Driving through flood water: what to do

October 05, 2022 by

Feeling wet behind the ears about whether you should drive through flood water? We have a crash course for you…

If you’re looking for a quick bit of advice about driving through flood water, here it is: don’t – unless you know – not think, know – that your car will be able to make it out the other side without sustaining damage – or worse, leaving you in a perilous situation.

Driving through flood water: what causes flooding

The phrase ‘flood water’ may bring to mind submerged roads and washed-away trees, but flood water doesn’t have to be so dramatic.

Burst river banks and severe storms can certainly bring significant flooding, but heavy rainfall, blocked drains, a loss of local wetlands and building on flood plains all have a part to play (alongside other factors) in the formation of flood water, meaning there need not necessarily be an extreme weather event for you to encounter this hazard.

Driving through flood water: heavy rain

The presence of flood water assumes there is or has been heavy rainfall, so it’s worth running through some quick tips on this topic first.

Wipers and tyres
Your windscreen wipers should be in good condition, being able to clear the glass without leaving streaks or squeaking annoyingly. A new set of wipers in the run-up to autumn never goes amiss.

Also be sure to check the condition and tread depth of your tyres: they’re the only thing connecting your braking and steering inputs to the road. The legal minimum tread depth for tyres is 1.6mm, but it’s generally recommended to replace them when they drop below 3mm – this is about the depth of the border on a 20-pence coin. Look for rips and tears in the rubber as well, plus any signs of perishing.

Slow down
Stopping distances in the rain will be at least double those in good conditions, so keep your speed down and be sure to allow a greater distance between your car and the vehicle in front.

Easy off the accelerator if you aquaplane
Aquaplaning is when a layer of water gets between the road and your tyre, essentially robbing you of grip and control. Taking your foot off the accelerator and not making ant steering inputs should help restore order.

Also be sure to have your headlights on in rainy conditions when visibility is poor, and don’t leave your bonnet open if you break down, as water can get into the electrics.

Driving through flood water: flowing water

Given the spray from this car, it is likely being driven too fast; further trouble will be added if the white car ahead gets stuck

If the water on the road is flowing fast, find an alternate route. It doesn’t take much depth for fast-flowing water to wash away a car (about 30cm will do it), and people do die in flood water.

Driving through flood water: standing water

If you are in any doubt about how deep the water is, don’t drive through it. Get out first and use a stick to gauge the depth, making a mental note of where on the stick the water comes up to, and comparing this to your car. You may need to get your feet or trousers wet to do this, but beware that even gradually flowing water can knock you off your feet, while flood water can be hiding sharp or dangerous obstacles and trip hazards.

If the water is higher than the lowest part of the car’s bodywork (generally the bottom of the front bumper or the sills beneath the doors), your safest bet is to not drive through it.

It is also worth highlighting that because water cannot be compressed, if any is sucked into your engine it will be the engine that bends and breaks, not the water. It takes very little water to ruin vehicle electronics, too.

Also bear in mind that if you can’t see the road through the water because it is too dirty or deep, you will have no way of knowing if there are drain covers or manholes that have been floated, presenting more potential perils.

If you’re comfortable with all of this and want to drive through flood water, bear the following in mind:

1. Go one at a time
Take it in turns to let cars go ahead of you: if a lead vehicle gets stuck or breaks down you may end up having to reverse back out of the flood – tricky at the best of times, and trickier still if there are other people behind you.

Apply the same logic to people coming the other way: oncoming traffic can create a bow wave, making the water deeper at points, and increasing the risk of your car being overcome.

2. Drive in the middle of the road
Roads are higher in the middle than at the sides, so use this to your advantage. Driving in the middle will also minimise the chances of you driving onto a soft verge, or hitting a kerb.

3. Keep your speed low, but not too low
Water is powerful stuff: drive too quickly and it could rip bumpers or other components off your car, your tyres could lose contact with the road, or water could be forced up into the engine bay, or behind wheel-arch liners and the like.

You do want to keep some speed up, though, as this will create a small bow wave that will keep the water lower than it might otherwise be. As with so many things in life, using your own judgement is best, but 3-4mph should be roughly right. Be prepared to slip the clutch in a manual car to keep the revs up, and don’t let the car stop moving, either.

4. Keep your revs high
Stick to first gear in order to keep the engine speed high; if in a car with an automatic transmission, lock into a low gear if the gearbox allows. Keeping the revs high will help keep water from entering the exhaust, while also reducing the risk of the car stalling.

5. Test/dry your brakes once on the other side
Once out of the flood water, apply your brakes to clear moisture from them, and to ensure they work effectively before you actually need to use them.

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