‘100-Year-Flood’ Doesn’t Mean What You Think It Means

May 15, 2018

When the weather gets dramatic, the big-picture descriptions come out: a 50-year storm, a hundred-year flood, a thousand-year floodplain. But it can get confusing when the numbers don't really mean what they sound like to the non-statisticians among us who hear them.

For example: a hundred-year flood doesn't mean that it's breaking a 100-year-old record.

Instead, it means that statistical analysis says that the event should recur every hundred years. Doing the math means that the likelihood of a hundred-year flood occurring in any one year is one in 100, or 1 percent. If you hear about a thousand-year floodplain, that means the chance of a flood in that area is one in 1,000, or .1 percent.

The USGS puts it this way:

“If we had 1,000 years of streamflow data, we would expect to see about 10 floods of equal or greater magnitude than the ‘100-year flood.’ These floods would not occur at 100 year intervals. In one part of the 1,000-year record it could be 15 or fewer years between ‘100-year floods,’ whereas in other parts, it could be 150 or more years between ‘100-year floods.’”

That’s because weather events are still unpredictable in lots of ways, especially on such huge time scales. Being called a hundred-year flood won’t stop one from happening sooner than it’s scheduled.

Breaking a 100-year-old record, which the National Weather Service says the Clark Fork River might do in the next few days, is a whole other calculation. Simply hitting that 100-year high doesn't make the current situation a hundred-year flood.