Detailed Accident Report

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Date: 2003-03-12
Submitted By: Stan Bones; Glacier Avalanche Center
Place: West of Marias Pass, MT; US HWY 2
State: MT
Country: USA
Summary: Large avalanche cycle along highway and railroad corridor

Early Wednesday morning, 3-12-03, 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, 3-9-03, a moist westerly flow collided with the

cold air. The air temperature shot up from around -15 F to near freezing

in approx. 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 however. 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 all finished 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 approx. 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

Stan Bones

USFS-Glacier Country Avalanche Center