Seven Mile

Recent dense windslabs reactive to snowmobile and skier triggering. Two size 1.5 slides failed on a thin layer of facets overlying a hard crust 20cm deep.

Two slides accidentally triggered. One remote triggered by snowmobile from the bottom of a SE slope, the other skier triggered while crossing the bottom 1/3 of an E facing slope (30deg at start zone, 35 deg above). Both crowns approx 20cm deep. SE slope slide was 20m wide and ran 200m. E slope slide was 40m wide and ran 30m, with extremely slow-moving debris.

The failure layer was remarkably thin. We were all surprised that only 1 - 2 mm of small facets (that displayed good clumping when squeezed) could produce such a sudden and planar failure.

We (group of 3) traveled by sled to the head of the drainage, with a plan to stick to mellow slopes based on the recent and ongoing wind activity. Winds in the drainage were predominantly from the NW (blowing upcanyon), but were swirling. After discussing the conditions and everyone's goals for the day, the group split up, with 2 travelling on sleds, and a solo person traveling on skis. In hindsight, this was a poor decision that is atypical for all members of this group. The sledders decided to explore around meadows to the south, while the solo ski tourer planned to lap low angle gladed trees. Within ten minutes of setting out, the sled group remote triggered a size 1.5 wind slab from the bottom of a SE facing slope. The crown was approx. 20 cm deep and 25 m wide, and ran on a windslab-crust interface. The sled group immediately turned around to rejoin the lone skier and notify them of the touchy conditions they experienced. During this same time frame, the skier ascended 20 - 25 deg NE facing slopes, and reached a bench below a prominent knoll. The NE slopes of the knoll looked extremely wind affected, and the skier decided to traverse across the bench to investigate whether E facing slopes had less wind effect, and whether it was possible to reach the top of the knoll via a sheltered route around the corner. Soon a well-treed gentle ridgeline came into view, which the skier decided would be a good route to ascend and ski back down from the top, without having to ski open steeper slopes. Between the skier and the ridge, however, was an open 30 - 35 deg slope that would need to be crossed. The skier knew the slope was a concern, and that crossing it violated all training and intuition. Against their better judgement however, the skier decided to cross this slope to avoid having to ski to the valley bottom and regain lost elevation. Multiple times during the short traverse, the skier thought to themselves that this was a bad idea, and contemplated turning around. Halfway across the slope, the skier heard a sharp and audible whoomph and immediately saw a large crack 5 m above the skier's position that began to propagate across the slope. The crown broke just below a slight localized convexity that was not noticeable at the start of the traverse, but was very noticeable when seen from below. There was a delay of 2 -3 seconds, and then the debris started moving slowly (walking pace), causing the skier (who had skins on and heels unlocked) to fall to their hip, but stay on top of the debris. The skier had their hand on the airbag handle in case it was necessary, but came to a stop only 1 - 2 m below the start zone, with skis resting on the bed surface. A ski pole was carried about 5 m, and the toe of the debris ran about 30m in total. The skier carefully skied away from the slope, and waited for the sledders in the valley bottom. After the group came back together, it was decided to avoid all avalanche terrain for the remainder of the day, and dig a pit at a protected location at the toe of the E slope. Pit revealed that the 20cm one-finger hard windslab had likely failed on 1 - 2 mm of small facets overlying a pencil-hard 1cm thick crust at 20cm deep. Compression test result was CTM (13) sudden planar at the windslab-facet/crust interface. Needless to say, lessons were learned by all members of our group today. We are experienced backcountry travelers with avalanche education up to AST2, and we knew that we were traveling in heavily wind-affected terrain. The plan was to stick to low angle slopes, yet one member of our group ended up crossing a slope that was outside that scope. The skier involved is shocked that they ignored red flags and their own intuition, purely to avoid losing elevation and a longer ascent. The biggest take away for all of us was a firm reminder to listen to your gut. If it feels wrong, it probably is.