What happened at Hebgen Dam
NWE answers why the dam failure occurred and how it is being addressed
NorthWestern Energy (NWE) held its annual Discussion of Madison River Topics April 26 in Ennis. This was the first in-person meeting in Ennis since 2019 due to the pandemic.
The topic at the forefront of everyone’s mind was the Hebgen Dam. Jeremy Clotfelter, Director of Hydro Operations for NWE, addressed the crowded hall, supplying answers. The sudden drop in water volume and flow on Nov. 30, 2021, which took forty-six hours to repair, left residents wondering how this could have occurred. It came as a surprise to the industry as well. An outlet gate dropped, and now they know why. The culprit, Clotfelter stated, was a component known as a coupling. The cylinder-shaped metal coupling, threaded on both ends, is approximately 12 inches high, eight to ten inches in diameter, and weighs approximately 125 pounds. “A square gate sits under water and closes to shut off flow. That gate is connected to a motor and gear box above the water by four pipes. Each pipe is connected by a coupler,” he explained.
There are several gates across the system, and of different varieties. The three gates on this particular structure serve different purposes; two feed the water in, one lets the water out. When that coupling failed, it affected the only gate allowing the water out. The gate, originally open to about eighteen inches, fell to just six inches. If the gate had closed completely the river would have been completely dry. Although, if it had closed all the way, an emergency gate could have been used to pass the flow more quickly.
Failure analysis showed that the coupling failed due to “stress corrosion cracking” which occurs within the metals themselves. This was not due to external stress. Imbedded stress within the materials can be created during the process of making it. It was of industry standard, manufactured in Massachusetts. Chemicals are likely the cause of this corrosion, likely ammonia, By Cori Koenig nitrates or biologicals. These can all be found in water, and it doesn’t take much, according to Clotfelter. It is unknown why this coupling at Hebgen, but not at hundreds of other installations in the country. There are still many questions to be answered.
Since it was not the design of the coupling that contributed to the gate failure, identifying a different material to create one was crucial. A material, one that is not susceptible to stress corrosion cracking, has been analyzed and sent to the Federal Energy Regulatory Commission (FERC) for approval. NWE is confident the approval will come soon as the seriousness of the situation is widely known.
Once the plan is greenlit by FERC, the repair can begin. NWE must also wait until there is enough reservoir volume to pass water. Clotfelter estimates this to occur sometime in June or July. The repair itself is going to take just one to two weeks. In the meantime, a steel coupling was put in place and the gate is being held up so it cannot fall again, a temporary but seemingly solid solution.
This event appears to have brought to light other issues that NWE did not foresee occurring. For example, because the gate did not fall completely shut, six inches from being fully closed, the “minimum flow alarms” did not sound. Additional alarms and cameras have since been placed at the dam in response. At the time of the incident they did not have, what Clotfelter refers to as a “rate of change alarm”. There is one now which is set at a rate of 5%. Therefore, if changes beyond the 5% occur, higher or lower, the alarm will trigger. These alarms are sounded not only to the Madison operators but also to the generation control center in Great Falls, which is manned twenty-four hours a day. Ideally, they would like cameras that can be manned 24 hours, but the bandwidth in the area does not currently support that.
Additionally, it was noted that at the time of the incident there were no signs providing the 24-hour emergency number for NorthWestern Energy. This too has been rectified.
Additional topics covered at the NWE discussion will be covered in a future edition.