NSW floods: What happens to a car if you drive through flood water

Driving through floodwaters the height of a pen are enough to put a motorist at significant risk. This is what can happen to a car in just 15cm of water.

The tragic death of a student after his hire car was trapped in floodwater at Glenorie on Wednesday has sparked renewed warnings not to drive in floodwaters.

An investigation will be held into the death of Ayaz Younus, 25, who died while trying frantically to escape the car as the water rose around him.

The SES has conducted 1013 flood rescues in the past week in NSW, a significant proportion involving vehicles, said SES spokesman Phil Campbell.

It’s not just currents, debris and damaged road surfaces that are deadly — an average vehicle that drives into water higher than 15cm will lose power as water enters the exhaust and washes into the air intake.

Modern cars with electrically-powered windows and locks are almost impossible to escape from once water finds its way into wiring and motors, disabling switchgear and pressing on bodywork, according to the NRMA.

Cars drive through floods at New South Head Road, Rushcutters Bay, during a heavy downpour of rain this week. Picture: Mark Evans

Mr Campbell said even off-road vehicles weren’t safe, as strong flows of water can disable four-wheel drive and other tools.

Groundbreaking research by the University of NSW in 2016 found as little as 15cm of water was enough for a small vehicle to be moved by water, reducing the driver’s control and endangering occupants. It completely floated away in 60cm of water.

The research, partly funded by the NSW SES, found a mid-sized car could be moved by water just 30cm deep and a four-wheel drive by 45cm, with fatalities associated with 4WDs in floodwater increasing significantly in the last 15 years.

Experiments found a 2.5 tonne Nissan Patrol 4WD could be rendered unstable by floodwater 45cm high, and a similar flow speed of 1 metre/second. Once the water reached 95 cm, it could completely float, and needed almost no force to move it by hand.

Modern cars also floated more easily because they were made airtight for comfort.

Mr Campbell said most flood deaths occurred in people close to home, often around sunset and sunrise, when visibility was poor, and all vehicle rescues put rescuers at risk.

“We work very much to try to discourage people who have that over-familiarity with the particular area,” he said. “The one’s who say I know this road in floods.

“You don’t know if this is that one time the road actually has been washed out and you can’t see it because the light levels are bad, it’s raining very heavily, it’s dark and the water is very dirty and murky.”

A Mercedes stuck in floods on the northern beaches this week after the driver attempted floodwaters on Boondah Rd Warriewood. Picture John Grainger

The other side of the coin, he said, were people with no experience of floodwaters and don’t understand how dangerous they can be.

Generally vehicle rescues peak when rain is heaviest, and while fewer rescues are expected as flood waters recede into the weekend, there was still a danger as evacuation orders lifted.

Any route with floodwaters, or a ‘road closed’ or a ‘road flooded’ sign should not be attempted.

Motorists shold find alternate routes or wait until the all clear is given for that road. It is safer to turn around than to enter floodwater.