Abstract
ABSTRACTThe thickness of supraglacial debris cover controls how it impacts the ablation rate of underlying glacier ice, yet this quantity remains challenging to measure, particularly at glacier scales. We present a relatively straightforward, and cost-effective method to estimate debris thickness exposed above ice cliffs using simplified geometrical measurements from a high-resolution digital surface model (DSM), derived from a terrestrial photographic survey and a Structure from Motion with Multi-View Stereo workflow (SfM-MVS). As the ice surface relief beneath the debris cover is unknown, we assume it to be horizontal and provide error bounds based on characteristic ice-surface slope at the visible debris/ice interface. Debris thickness around the three sampled ice cliffs was highly variable (interquartile range of 0.80–2.85 m) and negatively skewed with a mean thickness of 2.08 ± 0.68 m. Manual, and high-frequency radar, determinations of debris thickness in the same area show similar thickness distributions, but statistically different mean debris thickness, due to local heterogeneity. Debris thickness values derived in this study all exceed estimates from satellite surface temperature inversions. Wider application of the method presented here would provide useful data for improving debris thickness approximations from satellite imagery.
Highlights
Supraglacial debris cover alters the rate at which the underlying ice ablates in comparison with clean ice (e.g. Østrem, 1959; Nakawo and Young, 1982; Mattson and others, 1993)
We propose that measurements from scaled, high-resolution digital surface models (DSMs), generated from terrestrial or unmanned aerial vehicle (UAV) borne photogrammetry and Structure from Motion with MultiView Stereo (SfM-MVS) processing (Westoby and others, 2012; Smith and others, 2015; Carrivick and others, 2016), offer the geometrical detail to sample debris thickness exposed above ice cliffs, in a manner analogous to the theodolite survey methods presented by Nicholson and Benn (2012)
In this paper we (i) present a workflow to quantify debris thickness exposed above ice cliffs using measurements from scaled, high-resolution photographic DSMs, (ii) compare the derived debris thickness to available debris thickness collected by alternative means and (iii) discuss the utility and limitations of the new method of determining supraglacial debris thickness
Summary
Supraglacial debris cover alters the rate at which the underlying ice ablates in comparison with clean ice (e.g. Østrem, 1959; Nakawo and Young, 1982; Mattson and others, 1993). Simple trigonometry was used to approximate the debris thickness by determining the difference in vertical dimension of the two right-angled triangles formed between (a) a horizontal reference at the height of the survey position, the dip angle measured to the crest and the hypotenuse length of the distance to the crest line and (b) the same horizontal reference, the dip angle measured to the debris/ice interface This approach simplifies the three-dimensional (3-D) reality of the exposed debris layer, by assuming that the debris exposure is vertical and that the view from the fixed station is perpendicular to the debris face. In this paper we (i) present a workflow to quantify debris thickness exposed above ice cliffs using measurements from scaled, high-resolution photographic DSMs, (ii) compare the derived debris thickness to available debris thickness collected by alternative means and (iii) discuss the utility and limitations of the new method of determining supraglacial debris thickness
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