Dam, Tunnels, Gates Keep Downtown Safe and the River Walk Full

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Scott Ball / Rivard Report

The interior of the San Antonio River tunnel outlet station near the Lone Star Brewery.

Nefi Garza remembers the weekend of Oct. 18, 1998, when San Antonio was getting pummeled by one of its heaviest rains in modern history. 

An engineer working in the private sector at the time, Garza knew that shortly before the storm came, the City had just finished work on a tunnel 150 feet deep and 3 miles long meant to divert stormwater below downtown. He drove his father’s truck south, passing flipped cars on Broadway, thinking the flood would have devastated the River Walk downtown.

But when he peered over the side of the bridge at Houston Street, he saw a normal River Walk scene.  

“I looked over and people were literally having lunch on the River Walk,” Garza said. “I was like, ‘What the heck?’ They had no clue what was going on.”

Just over 20 years later, Garza is in charge of the flood management system that saved downtown that day and during every major flood since.

As assistant director for stormwater of the City’s Transportation and Capital Improvements Department, Garza manages a team of eight flood management personnel he refers to as “pre-first responders.” Any time a significant rain is set to hit San Antonio, these staffers are manning the controls of the complex set of dams, gates, and tunnels that control the San Antonio River. 

The tools available to his staff are some of the most advanced water-moving apparatuses available to any municipal government. The entire system along the San Antonio River from Olmos Basin to the start of the Mission Reach is designed to handle massive floods, as well as to keep a steady level of water lapping gently at the walkways of the downtown River Walk. 

Garza and his colleagues offered a tour of this system on Friday. As San Antonio continues to grow, stormwater engineers are using lessons learned on the San Antonio River downtown and applying them to other parts of the city drained by waterways such as Leon and Salado creeks, he said. 

“We’ve learned quite a bit,” Garza said. “Now, how do we take the technology that we’ve learned to other parts of the city?”

Olmos Dam

The oldest part of the system – and downtown’s first line of defense – is Olmos Dam, completed in 1926. The wall of concrete holds back water that drains from as far north at Loop 1604. 

All this water collects in Olmos Basin Park, which during dry times hosts soccer games and cyclists enjoying its paved and dirt trails. The floodwaters wash in trash by the ton, which collects behind trash screens placed over the dam gates. 

With the area along the river downstream of the dam home to University of the Incarnate Word, Brackenridge Park, the San Antonio Zoo, and the River Road neighborhood, there are strict rules about how much water can be let through. Only two of the dam’s six gates are allowed to be open two feet at a time. 

Like many aspects of the job, TCI staff have to make Olmos Dam a balancing act. The top of the dam is 16 feet higher than nearby U.S. 281, so a dam operator has to ensure that the major highway stays clear but also avoid causing flooding downriver.  

If Hurricane Harvey hadn’t veered back toward the coast and pummeled Houston in 2017, it could have filled the Olmos Basin with enough water to send four to six feet of it spilling over the dam, Garza said, citing modeling down by the San Antonio River Authority. 

After nearly 100 years of floods, water has never spilled over the top of Olmos Dam, Garza said. That could have been different if Harvey had continued on a track that forecasters at one point predicted would send the storm hurtling into San Antonio.

However, unlike in Houston, much of water would have drained within 48 hours, he said. San Antonio’s elevation change from north to south may seem subtle, but the city is built on a slope. 

That elevation change is what makes the second line of flooding defense possible, Garza said. 

San Antonio River Tunnel

Scott Ball / Rivard Report

The San Antonio River outlet tunnel.

The underground tunnel inlet at Josephine Street looks like a cross between a park and a bunker. Below the trees and water fixtures lies a massive concrete chasm that gulps river water deep into the earth. 

Garza and his staff gave a tour of the facility, where monitor screens show the readouts from sensors and flow gauges. From key locations like the tunnel inlet and the structure south of downtown known as Gate 5, operators can manipulate the flow of water throughout the system. 

Finished in March 1997, the San Antonio River Tunnel is an engineering marvel. Creating it involved drilling shafts a little more than 24 feet in diameter 150 feet deep, then using a gargantuan boring machine to tunnel through the dirt and rock below downtown to connect the two. That allows water to flow underground at Josephine Street, then back to the surface at Lone Star Boulevard. A smaller tunnel for San Pedro Creek was completed in 1991.

The whole thing works via gravity, Garza explained. The top of the inlet tunnel at Josephine Street is a mere 35 feet higher than the outlet at Lone Star Boulevard, but that elevation drop creates enough pressure that water flowing in can push the water in the channel until it can flow out later. 

After Harvey, Garza said they had two busloads of visiting stormwater engineers and other officials from the Houston area. 

“When I showed them the 35-foot difference, they were like, ‘Oh, we can’t get that [in Houston],’” Garza said. 

The tunnel also has a secondary function: keeping water in the River Walk during dry times. 

During wet years when water in the Edwards Aquifer is high, water gushes forth from the headwaters springs that feed the San Antonio River. When this isn’t happening, TCI staff can pump water stored in the tunnel back into the river, Garza said. Some recycled water from the San Antonio Water System’s sewage plants also supplements the system. 

“Once it does go through the River Walk itself, we are able to capture that water,” said Dave Carreon, a senior electronic technician who has worked for TCI for 14 years. “It’s just a big circle.”

At the tunnel outlet, TCI staff opened up the gates to a vast concrete room with ceilings at least 20 to 30 feet high. Much of the floor was taken up by a circular pool of water, its cloudy surface disguising its depth.

Even during the height of the 2010-2015 drought, the tunnel never went dry, Garza said. Because getting down there is so difficult, TCI will be purchasing an underwater drone with infrared scanners, typically used to inspect submarines. The drone can cling to the walls of the tunnel and scan the entire passage for structural defects, he said.

Flood Control Gates

Scott Ball / Rivard Report

Large machinery at the Josephine Street inlet rips debris out of the San Antonio River and slings it into a deck to be cleaned by City staff.

Garza, a San Antonio native who excels at breaking complex information into digestible pieces, explains that three concepts are essential to flood management. These are detention, or holding back water; diversion, or moving some of it to where it will do less harm; and channelization, or easing its pathway downstream. 

“We have a world-class flood control system because we employ all three methods, particularly in the downtown area,” he said. 

Olmos Dam is the epitome of detention, and the tunnel is a poster child for diversion. Channelization, Garza said, is the philosophy behind the bypass channel that sends floodwater flowing past the horseshoe-shaped curve of the downtown River Walk. 

Built piece by piece starting in the 1940s under the vision of architect Robert Hugman, the downtown River Walk lined by shops and restaurants follows a historical bend of the San Antonio River. 

In this area, the river is not actually a river, Garza said. In truth, it’s a lake. 

That’s because of Gate 5 on Nueva Street, the main gate in the San Antonio River that holds back the water downtown. On Friday, the gate was in a partially open position, with the river tumbling over the edge like a waterfall. 

Like the Josephine Street inlet, Gate 5 is a major node of the system. TCI workers have an office near the dam where they can operate the same controls they do at the inlet. 

Nearly as important are two other gates at the connection points where the river’s horseshoe bend connects to the main channel. Those gates – Gates 3 and 4 – can isolate the downtown loop from the rest of the river, protecting the River Walk from flooding. 

Local flooding officials are now applying similar concepts they’ve learned downtown to the rest of the city, Garza said. He cited an example on Leon Creek near Highway 151 where engineers are considering a bypass channel to protect homes from flooding, he said. As the city continues to grow, it will likely need continued investments in flood control to keep up with all the impervious cover – mostly asphalt and concrete – that is increasingly covering the local watershed and worsening downstream flooding. 

Garza is a proponent of so-called green infrastructure, or low-impact development, which can use plants and soils to slow down, spread out, and soak in smaller floods. But it’s the dams, channels, basins, pumps, and other “gray infrastructure” downtown that has made low-impact development possible on places like the Mission Reach, he said. 

“Green infrastructure by itself in Flash Flood Alley will get ripped out,” Garza said. “That’s why you need a good balance of gray and green to be able … to work together. Because we have so much impervious cover, we need both gray and green infrastructure to really have a much healthier ecosystem, a system that heals itself after a flood event, and ultimately provides a better place for us to live.”

Scott Ball / Rivard Report

Assistant Director for Stormwater Nefi Garza

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