PPL Montana has submitted a plan with the Federal Energy Regulatory Commission to isolate, rebuild and reinforce the intake structure at its Hebgen Dam near West Yellowstone.

Workers prepare equipment to isolate the intake structure at PPL Montana's Hebgen Dam near West Yellowstone.

The project will strengthen and stabilize the intake structure, which controls water flow from Hebgen Lake into the Madison River, and make it more resilient in the earthquake-prone Madison Valley. Work is expected to begin this summer.

“We took a very broad, comprehensive approach in our design that addresses safety and the seismic activity in the Madison Valley and protects fisheries and public recreation,” said David Hoffman, director of External Affairs for PPL Montana.

“The work will not affect the elevation of the lake or river flows, which are critical to aquatic resources and the summer recreation and tourism season in the Madison Valley,” he said. For public safety reasons, the U.S. Forest Service will close the area around the dam during construction.

Some of the design features of the new intake structure include a secondary emergency shutoff gate; a large gate at the bottom for emergency flow needs, topped by a normal operations flow-control gate; and a single bay replacing the current four bays. The structure itself will be anchored to the rock wall behind it.

The dam has consistently remained safe and stable, according to engineering inspection results and on-site monitoring equipment, Hoffman said.

Work on isolating the intake structure is expected to begin this summer and be completed by the end the year. Hoffman said that because of the short construction season at Hebgen, it appears that the project to rebuild and strengthen the structure will take two years to complete, beginning in 2010 and finishing in 2011.

PPL Montana employees, structural engineers and experts in dam construction began working at Hebgen on Aug. 31, 2008, when a problem developed in the intake structure, causing higher than normal river flows and lower lake elevations.

The cause of the problem is related to loss of stoplogs in one of the intake structure bays, but it has not been determined what caused the failure. Stoplogs, timbers similar to railroad ties, are stacked in the dam’s bays and used to regulate water flow from the intake structure.

Flow from the intake structure was controlled before winter, and then work began on a plan to isolate, improve and strengthen the structure. Isolation of the intake structure is required to perform additional inspections to determine the cause of the failure and to make the necessary repairs.

PPL Montana operates the dam to balance the needs of recreational users of Hebgen Lake, and the need to protect important river resources downstream on the Madison River.

Hebgen Dam creates Hebgen Reservoir just north of the Idaho border, which stores water for the Madison-Missouri river system. PPL Montana owns and operates eight hydroelectric facilities on the Madison and Missouri rivers.

PPL Montana provides safe, reliable energy from coal-fired power plants at Colstrip and Billings, as well as 11 hydroelectric plants along West Rosebud Creek and the Missouri, Madison, Clark Fork and Flathead rivers. It has a combined generating capacity of more than 1,200 megawatts and has offices in Billings, Butte and Helena. PPL Montana and its 500 employees are dedicated to Montana and its communities, supporting educational, environmental and economic development programs across the state. PPL EnergyPlus operates a trading floor in Butte that markets and sells power for PPL Montana in wholesale and retail energy markets throughout the western United States. PPL Montana and PPL EnergyPlus are subsidiaries of PPL Corporation (NYSE: PPL). More information about PPL Montana is available at www.pplmontana.com.