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Workers are racing to repair a damaged tunnel underneath downtown Santa Rosa that took a beating during this past winter’s record rainfall.

Since 1963, when the flood control project was built, Santa Rosa Creek has been confined to two underground concrete box culverts running for 1,400 feet under downtown.

The 14-foot-tall tunnels begin at E Street and run beneath the U.S. Bankruptcy Court building and Santa Rosa City Hill, where they are joined by a third tunnel conveying Matanzas Creek. All three emerge at Santa Rosa Avenue, at the entrance to the Prince Memorial Greenway.

But erosion has scoured out the bottom of the middle tunnel, exposing and corroding the steel reinforcement bars embedded in the concrete, and raising concerns about the safety of the structures.

“That’s obviously not something you want for a tunnel that’s holding up half of downtown Santa Rosa,” said Jimmie Griggs, the Sonoma County Water Agency engineer managing the project.

Erosion of the original concrete floor was identified in 1997 and repaired with a 3-inch-thick slab of concrete. But a recent inspection found much of the repair work had eroded away.

This past winter, nearly 58 inches of rain fell in Santa Rosa, breaking a 1902 record. Big storms like that generate huge flows through the culvert that act “like a sandblaster” and further scour out the tunnel floor, Griggs said.

Because of the winter storm damage and concern about more this winter, the $767,000 project was given a high priority. The work was green-lighted by the Sonoma County Water Agency earlier this year, and crews got underway about a month ago, Griggs said.

Third District Supervisor Shirlee Zane, who toured the site Monday, said at its core the project is about protecting downtown Santa Rosa from flooding, something everyone can relate to given the recent news out of Texas and Florida.

“It’s penny-wise and pound-foolish if we don’t invest in this aging infrastructure,” Zane said. “You can’t stop Mother Nature, but you can at the very least do your part to mitigate it and build in resiliency.”

The work is being done by T.P.A. Construction, of Rocklin, which specializes in large-scale concrete repair projects. The company was the only bidder for the work.

The location of the project makes it labor intensive, not to mention loud and dusty,

Workers have been using mini-excavators and hand jackhammers to bust out the degraded concrete, and haul the material out of the long tunnel in huge bags, which then have to be plucked out of the creek bed with a crane. Dozens of the 4-foot-high bags, each weighing about 4,000 pounds, are stacked along E Street awaiting removal.

A forklift hoists them into a dump truck where workers slash the bags with a razor and empty out the contents, which are being recycled at a local quarry.

The next phase involves the floor of the tunnel being sealed to prevent water intrusion and then resurfaced and re-leveled with a heavy-duty repair mortar to resist future scouring, Griggs said.

Because that material dries quickly, however, the concrete can’t just be whipped up in a big batch aboveground and pumped down with long hoses. It has to be mixed by hand down in the tunnel.

Work is expected to be completed Oct. 15.

Griggs, 26, said he is impressed with the engineering work that went into designing and building the tunnels originally, a massive, multiagency undertaking that was called the “Central Sonoma Watershed Project.” Even though today there is “negative stigma” attached to forcing a natural creek into a concrete channel, and it probably wouldn’t be allowed to be built the same way today, he said he’s proud to be a part of the repair project.

“I just feel like it’s our obligation to maintain this stuff,” he said.

You can reach Staff Writer Kevin McCallum at 707-521-5207 or kevin.mccallum@pressdemocrat.com. On Twitter @srcitybeat.