'100 year Flood'

Explained to you by Texas Monthly-

Yeah, yeah, I know. A 100-year-flood does not mean that these floods happen once a century. Per the United States Geological Survey:

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.

This attempt to simplify does so at the expense of offering any meaning whatsoever. You could have 100-year-floods three years running. You could have two 100-year floods in a single year. Or you might go 300 years or more without a single one. Probability is weird like that. Also of note from a probability standpoint: if you gamble with a 1 percent chance of losing once a year for thirty years, you end up with a 25 percent chance of crapping out at least once. Think about those odds the next time you sign a mortgage for a house in what is reassuringly described to you as located in a 100-year floodplain. 

- See more at: The Problems With “100-Year Floods”