Flood risk: what is a 'one in a 100 year' event?

In the wake of severe floods in Cumbria a number of public figures said defences were designed to cope with a one in 100 year event.

The Mayor of Keswick for example said his local defences were 'designed for a one-in-100-year event - and since it's six years since we had the last one, we are sort of surprised'.

So is this way of describing flood risk helpful or misleading?

Ruth Alexander from Radio 4's More or Less spoke to Eddie Mair.