The spillway safety project will be a massive undertaking. It will involve the overhaul of the dam gates, spillway, inlets and other aspects of the Garrison Dam.

The Corps recommended there be seven major elements of improvements to reinforce the spillway, according to a background paper. These improvements will armor against erosion damage, raise the spillway monoliths to prevent overflow, upgrade the deicing system and modify the spillway gates. Over the next six years, surveillance will occur over the planned construction area and detailed designs will be developed. Construction is anticipated in 2031.

Lake Sakakawea is the largest reservoir managed by the Corps, the third largest in the nation day-to-day, and is currently the largest active reservoir in the country, according to Jeff Greenwald, a planner and project manager with the Corps, in a briefing to Hoeven’s team. The North Dakota lake is the largest because Lakes Mead and Powell currently have lower than normal water levels.

There are 178 miles of reservoir upstream from the dam, and a breach would be catastrophic, “essentially unzipping the country down the Missouri River,” Greenwald said.

Ben Pifher/MDN Standing in front of a hydroelectric generator at the Garrison Dam Thursday are, from left to right, Phil Brown, Kim Thomas, Sen. John Hoeven, John Benson, Dave Sobczyk, Maverick Thompson, Josh Gormley, Aaron Dunlop and Jeff Greenwald. All except Hoeven are with the U.S. Army Corps of Engineers.

“Like something we’ve never seen,” is how Greenwald described a hypothetical failure of the dam.

Kim Thomas, chief of the Operations Division, said the country would be affected all the way to New Orleans if there was a failure.

“A lot of critical infrastructure, power sites, nuclear sites, two state capitals, and much of everything else would be under water, with Bismarck being under 90 feet of water, and an unprecedented domino effect would occur downstream,” Greenwald said.

The group from the Corps made comments describing affects akin to those of nuclear weapons.

“Over 2,000 river miles of flooding would be induced (by a failure),” said John Benson, the Engineering Division chief with the Corps’ Omaha District.

Ben Pifher/MDN One of two trunnions house the hinges for each flood gate. These trunnions will be replaced on each of the gates as part of the Garrison Dam Spillway Safety Project.

The spillway safety project originated following the 2011 flood, when the spillway was overtopped, changing the landscape downstream and washing away a road and recreation area which used to lie at the bottom of the spillway, according to Greenwald and others from the Corps.

“This is a big deal,” said Hoeven, referring to the dam as a whole, and mentioning the project is going to be one of America’s major construction undertakings.

After an overview of the project from the Corps, Hoeven and other guests were taken on a tour of the dam, spillway and power plant showing the concerns, highlighting the capabilities and explaining the plan for the $1.5 billion project.

Construction will occur in an active spillway, with the construction season and flood season overlapping from May to July each year. The flood risks will be monitored from the snowpack in the mountains to precipitation and reservoir inflows. The contractor work area will be limited and safety will be a priority, according to Greenwald.

The spillway is 1,300 feet wide and 3,000 feet long, transitioning down to 800 feet wide after a ramp which feeds to the Missouri River. A cement overlay 2-1/2 to 3 feet thick will be poured over the spillway. This slab will be up to 16 feet thick at the base of the spillway, near where immense baffle blocks are installed to prevent the base of the spillway from being washed away in a flood. More than 400,000 cubic yards of cement will be poured for the spillway modification, equating to as many as 90,000 truckloads of cement.

The gates holding back the reservoir are from the original construction and will remain after the project, with the trunnions holding the massive hinges on them being replaced. Stop logs will be used to allow contractors to work on the gates, while keeping the reservoir full of water behind them.

“The only concern is high water, and having to operate the gates,” said Thomas, referencing the safety of the workers on the project, and highlighting why water surveillance will be important through the project.

“This is a vitally important piece of infrastructure, not only for North Dakota, but really for the country, and we need to make sure that it’s sound in terms of its structure. This is why we have to make sure we are doing the necessary work,” Hoeven said.