Contributing writer, Ashley Denault, community relations senior specialist. 

As an Airbnb listing, a 2021 improvement project at Gross Reservoir northwest of Denver starts out great.

Spend weeks under Colorado’s brilliant blue skies in the tranquil foothills above the college town of Boulder. Enjoy as the leaves on the state’s famous aspen trees change from summer green to fall gold during September and October, the sun sparkles on the water of Gross Reservoir and snow dusts the top of the Continental Divide in the distance. Live where you work, work where you live. 

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For the uninitiated, questions likely would rise as the listing continues. 

Bunk with three of your highly trained co-workers ... (Um, what?) … in a streamlined suite ... (That’s not a suite.) … with an on-site support team … (But it has valet service!) … and catered hot meals from the on-deck galley … (Mmmmm, “catered.”) … while working under pressure, in near-freezing conditions, under nearly 300 feet of water. (Wait, how deep is the pool?) 

But that’s what four members of Seattle-based Global Diving & Salvage LLC signed up for in September and October 2021 at Gross Reservoir. The crack team of highly trained saturation divers — working with a large support team — ate, slept, lived and worked underwater for weeks.

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And for roughly eight weeks in fall 2021, Denver Water got a front-row seat watching the company’s close-knit team of 31 people as they replaced Gross Reservoir’s “trashrack,” the structure at the bottom of the reservoir that keeps large debris from getting into a dam’s outlet system. 

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The project used saturation diving to demolish the dam’s old trashrack and replace it with a new one due to the depth of the water and because Denver Water needed to keep water supply in the reservoir, rather than draining it.

Using divers to demolish the old trashrack and install a new one — while living in a pressurized chamber on the barge for just over a month including four days of decompression — helped ensure the project was completed as quickly and safely as possible. 

Read a previous TAP story about this project. 

First, the crew assembled a barge and loaded all the equipment needed for the project using a 110-ton crane perched on the top of Gross Dam. The equipment list for the project included the saturation living chamber where the trained divers spent more than a month, a diving bell, support crane, dive shack, hot water plant, air compressors, galley, tool cribs and diving gas. 

The diving gas, Heliox (canisters of which can be seen in the lower right of the picture above), is a mixture of helium and oxygen. The mixture does not contain nitrogen in order to prevent nitrogen narcosis and reduce decompression times for the divers. The helium in the mixture raises the pitch of divers’ speech, exactly like inhaling helium from a balloon. 

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The original trashrack was installed in the 1950s when the dam was constructed. Although the structure was sound, it needed to be replaced because it no longer aligned with new seismic regulations associated with the Colorado Division of Water Resources and the Federal Energy Regulatory Commission.

Dredging and demolition of the old trashrack took almost two weeks. The divers went to work on Sept. 20 and finished Oct. 25. Then they stayed in the chamber until Oct. 28 as it was slowly depressurized and they could emerge safely.

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Four divers worked in teams of two, one team resting while the other worked, to cover round-the-clock operations in extremely challenging conditions. 

Visibility was often poor due to the depth of the work and swirling silt at the bottom of the reservoir. 

And the divers worked in frigid conditions. Water temperatures averaged 38 degrees Fahrenheit. To combat these cold temperatures, divers had warm water continuously pumped into the body of their diving suits. 

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Before the trashrack arrived at Gross Reservoir, it was preassembled in Redmond, Washington, then dismantled for shipment. More than 50 truck trips were required to move the 25-foot by 25-foot device from Washington to Colorado. 

The pieces have a special coating that resists aquatic invasive species if they are present in the water, making it harder for the animals to attach themselves to the metal.

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In the end, the installation of the new trashrack finished ahead of schedule and on budget, with the company notching a perfect safety record during the project. 

And the new trashrack, secured in the inky darkness, under hundreds of feet of water? 

That’s good for another 70 years.