What is meant by the 100 year flood?

Question

What is meant by the 100 year flood?

Answer 1

The terms "10 year", "50 year", "100 year" and "500 year" floods are used to describe the estimated probability of a flood event happening in any given year.

Answer 2

The type of flood that would happen in 100 years.

Answer 3

refers to the flood levels of the highest flood in a 100 yr period.

Not to be taken literally, but used for planning purposes so bulidings, bridges etc are built with this flood level in mind.

Usually a map of this flood level is used when planning housing projects and determines flood insurance premiums.

Answer 4

The term "100-year flood" is misleading because it leads people to believe that it happens only once every 100 years. The truth is that an uncommonly big flood can happen any year. The term "100-year flood" is really a statistical designation, and there is a 1-in-100 chance that a flood this size will happen during any year. Perhaps a better term would be the "1-in-100 chance flood."

Answer 5

This refers to a level of probability.

There is a certain maximum size flood that can be expected on the AVERAGE of once every 100 years. The 100-year flood may not happen in a particular century or for several centuries, or it may occur several times in one century.

The point is that a flood of that size can be expected on the average of only once every 100 years.

A larger and more disastrous flood would be more rare. A certain larger size flood may be expected to occur only once every 1000 years on the AVERAGE.

The larger and more disastrous the flood, the rarer the event.

Answer 6

A 100 year flood level is a statistical estimation of the highest flood level that could be expected in any 100 year period; as other answerers have said.  However, it is a highly unreliable statistic; and there have been cases of '100 year floods' occurring three years in a row.  With increasing global warming and less predictable weather patterns, it has become more unreliable; especially if periods of drought are followed by unusually wet periods.

However, '100 year flood levels' are loved by organisations like insurance companies (who will apply higher premiums to policies, the closer a building is to the '100 year' level), and to Government building authorities, who may prevent residential buildings being constructed below the '100 year' level.  I have seen ridiculous cases where a residentiial subdivision has been given development approval, after the land has been raised one foot by filling with earth, to raise the land above the 100 year level.