100-year flood

For other uses, see 100-year flood (disambiguation).

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.

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