Advances in understanding the cooling rates and bending of needle ice: photogrammetric and thermal observations leading to the spatial distribution of needle ice creep

The freeze-thaw cycles are a process that dominate entire landscapes with different intensity and magnitudes that are strongly changing in the current climate change conditions. In particular, the diurnal frost heave (FH) and needle ice creep (NIC) are two poorly studied processes, especially in the field, despite their effects on the sediment transportation and vegetation colonization on Alpine slopes. In a site of the Central Alps, we applied photogrammetry, thermal imaging, manual measurements and thermistors logging to improve the knowledge of the driving parameter for the formation and accretion of needle ice, of the relations of their spatial distribution with cooling rates and the frost creep modelling change in the presence of the bending process. Among the driving factors we demonstrate that very local conditions govern the development of the needle ice, like the air cooling rates, the deepening of the freezing front and the surficial cooling rates. Especially low cooling rates are necessary for an intense process. Also the grain size influences the development of the needle ice with higher needle ice development under fine sediment than under small clasts due to a differential thermal conductivity. Moreover, the bending of needle ice has proved to be dependent on its length, age and type of heaved sediment. Therefore, potential frost creep formulas needed to be ameliorated considering the bending angle. Subsequently, a question about the role of slope gradient in NIC is treated.