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Over half of the world is covered with water, we need it to stay alive. When there's too much water, or water in the wrong place, it's a problem we call flooding.
In the UK people are sometimes forced out of their homes when lots of rain has fallen in a short space of time and caused a flood.
What is a flash flood?
It's where a river's level rises very fast, the flood may arrive instantly in the form of a wave or wall of water. People 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. 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, it's 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. 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.
Rivers are formed over a large number of years by the movement of water over and through land.
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