Slope Stabilization in Boomerang and Brownshirt

Those skiing at Lake Louise early last season may remember that we experienced a widespread avalanche cycle in much of our alpine terrain, losing much of the snow that had fallen to date in places like Brownshirt and Whitehorn II. This was in part attributable to heavier-than-average November snowfalls, which made it difficult for avalanche control teams to get into every piece of terrain before everything slid.

With lower snowfall so far this year, control teams, with the help of other patrollers and the trail crew, have done a great job of getting to where the snow is and using a combination of ski packing and bootpacking to break up the weak layers and temperature crust that exist. All of this work contributes to stability of the slope, and helps ensure the snowpack builds the way we want it to, giving it the best chance to survive the season. With more snow forecasted to fall early this coming week, crews will be constantly working these areas to make sure all new layers of snow are packed in as much as possible. It’s a big job, with huge amounts of terrain to cover, but also with equally huge rewards for all.

Boomerang is a perennial early season performer, as it is a flat, shale-covered surface that receives near-constant wind-loading. It’s usually the first alpine run to be skiable, and therefore often receives the bulk of attention from control teams. On the way to Boomerang is Windy Gap, which can be a vexing place to get snow to stick. I’ve lost count of the number of different ways teams have come up with to get wind-blown snow to stay put. The challenge comes from the wind, which usually blows strongly and can come from any direction (this is why early season skiers may have noticed that the snow fence there sometimes lays in a bunch of zig-zag patterns, rather than in long straight lines like everywhere else on the mountain). In any case, we can’t miss any opportunities to pack down any snow that gathers there, or else run the risk of having it all blow away again. Every flake counts!

Nov. 12 - Crews start to descend towards Windy Gap. (photo: Loisl Fortmuller)

Nov. 12 - Getting ready to hike to Upper Boomerang. (photo: Loisl Fortmuller)

Nov. 12 - Looking back towards Windy Gap. (photo: Kieran Diamond)

In Brownshirt, there’s enough snow for control teams to have begun ski-cutting, which is another slope-stabilization technique. Patrollers travel the run on skis, making big zig-zag tracks down the length of the run. This disrupts the continuity of layers, and breaks up what may be one big slab of snow on the slope into smaller pieces, reducing the likelihood and consequence of an avalanche. You’ll see these ski cuts in all of our avalanche terrain all season long, as it is the preferred method of avalanche control used by patrollers. In sketchier terrain, or when there’s more snow, patrollers will switch to the use of safer (and more expensive) explosives. Generally, frequent smaller dumps are easier to manage than occasional big dumps, which can slow things down as patrollers need to travel more slowly through hazardous terrain. Not that we ever complain about big dumps…

Nov. 12 - Patrollers ski cutting in Brownshirt. (photo: Rocket Miller)

Avalanche Control in Whitehorn II

Skiers visiting Lake Louise this week were treated to blue skies and sunshine, thanks to a high pressure system that’s settled in nicely in our area. And while skiers get to enjoy beautiful weather and great visibility on the mountain, this has also meant that our avalanche control team has been able to venture into and spend a lot of time in Whitehorn II.

Good visibility is always welcome during avalanche control work, and sometimes will be the deciding factor on whether the teams go into a certain area. Especially in a place like Whitehorn II, which is a huge piece of steep terrain with ridges, rocks, and cliffs, good visibility provides an extra measure of confidence for the crews traveling there. Stable weather generally results in little change to snow stability, allowing control teams to dedicate more of their time in places they had previously not spent much time in.

Looking back a bit, the week leading up to Christmas saw a few significant weather events, starting with about 20cm of snow that fell with little to no wind. As the snow petered out, the high pressure system moved in, announcing itself with west/northwest winds that redistributed most of that snow. This wasn’t the first time this season that winds came from other than the usual direction, and skiers familiar with how our mountain usually develops would have noticed drifting and scouring in unusual places, and Whitehorn II was no exception.

As a result of strong winds that blew in before the cold snap earlier in the month, the fans (areas at the bottom of slopes where avalanches “fan” out) of Whitehorn II had all their snow blown away, and it was only with the new snow before Christmas that these places had a chance to recover. The more recent winds resulted in hard slab conditions higher on the slopes, and because they came from the west/northwest, snow was distributed across the slope from the side, rather than down from above as is usually the case.

Wind slabs are the result of wind-transported snow. As the delicate snow crystals are blown across the surface, they are broken into tiny pieces. This means they can be packed into a tighter and denser layer of snow, a result increased by the packing action of the wind. Wind slabs often form the most destructive avalanches since they are harder to break apart than softer snow, and must be treated with caution.

Patroller stands next to upper edge of a slab avalanche. (photo - C. Sheppard)

One problem that can arise from a thicker and denser slab is that it can be for the most part supportive under the weight of a skier, which can give a false sense of security in the stability of the slope. Features on the slope (rocks, etc) can create weak spots, and even when the rest of a slope seems supportive, one turn in the wrong spot can cause the whole slope to avalanche. In the case of Whitehorn II, this slab was especially stubborn, due to its thickness and density.

When making snowpack observations, avalanche technicians use a scale to describe snow density. Rather than a number scale, as is used to describe sizes of avalanches for example, snow density is described by how easily a certain object will penetrate the layer horizontally (vertical penetration may go through more than one layer of snow, resulting in a variable and inaccurate reading). From least to most dense, the scale goes fist, four finger, one finger, pencil, and knife. The slab in Whitehorn II is pencil, which is about as dense as it gets before you get ice or a melt/freeze layer (knife). In the photo below, you can see that the patrollers’ ski cuts are barely visible at the top of the slope, due to the density of the snow and the inability of skis to penetrate that top layer.

While these slopes have reacted to explosives, the avalanching has not been widespread. In other words, rarely would one bomb be enough to get the whole slope. Snow that should have avalanched but didn’t is called hangfire, and enough hangfire remained in all gullies that many shots were required to clear each slope.

So – where does that leave us in Whitehorn II? It’s not ready to open yet, and while it may be easy to say we’re one or two snowfalls away from opening, there’s more to the picture than just snowfall. Wind and temperature have already played a big part in how the mountain has developed this season, and will also have a role in deciding what it’ll take to get Whitehorn II open. In the end, it’s a big piece of terrain, and lots of work by avalanche control teams will be needed no matter what the weather does.

Avalanches and ski cuts in Whitehorn II. (photo - C. Sheppard)

Video – Helicopter Avalanche Control

As promised, here’s a video from the helicopter avalanche control that happened at Lake Louise on November 27. I found a nice perch at the top of Whitehorn II and filmed the action below as the crew dropped bombs in Whitehorn II, Brownshirt, Upper North Cornice, Boundary Bowl (OOB), and along Richardson’s Ridge. Most shots produced little to no result, with the exception of Whitehorn II, Upper North Cornice, and Boundary Bowl. Of the two explosions in the video, the second is the one visible at the top of the gully in a photo from the original post, just from a different point of view.

***

More Powder at the Lake

The last few days at Lake Louise have seen some turbulent weather, as the latest system to roll through came in like a lion (high winds), and went out like a lamb (fluffy snow). Yesterday (Tuesday), most lifts on the mountain closed for at least part of the day as high winds played havoc with both our lifts and avalanche closure fences and signs.

Since the wind we get at Lake Louise usually comes from the south/south-west, the front side lifts are more susceptible to wind closures, and once we started getting gusts in excess of 80km/h at the top of the Grizzly Gondola, the time came to stop loading the lift and wait for the wind to subside a bit. Top of the World chair followed shortly after. Chairs can swing wildly in high wind, and the chance of a line derailment increases if the lift is moving. Also, if the lift were to stop, we have to think about any people who might be sitting on the chair and exposed to potentially unsafe conditions.

The other alpine lifts – Paradise and Summit – remained open, at least for the short term. Paradise is on the leeward side of the mountain, and riders don’t get a sense of the wind that’s pounding the front side until they get to the last few metres of the ride. The one thing we need to make sure of there is that people are able to unload the chair without the wind blowing them back into their seats. The Summit Platter is a surface lift, and therefore is not subject to the same issues as a chair lift in high wind.

At the same time, both Paradise and Summit did close, but not for lift-related reasons. While traveling down the fence that separates Paradise Bowl from Upper ER 5, and avalanche control team noticed that entire sections of closure fence and their signs were being blown away. Without the ability to maintain these vital fences during the wind, the avalanche forecaster decided it was best to close all alpine terrain until the wind let up and we could get a chance to ensure all avalanche closures were in place.

**********

This morning we arrived to a different scene. The winds had almost completely abated, and another 10cm of new snow had fallen during the night. The avalanche forecaster had a tricky morning of doing his stability checklist and deciding on the best course of action for the day’s control work. Not only were the winds from the previous day blowing strong, they were also coming from other than the usual direction, which means re-thinking every bit of avalanche terrain and having the teams ready for anything.

My team headed straight for Summit, and after hiking up to the peak, skied down over the Boomerang entrance traverse and made our way to the top of D Gully in Whitehorn II. We had 5 explosive shots with us – three single shots, and two nukes (two shots taped together). Due to a combination of high wind, moving snow, and the predominant rain crust and facet combination that has been causing natural and manmade avalanches all over the national park, we expected big things, especially in terrain like Whitehorn II, where there has not been extensive control work.

Making sure fuse is lit before throwing bomb.

Kaboom! - above the Boomerang traverse.

Once at the top of D Gully, I attached an igniter to the fuse of one of the nukes, lit it, and threw it down as far as I could so that it landed on the skier’s left flank of the gully, well above the narrow choke about halfway down and the part of the slope that generally produces results. The bomb went off, and while nothing but surface snow was affected around the shot placement, it did cause an avalanche to start about 30-50m downslope. All snow above the October crust ran, and it included the full width of the gully by the time it got to the choke, running far out onto the flats at the bottom.

Satisfied with that result, we began to make our way over the top of C Gully to do the same thing there. My partner began the traverse, and was no more than a few metres into it when suddenly there was a “woomph” as his weight caused the weak layer in the snowpack underneath him to collapse. he heard and felt this happen, and immediately looked around to see if the slope was avalanching. The snow around him stayed in place, but as we looked downslope, we saw that another large avalanche had started about 60 or 70 metres below, and, like in D Gully, was running on the crust and went pretty much side to side. This was a remotely triggered avalanche, since the start point was different from where the load (weight of skier) had been placed on the snowpack. In all likelihood, the weak layer of facets had collapsed under his weight and travelled horizontally to a point in the snowpack where there was enough tension, and possibly a weak point (boulder or other ground feature), resulting in the avalanche. This was similar to what we saw in Whitehorn I recently, when a patroller traveling well underneath the slope remotely triggered an avalanche far above him.

Remotely-triggered avalanche in Whitehorn II 'C' Gully.

With the avalanche starting far down slope, we were concerned about the snow higher up that did not slide (hang fire), and there was still quite a bit left. A single shot was thrown ahead of our progress, and when the explosion did not result in an avalanche, our worries eased.

Watching a slope get most or all of its snow taken away by an avalanche can be heartbreaking, but when you have such a suspect layer in the snowpack that’s producing avalanches all over the place, it has to be done in order to let the run rebuild itself. Otherwise, we’d spend the entire winter worrying about it. Now, at least in the places that have avalanched, that layer is no longer a concern, and the slopes have a chance to rebuild with a sturdier snowpack.

With most of the morning’s control work done, control teams can concentrate on ski cutting and getting ready for the next system to come our way. The forecasts are calling for more snow tonight and tomorrow, so the sooner we can rid ourselves of the weak layers, the sooner the slopes can begin the rebuilding.

ANFO in Whitehorn II

Another beautiful sunny day at Lake Louise today, and the avalanche control team made use of the good visibility to try and get some results in E, F, & G Gullies of Whitehorn II. Previous attempts to get results with our regular 1kg and 2kg shots were unsuccessful, so we decided to pull out the big guns – ANFO, that is.

ANFO stands for ammonium nitrate and fuel oil, and comes in large bags. It looks like lawn fertilizer, and can be used to make custom size shots to fit the needs of the control team. In this case, three 7kg bombs were made by placing the ANFO in separate bags, then inserting a 1kg shot with fuse and igniter.

We loaded the lift around 8:30 and arrived at mid-station just as the sun was beginning to peek over the mountaintops. By the time we got to the top of Summit, Whitehorn II and the rest of the backside was bathed in a beautiful early-morning light. I travelled around to Shoulder Roll on Boomerang so I could get some photos of the action (if there was going to be any). The first two shots were used in E and F Gullies, and both produced very minor results – just a little surface snow around the shot placement.

The shot in G Gully was a different story. As shown in the photos below, as soon as the bomb went off, fracture lines appeared along almost the entire vertical length of the run, and the slope avalanched down to the bottom and up the other side of the terrain slump that runs along most of the bottom of Whitehorn II. This was a size three avalanche, enough to bury or destroy a car. One patroller was asked why Whitehorn II hadn’t been open yet, and the reply was, “Go ski Boomerang, then look up to your right and you’ll get your answer”.

The yellow cloud of ANFO, and fracture lines appear at lowerl left.

Fracture lines continue downslope as snow begins to move.

Snow is on the move.

Avalanche widens lower on the slope.

Snow comes to a stop.