What causes flash floods?

Boscastle floods

Find out what causes flash flooding and why it can be so dangerous.

What are flash floods?

Flash flooding happens when rain falls so fast that the underlying ground cannot cope, or drain it away, fast enough. Roads can become like rivers and if there is a lot of water, it can flood buildings and carry cars away. So, if the rain is falling too fast for the ground or drains to cope, there is a risk of flash flooding.

What causes flash floods?

Most rivers flow fairly gently as they slope slowly towards the sea. Therefore, when a river floods it does so quite slowly as it takes time for the rain to percolate through the ground and into the rivers and out to sea − allowing time for some warning. With flash flooding there is often very little time between the rain falling and flash flooding occurring.

Flash flooding commonly happens more where rivers are narrow and steep, so they flow more quickly. It can also occur away from small rivers in built-up urban areas where hard surfaces such as roads and concrete don't let the water drain away into the ground. This leads to surface overflow and can often overwhelm local drainage systems leading to flash flooding.

An example of a flash flood

The village of Boscastle in Cornwall experienced flash flooding on the Boscastle flood 16 August 2004 (PDF, 91 kB)  when an exceptional amount of rain fell over eight hours. Heavy thundery showers developed over the moors to the south west of Boscastle. A small, steep river flows though Boscastle, and the thundery showers came one after another in a short space of time, over the same spot at the head of the river. There was simply too much water for this small river and the surrounding hillside to cope with, and the water gushed rapidly down the steep slopes into Boscastle.

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Last updated: 20 May 2014