Return period

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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.

[edit] Formula for working out return period or recurrrence interval

recurrence interval = n + 1 / m

n = number of years on record m = the rank of the flood being considered ( in terms of the flood size in cumecs)

[edit] Application

Within hydrology return period is important in relating extreme discharge to average discharge. The return period has an inverse relationship with the probability that the event will be exceeded in any one year. For example, a 10-year flood has a 0.1 or 10% chance of being exceeded in any one year and a 50-year flood has a 0.02 (2%) chance of being exceeded in any one year.

It is commonly assumed that a 10-year flood will occur, on average, once every 10 years and that a 100-year flood is such a big discharge that we expect it only to occur every 100 years. While this may be statistically true over thousands of years, it is incorrect to think of the return period in this way. The term "return period" is actually a misnomer.

The actual formula should be T = m/(n+1), where T= return interval, m= ranking, and n= equals number of occurrences.