Some areas are more prone to flooding than others. This problem is set to increase in the UK with the onset of global warming.
Floods can occur when rivers burst their banks, after a period of heavy rainfall. Large volumes of water can cause flash-floods, or floods in urban areas where the sewers and drains can't cope and there is nowhere for the water to soak away.
As with all driving emergencies prevention is better than cure; in the case of flooding this means watching the weather forecasts before you set out on a journey, if flooding is widespread you might be better off cancelling trips that are not absolutely necessary.
If you are in a flood affected area consider moving your car to a place of safety when you first hear the warnings, but also be aware that if flooding has started moving your vehicle could pose a serious risk - never underestimate the dangers of flood water.
Driving at speed into water that is more than about 15 centimetres deep can have dramatic effects - it could almost feel like driving into a brick wall with loss of control. This is why it's especially important to watch your speed on roads where there might be unexpected patches of water (perhaps hidden by a bend or a dip in the road).
I caught the cars on the right on camera driving into about six centimetres of standing water at around 25mph - an instant after this picture was taken the red car lost control, luckily an accident was avoided. perhaps next time the driver might not be so lucky. In 15 minutes I saw several near misses at the same spot.
If you intend to drive through a flooded section of road, your first task is to check the depth of the water. In normal vehicles you should never attempt to drive through water that is more than about 25 centimetres deep (or up to the centre of your wheels).
It's also worth checking where the air intake is on your engine. If water is sucked into the engine it will stall, but worse than this, it can cause severe damage that will require the engine to be stripped down in order to bring it back to life. Do not try to restart an engine that has sucked in water - the plugs or injectors should first be removed to allow the water to be expelled.
Some four-wheel-drive vehicles are equipped with high level air intakes allowing them to be driven through water several feet deep, however, you can say goodbye to your deep pile carpet and Gucci Sneakers if you attempt this! And as mentioned above - even 4x4 vehicles can be washed away in flowing water. If the water is fast-moving - even 30 centimetres depth of fast-moving water could wash your car off the road.
Where possible flooded roads are best negotiated by one vehicle at a time. wait for approaching vehicles to clear the water before you start to drive through.
Using first or second gear (L or 1 in an automatic) drive slowly to avoid creating a large 'bow wave' (a small wave can be helpful but too much and the water can wash back into the engine). Slipping the clutch and revving the engine will also help to keep the exhaust clear and keep the engine running if water splashes onto the electrics. In an automatic keep your foot on the gas in the lowest held gear and use the brake to control your speed (and hope for the best!).
In some cases a stalled engine can result in water being sucked back through the exhaust into the cylinders - this can cause extensive and expensive damage. Do not change gear because this can also cause water to be sucked back through the exhaust (due to the change in engine speed and manifold depression).
Another potential cause of damage in floods is a cracked catalytic converter ('cat'). The 'cat' is part of the exhaust system and works at high temperatures; if it comes into contact with very cold water there is a possibility that the rapid contraction of the metal could crack the welded sides - OK if you have plenty of money to replace it!
If your wheels start to lose grip partway through a flooded section it could be that the car is trying to float. To counter this, open a door and allow some water into the car, this will weigh it down, enabling the tyres to grip again - it's probably best to get a passenger to do this so that you can continue revving your engine and slipping the clutch.
After driving through a flooded section of road or a ford across a river, test your brakes (whilst still driving slowly) and be prepared to drive them off by touching the brake pedal very lightly with your left foot (practice this on an empty stretch of road next time you go out driving to discover what very lightly means!).
If your car has been abandoned and has stood in deep water for a long period (an hour or more) it's worth getting a mechanic to look at it before you try and start it. Alternatively, if you know a little about engines and have the appropriate tools, remove the spark plugs (or injectors) and turn the engine over to expel any water from the cylinders before trying to start the engine.
Stay calm - stay safe!