September 17, 2004
Associated Press

JACKSON -- An earthquake expert has raised new questions about the safety of Jackson Lake Dam, saying a strong quake could collapse the structure and cause catastrophic flooding in Jackson Hole.

Ralph Archuleta, professor of seismology at the University of California, Santa Barbara, speaking Monday at a meeting in Teton Village, said a quake on the Teton Fault could "liquefy" the dam and its foundation.

Liquefaction, in which seemingly stable substances dissolve as water is shaken from them, causes collapses of structures and foundations. 

"During his presentation he said his studies of the dam indicated that the dam and the dam foundation would most likely liquefy during a major event," said Jim Case, hazards geologist with the Wyoming State Geological Survey.

Archuleta inserted a caveat, Case said. "He said, 'I'm not an engineer,' but said, 'This is not good. It is not a good scenario."'

The dam, reconstructed in the mid-1980s, is about seven miles east of the Teton Fault, which geologists say is overdue for a quake. It could produce an event up to 7.5 magnitude, experts say, with ground shaking under the dam that would create forces equivalent to a magnitude 9 temblor.

About 18,000 people live in Teton County downstream of the dam, which holds back the Snake River.

Archuleta spoke to the Scientific Earthquake Studies Advisory Committee, a group that advises government agencies on quake danger. His comments came just as officials with the U.S. Bureau of Reclamation, which operates the dam, asserted the structure is safe.

Geologists and politicians have fought the bureau unsuccessfully to release a 2003 study of the dam's safety. The bureau cited national security reasons for rejecting release.

Case said he was stunned by Archuleta's statements.

"To say we had some upset people, myself included, is an understatement," he said. "I was incensed. I left the meeting room."

Teton County Commissioner Bill Paddleford, who has been seeking reassurances about the dam for almost four years, said Archuleta dropped a "pretty big bombshell."

"They've been withholding the dam safety study, the hazards report, from us for a number of years under the aegis of national security," he said.

Paddleford added it was his understanding that Archuleta was the person hired by BuRec to conduct the safety study. "If what I hear was correct, we're totally justified in asking for the study," he said.

The 65-foot-high dam holds back a 40-foot wall of water when the reservoir is full. It includes a concrete outlet structure, built in part on rock.

Bureau of Reclamation officials have maintained that the dam is safe, even though they acknowledge it would be damaged in a quake. That position was reiterated at the symposium by Bennett Raley, assistant secretary for water and science with the Department of the Interior.

Raley, who oversees the Bureau of Reclamation, left before Archuleta delivered his address.

"I would have no reluctance to live anywhere below the facility," Raley said in a later interview. "In fact, I would happily do so."

Such proclamations give little assurance to the group Geologists of Jackson Hole, which passed a resolution Tuesday calling for release of the bureau's study.

Archuleta "provided significant scientific evidence that the sediments beneath Jackson Lake Dam ... are likely to liquefy (like quicksand) at a seismic event of Magnitude 7," said the resolution, signed by the group's president, Wally Ulrich, and 12 members. "Liquefaction ... of the sediments is likely to cause the dam to fail."
The group also noted that risk to downstream communities in Teton and Lincoln counties could be reduced by lowering the water level behind the dam.

Case said Archuleta painted the following scenario on how the dam might crumble: During a quake, shaking would cause settling of foundation material, expulsion of pressurized water from the foundation, and cracking in the dam.

"The fractures (in the dam) would have over-pressured water shooting through them to the point it would take out big parts of the dam," Case said.

"What this brings up is the need, the serious need, for a group of engineers in the state of Wyoming to have access to the (bureau's) engineering study," he said.

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