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  State of Alaska    > Commerce    > DCRA     > Planning and Land Management   > Floodplain Management   > Local Residents   > 100-Year Flood  
"100 Year Flood"
Does Not Mean One Time in 100 Years!
  
100 Year Flood Definition
 
Copyright 2002 San Antonio Express-News, reprinted with permission.
 

Hydrologists and Floodplain Managers don't like to hear a term like "100-year flood" because, scientifically, it is a misinterpretation of terminology that leads to a misconception of what a 100-year flood really is.
Instead of the term "100-year flood" we would rather describe this hydrologic event as a flood having a 100-year recurrence interval. In other words, a flood of that magnitude has a 1 percent chance of happening in any year.
  

Recurrence intervals and probabilities of occurrence

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

 
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