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Another "biggest" being designed for dam

New bulkhead would keep lake off gates under repair

 

A preliminary design drawing shows the bulkhead structure attached to the dam on the upstream side of a drum gate. - courtesy USBR

A new project aimed at refurbishing very old structures atop Grand Coulee Dam, like so much of the dam, is likely to yield the largest such device ever built.

The 11 drum gates used to control the lake level, at 75 years old, are in need of major refurbishing, not just annual maintenance, explains Public Affairs Officer Lynne Brougher.

But that poses big problems without a way to plan long-term projects around the operation of a reservoir used to control the flow of the Columbia River, for fish, floods, farming and power production.

In order to schedule a major refurbishment, the bureau says it would have to plan on drawing down Lake Roosevelt for a dozen years.

But not if the bureau builds a big, new contraption that would do for each gate what the original coffer dams did when the dam was being built - provide a way to keep the water out.

Reclamation is proposing to construct a bulkhead structure that would attach to the dam and provide a water-free space between the bulkhead and the drum gate. This would allow both an overhaul of the drum gates and ongoing maintenance in dry conditions without drawing down the reservoir for extended periods.

"While there are similar structures in existence around the world, there is nothing on this scale," said Ian Turner, the manager of the project. "In true Coulee fashion, this would be the biggest, longest-spanning floating bulkhead that we're aware of. We are in pretty uncharted territory."

With about 65 percent of the design of the bulkhead done, the cost is estimated at $50 million to $80 million, a figure that could change as they get closer to a final design, notes Brougher.

The bulkhead will be designed to float into place, then attach to some equipment, also under design, to be installed on the dam itself.

The agency is taking public input on the idea, and on the alternative, doing nothing, which would necessitate that decade-plus drawdown. An environmental assessment is expected to be published in June 2016.

Brougher said the bulkhead would likely take about a year to build and get into place once the project is OK'd. Building it would take place on the shore. One place being considered is the concrete slab at the Crescent Bay boat launch area, built there to put together the new Keller ferry a couple of years ago.

When not in use, the bulkhead would be stored, either in the lake or on the shore.

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