N-year flood (Q5, Q20)

An N-year flood is a flood whose flow is reached or exceeded on average once in N years. The number after the letter Q gives this return period: Q5 once in 5 years, Q20 once in 20 years. For flood zones, Czech Decree No. 79/2018 Coll. works with four scenarios - Q5, Q20, Q100 and Q500 - and a map of extent, water depths and velocities is prepared for each.

How to read the number

The return period is a statistical average, not a calendar. The annual probability is the inverse of N: Q5 about 20%, Q20 about 5%, Q100 1% and Q500 0.2%. If the Q5 or Q20 extent reaches a plot, water will very likely get there repeatedly over the years.

Example

As an illustration for Q20: the probability that no 20-year flood occurs in 10 years is 0.95 to the power of 10, about 60%. At least one such flood in ten years can therefore be expected with roughly a 40% probability.

What to watch out for

• The values are based on standard hydrological data of the Czech Hydrometeorological Institute (ČHMÚ) and change with new data and river works, which is why maps carry a date.
• The flood zone boundary is Q100. The more frequent scenarios show where the risk is highest, and water depths and velocities at Q5, Q20 and Q100 are used to define the active zone.
• N-year flood maps only cover modelled river and stream sections; flash floods from fields are not included.
• For a property within the extent of frequent floods, find out when water last came and how high it rose. The municipal flood plan helps.