What is a return period?

The probability that events such as floods, wind storms or tornadoes will occur is often expressed as a return period.

The inverse of probability (generally expressed in %), it gives the estimated time interval between events of a similar size or intensity.

For example, the return period of a flood might be 100 years; otherwise expressed as its probability of ocurring being 1/100, or 1% in any one year. This does not mean that if a flood with such a return period occurs, then the next will occur in about one hundred years' time - instead, it means that, in any given year, there is a 1% chance that it will happen, regardless of when the last similar event was. Or, put differently, it is 10 times less likely to occur than a flood with a return period of 10 years (or a probability of 10%). 

Blocked culvert causes flooding after heavy rain. Ohariu Valley, Wellington, 2012. [Dave Allen]