What is a Flash Flood?
A flash flood is a sudden local flood, typically due to heavy rain. 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.
Here is an example of how quick a flash flood can happen.
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 and canals 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.