Glacier retreat, lake evolution, and the Glacial Lake Outburst Flood of 2017 in the Sulzenau valley, Tyrol, Austria

Mountain regions are well known for their sensitivity and dynamic response to ongoing climate change. Currently, many scientists are studying the impact of these changes on glacierized high mountain areas around the world. In the Stubai Alps (Tyrol, Austria), most of the glaciers are retreating, resulting in a reduction of the total glacier area and also the formation of glacial lakes in suitable topographical position. The Sulzenau Valley is facing a rapid loss of glacier ice. Since the early 2000s, the shrinking of one of the largest glaciers, the Sulzenauferner, has led to the formation and evolution of the proglacial Sulzenau Lake. In August 2017, the moraine part of the dam was breached and the lake released part of the retained water suddenly. Resulting glacial lake outburst flood (GLOF) damaged the power plant and water pipes of the downstream hut. Although the Stubai region is already well-investigated, the evolution of the very active Sulzenauferner Glacier and its proglacial lake are poorly documented. To fill this gap, the objective of this research is to collect multi-source data to reconstruct the evolution of the Sulzenauferner Glacier since 2000 and identify the preconditions, drivers and triggers that led to the 2017 GLOF. To this end, we combined multiple satellite and close-range remote sensing data to quantify changes of the glacier in space and time and the related growth of the lake. Firstly, based on optical images (e.g.: Sentinel 2, Google Earth images, in-situ pictures), we produced two detailed geomorphological maps of the Sulzenau valley, one before and another after the flood. The geomorphological maps illustrate the effects of the flood on the morphology of the lake and the channel. Secondly, we prepared the mosaic of very high-resolution images acquired with an Unmanned Aerial Vehicle (UAV) during a field campaign held in August 2021 and created a high-resolution Digital Elevation Model (DEM). The comparison of multi-temporal DEMs (i.e.: satellite and UAV-based) allowed us to map erosion and deposition areas and estimate the volumes of displaced material. Further, a Synthetic Aperture Radar Interferometry (InSAR) analysis and the computation of deformation time series is used to investigate the stability of the moraine dam and moraine slopes surrounding the lake. Although the 2017 GLOF of Sulzenau is considered rather a small-magnitude event, it is considered a representative study case of a dynamic high mountain environment affected by a rapid retreat of the glacier and associated processes and geomorphological responses.