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By Joyce Edlefsen
Standard Journal

REXBURG – Rocky Mountain Power continues its preliminary work prior to starting the physical rebuilding of the Ashton Dam.

After a lengthy review of the condition of the dam by its own engineers, consultants and the Federal Energy Regulatory Commission, the power company decided in February it would rebuild part of the dam.

Initially the company said the project was anticipated to take two years, but upon further analysis, the company has decided to try to complete the work by the end of 2012.

The first phase of the project is set to start in July, when the company will draw the water level down some and start work on a bypass tunnel.

But before that work could be started, several permits and preliminary steps had to be taken.

The company promised customers and stakeholders it would regularly provide reports on the status of the project. The forum for the reports has been Henry’s Fork Watershed Council meetings.

Mark Stenberg of Rocky Mountain Power reported to the council Tuesday that fieldwork has been completed on a sedimentation study that shows how much mud has accumulated since the dam was built nearly 100 years ago.

The sediment study was done to determine what types of mitigation measures might be needed to contain the mud in the reservoir during the reservoir drawdowns, and when certain work is being done on the dam, such as tapping a tunnel to let water pass around the dam.

If sediment escapes the reservoir in large enough quantities, it would muddy the fishery of the famed Henry’s Fork downstream.

“There are concerns for the fishery downstream,” Stenberg said. The study was needed because, “we were unsure of the amount of sediment in 100 years of dam,” he said.

The sediment was measured by a team in a boat using three frequencies of sonar that created 60,000 data points to develop a model that showed the depth and consistency of the mud.

For two or three days this spring the researchers boated back and forth across the reservoir in an effort Stenberg described as “monumentally tedious.”

Besides the sonar, the consultants took core samples of materials on the bottom, coming up with 43 representative samples.

The samples were taken back to a lab, where they were sieved for particle size. The data from the sonar and core samples was used to create color-shaded maps that show the contours of the reservoir floor and where the sediment types are located. “You can see the old river channel,” Stenberg says.

Here’s what the study found:

  • At the headwaters of the reservoir, where there was the most water movement, there were graded materials from cobblestone to gravel to sand, as anticipated.
  • Throughout the reservoir there was a uniform blanket of highly compacted silty clay, he said.
  • The surprise: The silt was 2 to 4 feet deep, much less than the depth of mud in some reservoirs.

“We were pleasantly surprised at the levels,” Stenberg says.

“If we drain it all the way, we would move a lot of sediment,” he said. “We’re looking at the elevation that will keep the most sediment in the reservoir.”

The next step in the process will be to hold a meeting (scheduled for next week) with the representatives of the Idaho Department of Environmental Quality, the Henry’s Fork Foundation and others to review the study results and modeling.

Stenberg also reported to the council that the company and DEQ have signed a consent agreement concerning turbidity levels that will allow some muddy water in the short term during construction. The agreement also calls for monitoring of fish-spawning gravels.

The company has had six companies submit bids to dig the bypass tunnel and is in negotiations with one of the firms to do the work, probably starting in July.

Ashton Dam facts

  • Owned by PacifiCorp Energy
  • Located on the Henry’s Fork of the Snake River about 2.5 miles west of Ashton.
  • Began operating in 1914 and was purchased and expanded in 1925 by Utah Power.
  • Consists of a dam and powerhouse with three generating units. The dam is a rock- and earth-filled structure, 60 feet tall and 226 feet long with a 70-foot-wide concrete intake and 82-foot-long spillway.
  • A roller compacted concrete cap was installed in 1991 to protect the embankment during flood flows.
  • Reconstruction scheduled after studies concurred it was necessary.
  • Project is planned for completion in December 2012.

Standard Journal
Rexburg, ID
June 19, 2010

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