Abstract
Abstract. Debris-covered glaciers in the Himalaya play an important role in the high-altitude water cycle. The thickness of the debris layer is a key control of the melt rate of those glaciers, yet little is known about the relative importance of the three potential sources of debris supply: the rockwalls, the glacier bed and the lateral moraines. In this study, we hypothesize that mass movement from the lateral moraines is a significant debris supply to debris-covered glaciers, in particular when the glacier is disconnected from the rockwall due to downwasting. To test this hypothesis, eight high-resolution and accurate digital elevation models from the lateral moraines of the debris-covered Lirung Glacier in Nepal are used. These are created using structure from motion (SfM), based on images captured using an unmanned aerial vehicle between May 2013 and April 2018. The analysis shows that mass transport results in an elevation change on the lateral moraines with an average rate of -0.31±0.26 m year−1 during this period, partly related to sub-moraine ice melt. There is a higher elevation change rate observed in the monsoon (-0.39±0.74 m year−1) than in the dry season (-0.23±0.68 m year−1). The lower debris aprons of the lateral moraines decrease in elevation at a faster rate during both seasons, probably due to the melt of ice below. The surface lowering rates of the upper gullied moraine, with no ice core below, translate into an annual increase in debris thickness of 0.08 m year−1 along a narrow margin of the glacier surface, with an observed absolute thickness of approximately 1 m, reducing melt rates of underlying glacier ice. Further research should focus on how large this negative feedback is in controlling melt and how debris is redistributed on the glacier surface.
Highlights
Glaciers cover approximately 110 000 km2 in high-mountain Asia (HMA) and as such constitute an important water storage of the region (Pfeffer et al, 2014; Immerzeel et al, 2010)
It is remarkable that the near-moraine glacier downwasting rate (−0.6 m year−1) is much lower than previously found downwasting rates over the entire glacier (< −1.3 and −2.18 m year−1; Nuimura et al, 2017; Immerzeel et al, 2014a). This indicates a possibly thicker debris cover near the moraines suppressing downwasting rates, which is in line with the lack of ponds and ice cliffs close to the moraine (Immerzeel et al, 2014a) and supports the debris supply rates presented in this paper. Considering these processes, our results indicate that lateral moraine mass transport can play an important role in debris supply to the margins of a downwasted glacier tongue with steep lateral moraines, where it offsets the downwasting of the ice with deposition of debris
A time series of 5 years of unmanned aerial vehicle (UAV) data is used to investigate the importance of lateral moraine mass transport to a debris-covered tongue and the following key conclusions are drawn:
Summary
Glaciers cover approximately 110 000 km in high-mountain Asia (HMA) and as such constitute an important water storage of the region (Pfeffer et al, 2014; Immerzeel et al, 2010). In contrast to transport from the rockwalls, which deposits material on specific locations, remobilization of lateral moraine debris can result in a much more spatially uniform debris supply to the glacier (Fig. 1Ac). On glaciers in the study area (Langtang Valley in Nepal), a supply rate of 0.4– 31 mm year−1 (Watanabe et al, 1998) was previously found These values are averages over a much longer time span (< 550 years versus < 247 years for studies in Norway and < 79 for the Alps; Curry et al, 2006) or include source areas beyond the moraines (Watanabe et al, 1998). We assess how important erosion from lateral moraines is in the formation of Himalayan debris-covered glaciers
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