Quantifying the morphological evolution and interaction of ice cliffs and supraglacial stream incision on debris-covered glaciers using high-resolution terrestrial lidar and UAV methods

Ice cliffs are known to enhance ablation on debris-covered glaciers and surface ablation. The upstream part of debris-covered glacier tongues is often characterised by downstream-widening supraglacial valleys with hummocky topography, arch-shaped ice cliffs alongside incised and meandering supraglacial channels. The incision of supraglacial channels has been suggested as a potentially important process for the formation of ice cliffs; however, the interactions between channel undercutting and ice-cliff formation are poorly understood and remain to be quantified. In particular, the stream undercutting cannot be observed from nadir-based satellite or UAV methods. In this study, we therefore use a more local approach to investigate these interactions by applying high-resolution terrestrial remote-sensing methods at the example of two debris-covered glaciers: Satopanth Glacier, located in the Indian Himalayas, and Zmutt Glacier in the European Alps. We combined (i) high-density point cloud data from a terrestrial laser scanner, (ii) drone imagery, (iii) time-lapse imagery, and in situ stake measurements of the channel overhangs and the debris and ice-cliff surfaces at daily and fortnightly intervals during the melting season. By differencing the point clouds and DEMs using a Lagragian reference system, we are able to calculate channel incision and melt rates alongside ice-cliff backwasting rates. We further constrain the evolution of these surfaces with the stake measurements and continuous time-lapse imagery of 30 (Zmutt) and 5 (Satopanth) minute intervals. Our results show that our approach, particularly the acquisition of point cloud data using terrestrial laser-scanning, offers promising perspectives for analysing channel incision and related ice-cliff backwasting. The dominating processes observed for the evolution of the surface morphology are the backwasting of the exposed ice cliff, the erosion of the stream in the undercut below, and the ablation of the debris-covered surface, which are exposed to a range of external factors (e.g., meltwater flow, air temperature, solar radiation, deposition, and debris thickness). We find that the sideway component of the channel incision usually exceeds the downward component and creates, depending on the size of the stream, undercuts of several 10s of cm (Zmutt) to several meters (Satopanth) in width. The related horizontal undercutting rates are generally comparable or more significant than ice-cliff backwasting and sub-debris ablation. However, we note that the incision and ice cliff morphology varies according to their location and orientation along the meandering meltwater stream. For deeply undercut ice overhangs, we are able to detect downward deformation that occasionally leads to a collapse of the ice cliff above and may thereby indirectly further enhance ice cliff backwasting.  Our results imply that stream incision is the driving process of undercutting and maintaining the ice cliffs, hence a crucial process for their formation and evolution. The integrated use of high-resolution field-based remote-sensing methods thereby contributed successfully towards a better understanding of the morphological evolution of surfaces with relatively thin debris and the related characteristic supraglacial valleys.