The average time interval between occurrences of a hydrological event of a given or greater magnitude, usually expressed in years.

Average period of time between occurrences of a given probability event.

The average length of time separating flood events of a similar magnitude: a 100-year flood will occur on average once in every 100 years.

This describes how often, on average, a flood of a certain magnitude is expected once. A flood with a 100-year return period would therefore be expected once in 100 years. However, this is just an average – it is possible to have 100-year floods in, for example, 5 consecutive years and not again for 500 years. Stretch A section of river along which there are no changes in water quality which are of practical significance and there is no obvious potential for such a change in the future.

ee recurrence interval.

The probable time period between repetition of extreme weather events, such as heavy rainfall or flooding, in a stationary climate (that is, a climate without global warming or other trends). In the case of rainfall, a return period is always related to a specific duration (e.g. 50-year return period of 24-hour extreme rainfall).

The average time interval between occurences of wave heights equal to or greater than the height associated with the return period. It is a measure of the infrequentness of higher wave heights.

The average time between events such as the flooding of a particular level. This information may also be expressed as the level which has a particular return period of flodding, for example, 100 years. The inverse of the return period is the statistical probability of an event occurring in any individual year.

return periods relate to the long term average time interval between events of a particular magnitude; thus a 1 in 100 year return period flood has a one per cent chance of occurring in any one year

can be defined generally as the average interval, in the long run, between events of a given size

The expected (mean) time (usually in years) between the exceedence of a particular extreme threshold. Return period is traditionally used to express the frequency of occurrence of an Event, although it is often misunderstood as being a Probability of occurrence.

Also called recurrence interval. The average time until the next occurrence of a defined event. When the time to the next occurrence has a geometric distribution, the return period is equal to the inverse of probability of the event occurring in the next time period, that is, T = 1/P, where T is the return period, in number of time intervals, and P is the probability of the next event's occurrence in a given time interval.

The projected period of recurrence of a particular event, such as a tropical cyclone of a particular intensity, severe flooding, or a defined wind gust; derived using either empirical or Monte Carlo methods and assumed distributions of extreme events. Great care needs to be taken in interpretation this statistical result; in particular a return period of, say, 50 years does not mean that the next event will occur in 50 years, it could occur next year, albeit with a low probability.

recurrence interval; a statistical parameter used in frequency analysis as a measure of the average time interval between the occurrence of a given quantity and that of an equal or greater quantity

Expected average interval between the occurrences of events at a particular threshold.

See recurrence interval.

The period within which it is mathematically probable that a natural phenomenon such as an earthquake of given intensity will recur.

A return period also known as a recurrence interval is an estimate of the likelihood of a flood or river discharge flow of a certain size. It is a statistical measurement denoting the average recurrence interval over an extended period of time.