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

The flooded Clark Fork River in the Schmidt Road area of Missoula, May 15, 2018.
Missoula County
The flooded Clark Fork River in the Schmidt Road area of Missoula, 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.

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