From volume 111, winter 2015-16

By Bree Stefanson

AVALANCHE CONTROL ON EAST STROHN PEAK. PHOTO: MOTI

I MOVED TO STEWART SEVERAL YEARS AGO to work in the BC Ministry of Transportation and Infrastructure Bear Pass Avalanche Program. The afternoon before my first day of work was my first time driving Highway 37A. My jaw dropped and my neck strained as my eyes tried to take in the complexity of the avalanche paths surrounding my truck. Within the first few hours on the job, I was in a helicopter finding out where my targets were and when to deploy the charges. Each of the sixty 25kg bags that went out of the door created avalanches, including numerous showy size 3s and a handful of movie-quality size 4s. It was like nothing I had ever seen. The large paths have a vertical fall of over 2,000m and avalanches can travel upwards of four kilometres before the mass crosses the highway. The mid-sized paths were a couple hundred metres higher than the vertical fall of Castle Mountain Resort, where I had worked as a ski patroller. The smaller paths can bury a vehicle or push one into a lake.

The pass taught me a lot my first season. For example, the center of an approaching low-pressure system can slip a little south and surprise you by sneaking in the backdoor with outflow winds. Amazingly, the large avalanche paths can retain what seems to be an infinite amount of load and are capable of producing size 5 avalanches. An impressive display of nature, the large avalanches command respect for the potential damage they can produce. It was hard not to get caught up in focusing on the large paths, but the “small” paths can still put a size 3 on the road. Also, the importance of clear, timely communication to the public became paramount when living in a community that becomes isolated once the road closes.

Coming into my second season I had a better idea of what to look for and what to expect. I also knew that I had just experienced an “average” season and hadn’t seen anything “above average.” I had come to appreciate the forecasting process which was well-established within the program. This process assesses the overall avalanche hazard for an unmodified snowpack and then applies that assessment to each individual path throughout the forecast area, taking significant occurrences into account. The paths are then individually ranked on the Ministry’s five level hazard scale to identify the paths of concern and dictate specific operational procedures that the maintenance contractor is required to follow while working within the avalanche area.

FRACTURE PROFILE ON ORE MOUNTAIN. PHOTO: MOTI

The 2014-15 season started warm and wet with average amounts of precipitation, but freezing levels were often above 1,000m. In the alpine, a significant instability was buried in the fall, and by Christmas a hard slab had developed over top. When we issued a Future Planned Event notifying our stakeholders that the highway would be closed for avalanche control, you can only imagine the feedback we got from surprised locals, as there wasn’t a flake to be found in town. The control mission was successful, with avalanches to size 4 crawling over nearly bare ground and terminating within 200m of the highway. This mission greatly reduced mass from our alpine start zones, and even though we had large deposits visible from the road, no one in town was buying my story.

The first time I heard the term “atmospheric river,” a significant storm that was forecast to track well to the south had shifted its course and was headed towards Stewart. The millimetres were stacking up on the XTs and we were all trying to forecast the effects of 100mm in a 30-hour period on our snowpack. We compared the various forecast models, attempting to pinpoint the peak of the storm. We applied the forecast to our current snowpack and attempted to hypothesize the timing and character of the expected avalanche cycles. Our theory was that there would be too much rapid loading for the paths to retain significant mass, and the paths would shed during peak loading. We anticipated the freezing levels to rise and induce a secondary wet cycle as the snowpack became saturated. With saturated runout zones, deposits from large avalanches initiating later in the storm would slow down, ideally stopping above the road. We planned a control mission for the peak of the storm, targeting rain-saturated paths below treeline. This closure would also empty the road of travelling public, allowing us to get a handle on the avalanche character and where the deposits were actually stopping without added pressure.

The storm was intense, starting with 3-5mm H2OE per hour, steady for hours. Twelve hours into the storm, snow levels were above 1,100m and precipitation rates had reached up to 7.4mm an hour. I was relieved once the road was closed for control, as the large paths were retaining their mass.

AN AVALANCHE DUSTS BEAR PASS. PHOTO: MOTI

Fortunately, the ceiling was high enough to access the below treeline start zones and the snowpack was saturated enough to release wet loose and slab avalanches. After a three hour mission, our BTL concerns were mitigated and we had plenty of daylight to fly through the pass to observe the natural occurrences. Sure enough, as we flew by, we saw every large alpine and mid-elevation path had healthy deposits below them, with all deposits stopping above the highway. We ran through our path hazard avalanche risk table and all of our paths of concern had released, with any residual hazard still falling well within our operational risk band. We made the call to open the road and continued to monitor avalanche activity.

The storm ended as fast as it came. When it was all said and done, Stewart had received 146mm over a couple of days with 110mm falling within a 24-hour period. This exceeded by twice as much the previously recorded maximum precipitation amount for a 24-hour period in Stewart in March.

At first light I drove through the pass with the clear morning sky showing crowns throughout the pass. By 10:00 a.m., the wind increased, grabbing all the new snow available for transport and quickly erasing the crowns.

Fortunately, the weather continued to improve and conditions were favourable for control the following day. We spent most of our mission above the treeline, producing numerous size 3 to 3.5 avalanches and a few size 4s. The deposits of these reloaded paths easily traveled over the debris piles produced during the storm, with some just stopping shy of the road. The avalanches were stunning. They were dry and moving fast until they hit the saturated snow, where they’d push a slow moving finger of wet mass through the run-out zone. It was impressive to see the power of the air blasts from the two plunging avalanches that dusted the road.

A PANORAMA OF THE FRACTURE LINE ON ORE MOUNTAIN. PHOTO: MOTI

Following the mission, I drove through the pass to capture the toe distance mass of the deposits, and I reflected on the storm, the natural cycles and the control missions. I thought about the various avalanche path characteristics over the elevation bands and the many avalanche problems I had just seen. In one storm there was storm slab, persistent slab on surface hoar, large plunging, loose dry, loose wet, and wet slab, as well as the potential for large avalanches to detach huge fins of glacial ice amplifying the deposit size. I was glad that we had eliminated the deep slab problem formed earlier in the season as it had become active in slopes adjacent to the forecast area, and this would have increased the magnitude of the impacts to the highway during the storm.

The Bear Pass is a wild place to work during a significant storm event, and the area provides a fabulous opportunity to learn a lot about avalanches. I am grateful to have seen an event like this and to have had such a solid team to work with through the season. I’m now in my third season in the pass and from the deposits I’ve seen in the archived photos, all I really know is that I have a whole lot more to learn.