Changes in sediment connectivity following glacial debuttressing in an Alpine valley system

摘要

Increasing air temperature and declining winter snowfalls are resulting in rapid glacier recession and the expansion of proglacial margins in Alpine regions. Such margins include substantial debris accumulations (e.g. frontal/lateral moraine ridges; till-covered and steep valley sidewalls) which may be unstable due to glacial debuttressing. Rainfall, snowmelt and ice melt out may then cause mass movements. Here, we quantify the decadal-scale erosion and deposition patterns and changes in connectivity for two valley sidewall geomorphological systems following retreat of the Glacier d'Otemma, Switzerland. We apply archival digital photogrammetric methodS to the period 1964 to 2009 to determine high resolution digital elevation models. These were differenced to calculate patterns of erosion and deposition and to quantify the evolution of sediment connectivity. We found that gully headward erosion (rates between ca. -10.6 mm/year and -1002.1 mm/year) was the main geomorphological process during glacier thinning but increasing depositional rates downslope of the gullies (ca. +21.3 to +298.5 mm/year) were recorded in the following years associated with significant alluvial fan growth at the slope base. While gullying enhanced connectivity by removing glacially conditioned sediment transfer buffers, so connecting side-slopes to upstream sediment sinks (the upslope contributing area between 1964 and 2009 increased by +73.8% and +195.1% in each subsystem), alluvial fans reduced the rates of sediment transfer to the rapidly enlarging glacial forefields. The detail of these responses is conditioned by three generic processes: (1) the wider geomorphic setting - here, the presence of a moraine bastion as a primary part of the sediment cascade strongly influenced gully morphology evolution and the likely length of the paraglacial period length; (2) the thickness of sediment left by the retreating glacier which controlled the influence of bedrock topographic buffers on connectivity; and (3) the extent to which diffusive drainage systems develop in response to the deposition at the hillslope base, which tend to disconnect sediment flux. Post-glacially, gully development has a self-limiting effect on sediment connectivity in that while gullying increases sediment connectivity, the eroded sediment leads to deposition on the alluvial plain that reduces sediment connectivity. (C) 2019 Elsevier B.V. All rights reserved.