Snow avalanche speed determination using seismic methods

Abstract

We present a new method to determine the average propagation speed of avalanches using seismic techniques. Avalanche propagation speeds can reach 70 m/s and more, depending on a wide range of factors, such as the characteristics of the avalanche track (e.g. topography) and the snowpack properties (e.g. density). Since the damage produced by the avalanche depends primarily on the size and on the speed of the avalanche, the knowledge of the latter is therefore crucial for estimating avalanche induced hazard in inhabited mountain areas. However, our knowledge of this basic physical parameter is limited by the difficulty of conducting various measurements in the harsh winter weather conditions that often accompany this natural phenomenon.

The method of avalanche speed determination presented in this paper is based on cross-correlation and time-frequency analysis techniques. The data used in this study come from the Ryggfonn (Norway) avalanche experimental site operated by the Norwegian Geotechnical Institute (NGI), and recorded by an array of 6 geophones buried along the main avalanche path during the 2003-2004 and 2004-2005 winter seasons. Specifically, we examine the speeds of 11 different events, characterized by size and snow type. The results obtained are compared with independent speed estimates from CW-radar and pressure plate measurements. As a result of these comparisons our method was validated and has proved to be successful and robust in all cases. We detected a systematic behaviour in the speed evolution among different types of avalanches. Specifically, we found that whereas dry/mixed type flow events display a complex type of speed evolution in the study area with a gradual acceleration and an abrupt deceleration, the speed of the wet snow avalanches decreases with distance in an approximately linear fashion. This generalization holds for different size events. In terms of time duration and maximum speed of the studied events, dry/mixed type avalanches lasted between 8 to 18 s and reached speeds up to 50 m/s, whereas the duration of wet avalanches ranged between 50 and 80 s and their maximum speeds were 10 m/s.