Recurrence interval is defined as which of the following?

• A. The probability that a flood of any size will occur in the next year.
• B. The average time between floods of any size.
• C. The probability that a flood of a certain magnitude will occur over a specific interval of time.
• D. The maximum flood magnitude recorded

Answer: (C)

Recurrence interval expresses how often floods of a given size are expected to occur, linking the magnitude of an event to its likelihood over time. In practice, it’s used to describe the average waiting time between floods that reach at least a certain magnitude, so the chance of seeing such a flood in any given year is about 1 divided by that interval. Extending this to a specific time window, the probability of at least one flood of that magnitude occurring within that interval is roughly the interval length divided by the recurrence interval (for small probabilities). So describing recurrence interval as the probability that a flood of a certain magnitude will occur over a specified interval of time captures how often—over time—you would expect such events to happen. This differs from floods of any size or from simply the maximum flood magnitude recorded, which do not directly express the likelihood over a time span.