General contractor Vegas Tunnel Constructors began sending the first 11 million gallons into the 3-mile tunnel beneath the bed of the lake Thursday afternoon, marking the latest milestone for a complicated and risky $817 million project on track for completion within the next month or so.
Before the valves were opened to let the water gush in, workers rode the elevator out of the intake tunnel for the last time. In the coming weeks, some 52 million gallons of water will fill their old work space, where they spent years digging and fortifying a 20-foot diameter tunnel from the shore to one of the deepest spots in the reservoir.
“This is a major milestone for everyone on the project,” said Erika Moonin, an engineer and project manager for the Southern Nevada Water Authority.
It’s especially big for the contractors. “They’re going to be done with the tunnel. Once they start filling, no one is going back into that space,” Moonin said.
The water authority considers the project critical drought insurance for a community that draws 90 percent of its drinking water from Lake Mead. The third intake — coupled with a $650 pumping station due to be built over the next five years — will keep water flowing to the valley even if the lake shrinks low enough to dry out the community’s two existing straws and prevent Hoover Dam from releasing water downstream.
So how does one fill 3 miles of intake pipe 600 feet underground? Slowly and carefully, Moonin said.
The process that officially began Thursday is expected to last several weeks.
The new deep-water intake is linked to the valley’s existing straws by a connector tunnel completed in 2013. A temporary bulkhead currently keeps water in the connector tunnel from seeping into the intake. To remove the bulkhead, workers must seal the tunnel off from the lake and drain it so they can work inside.
Moonin said a crew on a barge floating on Lake Mead finished the first part of the job last week by lowering a water-tight lid over the mouth of one of the existing straws used to supply water to the valley.
The roughly 11 million gallons left in the sealed connector tunnel is now being drained through valves in the temporary bulkhead and down into the new intake — the first significant amount of water allowed into the structure, though not enough to fill it.
Once the bulkhead has been removed, another 52 million gallons will be let in at a controlled rate using a mix of raw lake water piped in from above and groundwater from the surrounding rock. That seeping groundwater has caused fits for contractors since excavation began at the site in 2008, but now they can shut off the pumps and let it flood in, Moonin said.
As soon as the entire tunnel is full and the water pressure inside equals the pressure around it, a crane floating on the lake will be able to lift the cap — a 19,000-pound disc of stainless steel, 16 feet across and an inch and a half thick — from atop the new intake, opening its mouth to the reservoir above.
Moonin expects that to happen in early October. All that will be left to do then is take a drink.