Dry snow compaction: theory and French-press experiments

Compaction is the process by which snow densifies, storing water in high alpine regions and transforming snow into ice on the surface of glaciers. Despite its importance in determining snow-water equivalent and glacier-induced sea level rise, we still lack a complete understanding of the physical mechanisms underlying snow compaction. In essence, compaction is a question of rheology, where the rheology evolves with depth due to variation in temperature, pressure, humidity, meltwater, etc. The rheology of snow compaction can be determined in a few ways, for example, through empirical investigations (e.g. Herron & Langway,1980 J. Glaciol.), by microstructural considerations (e.g. Alley, 1987 J. Phys.), or by measuring the rheology directly, which is the approach we take here. Using a "French-press" compression stage, Wang and Baker (2013, J. Geophys. Res.) squished numerous snow samples of different densities. Here we derive a mixture theory for compaction and air flow through the porous snow to compare against these experimental data. We find that a perfectly plastic compaction law explains experimental results. Taking standard forms for the permeability and plasticity as functions of the porosity, we show that this compaction mode persists for a range of densities and overburden loads. These findings suggest that measuring compaction in the laboratory is a promising direction for determining the snow compaction rheology.