The impact of thermokarst development on flow and transport processes in alpine rock glaciers

Abstract

Active rock glaciers represent permafrost-affected aquifers that govern the response of many alpine headwater catchments. Their heterogeneous internal structure tends to channelize groundwater flowing along the permafrost table or within the frozen rock glacier core. This study derives the hydraulic properties of such thermokarst channel systems at three active rock glaciers in the Austrian Alps. Their basic configuration is assessed through spring flow analysis and dye tracer tests. Breakthrough curves are characterized by multiple peaks and strong tailing, implying flow path separation and partial retardation of the tracer cloud travelling through the rock glaciers. Individual channels can reach diameters up to several decimeters and are characterized by a convoluted, irregular geometry. Flow along the channels is fast, highly turbulent, and characterized by high frictional resistance. Heat transfer is predominantly advective, inducing a positive feedback loop that allows larger channels to grow at the expense of smaller ones, effectively increasing the hydraulic conductivity at the rock glacier scale. The preferential flow paths provided by the thermokarst channel networks dominate flow and transport through the rock glaciers in particular during the summer months, and thus govern spring flow dynamics, solute transport, permafrost degradation, thermokarst lake outburst hazard, and rock glacier front stability.