Why the SES's "15 to float" is wrong, but the message is still right

Just had a reader ask about a post from VicRoads saying that it only takes 15cm of water to float a small car. That seemed wrong to them, so I investigated and found the claim is misleading.

The underlying message is about driving in floodwater, which is extremely hazardous and should never be risked. The 15cm comes from a university study.

However, the results of the university study have been miscommunicated. The data is as follows; the actual test vehicle was a Yaris, weight 1045kg, ground clearance 15.5cm…so no way would it have floated in 15cm of water. The report states it took 60cm of water to float the car:

“In the testing conducted for this investigation, a Nissan Patrol 4WD with a kerb weight of greater than 2.4 tonne was shown to completely float in a water depth of 0.95 m. Smaller vehicles like the Toyota Yaris with weight of about 1.0 tonne are far more vulnerable due to their lower kerb weight. The Toyota Yaris tested in this investigation completely floated in water 0.6 m deep.”

The 15cm figure is the water depth at 12.6km/h required to move the car, not float it. Also, as you all are detail-oriented, it’s not just weight it’s design of the car; a 1000kg car with ground clearance of 10cm will float in less depth than a 1000kg car with clearance of 20cm.

The report goes on:

“In fast moving flows, which can be upwards of 3 to 4 metres per second when flows pass over bridges and causeways, this means that the smallest vehicles on Australian roads can become vulnerable when the flow is above the vehicle floor level in flood depths as shallow as 15 cm.”

And:

“As the flood water depth increases above 0.2 m, the force required to move the Toyota Yaris sideways rapidly decreases until the rear wheels are floating off the ground and effectively zero force is required to move the car sideways at a depth of 0.6m.”

The problem with saying “15cm to float” is that it doesn’t pass a smell test for anyone who knows anything about cars, and it’s not true as the car is being moved, not floated.

I feel this devalues the message, and it also doesn’t highlight the real risk of moving water. However, the public need simplistic messages so I guess it works from that perspective. It’s hard to balance accuracy with a cut-through message, but I feel this one is a bit lacking.

A good way to look at it would be that if it takes 0.6m to float a Yaris, then it only takes a quarter of that depth, 0.15m, of fast-flowing water to move it. I think that’s a pretty powerful stat.

We all know 1L of water = 1kg, so consider a car that’s 4m long…hit by a wall of water 4m long travelling at running pace, easily four tonnes plus. That’s a lot of weight hitting a nice flat surface so no wonder the car moves easily once the water gets onto the body panels.

The report says humans are better able to stand in floodwater than cars, which makes sense given we are far denser (80% liquid) and much less frontal area, no trapped air in the cabin, no tyres.

So to summarise:

do not drive in floodwater, no matter how shallow, it’s very likely there are unseen hazards not normally found in well-used water crossings
a small car will not float in 15cm of water, as that will barely reach its underbody.
the risk is proportional to water speed; a small increase in water speed can mean a BIG increase in being washed away, and that could be as little as 15cm of water for a small car subjected to high-speed water.
once moving water gets to the car’s ground clearance point risks escalate rapidly, 4WD or not
again, do NOT drive in floodwater, even if still, and moving floodwater is even more deadly due to risks such as rollover

Full report here:

https://www.wrl.unsw.edu.au/…/WRL-TR2017-07-Vehicle…