| Résumé: |
It is now largely recognized that current climate warming strongly impacts glacierized catchments. Increasing atmospheric temperature influence both the cryosphere and the hydrosphere in high mountain regions, and so drives glacier recession. The resulting deglaciated landscape is essentially composed of debris accumulations (e.g. frontal/lateral morainic ridges) and steep valley sidewalls usually till-covered (e.g. debris-mantled slopes). These landforms can be in an unstable state because glacier thinning causes glacial debuttressing, and subsequent failure of material. In our study, we used two distinct temporal scales to investigate the geomorphological evolution of the Glacier d’Otemma Little Ice Age moraine to rapid glacier recession at the paraglacial time scale. In an historical perspective between 1964 and 2009, and largely based upon archival aerial digital photogrammetry methods, we studied how the geomorphic response to non-glacial conditions, induced by glacial debuttressing, impacted hillslope sediment connectivity. We found that gradual moraine denudation, notably caused by gullying and headward erosion during the glacier thinning phase, promoted better sediment connectivity on the valley side slopes but reduced connectivity at the hillslope-alluvial plain scale due to the formation of large alluvial fans. We also highlighted that connectivity improvement through gully headward erosion is conditioned by the exposure of bedrock topographic buffers. Over a shorter time-scale, we conducted lidar surveys during the glacier ablation season in order to quantify shorter time scale geomorphic change at the hillslope-alluvial plain scale. We found that high magnitude events are essential for sediment reworking processes and consequently landscape evolution. |
Mémoire [10 Mo]