The region of Texas where flash flooding killed more than 100 people — including more than two dozen campers — is known for its tendency to experience flood emergencies, according to experts.
While local and state officials in Texas have said they were caught off guard by the severity of the flooding, the region is prone to extreme flooding, Marshall Shepherd, director of the Atmospheric Sciences Program at the University of Georgia and former president of the American Meteorological Society, told ABC News.
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Texas Hill Country is often colloquially referred to as “Flash Flood Alley” because the weather and landscape in the south-central Texas region work together to produce rapid flood events, according to the Texas Water Resources Institute (TWRI).
Flash Flood Alley is known as one of the most flood-prone regions in the U.S. due to its high susceptibility to flash flooding with steep terrain, shallow soil and repeated high rainfall events.
The “alley” stretches from Dallas to San Antonio and encompasses the Colorado and Guadalupe River basins, Carlos Javier Martinez, a senior climate scientist with the climate and energy program at the Union of Concerned Scientists, told ABC News.
The Guadalupe River Basin is one of the three most dangerous regions in the country for flash floods, according to the Guadalupe-Blanco River Authority.
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River streamflow records, which go back to the 1800s, show that major floods have occurred over nearly all sections of the Guadalupe River Basin, according to the U.S. Geological Survey.
One of the reasons why the region is so prone to flooding is because the Gulf provides an infinite source of moist air over the region, making high rainfall intensities a common occurrence. Moist air from the Pacific Ocean — frequently as monsoonal moisture — and cool air masses from the north often converge to produce extreme rainfall events in the region.
In addition, places like Hunt and Kerr Counties are situated on a floodplain between tall, rugged hills that funnel any rainfall down into rivers and creeks below. In this event, the extreme rainfall funneled into the Guadalupe River, which led to the rapid rise in water levels.
The region’s rocky topography makes it especially prone to flash flooding, according to the Texas Water Resources Institute. The type of clay-rich soil in the region contributes to the flash flooding because the clay soils do not easily absorb water and trigger high water runoff once wet.
“The shallow soil makes it difficult to absorb rainfall efficiently, so when intense precipitation events occur the fast-moving water runs off the soil and feeds into the river basins, leaving neighboring communities highly vulnerable to swollen rivers and flash flooding,” Martinez said.
“Flash Flood Alley” has a long and tragic history of major flooding events along the Guadalupe River and other nearby rivers. The river has flooded nearly once every decade over the past 100 years, Martinez said.
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July 1987 flood that killed campers near Comfort, Texas
One of the most infamous occurrences prior to the July 4 flooding occurred in 1987, when devastating flooding impacted the region around Comfort, Texas.
That event contained eerie similarities to the flood that occurred over the weekend, when slow-moving thunderstorms trained over a large area of the Hill Country during the late evening of July 16, 1987.
Between 5 and 10 inches of rain fell across the upper Guadalupe River basin, according to the National Weather Service, triggering a massive surge of water that traveled down the Guadalupe River during the early morning of July 17, 1987, through Ingram, Hunt — near Camp Mystic — Kerrville and eventually Comfort.
Evacuating campers and camp staff from the Pot O’ Gold Christian Camp were trapped and eventually swept away by overflowing floodwaters from the Guadalupe River, killing 10 campers, and injuring 33 others, according to the National Weather Service.
Summer 1978
Tropical Storm Amelia in the summer of 1978 also caused major flooding. The storm made landfall over Corpus Christi, Texas, and continued inland to bring extreme rainfall to south-central Texas from July 30 to August 5, according to the National Hurricane Center.
Medina, Texas, about 20 miles south of Camp Mystic, measured up to 4 feet of rain over the course of the event, according to the NWS.
Tropical Storm Amelia was responsible for 33 fatalities and an estimated $110 million in damages across Texas.
September 1921
The remnants of a hurricane that made landfall over eastern Mexico brought extreme rainfall to south central Texas from Sept. 8 to Sept. 10, 1921, including San Antonio and Austin.
An estimated 40 inches of rain fell near Thrall, Texas, with around 19 inches falling in Austin and 15 inches in San Antonio, according to the NWS. The flooding killed 215 people, including 159 along the Little and San Gabriel Rivers and 51 in San Antonio.
Deadly flooding was result of ‘worst-case scenario’
Over the weekend, heavy rain and other atmospheric conditions — combined with the topography of the land and drought — contributed to a “worst-case scenario event” for the region, Shepherd said.
The Guadalupe River rose 26 feet in less than an hour in an area near the site of Camp Mystic, the Christian girls camp that was flooded in the event, according to the NWS.
The catastrophic flooding killed at least 94 people — more than two dozen of whom were campers and counselors from Camp Mystic, a Christian girls camp situated on the Guadalupe River.
At least 11 campers were still unaccounted for, officials said on Monday.
ABC News’ Matthew Glasser contributed to this report.
