100-year flood

A 100-year flood, also called a 1% flood, or High Probability in the UK, is a flood event for a defined location at a level reached or exceeded once per hundred years, on average, but as there are many locations there are multiple independent 100-year floods within the same year. In the US, it is estimated on past records as having a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year.

The estimated boundaries of inundation in a 100-year or 1% flood are marked on flood maps.

UK planning guidance defines Flood Zone 3a "High Probability" as Land having a 1% or greater annual probability of river flooding; or Land having a 0.5% or greater annual probability of sea.

For coastal flooding and lake flooding, a 100-year flood is generally expressed as a water level elevation or depth, and includes a combination of tide, storm surge, and waves.

For river systems, a 100-year flood can be expressed as a flow rate, from which the flood elevation is derived. The resulting area of inundation is referred to as the 100-year floodplain. Estimates of the 100-year flood flow rate and other streamflow statistics for any stream in the United States are available. A 100-year storm may or may not cause a 100-year flood, because of rainfall timing and location variations among different drainage basins, and independent causes of floods, such as snow melt and ice dams.

In the UK, the Environment Agency publishes a comprehensive map of all areas at risk of a 100-year flood. In the US, the Federal Emergency Management Agency publishes maps of the 100-year and 500-year floodplains.

Maps of the riverine or coastal 100-year floodplain may figure importantly in building permits, environmental regulations, and flood insurance. These analyses generally represent 20th-century climate and may underestimate the effects of climate change.

A common misunderstanding is that a 100-year flood happens once in a 100-year period. On average one happens per 100 years, five per 500 years, ten per thousand years. Any average hides variations. In any particular 100 years at one spot, there is a 37% chance that no 100-year flood happens, 37% chance that exactly one happens, and 26% chance that two or more happen. On the Danube River at Passau, Germany, the actual intervals between 100-year floods during 1501 to 2013 ranged from 37 to 192 years.