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What is 100 Year Storm?

flooding lake deltonA 100-year storm refers to rainfall totals that have a one percent probability of occurring at that location in that year. Encountering a "100-year storm" on one day does not decrease the chance of a second 100-year storm occurring in that same year or any year to follow.[1]  In other words, there is a 1 in 100 or 1% chance that a storm will reach this intensity in any given year. Likewise, a 50-year rainfall event has a 1 in 50 or 2% chance of occurring in a year. In addition, each locality has its own criteria for how much rain must fall within 24 hours to classify as a particular rain event. See chart below for other rainfall events.


Likewise, there are similar classifications for 100-year flood events. The term 100-year flood is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year.[2]  The actual number of years between floods of any given size can vary because the climate naturally varies over time. Large floods or rain events can occur in successive or nearly successive years.[3] It should be noted that not every 100-year rainfall event leads to a 100-year flood. Local circumstances may allow for a less severe storm to lead to severe flooding.

Recurrence intervals[4] and probabilities of occurrences[5]

Recurrence interval, in years Probability of occurrence in any given year

Percent chance of
occurrence in any given year

100 1 in 100 1
50 1 in 50 2
25 1 in 25 4
10 1 in 10 10
5 1 in 5 20
2 1 in 2 50

Footnotes:

  1.  "100-Year Storm." 23 September 2003. Minnesota Climatology Working Group. 2 Jul 2008. <http://climate.umn.edu/doc/misc/100year.htm>.

  2. ibid

  3. Dinicola, Karen. "The "100-Year Flood"." U.S. GEOLOGICAL SURVEY. 22 August 2005. USGS. 2 Jul 2008 <http://pubs.usgs.gov/fs/FS-229-96/>.

  4. Recurrence interval: The average number of years between floods of a certain size is the recurrence interval or return period. The actual number of years between floods of any given size varies a lot because of the naturally changing climate.

  5. "Floods: Recurrence intervals and 100-year floods." USGS Water Science for Schools. 30 August 2005. USGS. 2 Jul 2008 <http://ga.water.usgs.gov/edu/100yearflood.html>.