Capital Weather Gang • Perspective
We still don’t know how to talk about floods
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Harvey is a 1,000-year flood event unprecedented in scale

(The Washington Post)

As Harvey’s rains unfolded, the intensity and scope of the disaster were so enormous that weather forecasters, first responders, the victims, everyone really, couldn’t believe their eyes. Now the data are bearing out what everyone suspected: This flood event is on an entirely different scale than what we’ve seen before in the United States.

A new analysis from the University of Wisconsin’s Space Science and Engineering Center has determined that Harvey is a 1-in-1,000-year flood event that has overwhelmed an enormous section of  Southeast Texas equivalent in size to New Jersey.

There is nothing in the historical record that rivals this, according to Shane Hubbard, the Wisconsin researcher who made and mapped this calculation. “In looking at many of these events [in the United States], I’ve never seen anything of this magnitude or size,” he said. “This is something that hasn’t happened in our modern era of observations.”

Hubbard made additional calculations that accentuate the massive scale of the disaster:

  • At least 20 inches of rain fell over an area (nearly 29,000 square miles) larger than 10 states, including West Virginia and Maryland (by a factor of more than two).
  • At least 30 inches of rain fell over an area (more than 11,000 square miles) equivalent to Maryland’s size.

A 1,000-year flood event, as its name implies, is exceptionally rare. It signifies just a 0.1 percent chance of such an event happening in any given year. “Or, a better way to think about it is that 99.9 percent of the time, such an event will never happen,” Hubbard said.

Apart from Harvey, there’s simply no record of a 1,000-year event occupying so much real estate.

Hurricane Harvey is the most extreme rain event in Texas history. But the city of Houston has some unique geographic and design challenges that have contributed to the flooding disaster. (Video: Daron Taylor/The Washington Post)

While no one questions the exceptional nature of Harvey’s rainfall, the concept of a 1,000-year flood event has been criticized by some academics and flood planners. For one, rainfall and flood data generally go back only 100 years or so, so statistical tricks must be applied to determine what 500-year and 1,000-year events actually represent. Furthermore, the climate is changing and precipitation events have become more intense in recent decades, so what constitutes different return frequencies (100-year, 500-year, 1,000-year and so forth) is probably changing.

Climate change studies have found that what’s considered a 500-year flood today may become much more frequent in coming decades.

But Hubbard, who analyzes geographic information to help decision-makers plan for floods, stands by the use of these return interval metrics despite their shortcomings. “For a community, they help put these events into perspective and understand the impact of a flood,” he said.

He added that they have “tremendous” value for flood planning and designing infrastructure to be able to withstand events up to a certain intensity. “Decision-makers have to be able to pick a number and say this is the number we need to be prepared for,” he said. “If we debate and belabor the accuracy of these estimates, the community will not have a value to plan for.”

Hubbard agrees that the climate is changing and precipitation is becoming more intense in some areas, but he said it would be complicated to adapt the flood return frequencies. “The challenge is trying to separate when you have these 500-year events happening all the time, what part is a changing climate, what part is changes in urbanization and agriculture and what part is the lack of understanding of what’s happened in the past,” he said.

In any event, Harvey puts an exclamation mark on the pattern of disastrous rain events in recent years and may be a harbinger of more such events in the coming decades.

“Expect #HarveyFlood record will be broken in 5, 15, 25 years from now — sooner rather than later,” tweeted David Titley, professor of meteorology at Penn State.

Read more

Harvey has unloaded 24.5 trillion gallons of water on Texas and Louisiana

Harvey marks the most extreme rain event in U.S. history

Houston is experiencing its third ‘500-year’ flood in 3 years. How is that possible?