Q500 - 500-year (extreme) flood

Q500 is a flood that is reached or exceeded on average once in 500 years, i.e. with about a 0.2% chance each year. Under Czech Decree No. 79/2018 Coll., a flood hazard map and water levels are prepared for it together with Q5, Q20 and Q100 when a flood zone is designated. It is the rarest scenario in the series of N-year floods.

How to read it

The legal flood zone ends at the Q100 line. Land reached only by Q500 is therefore not subject to the Water Act restrictions for flood zones and their active zones. Q500 is a scenario for exceptional events that go well beyond the 100-year flood. Calculated Q500 water levels are also listed in the table the river manager submits with the flood zone proposal.

Example

As an illustration: the probability that no Q500 flood occurs in 30 years is 0.998 to the power of 30, about 94%. At least one such flood in that time therefore has a probability of roughly 6%. For the 100-year flood it is about 26% over the same period.

What it means for a buyer

• The risk is low but not zero. It helps to know where water could reach, where the highest point of the plot is and whether the house has a cellar or equipment in the basement.
• A house outside Q100 but inside Q500 can be built and altered without flood zone restrictions, yet an extreme flood can still reach it.
• Q500 extents are shown in the HEIS map of the T. G. Masaryk Water Research Institute; for selected river sections the flood hazard maps of the Ministry of the Environment also show water depths.
• Floods outside river channels, such as flash floods from slopes and fields, are not covered by the Q500 map.