Sediment transport and bedrock erosion by wet snow avalanches in the Guggigraben, Matter Valley, Switzerland

Abstract

Wet snow avalanches breaking through to the base of the snowpack or overriding snow free surfaces can entrain basal material and act as important agents of sediment transport in steep alpine catchments. Here we present results from 4 years of measurements at the Guggigraben catchment (Matter Valley, Canton Valais), quantifying the volume of sediment transported by avalanches each winter season. Sediment load was estimated by measuring the debris content within representative areas and then extrapolating the cumulative volume. Results reveal a total transported sediment volume of 70 m3 in 2009, 23 m3 in 2010, 15 m3 in 2011, and 35 m3 in 2012, which correspond to catchment-wide erosion rates of approximately 0.05, 0.02, 0.01, and 0.03 mm/a, respectively. Sediment appears to be sourced predominantly from within the main channel or one its tributaries, originating first from rockfall or landslides. Avalanches thus play an important role at the Guggigraben in transporting loose sediment from temporary storage sites to the fan and main river system. Within the bedrock gully and in the avalanche source region above, signs of abrasive wear were evident on exposed bedrock surfaces. These included rounded and scoured bedrock, fresh boulder impacts, and scratch marks on the gully walls.