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Corps weighs options for Garrison Dam spillway after 2011 flood showed weakness
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Corps weighs options for Garrison Dam spillway after 2011 flood showed weakness

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The U.S. Army Corps of Engineers is considering how to make the Garrison Dam safer in future floods a decade after the use of the dam’s spillway revealed weaknesses within the structure.

The agency opened the spillway gates for the first time in the dam’s history on June 1, 2011, sending a cascade of water down the concrete chute. The water flowed into a basin and then a channel that connects with the Missouri River about 4 miles downstream of the hydroelectric power plant.

A wet spring and snowmelt from the Rockies during the months leading up to June contributed to the flooding that began in Bismarck-Mandan that month. Lake Sakakawea, meanwhile, swelled behind the dam.

Using the spillway helped the Corps manage the flood, but the spillway faced challenges as water rushed down its slope. Among them, at least two of 29 manholes positioned within the chute came loose.

The spillway has been used sparingly since then, but if manholes were to come loose again when water passes over them, there is a small chance it could set off a sequence of events causing the spillway to fail and a significant amount of water held back in Lake Sakakawea to barrel downstream. That could lead to flooding in Bismarck-Mandan 70 miles to the south, wash away bridges and roads, and create problems in as many as 12 states along the route of the river, according to the Corps.

“We’re trying to prevent an uncontrolled release of water,” said Jeff Greenwald, lead planner and project manager with the Corps on the Garrison Project Dam Safety Modification Study. “Garrison is a safe dam. We are looking at very rare events and very low-probability events. Because the consequences are so high, we think this warrants investigation and potential modifications in order to protect the public.”

The study Greenwald is involved in aims to figure out what to do to prevent that unlikely scenario and address other less-pressing issues at the dam, such as erosion along its western slopes.

The manholes are part of a drainage system built into the spillway. They provide access to a drainage system meant to collect seepage from upstream that makes its way beneath the concrete and water that moves through seams of lignite coal in the area, Greenwald said.

The drainage system could become overwhelmed by water if manholes were to come loose again while the spillway is used. Ultimately, concrete slabs that make up the chute could dislodge and cause the spillway to fail.

The Corps is studying the issue and potential fixes. Options include replacing the chute with various types of concrete, building a new drainage system without manholes, constructing a deep concrete wall below the chute, retrofitting the manhole covers and frames or repairing issues in the existing drainage system. The Corps also could take no action.

The agency also is studying a secondary issue that poses a small but serious risk to the dam: instability along the western slopes near the power plant. The area is made up of compacted sands that are prone to erosion, shallow slides and sinkholes, Greenwald said.

A slope failure is unlikely, but it could take out relief well infrastructure if it were to happen, he said. The Corps is considering adding additional drains along the slope or steel piles to improve stability.

As the Corps studies the issue, it will produce a document known as an Environmental Assessment that identifies the best path forward and evaluates alternatives.

The agency anticipates pinpointing its preferred option this year. Eventually a draft of the Environmental Assessment would be made available for public comment, likely next year, Greenwald said. He anticipates the process would wrap up later in 2022.

Congressional funding ultimately would be required to make physical changes to the dam once the review is complete.

Reach Amy R. Sisk at 701-250-8252 or amy.sisk@bismarcktribune.com.

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