Snowbound Solutions was very excited at the prospect, as we had worked on similar projects in the past. We realized immediately the project potential and how a multi-tenancy database pattern in the avalanche forecasting arena could impact and add value to all backcountry avalanche centers. Snowbound Solutions role in the multi-year project has been to:

The avalanche cycle on 19 January 2012 proved to be one of the largest cycles that the Highway 21 corridor has ever experienced. Two separate avalanche cycles occurred during the seven day storm. The first avalanche cycle produced large dry avalanches while the second avalanche cycle produced medium wet avalanches. An automated weather station (10 mi north at Banner Summit) recorded 2.75 inches of water and approximately 22 inches of snow two and a half days prior to the first avalanche cycle.

After the storm cleared, 57 avalanches were reported with storm totals equaling 53 inches of snow and  5 inches of water. Thirty seven avalanches left debris on the highway with debris pile depths ranging from 5 to 26 feet deep. Figure 1a provides an overview of the highway corridor and the avalanche paths that were active during the avalanche cycle.

The signal amplitude shows the classic infrasound signal characteristics of an avalanche. The avalanche gradually increases (t = 30 s) amplitude as it gains momentum and size. As it approaches the highway it begins to decrease amplitude (t = 60 s) and eventually stops. A maximum pressure of just under 1.5 Pa (t = 55 s) was recorded at a distance of approximately 1000 feet.

A small precursory signal was recorded, 14 s prior to the signal within the avalanche start zone. We hypothesize that the signal was from the snowpack fracture propagation which displaced the snowpack and caused the pressure wave in the air (i.e., a “whumpf”). We believe that this is the first published example of a potential snowpack failure event captured remotely on an infrasound array.

The average velocity of the avalanche was 33 mph from the first major signal at 24.2 seconds to the impact with the highway at 54.0 seconds. The average velocity was of reasonable magnitude, however, the velocity of the avalanche was highly path dependent. The avalanche most likely did not have enough time to reach a steady terminal velocity. It is also possible that the avalanche was still accelerating when it impacted the highway.

The measured avalanche velocity can be described by the velocity of a mass-gaining momentum as it moves down slope. Initially, the velocity was small as the initial snow mass began to move. As the avalanche gained momentum, a suspension layer likely formed and produced a high-amplitude infrasound signal with high signal correlation. The avalanche accelerated quickly to a maximum velocity of 80 ± 17 mph within approximately 1000 feet of the highway as more snow was entrained right before it impacted the highway.