100-year flood
Indication of the likelihood of a flooding
Summary
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 of being equaled or exceeded in any given year.
Originally created by Jaknouse
8/13/2002, 7:33:18 PM
Modified
12/1/2025, 1:21:16 PM
Recent revisions
Typo/[[MOS:CURLY|quotemark]] fixes, replaced: ’s → 's, Anaysis → Analysis
HTTP to HTTPS for [[United States Geological Survey|USGS]]
/* top */ =High Probability in the UK, FEMA’s flood maps often miss
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[[Wikipedia:OABOT|Open access bot]]: url-access updated in citation with #oabot.
Added more examples
added explanation of basic formula & examples
Reference edited with [[Wikipedia:ProveIt|ProveIt]] #proveit added UK & Dutch links, consolidated some topics
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[[WP:CLICKHERE]] and [[WP:NOELBODY]] (If somebody wants to learn about NFIP, they can Google it. They don’t NEED a link, since they’ll surely sooner visit the page directly instead of through a random EL. Plus, this page isn’t about insurance anyway.)
/* top */ not a specific flood event
move external link
v2.05b - [[User:WikiCleanerBot#T5|Bot T5 CW#17]] - Fix errors for [[WP:WCW|CW project]] (Category duplication)
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Add: s2cid, doi, series, chapter. | [[:en:WP:UCB|Use this bot]]. [[:en:WP:DBUG|Report bugs]]. | Suggested by Лисан аль-Гаиб | #UCB_webform 3/380
/* Regulatory use */ alignment
Shorten short description per [[WP:SDSHORT]]
see [[Talk:100-year_flood#Definition_inconsistent_(multiple_events_within_the_same_year)]]
