Hebgen dam trout Madison River

A NorthWestern Energy truck blocks access to Hebgen Dam on Wednesday.

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Flows from Hebgen Dam into the Madison River have returned to normal following a component failure this week that led to a drastic drop in water levels.

NorthWestern Energy fixed the broken component late on Wednesday, using a new part from a company in Anaconda. But how the component failed is still murky.

“Nothing tangible, and any guesses at this point would be awfully premature,” said Jeremy Clotfelter, the director of hydro operations for NorthWestern Energy.

What failed, however, is clear. The gate allows water from Hebgen Lake to flow downstream, and can be lifted by using a gate stem. That stem is a long metal shaft composed of four sections, each interlinked by a coupler, and then operated by an electric motor at the top of the dam.

There were three couplers, and one on the uppermost section of the gate stem broke, Clotfelter said. The coupler was a one-piece, machined component, and when it broke, it could no longer bear the load of the gate several feet below.

The gate dropped down, shrinking the opening for water to flow from Hebgen Lake into the upper Madison River.

But monitoring equipment at the dam gave no sign that water flows from downstream began to drop after the coupler failed.

At the deck level above the water on the dam, there is a position indicator, Clotfelter said. That position indicator tells dam operators how far up or down the gate stem is positioned, and that the position of the stem at the top of the dam indicates the position of the gate below.

That position is used to calculate and control flows from the dam, he said.

When the gate stem failed, it was no longer connected to the gear box above. However, the uppermost part of the stem — which was above the broken coupler — was still intact and connected to the motor and monitoring equipment.

“So it truly never moved, it never saw any change or any calculated flow change or anything,” Clotfelter said.

Clotfelter said that it could take months to figure out exactly how the coupler broke. He said part of the process that the utility company will go through in figuring out how the piece failed included a “finite element analysis.”

That type of analysis shows how a mechanical component will react to different stresses and forces, he said. For example, if the results show that the failure occurred due to a torsion force, or a torque, that would lead to a root cause that is much different than an analysis that showed a bending moment, and would require a different approach, Clotfelter said.

Another avenue would be through metallurgical analysis, which could identify if the metal coupler failed due to stress over time, or an acute failure — like a large force suddenly breaking the metal component.

“With that information in hand, that begins to guide us to a root cause, and once we have a root cause then we are in a much better position to implement corrective actions,” Clotfelter said.

The last upgrades to the dam were completed in 2014 after stoplogs failed a few years earlier, sending over 3,000 cubic feet per second blasting downstream on the Madison River. The Federal Energy Regulatory Commission — the federal agency that provided a license to the utility company to operate the dam — was also revamping its seismic standards when the stoplogs gave way in 2008.

Both events ran parallel, and led to about $40 million in upgrades to the dam to get rid of the old wooden flow controls and meet the federal agency’s new standards, Clotfelter said.

The agency routinely inspects licensed dams, and completed an inspection on Hebgen Dam this summer. Clotfelter said that inspection did not find anything that would indicate a failure in the coupler on the gate stem would occur.

“It was not a ‘we are going to inspect in-depth the gate stem couplings,’ it’s not that narrow, they’re looking at the overall health of the system,” Clotfelter said.

The federal agency also annually certifies the gates for operation, he said.

NorthWestern Energy has to file two reports with FERC, one for impacts to the Madison River, which has to be filed within 10 days of the incident. The other is for dam safety, and the utility company has 14 days from the incident to file its report.

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Alex Miller is the county and state government reporter and can be reached at amiller@dailychronicle.com or by phone at 406-582-2648.

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