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Driving through water: is it safe to do so?

Is it safe to drive through water?

Martynn Randall is technical editor at Haynes and has been with us for approaching 30 years. He's written more than 60 Haynes publications and has owned more than 85 cars and 60 motorbikes... so far!

Flood water

As someone who has lived on the edge of the Somerset levels for many years (this image shows the view from my gate after days of rain), I believe I have some wisdom to dispense concerning driving through surface water and floods.

And having seen a few YouTube videos of people driving through deep fords at top whack, with predictable results, it seems there are plenty out there who would benefit from the advice.

Some of it is common sense, some of it may not apply so much if you have a high-riding vehicle, but it's a useful refresher on how to drive safely when extreme weather events occur.

How to drive in floods?

The best advice about driving through a flooded section of road is… don’t. It’s extremely dangerous and it does your car no good at all. The roads often have ditches running parallel to them, and if the water’s deep enough, it’s not possible to tell where the road stops and the ditches start. And you do not want to drive into a flooded ditch.

As well as the immediate danger, flood water will wash any lubrication from the suspension joints and bearings. It can remove the grease between the brake pads, the calipers and the CV joints if the boots are damaged. And if it gets into the transmission bell housing, it can cause the clutch to fail.

Apart from being swept away by the flood waters, the main danger of driving through them is ingesting water into the engine. Fluids don’t really compress, so if water gets into the combustion chamber, the pistons will hit the water and something will break. It's called hydraulic locking, or hydrolocking, and it could be the piston, connecting rod, crankshaft, cylinder head, etc. Basically, the engine will be wrecked.

If you absolutely have to drive through that flood, here are a couple of points to bear in mind:

  • Keep the car in a low gear (first, preferably) and go slow to keep the bow wave as small as possible. The engine air intake is usually just below the bonnet line at the front.
  • Unless you can see that the water is shallower on one side, try to stick to the middle of the road. The camber often makes the water deepest at the edges. Obviously, you'll have to rely on there being no oncoming traffic, or hope that vehicles on the other side of the flood will wait for you to come through. If they don't, their bow wave could well swamp you.
Car driving through water

Floodwater (near) disasters

I’ve had a couple of floodwater-related driving incidents over the years:

The first was many years ago, driving home from work in a Datsun 280ZX. As I drove into the flooded road, I could see the current going from left to right over a flooded bridge. When I got to the bridge, the water lifted the car off the ground and put it down again about a metre to the right – just next to the river bank and deep water. Needless to say, I was shaken.

The second was only a couple of years ago. Driving home again, but this time in my BMW Z3. I’d already tried two routes to get home, but was turned back by floods that I thought were too deep to attempt to drive through.

Using a third route, I got to within about half a mile of my house, when I came across a flooded section. It didn’t look too deep or too long, so I thought it was worth the risk. I slowly made my way through with the driver's door open to judge how deep it was. As soon as it came to the top of the door sill, I was going to reverse out. However, about two thirds of the way through, the engine just stopped. I tried to restart but nothing electrical worked.

I had no choice but to get out and push the car out of the flood. As I was doing this, the cause of the failure dawned on me. On this engine, the starter motor is located at the bottom of the engine block, and the battery lead attached would have shorted out when submerged. Sure enough, as soon as I got out of the water, the car started up again.

car driving in puddle

How to drive on surface water

Driving slowly through puddles can be kind of fun in a childish way, but hitting surface water at speed can be terrifying. Above about 30mph, modern low-profile, wide tyres will begin to aquaplane when they encounter standing water. At 70mph it’s a guarantee.

Aquaplaning is when the tyres hit the water so hard that they are lifted upwards and lose contact with the road beneath. The steering and brakes are effectively useless and the vehicle is rushing to the scene of the nearest accident.

If just the nearside or offside wheels come into contact with standing water at speed the car could be forced to turn sharply left or right. Scary stuff – best avoided by slowing down lots.

Wet weather driving tips

  1. Check the weather forecast Before you even start your trip, check the weather forecast to be aware of any impending storms. If the conditions are severe, consider postponing your trip if possible.
  2. Emergency kit Pack an emergency kit in your vehicle in the unlikely event you get stranded during the storm. Be sure to include some warm, waterproof clothing, extra food and drink, as well as a good quality, waterproof torch to help with visibility for when you leave your vehicle.
  3. Prepare your vehicle Ensure your vehicle is in good working condition. Check the tyres' tread depth and pressures, brakes, lights and windscreen wipers. Make sure your vehicle has enough fuel.
  4. Drive slowly Reduce your speed and increase your following distance. Wet or icy roads reduce traction and increase braking distance, so leave plenty of space between you and the vehicle in front of you.
  5. Use headlights Turn on your headlights, even if you have daytime running lights. This increases your visibility to other drivers. Dipped headlights will improve visibility in bad weather conditions such as heavy rain and fog – and remember to use your rear fog lights if visibility worsens.
  6. Stay in your lane Avoid sudden lane changes and erratic manoeuvres. Stick to your lane to reduce the risk of aquaplaning.
  7. Brake gently If you need to brake, do so gently and gradually to avoid skidding.  If you start to skid or slide, remain calm. Steer in the direction you want to go, and avoid overcorrecting.
  8. Reduce distractions Minimise distractions in the car. Avoid using your phone and stay focused on the road.
  9. Stay aware Even after a storm has passed, road conditions can remain hazardous due to debris, fallen branches, and flooding. Exercise caution until it's clear that the roads are safe.