Is a massive tunnel system the answer to Houston’s flood woes?

Picture this: Another massive rainstorm overtakes Houston, but instead of inundating homes and businesses, much of the floodwater funnels into a massive underground tunnel system that whisks it away to the Houston Ship Channel.

It may sound far-fetched, but it's one concept on the table as the Harris County Flood Control District begins to explore more creative flood control methods after Hurricane Harvey, when the Houston area became the site of the worst rainstorm in American history. 

Major rainstorms that brought widespread flooding to the region on Wednesday served as the latest reminder of how flood-prone the area is. 

The district hasn't offered many specifics on the proposal; it only just got the green light from county commissioners to apply for a federal grant to study the feasibility of digging miles of channels underneath the nation's third largest county.

While it's far from clear whether it will ever happen, the concept almost immediately generated widespread response when it was announced earlier this spring. Local officials told the Houston Chronicle it’s outside-the-box thinking with benefits that could outweigh the heavy price tag. Residents reading about the project on social media have expressed fears of sinkholes from the underground construction. Even entrepreneur Elon Musk, who owns tunnel construction company The Boring Company, jumped into the conversation on Twitter.

So would such a tunnel system really be a logical solution for Houston’s flood woes? 

Drilled 100 to 200 feet underground, the underground channels act as temporary storage for floodwater during intense rainstorms, said Larry Larson, a senior policy adviser at the Association of State Floodplain Managers. Once the rain has stopped, the stormwater can be used for a variety of purposes. It can be pumped back to the surface into a river or wetlands or even used to recharge aquifers. 

If cities have a section of river that regularly overflows, a tunnel can convey extra water underground and help reduce the amount of water that flows onto land during storms, said Christof Spieler, project manager of the Greater Houston Flood Mitigation Consortium. Large-scale tunnels can also act as an additional set of waterways, taking pressure off undersized drainage networks, he said.

But Larson and Spieler said it's hard to tell if such a system would make sense for Houston — a low-lying coastal city that's experienced three 500-year floods in the past three years. 

The area's soft soil and high groundwater table would likely complicate construction, Larson said. And while most cities have flood zones concentrated around rivers and other bodies of water, Spieler said Houston experiences more dispersed flooding 

"It's not one big waterway, but dozens of them, and no clear floodplains," he said. 

Because flood patterns vary across watersheds, Spieler said what might be a good solution in one part of the county wouldn't necessarily be the best choice in another.

“[Houston has] the worst of conditions — huge development and you’re in terrain that’s flat as a pancake. You can’t get the water out,” Larson said. “You’re even subject to coastal storm surge. Houston’s really in a bullseye there.”

But, Larson said, that doesn’t mean you don’t explore your options.

“When you look at potential solutions, what you’ve got to look at is what’s the cost going to be and what are the benefits going to be. And you won’t do that unless you do a study,” Larson said. “From the standpoint of the billions of dollars in damage in the Harris County area [after Harvey,] $400,000 [for a feasibility study] is kind of a drop in the bucket.”

Spieler said it's hard to tell whether such a tunnel system would have greatly minimized the historic flooding Houston saw last August. 

The county had effective measures in place before the storm — newly widened bayous and lots of detention ponds — but Spieler said “if we had spent more on all sorts of flood mitigation, you would have seen fewer houses flood in Harvey.”

"The terminology is bad, because you cannot control floods," Larson said. "We tried that for years. You can’t engineer your way out of this problem, and Houston is finding that out."

Regardless of what the district decides, Spieler said, it’s important to remember the limitations of flood mitigation. Good flood planning requires a multipronged approach, he said, and good flood planners realize that you can’t solve flooding, you can only reduce it.

“[Tunnels solve] a very particular problem in flooding,” Spieler said. “There’s a natural tendency to think we can solve flooding, that just by building this infrastructure, this problem will go away. And that doesn’t work. Whatever storm you design this for, there’s always a possible bigger storm.”

Flood control tunnels are nothing new to Texas — San Antonio built the San Pedro Creek Tunnel in 1991 and completed the longer San Antonio River Tunnel in 1997. Austin continues to put the finishing touches on the Waller Creek Tunnel and a tunnel in East Dallas received the long-awaited go-ahead in February. 

Should the district choose to pursue the project, tunnels could cost up to $100 million per mile, Steve Costello, the city’s chief resilience officer, told the Houston Chronicle. 

On Aug. 25, Harris County voters will decide on a $2.5 billion bond proposal to fund a slate of yet-to-be-determined flood protection projects that could include a tunnel system. Preliminary engineering for the underground channels are included on a list of eight possible countywide bond projects — along with a $20 million budget line.

The feasibility study for the tunnel system is expected to be completed in October. The study will likely examine whether such a project is possible given the geography of Harris County, said district spokesperson Karen Hastings.