City gets some protection for its power lines

Utility hopes to fix a weak spot in the 43-mile transmission line

Posted: Friday, October 02, 2009

Alaska Electric Light & Power is building an avalanche "diverter" at a cost of as much as $2 million to protect its most vulnerable transmission tower.

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It was during 2008 that an avalanche of historic proportions swept down a slope demolishing towers and cutting Juneau off from the Snettisham Power Project's supply of cheap hydroelectric power.

This year that tower, called 4/6 by the utility, will be protected, said Scott Willis, power generation manager for AEL&P.

"This is one of two (that went down) in the big '08 avalanche," he said. "It's one that we considered particularly vulnerable so that's where we're building the first diversion structure."

The diverter is a "V" shaped structure, designed to direct the force of an avalanche away from the base of the tower.

The second destroyed tower was bypassed, so Willis said the two biggest weaknesses in the transmission line have been addressed, though there are no guarantees the line is safe.

The switch to diesel backup generators at a time when oil prices were rising traumatized Juneau in the spring of that year, and left the city on backup for weeks, paying rates that jumped from about 10 cents per kilowatt hour to 52.5 cents.

The crisis prompted extensive community discussion of what could have been done to avoid the loss of power, including buried lines, different routes and submarine cables, as well as armoring the tower bases against avalanches.

Protecting the towers with diversion structures turned out to be the cheapest way to go. Cheap is relative, however. The diversion structures cost millions of dollars, instead of tens of millions of dollars for the alternatives, if they were even possible.

Public comments at the time suggested such structures would cost a few hundred thousand dollars, while Willis said the one under construction would cost between $1.5 million and $2 million.

The cost comes largely from the location, he said. Crews have to be helicoptered to the steep mountainside along Port Snettisham's Speel Arm daily. This week a heavy-lift Chinook helicopter was brought in to lift the structure's biggest components to the site.

"It's a very expensive structure," Willis said. "Remote construction in a very steep location is expensive."

Willis said he's been asked why they're not putting one on each tower, and the reason is the cost.

"The cost of these avalanche protection structures will be rolled into rates," he said.

AEL&P's power sales in 2007 totaled $30 million, and the cost of avalanche protection will be spread among ratepayers in a manner approved by the Regulatory Commission of Alaska, which regulates state-sanctioned monopolies.

At the time of last year's slide, it was estimated it would take years to figure out what needed to be done and make it happen. Now the protection measures are expected to be in place before winter.

Willis said the tricky part was figuring out what needed to be done. Juneau avalanche expert Bill Glude's consulting firm provided crucial information about the risks the line faced.

"First we had to have an avalanche expert tell us how large an avalanche could happen here and what are the forces an engineer would have to design for," Willis said.

Willis said it was too difficult to design for a 100-year avalanche, so they instead designed for a 50-year event.

"With a 100-year, the forces are just way to large" to protect against, he said.

"This is going to cover most of the likely avalanches that may happen," he said.

It is not clear whether the 2008 avalanche makes future slides more or less likely, Willis said. That slide scraped the slope clear of much of the vegetation that previously held snow in place.

"I don't know if we can say if it is more likely or not, its how much snow you pile up before it cuts loose," he said.

Willis said Glude estimated the 2008 avalanche at somewhere between a 100-year and 300-year event.

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