Monitoring the dynamics of an alpine rock glacier with repeated UAV and GNSS data

Time series of rock glaciers (RG) movements in the European Alps indicate an acceleration in permafrost creep in recent decades in relation to an increase in ground temperatures and water content. In this work, we analyse the geomorphological changes of an active RG located in the Western European Alps, in Valtournenche Valley (AO, Italy).

Five photogrammetric surveys were realized on the RG between 2015 to 2019, using a senseFly eBee RTK and a DJI Phantom 4 UAVs. During UAV acquisitions, 21 ground control points were placed all over the study area and their coordinates were measured in GNSS RTK mode, for georeferencing each photogrammetric model. The monitoring activity also includes GNSS campaigns, carried out annually since 2012, which provides high accurate surface displacement measurements but limited to 54 points. In addition, in July 2015 two Electrical Resistivity Tomography profiles were performed, with the Wenner-Schlumberger configuration, to identify the internal structure and potential ground ice content inside the main body of the RG.

The Structure-from-Motion technique was used to generate orthophotos and digital surface models with a resolution of 5 cm/px. Successively, we estimated the three-dimensional change of the surface displacements (surface lowering and accumulation processes) of the RG comparing pairs of point clouds, using the Multiscale Model to Model Cloud Comparison (M3C2 plug-in). A first evaluation of the horizontal surface velocity was computed identifying corresponding features manually on the orthophotos through time and a second assessment was performed based on repeated GNSS campaigns. Surface velocity obtained by orthophotos manual identifications is validated against repeated GNSS measurements. The analysis shows a good correlation at all magnitudes with a R² equal to 0.988 and RMSE of 26 cm.

The RG shows a clear distinction in creep dynamics between a faster western part (values up to 1.8 m/y) and a slower eastern part, with values below 0.1 m/y in the most upstream part. Considering the period 2012-2020, maximum peak of surface velocity is reached in 2015, followed by a velocity decrease until 2017-2018 when the smallest movements are recorded. However, the following two years (2018-2019 and 2019-2020) are marked by a gradual increase in surface horizontal velocity. The absence of significant of any significant movement in the upstream part is related to the lack of permafrost consecutive to the development and advance of a local glacier during the Little Ice Age. The slower eastern part is almost gently inclined and corresponds to a currently degrading part of the RG, with an ice melt-induced subsidence of up to 5 cm/year. The faster area is also the steepest, where the driving stress is also the largest. The presence of the frozen ground at depth, probably its structure and thermal state, but also the topographical settings are the main factors explaining the current RG flow pattern.