Development of the morphodynamics on LIA lateral moraines in ten glacier forefields of the Eastern Alps since the 1950s

Abstract. Since the end of the Little Ice Age (LIA) in the middle of the 19^th century, Alpine glaciers have been subject to severe recession that is enhanced by the recent global warming. The melting glaciers expose large areas with loose sediments, amongst others in the form of lateral moraines. Due to their instability and high slope angle, the lateral moraines are reworked by geomorphological processes such as debris flows, slides or fluvial erosion. In this study, the development of the morphodynamics and changes of geomorphological processes on lateral moraines were observed over decades, based on a selection of 10 glacier forefields in the eastern Alps. To identify geomorphological changes over time, several datasets of archival aerial images reaching back to the 1950s were utilized in order to generate digital elevation models (DEMs) and DEMs of difference. The aerial images were complemented by recent drone images for selected moraine sections, enabling a high-resolution analysis of the processes currently occurring. The results concerning the development of morphodynamics on lateral moraine sections are diverse: some slopes display a stagnation of the erosion rates, the rates on one section increase significantly and the majority of the slopes show a decline of the morphodynamics over decades, which, however, stay on a high level in many cases. In particular, moraine sections with high morphodynamics in the beginning of the observation period mostly show high erosion rates up until now with values up to 11 cm per year. These moraine sections also feature heavy gullying on their upper slopes. A correlation between the development of morphodynamics and the time since deglaciation could scarcely be established. In fact, the results rather indicate that characteristics of the lateral moraines such as the initial slope angle at the time of deglaciation have a significant influence on the later morphodynamics. These observations raise concerns whether often conducted analyses based on the comparison of lateral moraine sections with different distances to the glacier terminus, assumed to represent varying time spans since deglaciation, can provide sound evidence concerning their process of stabilization.