CAN SNOW AVALANCHES CARVE STEEPLAND CHANNELS? EVIDENCE FROM CHANNEL MORPHOLOGY, DEPOSIT STRATIGRAPHY AND DISCRETE ELEMENT MODELING

There is mounting evidence that episodic scour by debris flows can be a significant transport and erosional process in high-gradient valleys. However, many of these steep valleys are at high elevation where seasonal snow accumulates, becomes unstable and flows down slope as a snow avalanche. The coincident location of these distinct granular flows and evidence of rock wear in the aftermath of both raises the question of what carves steepland channels: debris flows or snow avalanches? Clearly snow avalanches are not a requisite process for valley formation in steep terrain, yet in high-gradient high-elevation areas with considerable snowfall, the relative contribution of each process in setting channel form and rates of downcutting remains unknown.

As an initial attempt to make progress on this question we report on field observations made a few days after a spring avalanche travelled down a steep channel on Monte Velino, in the Italian Apennines and on complementary discrete element simulations of the unique bimodal grainsize distribution observed in the deposit, with the goal of quantifying the erosive potential of snow avalanches relative to debris flows. This particular spring snow avalanche is illuminating as it traveled over both a snow-covered bed in the upper catchment and over snow-free channel sediment in the last 1.0km of run-out. No erosion of sediment occurred where the channel sediment was covered by the established snowpack. Below snowline where the snow avalanche flowed over unfrozen ground entrainment of bed sediment was generally limited, but evidence for various forms of sliding wear were prevalent. This superficial bed wear was in strong contrast to the near complete destruction of vegetation above and below snowline. These observations indicate that the impact of snow avalanches is primarily restricted to the removal and transport of vegetation rather than the entrainment of bed sediments. However, such avalanches may play an important role in ‘priming’ the bed for future debris flow scour by removing stabilizing vegetation. Trenching of the snow avalanche deposit confirmed that little coarse-grained sediment was entrained and revealed that the deposit was vertically stratified. The basal layer was composed of individual snow grains, while the upper thicker layer was made of cm scale snowballs.