Understanding what a 100-year flood really means

Axios

6 days ago

Trump administration officials called the July Fourth Guadalupe River flooding a "100-," "500-" or "1,000-year flood" during a Friday visit to Kerrville, prompting questions about the meaning of the term.
The big picture: Such phrases shape how the public understands risk. But these events are not unprecedented — in fact, they can occur regularly.
We break down what these terms mean.
What is a 100-year flood?
A 100-year flood is one that has a 1% chance of occurring in any year. It can happen more than once a century, and it's not related to the death toll of a flood, Texas state climatologist John Nielsen-Gammon tells Axios.
A 1,000-year flood has a .01% chance.
The designation is determined by how often, historically, a river is expected to reach a certain height. It's different for each river basin.
What they're saying:"Really you should expect to see several of these in any given year just because there are lots of places," Nielsen-Gammon says.
"The odds of flooding the following year aren't affected by whether or not it just flooded."
Was the July Fourth flood a 100-year, 500-year or 1,000-year flood?
Simply put, it's too early to tell.
Is a 100-year flood different from a 100-year rainfall event?
Yes — a rainfall event is determined by looking at how often high intensity rainfall occurs in an area.
By the numbers: Kerr County received 10-12 inches of rainfall in just a few hours on July Fourth, per KSAT.
That could make it a 1,000-year-rainfall event for the area, Nielsen-Gammon says.
The Guadalupe reached an all-time high of 37.5 feet in Hunt, per KHOU.
Is extreme flooding caused by climate change?
While climate change is linked to more intense rainfall, it's uncertain whether that means more extreme flooding events, Nielsen-Gammon says.
Still, some climate scientists say the data that 100-year floods are based on is outdated.
"When you start to do the calculations for today's climate, you find that events that you might expect to happen once every hundred years might happen once every 20 years," Andrew Pershing, chief program officer at Climate Central, told Time.
What types of floods has Central Texas experienced?
On Memorial Day 1981 in Austin, 13 people died in a 100-year flood.
The Blanco River flood in Wimberley over Memorial Day weekend in 2015 led to 13 deaths. It's considered a flood of record for the town.
An October 1998 flood in and around San Antonio took 31 lives statewide. It's considered a 500-year flood.
Could a catastrophic flood happen again?
How officials communicate about floods can affect the public's response to risk, Lucy Atkinson, a University of Texas at Austin professor who has researched environmental communication, tells Axios.
"If you think about a 1% chance of anything happening, we think, oh that's highly unlikely," Atkinson says. "So the question becomes — how do we get people to think ... that risk is very real."