Marias Pass Avalanche Cycle - March 12, 2003
(NW Montana, Glacier Park area)
Keywords: avalanche cycle, cold front, air blasts, snow sheds, avalanche path, wet avalanche, avalanche track, run-out, fracture line profile, melt-freeze ice layer, faceted grains, percolating water, bed surface
Early Wednesday morning (03-12-2003) an extensive avalanche cycle began along the US Highway 2 and Burlington Northern Santa Fe Railroad corridor on the southern edge of Glacier Park. This area is west of Marias Pass in NW Montana. The avalanche activity forced the closure of the highway until Friday morning, delayed rail freight traffic, and caused suspension of Amtrak passenger rail service over the pass for over 48 hours. Train passengers were bussed south between Whitefish and Shelby over Highway 200 through Great Falls.
Leading up to the cycle, the weather over the region had been extremely cold and snowy. An extensive cold front along the eastern front of the Rockies produced air temperatures well into sub-zero readings. Coupled with the cold were northerly winds and a significant amount of new snowfall. Then on Sunday (03-09-2003) a moist westerly flow collided with the cold air. The air temperature shot up from around -15 F to near freezing in approximately 15 hours and remained there for the next two days. This warming was accompanied by heavy snow mixed with rain.
The initial avalanches tended to be dry and produced significant air blasts. Nine snow sheds protect the railway and these gave protection to the main flow of most of the major paths. Several of the larger slides, however, overran the ends of some sheds while others hit the rails in unprotected locations. Sections of power line and signal fence were damaged. No derailments occurred. One avalanche topped a snow shed, flew over Bear Creek, and pancaked onto the highway, ripping out a section of guardrail. Another slide stopped short of the road, but put an air blast across the road damaging trees on the opposite side. With warming temperatures and rain the debris flows began to change to wet and slowed. When it was all over, only one avalanche actually put debris on the highway. Located at the toe of the south facing major avalanche paths, the railroad took more of the beating, but even that could have been worse had the region received something close to a normal snowfall this season. Instead a shallow snow cover in the tracks and an abundance of vegetative and terrain features helped in impeding the flow and reduced the run out distance.
Fracture line profiles after the event show that the failing layers could be traced to three locations within the snow pack. Closest to the surface was initial failure upon a melt-freeze ice layer approximately one third of the way down into the snow cover. The major weakness however, was sandwiched between this ice layer and another 6-10-inches further down. This weak layer was predominately faceted grains and very weakly bonded and easily stressed once the upper melt-freeze ice began to weaken with warm temperatures and percolating water. The final bed surface in some of the releases was the ground, with failure within a layer of old facets dating back to last Nov-Dec. Significant in this event was the loss of strength as the snow pack warmed with rising and prolonged temperatures. Luckily no one was caught, injured, or killed in any of the slides
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