With engineers working at a feverish pace to get more than 200 projects in its $2.5 billion bond program moving, much of the Harris County Flood Control District’s efforts are focused on nuts-and-bolts improvements — including widening bayous, digging detention basins and purchasing flood prone homes.

From his cramped office at district headquarters, however, engineer Scott Elmer is pursuing the most ambitious project the agency has ever conceived: massive tunnels that could funnel stormwater beneath the region’s bayou network to the Houston Ship Channel.

The tunnels could provide a crucial new tool to complement existing flood control methods, as new development in fast-growing Harris County and more intense storms wrought by climate change place additional pressure on infrastructure.

“When you look at events such as Hurricane Harvey and Tropical Storm Imelda, it’s time for that type of out-of-the-box thinking,” Elmer said.

The flood control district has considered tunnels since the 1990s, though plans have never advanced beyond paper. Since Harvey in 2017, which flooded more than 200,000 county residences and damaged many of the district’s defenses, the county has revisited the idea.

A study engineers completed in October reached two important conclusions — that tunnels feasibly could be constructed and that they could move substantial amounts of stormwater that otherwise could pool in neighborhoods or push bayous over their banks. Encouraged by the results, the district has begun a second phase of research, which over the next year will map one to five possible routes. A third one-year phase would include a geotechnical analysis to evaluate construction challenges.

Elmer said the district will look for paths along public rights of way, including freeways and utility corridors, since buying private land is time-consuming and expensive. He estimated the tunnels would be 25 to 40 feet in diameter.

Tunnels have worked to protect other cities. Austin and San Antonio have them; Dallas is building one. Harris County engineers are studying the new 2.3-mile Anacostia River Tunnel in Washington, D.C., noting that city also sits on a coastal plain.

The sheer volume of water Harris County’s tunnels would need to transport, however, sets the Houston area apart. Harvey dumped 51 inches of rain on some parts of Harris County, more than Washington receives in an average year.

Elmer said his team has yet to determine precisely how much of a benefit tunnels would provide, though he cautioned there is no flood protection measure that can fully protect the county.

“We don’t view these tunnels as a standalone,” he said. “We believe there will always be a role for some of the more traditional methods. The tunnels would be a benefit in some areas.”

The biggest obstacle to building tunnels is likely to be the cost.

At a price tag of about $100 million per mile, even a single 20-mile route connecting the Addicks and Barker reservoirs to the ship channel would cost around $2 billion. Elmer said Harris County would have to seek funding from the state and federal governments, perhaps as a supplement to a bond passed by voters.

Those potential partners, however, have a mixed record when it comes to helping Houston protect against flooding. The state refused to immediately tap its $10 billion Rainy Day Fund after Harvey. Congressional funding lapses for Project Brays caused significant delays to improvements along that bayou.

Should the flood control district solve the funding problem, Elmer said, engineers could submit a tunnel construction plan to Commissioners Court within two to three years.

The cost of flood tunnels draws comparisons to the Ike Dike, a 70-mile coastal barrier aimed at protecting the Houston-Galveston region — particularly Houston Ship Channel industries — from Gulf of Mexico storm surges. The Army Corps of Engineers in October proposed a version of the plan that could cost as much as $31 billion, two to three times more than initial estimates. The plan did not explain how the massive endeavor would be funded.

Elmer said tunnels could provide more immediate benefits for Houston and Harris County home and business owners. For one, tunnels can be completed in stages, and new branches can be added as needed and when funding becomes available.

Local leaders have given the flood tunnel idea qualified support. Harris County Judge Lina Hidalgo, who has urged departments to innovate, said she would support the project so long as it remains practical and funding partners can be secured. Mayor Sylvester Turner has said local funds are not enough for robust flood protection, noting how support from the Federal Emergency Management Agency is critical to building the North Canal project.

Experts also offer cautious approval. Jim Blackburn, co-director of the Severe Storm Prediction, Education, and Evacuation from Disasters Center at Rice University, long has urged Harris County to more aggressively approach flood control. Tunnels are a bold idea, he said, so long as they do not exacerbate flooding downstream.

“What I’m concerned about is that in an effort to keep the cost down, they may attempt to terminate it in an area that may already be congested, from a water standpoint,” Blackburn said.

Susan Chadwick, executive director of advocacy group Save Buffalo Bayou, panned the tunnel idea. She said the enormous sum of money needed to construct them would be more wisely spent purchasing undeveloped land in west Harris County, which acts like a sponge during rainstorms.

Cynthia Neely, an advisory board member with Residents Against Flooding, echoed Blackburn’s comments about ensuring tunnels do not make flooding in other areas worse. She said tunnels may be expensive but would be a worthwhile investment over the long term.

“Anything we do is going to be far less than what we pay, and what all Americans pay to bail us out, over and over again,” Neely said.

zach.despart@chron.com