Avalanche Triggered, Snowboarder Caught on Four Gables
Here is his account:
On Sunday 4 of us went out to Four Gables Peak to do the North Couloir.
We climbed the gradual ridge to the north. We were able to skin most of it, and didn't get any sign of instabilities or windloading. When the snow ran out we got on the rock and followed that to the lower of 2 entrances into the couloir. There was another shorter entrance to our left that we avoided, due to obvious signs of windloading and a short convex rollover into the chute.
I dropped in first and made 3 ski cuts, 1 little one at the top, and then two more across the entire width of the chute. I stomped a little bit on the first two, and didn't get any results or signs of instability. On the third cut I built up a little speed and put as much force on the snow as I could. And still no results.
At that point I was about 100 feet below the ridge. I decided to start making some turns. After about 2-3 turns I made another turn on a wall of snow that had drifted onto the rocks on the side of the chute. I believe that's where the slide started from. The other guys were watching from above and can probably clarify exactly what happened there.
I was looking downhill, and was back in the middle of the chute aiming for the other side when the snow around me broke up and started moving. I glanced back and noticed that the slope behind me, as well as the snow below me was all moving, and I was in the middle of it.
I was pushed forward, straight down the fall line. I slid in it for approximately 700-800 vertical feet. On two occasions I was able to get myself upright and surface, only to be pushed back under the snow. Towards the bottom of the chute the angle kicks back a little bit. This caused the sliding snow to slow down enough for me to get get myself upright again, and out of the slide path.
I didn't get a look at the crown, though I don't think it was that big. I think the only reason the slide went as far as it did was because it was sliding on an old layer of firm windboard.
They descended the couloir slowly, without incident.
The crown was about 10 inches
The crown was about 10 inches tall and about 10 to 15 feet wide. The windslab that released was of a NW aspect. It broke free when Dan rode slightly up onto the skier's right sidewall of the couloir. It was frightening seeing how fast the slide accelerated. Dan was probably moving at almost 40mph down the couloir.
analysis
The boarder involved was one of the most experienced back country travellers in the area. He had his eyes open, and was aware of the potenetial for dangerous windloaded areas. The fact that he got caught shows how tricky it can be to recognize areas that are dangerous and areas that are safe.
The area where he triggered this slide is exactly the kind of feature that Sue is talking about that could produce an avalanche: "steep, shallow snow pack, rocky, sides of gullies." Wind slabs usually have varying thickness accross a slope. It is usually towards the edges where it is thinner and more sensitive where a skier or boarder's weight can penetrate through and cause a failure in the weak layer underneath. This failure could then propagate under the thicker areas of the slab to cause a larger release. In addition to the windload at the edge of the gully, the previous snow load at the edge of the gully is usually also the thinnest. When thin snow persists near rocks, it tends to transform into weak sugary snow. Now, put this dense wind slab over weak sugary snow, you have prime setup for an avalanche waiting for a trigger.
We usually feel good about stability after making a 2 or 3 good ski cuts at the top of a slope. This is generaly true when the wind has been previously blowing into the slope from the top. With cross winds however, especially accross gullies, wind slabs could form far down the slope. A patroller triggered a similar slide on Mammoth Mtn last saturday just above the throught in Avalanche 2, and was swept down 500 feet, losing both skiis and suffering some facial trauma.
Be aware of the power and affects of the WIND! Even when it is calm, has the wind been blowing recently?? One question remains ... how long does it take for a windslab after it forms to go from sensitive to stable?? Hours?? Days?? Longer?? .