HOUSTON — It seems Houston is hit with so-called 100-year floods every few years. So why is it called a 100-year flood? The KHOU 11 VERIFY team sets the record straight on what the term actually means.
There was Hurricane Harvey in 2017, Tropical Storm Allison in 2001 and Tropical Storm Imelda in 2019 -- three storms brought historic flooding to the Houston area in just the last couple of decades. All three were 100-year flood events, if not worse. So how can that be? Does a 100-year flood mean that it happens only once a century? We went to our sources: FEMA, the US Geological Survey and Harris County Flood Control District.
"So probably this 100-year flood, that phrase, was really pretty quickly seen to be a misnomer," USGS National Flood Coordinator Karl Winters said.
Instead of thinking about it as a once-in-a-100-year event, Winters said it's about the average chance.
"So what we do now is we try to put it in terms that describe the random nature of a series of annual flood peaks," he said. "We say it's the flood that has a 1% chance every year of being equaled or exceeded."
Jeff Lindner with the Harris County Flood Control District explains it this way: "If you had water in your house today, you don't have 99 more years to go until potentially you'd see that again, it resets immediately. And so tomorrow, you can once again have a 1% chance of having water reach that elevation or potentially get inside your house. And if you expand that to a 30-year period, a life of most mortgages, it's a 26% chance that you will see water in that 100-year flood plain, or 1% chance area over the span of 30 years."
So no, a 100-year flood doesn't mean it can only happen once in a century. And with all the bayous, creeks and rivers in the area, our experts say don't be overconfident.
"You may hear on the news that a 1% chance one event happened," said Larry Voice with FEMA. "That might not be your watershed, you know, and you're thinking, hey, water didn't come near my house. I'm safe. It might not have been your watershed that had that 1% no chance flood and that's why I didn't come close to you."
Also, everyone should have flood insurance.
"Flooding is our No. 1 natural hazard here, and we can flood 12 months out of the year," Lindner said.