洪水是这世界上最具杀伤力的自然灾害之一，这不仅仅是因为洪水本身的破坏性还因为由洪水所带来的各种伴随灾害，像是泥石流，瘟疫等等。那么什么是洪水，洪水是怎么形成的呢？本文为大家带来的托福阅读材料就来讲解这个问题。
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.
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.
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.
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. The river forms so that the water will have somewhere to go. Floods happen when more water tries to fit into a river than there is space for. The river bursts its banks.The ground in different bits of the country is made up of different types of soil and rock. Some combinations are able to soak up more water than others.
If a place that cannot soak up much water is rained on heavily, there is nowhere for the water to go and flooding happens.
The ground can only soak up so much water, to a level called the water table. The water table is higher in certain areas of the country, making those locations much more at risk.
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