What is a hundred-year flood?

In connection with flood protection projects one often hears of a hundred-year flood. Here is a short attempt of explanation.

Many flood protection structures are built in such a way that they protect against flooding up to a centennial flood. Technicians use the abbreviation HQ100 for the amount of water that flows during such a flood. A centennial flood is high water which occurs statistically once in a hundred years. This sounds very rare. What is the probability of experiencing a flood of this magnitude?

A flood that occurs statistically only once in 100 years is a rare event, but the probability to experience it is relatively high.

The following table and graph show how high the probability is to experience a flood event of a certain annuality. The specified probabilities are based on the assumption of a life expectancy of 80 years.






Probability to experience such a flood event

93 %

55 %

23 %

8 %

Probability to experience a flood of a certain annuality
(hydrological risk, assumption age 80 years, according to Kirnbauer 1981)

The probability of experiencing a centennial flood is about 55%, which is relatively high. Who lives in an area that is protected for floods up to this annuality, also sees that the probability to experience a bigger flood, for which the protection effect is no longer given, is also not small. For this reason, it is important to be aware of the risk and to be prepared for a possible emergency.

For people interested in mathematics, who want to do the calculations themselves:

The calculations are based on the following literature: Kirnbauer, R. (1981). Basics and statistical methods for the determination of design floods. Vienna University of Technology.

The formula used is:

R … hydrological risk (= probability of experience)
T … Annuality of the event
n … lifetime in years (plant, human etc.)