A field-based model of permafrost-controlled rockslide deformation in northern Norway

Abstract

Knowledge about the detailed processes linked to the existence of permafrost in rockslide fractures is sparse. Large parts of the Jettan rockslide are located right below the discontinuous permafrost limit in the arctic part of the alpine landscape of northern Norway. Combining four years of meteorological and rockslide deformation data with temperature measurements from different parts of open fractures, shallow bedrock boreholes and air, as well as daily snow cover observations, allows a detailed identification of the key processes involved. These field data are the basis for the development of a permafrost controlled rockslide model. Seasonally, the deformation has a very distinctive pattern with high deformation starting abruptly right after snowmelt in May, and lasting until snow isolation in winter. Then there is a gradual transition to medium deformation as the ground is cooled further for another 1–2months. Finally, the winter period, when maximum snow occurs in the fractures, is characterized by limited or almost no deformation. The primary controlling deformation process is meltwater percolation into fractures in summer with significant refreezing, ice formation and temperature increase in the lower part of the fractures from −1°C to 0°C. Sporadic permafrost exists below the discontinuous permafrost limit, and may extend into open fractures and sliding planes. Another primary process is the significant cold air accumulation in fractures in early winter, due to the Balch effect, which significantly cools the fracture and surrounding rock promoting permafrost development. Finally, the cold air effect is stopped by snow isolation once enough snow has accumulated in the fracture by late winter. The deformation itself is thought to be controlled by changing shear strength of the brecciated sliding planes due to either changing ice temperatures and/or variations in water infiltration to the unsaturated sliding zones. The overall system is very locally controlled driving itself, and the effect of a future climate change can thus be of minor importance.