Water Q&A: We had a "100-year flood" two years in a row! How can that be?

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Learn what hydrologists mean when they say "100-year flood".

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We had a "100-year flood" two years in a row! How can that be?

Water Questions and Answers

The term "100-year flood," is used to describe the recurrence interval of floods. As the table below shows, the "100-year recurrence interval" means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year event will occur.

Recurrence intervals and probabilities of flood occurrence
Recurrence interval, in years Probability of occurrence in any given year Percent chance of occurrence in any given year
100 1 in 100 1
50 1 in 50 2
25 1 in 25 4
10 1 in 10 10
5 1 in 5 20
2 1 in 2 50

But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood you know that it means one even BIGGER! As an example, in July of 1994, some places in south Georgia received more than 20 inches of rainfall in a few days -- the floods they produced were tremendous... way over the 100-year flood. At Senoia, Georgia, the maximum amount of water flowing by the Line Creek gage was 2.4 times greater than the 100-year flood level.