Vehicle damage and transport and possible shock wave formation in the Flateyri avalanche in January 2020.

Two large dry slab avalanches from the 700 m high mountain above the village of Flateyri, NW Iceland, partly overflowed two deflecting dams on the eve of the 14th of January 2020. While most of the avalanche snow was deflected from the village by two 15-20 m high deflecting dams, approximately 10 % of the total mass overflowed the dams, injuring one person and causing damage to one house, three vehicles and breaking a steel mast, a timber shed and shrub/bush. Radar measurements of the speed of the avalanche, the density of the avalanche deposit, damage to structures, and witness accounts suggest that the avalanche was a transitional avalanche with a 400 m long fluidized head, followed by a denser core, upstream of the dam and the overflow belonged to a fluidized region of intermittent density. It is believed that the sonic speed in such fluidized regions can be one order of magnitude lower than that of air. In such a case the speed of the fluidized region can be comparable to the sonic speed within it, giving rise to the possibility of supersonic shock wave formation.

The aim of the current study is to analyze the dynamics of the part of the avalanches that overflowed the deflecting dams, based on the in-situ damage that the overflow caused, and attempt to distinguish between damage that a shockwave would cause and damage that the kinetic energy in the overflow may cause. The focus is on the three vehicles that were hit by the avalanche overflow and transported 13 to 20 m horizontally, two of them over a three to four meters high pile of snow. We find that aerodynamic forces caused by the overflow could have been large enough and lasted long enough to transport the cars the observed distances. The calculations are simplified and effects of an uneven lateral density distribution are omitted. Supersonic conditions in the overflow are considered unlikely.

A moving supersonic shockwave would not have lasted long enough (order of 10-100 ms) to transport the cars. Weak compression shocks in subsonic avalanche flow may form upstream of stationary obstacles, due to the compressibility of the avalanche front. Those would also be too short-lived to contribute to the transport of the vehicles. Damage to brittle objects (with a resonance period in the same range as the pressure wave period, 2 to 100 ms), such as window glass, doors, and non-reinforced concrete walls, can however be contributed to such short-lived impacts and interactions with moving pressure waves.

Shockwave-turbulence interactions are generally known to cause high-intensity noise radiation of various characteristics. Standing shocks, due to supersonic flow upstream of stationary obstacles (e.g. buildings or high cliffs) could be a cause for impulsive sounds observed prior to the arrival of the avalanche.