Multi-temporal analysis of morphological changes in an Alpine proglacial area and their effect on sediment transfer

Proglacial areas in high-mountain environments are among the most dynamic landscape zones in terms of morphological, sedimentological, and hydrological changes. In fact, in such areas both glacial and paraglacial dynamics exert a strong control on water fluxes (and thus on bedload transport capacity) and on sediment supply. Because of the rapid changes undergoing in these environments, the understanding of the dynamics of sediment transfer and the prediction of how this will change in future still remain a challenge. In this study, we used four different high-resolution topographic surveys to quantitatively analyse the spatio-temporal patterns of sediment transport that occurred in the proglacial area of the Sulden glacier (South Tirol, Italy). The topographic surveys covered the period 2013-2019 and - by building DEMs of difference (DoDs) - we calculated the volumes of erosion and deposition occurring at the glacier terminus and along its outflow channels. Our results indicate that, within the analysed time-period and for different areas, erosion and deposition generally show similar values, suggesting that the analysed proglacial areas has been acting both as a sediment trap and as a sediment source for the downstream river reaches. In addition, the results indicate a complex morphological dynamics, which have strongly impacted coarse sediment connectivity in the upper catchment. In particular, high tem-peratures and the recent and rapid glacier melt, have promoted a direct control on the structural connectivity within the proglacial area. Morphological changes have increased or decreased sediment connectivity depending on pre-existing morphological features. In contrast, extreme precipitation events, both in term of rainfall and snowfall, have locally and temporally increased the functional connectivity, sometimes leading to long-term modifications in the proglacial area. Overall, in the analysed time-period sediment transfer has been extremely dynamic, suggesting that sediment in the downstream river channel may have been supplied with irregular pulses associated with the morphological changes taking place in the proglacial area.