T&D World Magazine
Alaska Electric Light  Power Co Avalanche mitigation

A heavy-lift helicopter removes tower 4/6 wreckage from the mountain.

The Next Avalanche

Learning from the past, Alaska Electric Light & Power has become a leader in avalanche mitigation and snow science.

A sudden, sharp pain forms in the pit of employees’ stomachs when the lights flicker ever so slightly at the Alaska Electric Light & Power Co. (AEL&P) in Juneau, Alaska, U.S. Immediately, they remember back to a cold, rainy morning on April 16, 2008, when a monstrous avalanche hurled into the Snettisham transmission line. The lights flickered, and the events that followed will be forever etched in their minds.

Important changes have occurred at AEL&P in the nearly nine years since “The Avalanche,” which is how the utility’s employees refer to it (see “After the Avalanche,” T&D World, March 2009). Since then, as a result of innovation and teamwork, AEL&P has become a leader in avalanche mitigation and snow science.

The Snettisham Transmission Line

The 138-kV Snettisham transmission line runs 44 miles (71 km) from Juneau to the 78-MW Snettisham hydroelectric plant, which went into service in 1973. Ever since, the transmission line has been Juneau’s primary power source, serving 33,277 residences. It is truly a wonderful place inside the Tongass National Rain Forest, where nearly a million cruise ship visitors arrive every summer to experience breathtaking views of the ocean, glaciers, wildlife and mountains. It is a wet place. The power plant location receives three times as much rainfall as Juneau airport’s average of 62 inches (1575 mm) annually.

The same ingredients supporting hydroelectricity also are part of the recipe for creating avalanches. The transmission line is draped along steep coastal mountains between snowcapped peaks and a navigable portion of the Pacific Ocean known as Stephens Passage. Many sections of the line are subject to avalanche hazards, but it is an area approximately 4 miles (6.4 km) from the plant that is particularly prone to big avalanches.

Avalanches occur every year along the Snettisham line; however, that cold rainy morning in 2008 was the perfect storm to produce a monster avalanche. The Avalanche — categorized as the largest and most destructive class R5 D5 avalanche, capable of sheering off huge Sitka spruce trees more than 400 years old and gouging solid granite rock — came crashing down the mountain, first destroying the 148-ft (45-m) aluminum lattice tower 4/6. When the snow finally settled, nine towers had been destroyed or damaged, and a 10,000-ft (3048-m) section of powerline was entrained into a debris field of trees, rock, ice and metal. The Avalanche’s path started at the mountaintop and went down a runout, dropping off into the ocean 3000 ft (914 m) below.

Alaska Electric Light & Power Co., Avalanche mitigation
Alaska Avalanche Specialists provides an overview of the April 16, 2008, avalanche spanning 10,000 ft of the Snettisham transmission line.

Emergency Response

At first light, a line crew was dispatched in a helicopter to assess the problem. Behind a rain-streaked windshield, the crew strained to focus on bits of flotsam and slabs of ice floating in the ocean. As the crew followed the debris path upward to where the powerline had been, a harsh reality came into view: Juneau was going to be disconnected from its main power source for a long time.

The crew made its way back to AEL&P’s main office. Employees huddled around, anxious to understand the situation. Phones rang as family, friends and everyone else looked for answers. For certain, when news is really bad, the news will travel fast. Soon calls from other towns, states and countries followed. It was an emotional moment for AEL&P employees. How they responded next was the determination for success or failure, as in any emergency.

Alaska Electric Light & Power Co., Avalanche mitigation
The crew foreman, known as Bam Bam, overlooks the challenge ahead.

A brief period of astonished dismay turned into a strong conviction to recover from the emergency. AEL&P initiated a response plan. Individuals assigned to lead strategic functions jumped into action as one team. Lines of communication were established to all stakeholders, including employees and the community. AEL&P executed predrafted contracts to pull utility line contractors and consulting engineers into the team, which assembled in the utility’s war room for an assessment of the damaged line and available resources to brainstorm a solution. What followed was a 45-day challenge to repair the Snettisham line with an inventory of spare parts.

Innovation is a wonderful product of effective teamwork. For example, tower 3/5 is a three-pole dead-end structure with 12 guy wires extending down to associated anchors. A total of 15 objects were buried under 20 ft (6 m) of snow across a 300-ft (91-m) site layout of tangled debris. The objects had to be found so a new tower could be erected with spare parts.

This is where innovation came into play. Home Depot had received pallets of Kingsford briquettes for the upcoming summer barbecue season. The utility had all of the pallets flown to the site in Ward Air’s turbine-powered de Havilland Otter amphibious airplane. Dowl Inc. surveyed and staked the locations of the 15 objects in the snow. Line contractor City Electric set 55-gal (208-l) barrels on the staked locations and filled them with the briquettes. Once lit on fire, the hot amber glow inside the barrels was left to work. The next day, a combination of 15 anchors and foundations were found at the bottom of each 20-ft-deep hole with smoldering barrels resting alongside them.

Alaska Electric Light & Power Co., Avalanche mitigation
A City Electric crew works to salvage a damaged tower and release the conductor entrained in avalanche debris.

Post-Emergency

Crews worked into the unusually hot evening of June 1, 2008, 45 days after The Avalanche. At about 9 p.m., an exuberant radio announcement confirmed the Snettisham line was re-energized!

But AEL&P was not done. Immediately, the utility started working to capture lessons learned and develop short- and long-term goals. Winter would be delivering snow again in six months, and AEL&P had to be prepared. Lessons learned were incorporated into the utility’s emergency response plan, including improved service agreements, communication methods, resource inventories, technical data and training.

Spare tower parts that had not been manufactured in more than 35 years were replenished. A light detection and ranging (LiDAR) ground survey of the entire 44 miles was conducted to help engineer new solutions if a tower site were buried under a mountain of snow again. An updated assessment of avalanche hazards was conducted using the new survey data and current avalanche knowledge. Hazards were prioritized from highest to lowest to guide the long-term goal of improved reliability.

Unfortunately, the Snettisham line did not receive a break from Mother Nature. On Jan. 12, 2009, tower 3/5 was hit again by an avalanche, damaging it and the neighboring towers. Winter conditions were in full force. None of the damaged tower sites could be accessed safely. However, AEL&P had completed its short-term goals and was prepared. It immediately leaped into action, establishing strategic teams and communication channels. The utility started digging into the problem, and 21 days later, the line was re-energized. The lessons learned helped, and the utility was able to refocus on its long-term goal.

Alaska Electric Light & Power Co., Avalanche mitigation
Tower 3/5 wreckage is scattered across the mountain by the Jan. 12, 2009, avalanche.

Avalanche Forecast and Control

The first change toward achieving improved reliability was implementing an active avalanche forecasting and control program. Avalanche forecasting is an understanding of weather, snowpack and terrain. Trained forecasters input terrain survey data, field-acquired snowpack data and weather data from strategically located weather stations into forecasting software. A daily forecast is produced to make the decisions on-site accessible to employees working in the field as well as to make a decision so active control measures can be implemented. Active avalanche control means dropping explosives from a helicopter onto the snow to trigger a controlled avalanche. Yes, it is adventurous!

Deciding when to send people out in the field to drop explosives out of a helicopter in winter conditions is not an easy decision because of inherent risk. It also is expensive and time intensive to comply with regulatory requirements.

Of course, there are not many volunteers. It is a hard sell and change was necessary to make the program sustainable. As the forecasting program developed, another innovation by AEL&P included the introduction of the Daisybell to North America. Manufactured by TAS Inc., the Daisybell is a bell-shaped combustion chamber slung under a helicopter. A large thump can be heard when hydrogen and oxygen mixed inside the chamber are ignited. The combustion then directs a compressed wave of air down onto the snow. What follows is an accelerating slab of snow gaining momentum down the mountain. The Daisybell dramatically reduced the risk and cost while also improving the effective results of active avalanche control.

Alaska Electric Light & Power Co., Avalanche mitigation
Coastal Helicopters and crew prepare to take off for a Daisybell avalanche control mission.

Long-Term Focus

AEL&P continued working toward its long-term goal by accomplishing milestone after milestone of prioritized objectives. Understanding the potential destructive forces involved meant a better understanding of the dynamic nature of avalanches was needed. Field data was collected, including tree-core sampling and testing the strength of the gouged granite rock. With the help of Wilbur Engineering and Dryden & LaRue Inc., the dynamic forces generated from monster avalanches could be modeled using the Swiss avalanche simulation programs Aval-1D and RAMMS. The model was calibrated through reenactments of past avalanche events to make it ready for future mitigation structure designs.

Among the highest priority, tower 4/6 was the first to receive a wedged-shape diversion structure. More than 200,000 lb (90,000 kg) of steel delivered by barge was slung up the mountain with a heavy-lift helicopter in 20,000-lb (9072-kg) bundles waiting to be erected. The terrain was so steep, equipment like the Ingersoll Rand LM-100 drill rig had to be tethered to the mountain. The miners working on the foundation used the aid of ladders and ropes to move around. Iron workers assembled the 40-ft (12-m) structure capable of withstanding forces up to 5221 lb/sq ft (25,491 kg/sq m). Once completed, crews proceeded to construct diversion structures in front of towers 4/4 and 4/5.

However, the potential forces at tower 3/5 exceeded any practical diversion structure design. Therefore, the decision was made to bypass this structure with a 2500-ft (762-m) span across East Crater Bowl, while other towers received structural strengthening modifications. A unique breakaway system also was installed on each segment of conductor between the nine towers. The idea was to prevent towers or conductor from being pulled down by an avalanche as a result of cascading damage into adjacent towers, thereby containing — or boxing in — the problem to reduce impacts. Today, every segment and structure along the 10,000-ft (3048-m)-high avalanche-prone area has been studied and mitigation implemented. Some people ask if it will work; other people hope never to find out.

Alaska Electric Light & Power Co., Avalanche mitigation
On steep mountainous terrain, a crew member inspects the construction of a diversion structure for tower 4/5.

Challenge to Success

While important changes have occurred in the nearly nine years since The Avalanche, one thing has not: the Snettisham transmission line will always be exposed to harsh conditions that have the potential to cause significant damage. In March 2012, a large avalanche came down above tower 4/6 and was diverted. It was determined to be comparable to the 2008 avalanche, reaching about 70% of the diversion structure design load. The diversion worked. The tower and line would have been knocked down without the structure.

Alaska Electric Light & Power Co., Avalanche mitigation
Diversion structure stands guard, protecting tower 4/6 from avalanches.

What was an overwhelming challenge has turned into an overwhelming success. With the introduction of several innovations resulting from teamwork, AEL&P is a leader today in industrial avalanche mitigation and snow science. Both the utility and the community of Juneau were recognized with the Edison Electric Institute Emergency Response award for their efforts. 

Alaska Electric Light & Power Co., Avalanche mitigation
Diversion structure stands guard protecting tower 4/5 from avalanches.

 


Eric Eriksen is vice president of transmission and distribution for Alaska Electric Light & Power, an Avista Utilities company. In his 17-year career, Eriksen has worked on such projects as the Taku Inlet submarine cable and cruise ship shore power, and served as project manager for the Snettisham avalanche repair and mitigation projects. He holds BSEE and MBA degrees from the University of Alaska. He volunteers time on the Alaska professional licensure board for Architects, Engineers and Land Surveyors, as well as the Alaska Emerging Energy Technology Fund advisory board. Eriksen is currently president of the Juneau Chamber of Commerce and a private bush pilot in Alaska.

 

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