would drive into water in my URQ, the low engine air intake would cause a hydro lock on the engine, I would never forgive myself.