Thankfully, they remain rare and, when they do occur, often drain away before anybody realises what has happened.
Most inland floods occur relatively slowly with water levels reaching a peak over a period of days.
It is this unpredictability and rapid change that makes flash floods extremely dangerous. As the water builds up, it displaces into tributaries and streams with an accelerated run-off from the land. Within minutes, the water level will rise dramatically, bringing with it mud and natural debris from the land. In the worst cases, areas which are normally quite dry can be suddenly overrun with waves of rapidly moving muddy water. Hydrologists have measured almost instantaneous rises in flood waters from virtually nothing to up to 10 metres in the worst cases of flash floods.