Texas' 'flash flood alley': For centuries, a 'bull's-eye' when epic rain falls

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2 days ago

The catastrophic flooding in the Hill Country of south-central Texas on July 4 took place in a region that's known as "flash flood alley," a geographic area that also includes many of the state's major metropolitan areas, such as San Antonio, Dallas, Austin and Waco.
This region is among the nation's most prone to flash flooding, known for its propensity for fast and furious flooding when extreme rain falls, Alan Gerard, CEO of weather consulting company Balanced Weather said. As bountiful moist air from the Gulf of America, renamed from the Gulf of Mexico, moves over the steep hills, it can dump heavy rain.
Experts say the flooding was not a surprise, based on historical and prehistorical data:
"The flooding was certainly extreme but it should not have been historically unexpected," said political scientist Roger Pielke, Jr., in an email. "The documented record of extreme flooding in 'flash flood alley' goes back several centuries, with paleoclimatology records extending that record thousands of years into the past," he said.
Extreme flooding began in the Texas Hill Country around 4 a.m., on July 4 as thunderstorms dropped more than 10 inches of rain on the region. The rain overwhelmed the Guadalupe River, causing it to quickly rise.
Scores of people have died in the devastating floods, including 27 campers and counselors from Camp Mystic, an all-girls camp in Central Texas.
Geology is a key factor in the designation of flash flood alley. The "Balcones Escarpment," a geologic fault line that roughly parallels Interstate 35, marks the location of flash flood alley. This inactive fault zone formed a rise in the topography in the area, which enhances storm systems that pass over it, causing them to dump more rain there than they might elsewhere, according to AccuWeather.
"We're going from the coastal plains right into the Hill Country. There's a rise of at least about 500 feet in elevation," Pete Rose, a meteorologist with the Lower Colorado River Authority, told AccuWeather in 2022.
"Along with that, you have a lot of your hills and valleys that go along with that type of topography, and these hills don't contain a lot of soil; they have very thin soil. So when rain does hit them, not much of it gets absorbed," Rose said, noting that water will rush down the valleys and pile into creeks and streams.
Warm, moist air from the Gulf helps fuel storms as well, giving them ample moisture to dump lots of rain in a short amount of time across the dusty Texas soil.
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Pielke points to a classic 1940 historical text on U.S. floods, which shows that "the same region of Texas that experienced this week's floods has long been known to be a bull's-eye for flash flooding." In fact, almost a century before that book was published, Texas experienced one of the greatest losses of life in U.S. history related to extreme weather.
In 1846, in the months after Texas became a U.S. state, massive flooding compounded the many problems facing thousands of recent immigrants from Germany who had been settled in New Braunfels, Texas, which was significantly impacted by this week's floods, Pielke said.
According to a 1846 account, cited in a 2006 PhD dissertation on flooding in Texas by William Keith Guthrie, at the University of Kansas:
"The Guadalupe (River) would often rise 15 feet above its normal stand after these heavy rains, carrying with it in its swift torrent a number of large trees, uprooted farther up the hills. Smaller brooks, ordinarily not containing flowing water, became raging torrents which could be crossed only by swimming."
According to Guthrie's dissertation, "archaeological evidence indicates that Paleo-Indians adapted to the region's flood regimes by careful placement of campsites and nomadic lifestyles. Native Americans during the earliest periods of contact with Europeans in Texas, according to Spanish records, also adapted to the prevailing cycles of seasonal flooding in the state's interior by modifying their economic trade cycles."
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Flood experts believe that the future will bring an increased risk of flash flooding to this already flood-prone area as more development in the region creates more impermeable surfaces and thus more runoff, AccuWeather said. Bigger storms, enhanced by a changing climate, may also lead to more flash flooding as a warmer atmosphere allows storms to hold more water.
"Cities such as Austin have been taking preventative measures to warn locals about the threat of flash flooding, putting up signs in areas that flood frequently. Austin also has the Flood Early Warning System, a network of rain gauges, barricades and cameras that monitor the threat of flooding in the city," according to AccuWeather.
"This tragedy occurred in a location that has among the greatest risks in the nation of flash flooding, where kids in summer camps have previously been swept away to their deaths, and where warning systems are (apparently and incredibly) not in place," Pielke said. "This tragedy never should have happened and it should never happen again."
Flooding in Texas could be among the worst on Earth, Rose said, noting that "we're in a very, very flash flood-prone area, not only of Texas but out of the country and even the world."
Contributing: Dinah Voyles Pulver, USA TODAY
This article originally appeared on USA TODAY: Why Texas' 'flash flood alley' is a 'bull's-eye' when epic rain falls