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LAKE SONOMA – It was mostly quiet Wednesday in the bowels of Warm Springs Dam but for the distant roar of water as a team of engineers aboard a 3½-ton flatbed vehicle entered the concrete outlet tunnel.

A swift current flowed around the vehicle’s four huge tires, though the release of water had been reduced by half to accommodate the dam’s routine, two-day examination.

Standing high enough aboard the metal vehicle that their hard hats scraped some of the lime and calcium stalactites hanging from the curved ceiling 14½ feet above the tunnel’s inundated floor, the half-dozen Army Corps of Engineers personnel harnessed to the cart’s railing looked more like miners than engineering types as they started the 2,400-foot trip through the tube.

If there is indeed substantial rain this winter, as surely everyone in California must hope, Warm Springs Dam appears ready for it.

But for a sticky valve requiring routine maintenance, inspectors of varied specialties gave the thumbs up at the close of the annual inspection, which is designed to uncover any structural, mechanical or electrical deficiencies that might prevent the dam from fulfilling its flood-control function in the event heavy rains return.

Having gone several years without the strain of holding back substantial rainfall in Lake Sonoma, the dam must nonetheless undergo yearly assessment to “identify any last-minute repairs we need to do before flood season,” said Mike Dillabough, chief of operations and readiness for the Army Corps’ San Francisco District.

Inspectors started Tuesday by walking the earthen dam built across Dry Creek to create the lake in 1983, searching for signs of seepage, burrowing rodents or other signs of wear and tear that might signal structural problems in the compacted earth that supports the inner, impermeable core of the dam.

Specialized crews also descended 300 feet into the dam tower to ensure the inner mechanical and electrical works that govern dam releases were in proper working order. There are 390 different instruments used in dam operations, Dillabough said.

Workers also inspected the concrete spillway that would allow the escape of lake overflow if there was a catastrophic flood and examined the access road for structural integrity.

But it’s the trip into the tunnel — albeit a slow one — that may offer the most thrills, beginning with a brief ride dangling below a crane used to lower the specially made vehicle down to the mouth of the tunnel, where the water spills out from Lake Sonoma over a terraced surface designed to slow the current and dilute its energy so it is safe for the fish and birds downstream.

Those on board wear life jackets and body harnesses hooked to the railing of the vehicle, which was designed a decade ago for inspection of the tunnels at both Warm Springs and Coyote Dam, which was built to create Lake Mendocino.

The tunnel inspectors have a monitor with them to ensure the oxygen level is good and carbon monoxide is in check as they enter the dam.

The vehicle, its wheels canted to accommodate the circular shape, has mounted bright lights that illuminate the rounded walls of the tunnel to permit inspection of seams and patchwork and ensure it can withstand high pressure. A special inspection conducted every five years includes measuring the walls for stability down to a thousandth of an inch, Dillabough said.

“Our job,” Dillabough said, “is to make sure this is still working 100 years from now. So, a little tender loving care.”

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