ALBERT

Description: Debris-covered glacier tongues 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 tongues, yet little is known about the relative importance of the three potential sources of debris supply to those glaciers: the headwalls, the glacier bed or the lateral moraines. In this study we hypothesize that erosion from the lateral moraines is a significant debris supply to the debris-covered tongues, in particular when the tongue is disconnected from the headwall due to glacier downwasting. To test this hypothesis eight high-resolution and highly accurate digital elevation models for the lateral moraines of the debris-covered Lirung glacier in Nepal derived from an unmanned aerial vehicle between May 2013 and April 2018 are used. The analysis shows that the lateral moraines erode at an average rate of 0.31±0.26myr−1 during this period driven by different erosion processes. There is also a higher erosion rate observed in the monsoon season (0.42myr−1) than in the dry season (0.25myr−1). In addition the loose lower parts of the lateral moraines erode at a faster rate during both seasons. These erosion rates translate into an annual increase in debris thickness ranging from 0.17myr−1 when the eroded material is distributed over the entire glacier to 0.29myr−1 in case the material is deposited in a narrow runout zone. It is concluded that the lateral moraines provide an important source of surface debris for glaciers in an advanced state of mass loss, and the source needs to be incorporated into models of glacier evolution. Further research should focus on how large this negative feedback is in controlling the melt of the tongues and a better understanding of the redistribution of debris on the tongue is therefore required.