Abstract
ABSTRACTMeasurements of glacier ice cliff evolution are sparse, but where they do exist, they indicate that such areas of exposed ice contribute a disproportionate amount of melt to the glacier ablation budget. We used Structure from Motion photogrammetry with Multi-View Stereo to derive 3-D point clouds for nine ice cliffs on Khumbu Glacier, Nepal (in November 2015, May 2016 and October 2016). By differencing these clouds, we could quantify the magnitude, seasonality and spatial variability of ice cliff retreat. Mean retreat rates of 0.30–1.49 cm d−1 were observed during the winter interval (November 2015–May 2016) and 0.74–5.18 cm d−1 were observed during the summer (May 2016–October 2016). Four ice cliffs, which all featured supraglacial ponds, persisted over the full study period. In contrast, ice cliffs without a pond or with a steep back-slope degraded over the same period. The rate of thermo-erosional undercutting was over double that of subaerial retreat. Overall, 3-D topographic differencing allowed an improved process-based understanding of cliff evolution and cliff-pond coupling, which will become increasingly important for monitoring and modelling the evolution of thinning debris-covered glaciers.
Highlights
In the coming decades, ongoing mass loss from Himalayan glaciers and changing runoff trends will affect the water resources of over a billion people, including those who require it for agricultural, energy production and domestic usage (Immerzeel and others, 2009, 2010; Lutz and others, 2014; Mukherji and others, 2015; Shea and Immerzeel, 2016)
Raster-based DEMs generally give a poor representation of steep slopes or steeply-sloping topography (Kolecka, 2012) and their differencing incorporates a mixed signal containing surface elevation change related to debris cover, ice cliff dynamics, supraglacial ponds and glacier emergence velocity (Vincent and others, 2016)
Maximum cliff height reduced for all cliffs over the study period, this change was generally small for those cliffs that persisted through the study
Summary
In the coming decades, ongoing mass loss from Himalayan glaciers and changing runoff trends will affect the water resources of over a billion people, including those who require it for agricultural, energy production and domestic usage (Immerzeel and others, 2009, 2010; Lutz and others, 2014; Mukherji and others, 2015; Shea and Immerzeel, 2016). Debris-covered glaciers have a hummocky, pitted surface, caused by variable melt rates under different debris thicknesses, and include extensive coverage of ice cliffs and supraglacial ponds (Hambrey and others, 2008; Thompson and others, 2016; Watson and others, 2016, 2017). Studies using DEM differencing to quantify elevation change over debris-covered tongues have revealed an association between glacier surface lowering and the presence of ice cliffs and supraglacial ponds (Immerzeel and others, 2014; Pellicciotti and others, 2015; Ragettli and others, 2016; Thompson and others, 2016), confirming historical ice cliff observations Raster-based DEMs generally give a poor representation of steep slopes or steeply-sloping topography (Kolecka, 2012) and their differencing incorporates a mixed signal containing surface elevation change related to debris cover, ice cliff dynamics, supraglacial ponds and glacier emergence velocity (Vincent and others, 2016)
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