Analyses of UAV and GNSS based flow velocity variations of the rock glacier Lazaun (Otztal Alps, South Tyrol, Italy)

Flow velocities were measured on the active rock glacier Lazaun by eleven GNNS (2006-2018) and six UAV campaigns (2016-2018) to better understand the flow pattern and its possible causes. This medium-sized, active rock glacier is located W of Kurzras/Maso Corto in the Schnals/Senales Valley in the southern Otztal Alps (South Tyrol, northern Italy). The UAV data were used to generate displacement maps by image correlation techniques, to analyse surface lowering and accumulation processes, and to interpret the geomorphology base on shaded relief images. The spatial information from UAV contributes much more to the understanding of the rock glacier flow patterns as the point based GNSS measurements even when the UAV based horizontal displacement analyses are less precise. The level of detection (LoD) for the absolute horizontal displacements derived by image correlation from UAV data ranges between 0.05 and 0.15 m. A comparison of the displacement lengths shows that the UAV derived displacement vectors deviate in average about +/- 0.05 m from the GNSS measured displacement lengths. The deviation in the displacement directions is smaller when the absolute displacement is higher and ranges in zoneswith an absolute displacement higher than 0.3 m in dependence to the data quality between 0 and 4 degrees. Consequently, when planning a UAV based rock glacier monitoring, the achievable data accuracy, rock glacier flow velocity and the time span must be considered. Analyses of the rock glacier velocities from GNSS and UVA data show that the calculated horizontal mean velocity increased significantly from 2 mm/day in 2006 up to 6 mm/day in the period 2012 to 2016 and then decreased to values of 3-4 mm/day until 2018. We assume that internal structures and hydrogeology are essential parameters that control the flow mechanism of rock glacier Lazaun, particularly the shear horizon composed of debris and banded ice that is present at the base of the permafrost body, and the unfrozen, groundwater-bearing sediment layer between the shear horizon and the bedrock. We conclude that climate warming caused increased infiltration of meltwater into the shear horizon and that this is the main driving force for the movement of the rock glacier. Increased meltwater infiltration as result of climate warming caused an increase of flow velocity until 2016. The observed thickness loss near the front of the rock glacier indicates increased melting of permafrost ice resulting in a significant loss of permafrost ice during the last years. This process that particularly occurs in the lower part of the rock glacier is believed to be responsible for the decrease in flow velocity since 2017. (C) 2020 The Author(s). Published by Elsevier B.V.