Sediment export from an Alpine proglacial area under a changing climate: Budgets, rates, and geomorphological processes

Proglacial areas in the European Alps and other high-elevation mountains are currently undergoing rapid change due to global warming. Because of rising temperatures, glaciers and glacier forefields are subjected to increased melting and associated sediment export. This observation is increasingly important with respect to high-elevation geomorphological and ecological dynamics, emerging natural hazards and mitigation efforts, and hydropower plant management. It is therefore crucial to analyze the factors and feedback mechanisms governing sediment production, transport, and deposition in these rapidly changing areas. In this study, we investigated the sediment dynamics of a proglacial area located in the Eastern Italian Alps over the period 1969-2021 with the aims of: i) identifying the areas of sediment production; ii) quantifying volumes and rates of bedload sediment transport; and iii) determining the relative contribution of glacial export and fluvial erosion to the total sediment budget. We found that i) apart from glaciers, moraines and fluvial channels have been the most important sediment sources, albeit with substantial differences in terms of connectivity and thus supply rates; ii) the volumes and rates of sediment erosion varied by one order of magnitude (between tens and hundreds of mm per year), and were generally higher along the channel network; and iii) for a relatively shorter time interval between 2005 and 2021, the relative contribution of glacial bedload input with respect to the total sediment budget ranged between 34 % and 37 %, whereas 45 % to 59 % was derived from lateral fluvial erosion. Only a relatively small sediment volume was generated by net channel bed incision. These results imply that most of the sediment released from the proglacial area of the Sulden glacier is progressively transferred to the downstream sector of the channel network, with volumes that range between 931 and 1017 tons yr(-1) km(-2). These values are in the typical range of sediment export volumes from glaciated basins and highlight the high dynamicity of this region of the Alps. In general, our results confirm the complexity - in terms of spatial and temporal variability - of Alpine proglacial systems and highlight the need to systematically study these areas on a wide spatial and temporal scale, since the information provided by single locations or individual sectors of the sediment cascade, may not be adequate for understanding the dynamics acting in the entire proglacial regions.