Quantifying and characterising contemporary rockfall supply to a debris-covered glacier catchment

Abstract

<p>Supraglacial debris extent and thickness is an important control on the ablation rate of a debris-covered glacier. Debris is supplied to the surface of a debris-covered glacier through several pathways with the primary source of this debris originating from rockfall in both the accumulation area, where debris is transported englacially downglacier, and the upper ablation area, where debris remains in the supraglacial environment while transported downglacier. Current quantification of debris supply to debris-covered glaciers is limited to headwall erosion rates determined through the dating of headwall derived supraglacial debris using <sup>10</sup>Be concentrations, or estimations of these rates using a ratio of supraglacial debris flux to the headwall catchment area. To increase the knowledge of the contemporary short-term estimations of these processes, repeat LiDAR scans of debris-contributing slopes were acquired during a single ablation season in both July and September at Miage Glacier, Italy. An area of ~7.7 km<sup>2</sup> comprising > 1.8 billion 3D points was scanned per survey epoch, covering ~33% of the glacierised area of Miage Glacier. Sequential scans were co-registered using an iterative closest point adjustment algorithm within CloudCompare. Manual filtering was used to remove snow, artefacts, and the glacier surface from the raw point clouds. To ease processing, the rock walls were segmented both horizontally and vertically within the catchment. Change detection was carried out using the M3C2 algorithm at a projection scale of 0.3 m and point clouds representing areas of significant change within the segment were obtained using a distributed 95<sup>th</sup> percentile confidence interval. The DBSCAN clustering algorithm was used to identify individual rockfall clusters, and the volume of each rockfall was calculated using both an iterative alpha-shapes approach. Finally, a bounding box approach was used to estimate the a, b and c axes and therefore shape of the individual rockfalls. Increasing the projection scale used within the M3C2 algorithm decreases the frequency of significant rockfalls found exponentially, and an iterative alpha shapes approach is the most computationally efficient volume estimation method. Our results show that the Miage Glacier catchment is dominated by small scale rockfall events, although at least one large-scale rockfall event is evident in the upper ablation area (validated by time-lapse imagery). This failure on a recently deglaciated area of rock wall highlights that slope response to glacial erosion can be rapid following periods of deglaciation.</p>