It doesn’t seem to matter how ardently or often authorities urge caution – motorists still insist on ploughing their cars into floodwaters, often with disastrous consequences.
Three men died recently after being swept away while trying to drive through floodwaters in separate incidents in the ACT, the NSW southern highlands and Sydney's southwest, and the NSW State Emergency Services launched more than 80 rescues of stranded cars.
NSW SES Acting Commissioner Greg Newton says the high number of flood rescues is distressing.
“People need to re-think their actions and not drive into floodwater, because by doing this they are not only placing their lives at risk, but the lives of our volunteers who have to go out and rescue them,” he says. “Entering floodwater is the number one cause of death and injury in a flood, so everyone should stay out and stay alive.”
Emergency workers in south-east Queensland have been forced to rescue multiple drivers who became stranded after driving into floodwaters, caused by a weekend of torrential rain.
Brisbane chief fire officer Steve Hollands was exasperated by the cavalier attitude of some drivers.
"It is beyond me that people still enter water — some of our vehicles had to park across the road to stop more people from driving into where they had just rescued someone," he told ABC.
Now, world-first testing from engineers at University of New South Wales proves just how easily cars can be washed away by even the smallest currents – findings that will hopefully deter drivers from testing their luck against floodwaters.
Previous experiments to understand the force of floodwaters have relied on using vehicle miniatures, rather than actual cars.
The researchers themselves were surprised by just how little water was required for motorists to get into a sticky situation, principal engineer and research leader Grantley Smith says.
“What was surprising was just how little water it took to make even a large vehicle unstable,” he says. “They became vulnerable to moving floodwaters once the depth reached the floor of the vehicle. Even in low water depths and slow flow speeds, floodwaters had a powerful enough force to make them float away.”
The researchers found a small car like a Toyota Yaris, weighing 1.05 tonnes, became buoyant in water only 15cm deep and with a flow speed of just 3.6km/h. When the water reaches 60cm high, the car completely floats away.
Even a 2.5 tonne Nissan Patrol 4WD can be rendered unstable by floodwater 45cm high, and a similar flow speed. Once the water reaches 95 cm, the four-wheel drive can completely float, and needs almost zero force to move it by hand.
By contrast, an able-bodied adult is much more stable in flowing water than the 4WD.
Why?
Modern cars are manufactured to be very airtight, which makes them more comfortable. Unfortunately, it also means they float more easily than their older counterparts.
Smith says people also underestimate the power of floodwaters.
“People don’t realise that even slow-moving water packs a powerful punch,” he says. “Water is heavy: each cubic metre weighs about 1000kg.
“If a house is exposed to floodwaters two metres deep and 20 metres wide – travelling at a steady 1 metre/second – the force is equivalent to being hit by a 40-tonne semitrailer every 15 seconds.”
The experiments were funded by UNSW and the NSW State Emergency Service, the NSW Office of Environment and Heritage, with IAG providing the cars.
Image: Principal engineer Grantley Smith, of the UNSW Water Research Laboratory. Photo:Grant Turner/Mediakoo/UNSW.