San Antonio Wasn’t Built For Rain, But This 3-Mile Underground Flood Tunnel Was

“When there’s a heavy rain, the water just pours into the Olmos Basin and is diverted into the San Antonio River, which rises up those 20 feet and sometimes higher above the banks and causes some real problems.”

By Jack MorganJuly 13, 2021 9:40 am, , ,

From Texas Public Radio:

One of San Antonio’s biggest and most important creations is something those who live here have never seen. That’s because it’s far below the city, and it was created to keep us safe.

This hidden mega-structure was created with a very specific purpose: to prevent flooding. The city’s downtown suffered repeated major floods over the decades, but author and historian Lewis Fisher said the one in 1921 changed everything

“This time, San Antonio was a large city and it was a great disaster for downtown and for residential areas downstream,” he said.

Fisher is the author of three books about the River Walk, and many more on San Antonio history. As many as 80 San Antonians were killed in the 1921 flood and all of downtown was left deep under water. The reason for that flood was the heavy rains that fell in Olmos basin, a 36-square mile largely limestone area that all drains towards downtown.

Olmos Dam
City of San Antonio Public Works Dept.

“And so when there’s a heavy rain, the water just pours into the Olmos Basin and is diverted into the San Antonio River, which rises up those 20 feet and sometimes higher above the banks and causes some real problems,” Fisher said. “As early as the 1840s, there were proposals for a dam at Olmos Basin.”

Building that dam was a substantial effort in the 1920s because the federal government didn’t help pay the cost for major public works then like it does now. It was up to the city alone to pay for and build Olmos Dam. It was finally completed in 1928, and built to withstand a 100-year flood — even two back-to-back. But 100-year floods aren’t the worst case scenario. Public Works Assistant Director Nefi Garza said there are 200-, 300- and even 500-year floods.

“There’s even something known as the probable maximum flood, which is what’s the worst flood that can possibly occur in this area given the climate, the terrain and so forth,” Garza said.

Nefi Garza at the Flood Tunnel Intake
Jack Morgan

Planning for a worst case scenario is a complicated process.

“There’s many storms that can be worse than a 100-year,” he said.

Because of that, a massive flood tunnel was devised and then built by the U.S. Army Corps of Engineers.

“We’re at the tunnel inlet of the San Antonio River Tunnel. This inlet is right on Josephine Street. You can actually see it right off 281,” he said, standing where the flood tunnel begins.

The intake sits a couple of hundred yards north of the Pearl development on the east side of the river. It looks like a massive concrete bunker, 120 feet across and 30 feet high. In front of the tunnel intake ports are metal grates that grab whatever solid objects are pushed into them. The water is allowed to pass through, but debris is removed before falling into the tunnel.

“Garbage cans, everything you can imagine,” said Garza. “We’ve found couches. We have found all kinds of things that just gets washed away in a flood.”

Nefi Garza points out the track the Flood Tunnel takes through downtown.
Jack Morgan

The flood tunnel’s scale is mind boggling: It’s 24 feet wide, drops straight down 150 feet, turns 90 degrees, then goes south for more than 3 miles before turning upwards again for 140 feet. The water is discharged into the San Antonio River at Roosevelt park about a mile south of downtown. The project to bore that 3+ mile tunnel began in 1993. Four years into it, there was a big problem.

“They were probably 95% complete. The boring machine was right underneath the football field there at Brackenridge High School,” he said.

The borer’s bit broke on some shale. But how to repair a machine 140 feet below the ground, especially when the broken part is at the front?

The Flood Tunnel outlet at Roosevelt Park
City of San Antonio Public Works Dept.

“Right, so that’s why they had to back it up, take it apart, to put in that bit so that they could move forward on it,” he said. “It was a big ordeal.”

The tunnel was less than a year old in October of 1998 when Garza, an engineer at the time but not yet working for the city, finished work midday on a very rainy Saturday. Driving down Broadway Street, it became clear what had happened while he was at work.

“There was debris everywhere. Cars were flipped over,” he said. “I was thinking, ‘Whoa, what’s going on?’ I realized, ‘OK, there’s a flood happening. I need to see what’s going on with the tunnel.’”

He raced down to the intake ports but police had cordoned off the area, so he headed downtown to the River Walk.

“And to my surprise, people were having lunch on the Riverwalk, no clue of what was happening,” he said.

More than 16 inches of rain had fallen in that flood — now called the 1998 Flood — yet tourists were still enjoying their margaritas just feet from the San Antonio River, which had stayed at a constant level. The flood tunnel had worked! Garza then drove down to the tunnel outlet at Roosevelt Park, where the water was shooting out of the concrete structure.

“The water that was coming out was coming out with such force out of that outlet that you could feel the heat of the water. Mist everywhere. It was incredible to see,” Garza said. “Downtown had completely been protected. And the people, the tourists, our financial institutions, our governmental institutions, all were kept safe.”

Graphic representation of how the Tunnel works, with input at center left and outflow at far right.
Jack Morgan

San Antonio’s flood tunnel is a gargantuan engineering feat that’s made to function without you even knowing it’s doing its job. A job it’s done without fanfare for nearly a quarter century.

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