The 100-year floodplain is the area of land that has a predicted flood risk exceeding 1 percent in any given year. It's called the "100-year flood" because a 1 percent annual risk is the same as "one in 100."

It's a common, yet often misunderstood phrase. It does not mean it will be 100 years before the next flood or that 100 years must elapse between floods. It doesn't imply any one type of rainstorm. The flood of October 1998 came close to being the "100-year storm" in a few areas of the metropolitan area, but even it wasn't the "100-year flood" for most of us.

A 1 percent annual risk translates into a 26 percent chance of having at least one flood during the life of a 30-year mortgage.