## Recurrence interval

Reports of flooding sometimes use the phrase “a 100-year flood”. This does not mean that such floods only happen once every 100 years. Instead these terms mean that, on the basis of past records, the probability is that such a flood will only happen once in any given 100 years.

A recurrence interval how often a river is expected to reach a particular level of flow.

$$\mathsf{recurrence\;interval = \frac{(N+1)}{M}}$$
• $$N$$ = number of years for which data has been collected
• $$M$$ = rank (known as the magnitude number)