Driving through floodwater and heavy rain can feel quite perilous at the best of times, often catching out newer drivers or those who don't have any experience with driving in adverse conditions.
With our quick and easy guide though, you'll feel better-equipped to navigate these tricky driving situations.
How to drive in heavy rain
- Turn your headlights on – the Highway Code says you must use them when visibility is seriously reduced (less than 100m).
- Use fog lights if you like, but switch them off when visibility improves.
- Leave twice as much space between you and the car in front – it takes longer to stop in the wet.
- If your steering feels light due to aquaplaning, ease off the accelerator and slow down gradually.
- If you break down don't prop the bonnet open while you wait. Rain-soaked electrics can make it harder to start the engine.
How to drive in floods and standing water
If you hear there's flooding on the way, try to move your car to higher ground to stop it from getting damaged. Water plays havoc with vehicle electrics and could cause a number of issues (like your airbag suddenly deploying for no reason). Otherwise, follow our tips below:
- Try to avoid standing water if you can.
- Don't drive into flood water that’s moving or more than 10cm (4 inches) deep. Let approaching cars pass first.
- Drive slowly and steadily so you don’t make a bow wave.
- Test your brakes as soon as you can afterwards.
- Fast-moving water is very powerful – take care or your car could be swept away.
If you do get stuck in flood water, it's usually best to wait in the car and call for help rather than try to get out.
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Why slow down in flood conditions?
Driving fast through water is dangerous, inconsiderate and can end up being very expensive.
Your tyres can lose contact with wet roads, causing you to lose steering control – this is called aquaplaning. If you feel it happening, hold the steering lightly and lift off to slow down gently until your tyres grip again.
At anything above a slow crawl you’ll cause water to splash onto pavements, soaking pedestrians or cyclists. You could be fined and get points on your licence for this.
It only takes an egg cup full of water to be sucked into your engine to cause significant damage. For many cars, the engine’s air intake is low down at the front, making it quite susceptible to water damage.
If you need to leave the vehicle:
- Look out for slip and trip hazards like kerbs under the water.
- Be aware that maintenance hole covers can get lifted and moved.
- Water levels can change quickly.
- Assume that flood water is contaminated.
Types of floodwaters and their effects:
- Urban flood water can carry dangerous bacteria from drains and sewers that could cause disease.
- Rural flood water is more likely to be contaminated by agricultural chemicals and animal waste.
Just because the road goes into the river on one side and comes out on the other, that doesn’t mean a ford is safe to cross. The depth and speed of the water changes with the weather.
More on safely fording a river.
- Most drowning deaths happen within only 3m of a safe point.
- Two thirds of those who die in flood-related accidents are good swimmers.
- A third (32%) of flood-related deaths are in vehicles.
- Cold water reduces your muscle strength – 20 minutes in water at 12C lowers muscle temperature from 37ºC to 27ºC, reducing strength by 30%.
- Just 15cm of fast-flowing water can knock you off your feet and be enough for you not to be able to regain your footing.
- It's a challenge to stand in waist-deep water flowing at only 1m/s. By 1.8m/s (4mph) everyone is washed off their feet.
- If the speed of the flood water doubles, the force it exerts on you or your car goes up four times.
- 60cm of standing water will float your car.
- Just 30cm of flowing water could be enough to move your car.
- A mere egg cup full of water could be enough to wreck an engine.
- Floodwater can be contaminated and carry diseases.
- Culverts (tunnels carrying water under a road) are dangerous when flooded – the siphoning effect can drag in pets, children and even fully grown adults.
What might happen if you drive into floodwaters?
Depending on the level of the floodwater, it's possible that your car gets swept away, causing irreversible damage to the integral parts of the vehicle.
Some other things that could happen include:
- Damage to your car's electrical systems.
- Reduced or no braking.
- Aquaplaning, which can cause you to lose control of your vehicle and crash.
- You can hit objects hidden under the water (e.g. maintenance hole covers that have come loose in the flood).
- At lower water levels, you might splash pedestrians, which could get you fined.
How do I know if water got into my engine?
There are a few key symptoms to look out for if you think water has gotten into your engine. These are:
- A fast cranking sound or whirring when you turn the key in the ignition.
- An abnormally strong smell of petrol, especially near the exhaust pipe.
- Your car won't start at all, or starts briefly and stops again.
Checklist for driving in heavy rain & adverse conditions
If you know you need to drive in heavy rain or bad weather conditions and you can't avoid it, make sure to carry out the following checks on your vehicle before you get going:
- Check the equipment of your car (e.g. windshield wipers, headlights, fog lights, tail lights).
- Check the tread of your tyres if you haven't already (if you've lost a lot of tread, it'll be more difficult to grip on wet surfaces).
- Ventilate your car if the windows are fogged up. Visibility is key.
- Test your breaks if possible before joining a busy road.
- Drive slowly to avoid losing control on wet surfaces – your reaction time may not have changed, but the reaction time of your car increases in wet weather (e.g. braking takes longer, which is why it's important to maintain a larger distance between yourself and the car in front).
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How should you drive through floodwater?
If you can't avoid driving through floodwater and it's not so high that it will damage your engine, then try to drive through it slowly in first gear, around 3-4mph, but with higher than usual revs. It's important that you keep the car moving and avoid stalling, so be prepared to engage the clutch and accelerate to stop this from happening.
What depth of water can a car drive through?
Generally, it's not advised to drive through water deeper than 10cm, unless you have a 4x4 that sits slightly higher than the average vehicle.
What should you do after driving through a flood?
First off, check your brakes – they can become clogged after driving through water, especially if you've driven through a muddy area. In fact, the underside of your car could have debris stuck to it, so it's best to have a look underneath if you can.
If you've driven through quite high floodwater, then check your lights and car interiors for damage. You may also want to ensure that your car's fluids haven't been contaminated e.g. the engine oil. You can use the dipsticks to check for discolouration of the liquid (if it looks milky or diluted then you may have to get the fluids replaced).
Can heavy rain damage a car engine?
Most cars are designed to withstand regular amounts of rain, but if flooding happens then it can be a big problem for your car's engine. If flooding occurs and there's standing water, for example, this can get into your engine and cause critical damage. If possible, always avoid driving in heavy rain or floods.
01 March 2023