Five years ago, NorthWestern Energy began reconstruction on the intake of the Hebgen Dam, which controls the flow of the Madison River – that construction was completed this summer.
“We’re still working on demolishing the cofferdam, which is out in front of the intake,” said Benjamin Cope, NorthWestern Energy’s project manager for the dam rehabilitation. “When that gets to a point where it’s safe, we will release water through the new intake.”
A cofferdam is a temporary enclosure that diverts the water from where construction is happening. Divers are used to demolish the cofferdam, according to Cope, which is why it would be unsafe to open the intake before the cofferdam is taken down.
“It’s looking like it will be within a month or a month and a half,” he said.
Cope said the new intake is located at the same elevation as the original gates on the old intake.
“The depth from where it pulls water will be almost exactly the same,” he said. “But that depth is deeper than where we have been discharging water for the last five years (during construction).”
That means the water in the river will initially be colder than it has been, Cope added.
As for any issues with sediment, Cope said those should be nonexistent.
“We’re not pulling directly from the bottom (of the reservoir) so we won’t have any lake sediment,” he said. “No floating debris.”
After the first few hours of releasing water from the new intake, Cope said there will not be any noticeable changes to the Madison River.
“Aside from the short term, the first few hours after its open, it will be normal,” he said. “There will be no more sediment than what we usually see.”
Project history
In 2005, an analysis of the dam indicated the need for a new intake, one that could survive a maximum earthquake. Then, in 2008, the stop logs in the dam failed and there was an uncontrolled release of water, which made the need for the project more immediate. Construction on the intake began in 2009.
The dam needs to be able to withstand an earthquake because the area is prone to seismic activity. In 1959, a 7.5 magnitude earthquake caused a huge landslide that blocked the flow of the Madison River and damaged the dam – that is how Quake Lake was created.
Now, the structure will be able to not only survive, but continue working during another similar earthquake.
The spillway
Following the intake rehabilitation project, NorthWestern Energy plans to begin replacing the dam’s spillway. A new cofferdam will be constructed in front of the spillway, which is at the end of the dam closest to the highway.
“Much of the materials from the intake cofferdam will be reused to construct a new cofferdam in front of the spillway,” according to a NorthWestern Energy press release. “The spillway cofferdam is needed to isolate the spillway from the reservoir for 2016.”
According to Cope, the new cofferdam for the spillway will be constructed within a year, and then work will begin on the spillway. While that work takes place, the dam will continue releasing water normally from the new intake.