Q100 - 100-year flood

Q100, known in Czech as „stoletá voda“ (the hundred-year water), is a flood with a 100-year return period. Decree No. 79/2018 Coll. describes the return period as the occurrence of a flood that is reached or exceeded on average once every hundred years. The Q100 flood line defines the flood zone in the legal sense.

How to read the number

“Once in a hundred years” does not mean a hundred quiet years after a big flood. The probability is the same every year, about 1%, and two 100-year floods can come shortly after each other. The Q100 value is based on standard hydrological data of the Czech Hydrometeorological Institute (ČHMÚ), so it may change with new data or river works.

Example

As an illustration: the chance that no 100-year flood occurs during 30 years of owning a house is 0.99 to the power of 30, about 74%. The probability of at least one such flood in that time is therefore roughly 26%, about one in four.

What it means for a buyer

• Within the Q100 extent the Water Act restrictions for flood zones apply, and in the active zone building is banned with few exceptions.
• The maps also show the more frequent Q5 and Q20 floods and the rarer Q500. If the Q5 or Q20 extent already reaches the house, the risk is much higher than with Q100 alone.
• A digital terrain model helps to judge how high the plot and the house sit compared with their surroundings.

Frequently asked questions

Where can the Q100 boundary be found?

In the flood zone map of the T. G. Masaryk Water Research Institute (HEIS) and in the measure of general nature by which the water authority designated the flood zone. For selected river sections, the flood hazard maps of the Ministry of the Environment also show water depth and velocity during Q100.

Does a house outside Q100 mean water will never reach it?

No. A flood larger than the 100-year one is less likely but possible. River flood maps also do not cover floods running off slopes and fields after intense rain.