Car Care

Driving in heavy rain or floodwater


With the recent flooding this week in Canterbury and surrounding areas on the South Island, it is a timely reminder that flooding can happen at any time of the year, but especially in our winter months.

Flooding occurs when the input of water is greater than drainage can take away. This can be caused by a significant amount of rain, or by a river bursting its banks, or even a burst water main.

The key advice for motorists is to avoid travel in situations where you could encounter flooding or other road hazards unless it is absolutely essential. Always follow the advice of Civil Defence and emergency services and check for road closures before you head off. You could also run into insurance hassles, so it’s best to check your (or your company’s) vehicle usage policies before setting out.

Don’t, but if you do…

If you have to drive when there’s been heavy rain and flooding, make sure your vehicle is capable of the task. If you have an SUV, Ute or vehicle with raised suspension you could be able to pass through water that is deeper than a standard car will pass through. The standard car sill height can be around the 15-20cm and so water could be nearer your doors as it rises. In water a foot deep, most cars would start to float or feel a bit light, and double the depth could be in danger of being swept away in flowing waters.

It is recommended not to exceed 5kmh when driving through water as to not overtake the bow wave generated at the front of the vehicle. If a vehicle is coming in the other direction, let it past first so that your two bow waves don’t meet. If you’re following another vehicle, wait for it to clear the flood, because if it becomes stranded you don’t want to be trapped behind it as you could run into trouble if you lose momentum.

The aftermath

If a vehicle drives through shallow flood waters, then it usually will dry out by itself after a few kilometres of driving. The brakes will dry off with heat, so the driver would need to lightly apply them after exiting the water to ensure they will operate safely next time they are applied.

The AA recommends that you have your air filter checked by your automotive vehicle service and repair provider if the pressure wave in the front of the vehicle was near bonnet level, as this is usually where the engine air intake is.

If a vehicle sits or drives in deep flood water, it’s a different story

Driving into deep water at speed is like driving into a wall. The resistance of the water will slow your vehicle down extremely quickly. Water will be pushed in a wave ahead of your vehicle, entering into the bonnet and could be forced into the engine, damaging parts (water doesn’t compress, so even a small amount inside can be enough to bend internal components).

If you find your vehicle has been caught in floodwaters

Even if water hasn’t seeped inside the vehicle, it is not recommended the vehicle be driven.

Contact your insurance company and seek advice. The vehicle will need to be sent to your automotive vehicle service and repair provider to have all the driveline components checked and may require transmission and differential fluids replaced.

In some instances, even the CV joints in the axles of some vehicles may need to have the grease replaced as water may have seeped in through any cracks/holes in the rubber boots or ventilation valves. The brakes may have trapped mud or silt and require a thorough check and clean to ensure they work safely.

In the unfortunate event a vehicle is trapped in flood waters and has water inside, do not attempt to power up the system or start the vehicle as water could be inside the engine and electrical systems. We recommend calling your insurance company and follow their recommendations.

There is a high chance of a submerged vehicle being written off as the electrics, safety systems, engine and drive train would be full of water and may not recover even if dried out. The cost of replacing all the electrical and safety systems alone could cost more than the value of the vehicle.  

Check Waka Kotahi NZTA’s website for road closures, and visit AA Insurance and AA Auto Centre for advice.

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