Assessing mass change and surface velocity of the debris-covered tongue of the Lirung Glacier, Nepal, over the winter period between October 2013 and May 2014 by means of Unmanned Airborne Vehicle imagery

Abstract

Himalayan glaciers often feature a mantle of supra-glacial debris which greatly influences their dynamics in terms of flow and mass balance. The glaciers are an important source of fresh water, and data on the response of these glaciers to climate change is very valuable (Brown et al., 2010). However in situ research is hampered by inaccessibility of the glaciers due to their remoteness and difficult circumstances for field work caused by the rugged debris covered terrain. Space born remote sensing methods cannot supplement the scarce field data due to generally coarse resolutions or otherwise high monetary costs (Vincent et al., 2013). This provides opportunities for the novel technique using images acquired by Unmanned Aerial Vehicles (UAVs). UAVs can obtain images at a high spatial and temporal resolution, and are therefore very well suited to fill the gap left by field work and space born remote sensing. For this thesis this new method has been applied the debris-covered tongue of the Lirung Glacier, located in the Nepalese Himalaya. Using UAV images acquired in May 2014, an orthorectified mosaic image and Digital Elevation Model (DEM) of part of the glacier tongue were constructed, by means of stereo-imagery and the Structure from Motion algorithm. During processing it became clear that the image dataset was not of optimal condition, but valuable information could still be extracted. Surface feature evolution, surface elevation change, surface velocity, melting and mass balance have been assessed for the winter monsoon period between October 2013 and May 2014. The ortho-mosaic and DEM have been compared to those of May 2013 and October 2013, created by Immerzeel et al. (2014). A difference DEM computed for the period between October 2013 and May 2014 indicated that surface elevation decrease and melt continues during winter, but less than during summer. Supra-glacial ice cliffs were visible as regions of enhanced melt, due to large backwasting rates. By means of manual feature tracking between the ortho-mosaics of May 2014 and October 2013 the surface velocity of the glacier was assessed. This showed homogeneous, slow velocities across mainly the middle and lower part of the glacier tongue. This in contrast to the summer period, which showed a more heterogeneous surface velocity field. The main finding of this research is that during the winter period the debris-covered tongue of the Lirung Glacier continuous downwasting, but far less change occurs than during the summer period. This is most likely due to the higher temperature and larger amounts of precipitation in the form of rain that occur during the summer monsoon period. It has been shown that UAVs can contribute greatly to the research concerning these type of glaciers, due to their high spatial and temporal scale possibilities. Future research, and obtaining a longer time data set thing can prove of great value to the research into the dynamics of debris-covered glaciers.