Monday 4/18/2011 Ned Bair Mammoth Mountain Ski Patrol Ph.D. Candidate, Donald Bren School of Environmental Science and Management, UC – Santa Barbara US Army Corps of Engineers Cold Regions Research and Engineering Laboratory

Climax post control 2

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R4D2 Average crown depth: 28”, maximum: 48”. 755 ft wide, ran 710 vertical ft. Bed surface: 1F melt freeze clusters 0.5-1mm. Slab: mm wind slab, broken particles, mostly rounded forms (typical) Step down failure (atypical) 2 nd post control of the season on Climax. On 12/29/2005 a cornice collapsed and caught 6 people in an R3D2 hard slab. 5 skis lost, no injuries. 4

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8 people caught, 1 skied out, 5 dug themselves out 2 partially buried 2 non-critical injuries, none went to hospital 1 patient found from a glove sticking out of debris. She was face down and was resuscitated with rescue breathing. The other patient had a minor knee injury. 6

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9” inches of new snow, 1.23” SWE, 14% water. Base depth 1400 hrs on day of incident Temperature change from low of 0600hrs on 4/17 to 28° 1400 hrs at Sesame Street Snow Plot (el 9000 ft.). Time of accident 1352 hrs. Temperature change from low of 0600 hrs on 4/17 to 1400hrs at the Top Weather Station (el. 11,052 ft.). 10

The top had not been open to the public since 4/13. Climax was ski cut on 4/14, but not opened to the public. Guns 1 and 2 blind fired that morning. Climax was shot with 4 1kg hand charges and ski cut, then ski cut on a 2 nd lap. Only a small R1D1 (4” crown, ran 75 ft) was recorded. 32 other avalanches. Max. class size R4D3, max. crown size 60“(Dave’s Run). 400’path width, 1000’ path length. Triggered by Gun 2 and cleaned up with hand charges. 5 avalanches that were class 3 and above with ≥ 30” crown sizes. 11

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The top was opened to the public at The first guests would have skied Climax around By the time the post control occurred at 1352, at least 100 skiers had been in the starting zone. 13

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Climax cannot be shot by the 105mm Howitzer. Hand charges were likely not a powerful enough control measure. It is possible that solar radiation destabilized the slope. Low thermal conductivity of snow makes it unlikely that a small increase (6-11°F) in air temperature affected the weak layer. It is also possible that hand charging and ski cutting left the slope in a weakened state, but this explanation suggests the avalanche would have been triggered soon after opening. 22

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Warming effects generally destabilize slopes in the short term and increase stability over longer timescales (McClung and Schweizer, 1996): Short term: Decreased slab stiffness (elastic modulus).Deeper and shallower stress bulb. Increased strain rate (deformation). Faster glide and settlement. Increased energy release rate. The propagating collapse wave can travel more easily. Long term: Increased rate of bond growth. Weak layer and slab bonding. Increased settlement causes densfication and strengthening. Decreased temperature gradient 24 McClung, D. and J. Schweizer Effects of snow temperatures on skier triggering of dry slab avalanches. International Snow Science Workshop, Banff, Alberta, Canada,

A classic non-persistent weak weak layer. It had healed by the time I did a crown face profile less than 24 hours later. Climax is always a problem because it cannot be shot with the gun (blind or direct). Hand charging with surface shots and ski cutting are probably not always adequate control measures. It is not possible to leave Climax closed and open the rest of the top. Opening the top easily doubles the amount of skiable terrain, so there is pressure to get it open. Since this incident, we have been more conservative in opening the top and more proactive with shooting Climax, including hand charging during during storms and throwing hand charges from a gondola cabin to reach further down into the path. 25