Abstract
Abstract. Shallow ground-penetrating radar (GPR) surveys are used to characterize the small-scale spatial variability of supraglacial debris thickness on a Himalayan glacier. Debris thickness varies widely over short spatial scales. Comparison across sites and glaciers suggests that the skewness and kurtosis of the debris thickness frequency distribution decrease with increasing mean debris thickness, and we hypothesize that this is related to the degree of gravitational reworking the debris cover has undergone and is therefore a proxy for the maturity of surface debris covers. In the cases tested here, using a single mean debris thickness value instead of accounting for the observed small-scale debris thickness variability underestimates modelled midsummer sub-debris ablation rates by 11 %–30 %. While no simple relationship is found between measured debris thickness and morphometric terrain parameters, analysis of the GPR data in conjunction with high-resolution terrain models provides some insight into the processes of debris gravitational reworking. Periodic sliding failure of the debris, rather than progressive mass diffusion, appears to be the main process redistributing supraglacial debris. The incidence of sliding is controlled by slope, aspect, upstream catchment area and debris thickness via their impacts on predisposition to slope failure and meltwater availability at the debris–ice interface. Slope stability modelling suggests that the percentage of the debris-covered glacier surface area subject to debris instability can be considerable at glacier scale, indicating that up to 32 % of the debris-covered area is susceptible to developing ablation hotspots associated with patches of thinner debris.
Highlights
Debris-covered glaciers are the dominant form of glaciation in the Himalaya (e.g. Kraaijenbrink et al, 2017) and are common in other tectonically active mountain ranges worldwide (Benn et al, 2003)
The location of the ground-penetrating radar (GPR) system was recorded simultaneously at 1 s intervals by a lowprecision GPS integrated with the IDS, which assigns a GPS location and time directly to every twelfth GPR trace and by a more accurate differential GPS system consisting of a Trimble XH and Tornado antenna mounted on the GPR and a local base station of a Trimble Geo7X and Zephyr antenna
The debris thickness data presented here suggest that the local debris thickness variability may show characteristic changes in skewness and kurtosis associated with progressive thickening and/or reworking of debris cover over time
Summary
Debris-covered glaciers are the dominant form of glaciation in the Himalaya (e.g. Kraaijenbrink et al, 2017) and are common in other tectonically active mountain ranges worldwide (Benn et al, 2003). Prevailing weather conditions and local debris properties, such as albedo, lithology, texture and moisture content, influence the amount of energy available for sub-debris ablation and modify the exact relationship between debris thickness and ablation rate, but the general characteristics of the so-called Østrem curve are robust, further demonstrating the dominant role of debris thickness in this relationship (Fig. 1). Both theory and observations indicate that the spatial variability of supraglacial debris thickness typically has both a systematic and a non-systematic component. To show how debris thickness varies with topography, radargrams were topographically corrected for display purposes after the ice interface had been picked
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