Needle ice formation, induced frost heave, and frost creep: A case study through photogrammetry at Stelvio Pass (Italian Central Alps)

Needle ice growth is one of the more widespread and easily visible, but less studied, climate related processes
shaping soil evolution, surface dynamics and ecosystem changes in the alpine environments. Here, we show the
results of the monitoring of needle ice development at four plots located at 2670m a.s.l. close to the Stelvio Pass
in the Italian Central Alps during 2016. Needle Ice formation and evolution with time was monitored through the
photogrammetric technique of the Structure from Motion (SfM). Our monitoring data included also quantitative
measurements of some selected physical and climatic parameters like air temperature, ground temperature and
ground water content at depths of 2 and 5 cm. Our data demonstrate that needle ice can develop with a relatively
low ground water content (13.2%), at a relatively high minimum ground temperature (−0.3 °C) and with a low
cooling rate (< 1.8 °C h−1). Moreover, for the first time, we observed that needle ice can form below a thin snow
cover (< 25 mm) that can enhance the sensible heat flow from the ground to the atmosphere and, therefore,
promote the cooling of the near surface ground. Statistically, the minimum air temperature results in the leading
factor for the needle ice growth.
The total frost heave seems to be related to the abundance of fine material (although we couldn't demonstrate
it statistically). The absence of statistically significant relationships between frost heave and frost creep could be
probably due to the importance of the observed needle ice toppling and the possible sliding of the clasts during
the melting phases.