"A common misunderstanding is that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or

"A common misunderstanding is that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period." [1] Which is different than "You'd expect to see it at least once in 100 years." However, yes, the expected value of the number of 100-year floods occurring in any 100-year period is 1. [1] https://en.wikipedia.org/wiki/100-year_flood#Probability https://en.wikipedia.org/wiki/100-year_flood#Probability