Adjective to describe a significant natural disaster "which only occurs once every century/millennium"?

I'm looking for an adjective that expresses the rarity in terms of scale or impact of an event (especially a natural disaster like a flood or an earthquake). Words like disastrous, catastrophic, and devastating all describe the severity of impact, but don't stress the chronological rarity. In Chinese, there is an expression 百年一遇 which literally means "seen only once every century". I'm wondering if there is an English word or expression for that.

Solution 1:

There are various expressions like "50-year storm" or "100-year flood" that refer to events only expected every 50 or 100 years. 50 year storm is a surfing term popularised by the film Point Break. Wikipedia has an article on 100-year flood which it defines as "a flood event that has a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year". The NOAA even talks about a "thousand-year storm". It seems that this is an expression that can be modified to your event or time period.

You can write "100-year flood" or "100 year flood" depending on your preference for hyphenation; the forms with a hyphen seem more common, but it's not a hard and fast rule.