BBC Home
Explore the BBC



Last Updated: Monday January 10 2005 15:11 GMT

What is a flash flood?

Floods after the River Severn burst its banks in Gloucester

It's where a river's level rises very fast, and sometimes the flood can arrive instantly in the form of a wave or wall of water.

People often have no time to get to the safety of higher ground.

Flash floods are unusual in the UK. We do get a lot of rain and sometimes it's really heavy but normally the soil and plants act like a giant sponge.

Flood damage

They soak up the rainwater and slow it down on its journey into our rivers.

Sometimes the sponge effect doesn't work because the ground is already very wet.

When that happens all the rain tries to get into the river at the same time. If the river isn't big enough to handle the water there will be a flood.

Flat area

In a flat area the rainwater sits around in temporary ponds and lakes until there is room for it to get into the river - like a rainfall traffic jam.

Hilly area

Where the ground is steep you could be looking at a dangerous flash flood. Gravity pushes all the rainwater into the river at the same time.

Flash floods in Funchal, Madeira

The river level rises really quickly, and the water moves downhill fast. As it travels it picks up objects in its path.

They might be cars, trees or even giant boulders.

Steep narrow valleys are the most at risk, a lot of water is trying to push through a small gap.

This causes a 'bottleneck' and makes the flooded river even deeper.