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Expert: $100,000 in concrete could have protected Snettisham line

Posted: Sunday, May 04, 2008

Shortly before 4 a.m., several feet of new snow on steep terrain gave way and raced for the water below. Scraping down nearly vertical rocky chutes and snapping trees, the avalanche built up tons of force before crashing into an aluminum tower placed in its path 35 years ago.

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Michael Penn / Juneau Empire
Michael Penn / Juneau Empire

It was the first of several April 16 avalanches to let go of the mountain.

By the time the cycle was over, five towers were damaged, two were destroyed and Juneau faced an energy crisis that could cost the community more than $25 million before it's over.

In some places, the avalanches were full-size and took the entire year's snowpack. One avalanche wrapped around a fanning ridge and consumed most of the mountainside below. After sunrise, at least one "secondary slide" pushed tower 3-5, already damaged and teetering, into Speel Arm.

Juneau Mayor Bruce Botelho called it an "act of God."

Alaska Electric Light & Power Co. President Tim McLeod said the electric company never imagined avalanches so big would hit the line.

Avalanche diversion expert David McClung said a relatively cheap concrete wedge structure at the base of each tower could have deflected the tons of force delivered by the avalanches, protecting the line.

McClung estimated the cost to protect the exposed towers with diversion structures to be $100,000. It's a negligible cost compared to the millions the energy crisis will cost Juneau, he said.

McClung holds a Ph.D. in geophysics and avalanche defense mechanisms and is the co-author of the definitive technical textbook on avalanche science in North America. He also is a professor of civil engineering at the University of British Columbia.

Much of the decision to protect towers rests on the expected frequency of destructive avalanches and the potential size of those avalanches, McClung said. On a 1 to 5 scale, avalanches larger than size 2 have destructive potential, he said.

McClung said he was not aware of any tower with a diversion wedge being destroyed in British Columbia.

"They protect their towers," he said.

If the potential for a class 3 avalanche exists and happens more than once in 100 years, McClung recommends a concrete diversion be placed on the tower base.

Scott Willis, AEL&P generation engineer, said the main avalanche was a class 4 on the destructive scale.

The Northwest Avalanche Center in Seattle said a class 4 avalanche will mangle a train car.

AEL&P Engineer Erick Erickson on Friday said the company was gathering size classification information for the avalanches that hit the line towers.

Guidelines for tower placement in avalanche zones don't exist in the United States, McClung said. British Columbia has a published standard for placement, which is used by some utilities in the Western states, he said.

No Alaska statutes or regulations address the placement of transmission towers, said Anne Wilde, a spokeswoman for the Regulatory Commission of Alaska. She couldn't explain why none exist other than to say the commission's power comes from the Legislature.

"We're not in the business of telling utilities how to build," she said.

"I kind of have the feeling that in Alaska the standard is more liberal for this sort of thing," McClung said.

As AEL&P races to repair the Snettisham line, new and rebuilt towers will be set on the same pads in the same avalanche paths. According to Scott Willis, it's the most economical thing to do for the company and ratepayers.

Once the line is restored, AEL&P will commission a long-term study to determine whether protective measures can be added to the towers in the future, Willis said. He expects Juneau avalanche expert Bill Glude to explain avalanche potential and possible destructive force to engineers. Willis said the engineers would be "turned loose" to conceive of ways to protect the line.

Any recommendations and cost estimates would be forwarded to the state agency that owns the line, Willis said.

By agreement, AEL&P would be financially responsible for any improvements.

Willis said he was not specifically aware of other utilities' practices to protect transmission towers in avalanche terrain. He believes protective diversion structures were passed over by the U.S. Army Corps of Engineers as "cost prohibitive" when it built the $170 million project in the early 1970s.

McClung said experts with knowledge on diversion structures have been around since the 1960s, when avalanche science began in earnest in North America.

A lot of time and energy went into placing the towers to mitigate avalanche damage, Willis said. Finding the cheapest available structure to support the lines was the goal in choosing tower type for each location, Willis said.

Since construction, avalanches have damaged Snettisham towers, but Willis said he could not recall any recommendations to build diversion structures on the towers hit by the April avalanches.

"I've heard it's done in places," Willis said.

• Contact reporter Greg Skinner at 523-2258 or greg.skinner@juneauempire.com.



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