Quantifying debris supply and debris flux in deglaciating mountain glacier catchments

Stewart, Rebecca Louise (2022) Quantifying debris supply and debris flux in deglaciating mountain glacier catchments. Doctoral thesis, Northumbria University.

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Abstract

Debris-covered glaciers are found in most glaciated regions on Earth. The presence of a supraglacial debris cover enables these glaciers to persist for longer periods of time at lower elevations than their clean-ice counterparts. While the processes that govern the behaviour of debris-covered glaciers have been the focus of contemporary research, there exists scant quantitative information pertaining to the supply of sub-aerial debris and the subsequent evolution of supraglacial debris cover thickness through time. This thesis therefore aims to quantify sub-aerial debris supply and supraglacial debris thickness evolution in a deglaciating Alpine catchment. This is achieved through in situ fieldwork and remote sensing workflows, paving the way for facilitating a quantitative understanding of key debris covered glacier processes, and remote estimation of debris thickness changes at glaciers where field studies are sparse. This study also contributes knowledge towards the development and refinement of next-generation debris-covered glacier numerical models.

Firstly, the sub-aerial debris supply was quantified using high resolution terrestrial laser scanning obtained over two field campaigns at Miage Glacier. Approximately 700,000 m2 of rock slope was scanned at both the start and the end of the 2019 ablation season, representing the most extensive known rock wall survey undertaken in a deglaciating catchment. Over a survey interval of ∼90 days, which encompassed most of the 2019 ablation season, 2,582 rockfalls across eight orders of magnitude (10−3–104) were detected using a novel processing workflow. In sum, these rockfalls represented a potential debris addition to the glacier surface of 29,271.11 m3, with ∼28,018 m3 originating from a single rockfall event. When evenly spread out over the predicted rockfall runout area, this equates to a local thickening of 0.04 m m−2 (inclusive of the large rockfall event), or 0.002 m m−2 (not considering the large rockfall event). A survey area erosion rate of 0.462 mm a−1 was estimated, and the impact of volume estimation method was shown to impact this rate significantly, producing estimates between 0.115–1.145 mm a−1. There was a strong preference for rockfalls from a NE aspect, likely reflecting aspect-related controls on total solar radiation receipt, which in turn dictate rock wall temperature fluctuations, and ultimately rock damage. Magnitude-frequency analysis shows that smaller rockfalls are a more significant contributor to the sub-aerial debris supply throughout the study area, and slopes located within 50 m vertically of the contemporary glacier surface generate more high-magnitude events than higher-elevation slopes.

Secondly, reanalysis data was used in conjunction with a supraglacial energy balance model to estimate statistically significant debris thickness. ERA-5 reanalysis data were shown to be superior to NCEP/NCAR reanalysis data in forcing the energy balance model to estimate debris thickness, when compared with automatic weather station data at Miage Glacier and Khumbu Glacier. Using this remote sensing-based workflow, statistically significant debris thickness changes over an ∼20 year period were reconstructed for three debris-covered glaciers at different stages in their so called ’life-cycle’. These results represent the first known remotesensing- derived estimate of glacier-scale debris thickness changes.

Finally, a conceptual model of a debris-covered glacier life-cycle was proposed, which is supported by novel quantitative estimates of key processes and fluxes. A median debris thickness increase of 0.01 m per pixel was estimated at Miage Glacier between 2005–2016. This increase equated to an addition of ∼2,165 m3 of debris per year over the ablation zone. Excluding the large rockfall event that alone contributed over 28,000 m3 of debris to the surface of the glacier, reconstructed rockfall events contributed up to 1,252 m3 of debris to the surface of the glacier over the 2019 ablation season. By reconciling these two numbers, rockfall from bedrock slopes was estimated to contribute ∼57% of the total debris flux directly to the ablation zone of Miage Glacier, whilst so-called ’indirect’ sources such as englacial melt-out and debris remobilisation are hypothesised to contribute the remaining total debris increase. These results form the beginning of a new quantitative framework which uses novel data derived from cutting edge and complementary Earth observation technologies, implemented within a robust and transparent workflow, to shed light on key sediment fluxes that are crucial for improving understanding the evolution of debriscovered glacier landsystems.

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