On the morphological characteristics of overdeepenings in high‐mountain glacier beds

Abstract

Overdeepenings, i.e. closed topographic depressions with adverse slopes in the direction of flow, are characteristic for glacier beds and glacially sculpted landscapes. Quantitative information about their morphological characteristics, however, has so far hardly been available. The present study provides such information by combining the analysis of (a) numerous bed overdeepenings below still existing glaciers of the Swiss Alps and the Himalaya‐Karakoram region modelled with a robust shear stress approximation and (b) detailed bathymetries from recently exposed lakes in the Peruvian Andes. The investigated overdeepenings exist where glacier surface slopes are low (< 5°–10°), occur in bedrock or morainic material and are most commonly a fraction of a kilometre squared in surface area, hundreds of metres long, about half the length in width and tens of metres deep. They form under conditions of low to high basal shear stresses, at cirque, confluence, trunk valley and terminus positions. The most striking phenomenon, however, is the high variability of their geometries: Depths, surface areas, lengths and widths of the overdeepenings vary over orders of magnitude and are only weakly – if at all – interrelated. Inclinations of adverse slopes do not differ significantly from those of forward slopes and are in many cases higher than so far assumed theoretical limits for supercooling of ascending water and corresponding closure of sub‐glacial channels. Such steep adverse slopes are a robust observation and in support of recently developed new concepts concerning the question about where supercooling of sub‐glacial water and closure of ice channels can or must occur. However, the question of when and under what climatic, topographic and ice conditions the overdeepenings had formed remains unanswered. This open question constitutes a key problem concerning the interpretation of observed overdeepenings, the understanding of the involved glacio‐hydraulic processes and the possibility of realistic predictive modelling of overdeepening formation. Copyright © 2016 John Wiley & Sons, Ltd.

Compiling an Inventory of Glacier-Bed Overdeepenings and Potential New Lakes in De-Glaciating Areas of the Peruvian Andes: Approach, First Results, and Perspectives for Adaptation to Climate Change

“…It illustrates that the results of the here-applied procedures are robust with respect to the general existence/location of overdeepened bed parts and their approximate shape but may only provide a rough order-of-magnitude estimate on potential lake depths and volumes cf. [9]. This three-step procedure was chosen to enable the combination of independent information from visual inspection and numerical modelling.…”

“…It illustrates that the results of the here-applied procedures are robust with respect to the general existence/location of overdeepened bed parts and their approximate shape but may only provide a rough order-of-magnitude estimate on potential lake depths and volumes cf. [9]. An important aspect concerning lake safety and potential outburst mechanisms concerns the question of whether potential lakes will become dammed by bedrock or till/moraine would be blocking the potential lakes.…”

Multistage Rock‐Slope Failures Revealed in Lake Sediments in a Seismically Active Alpine Region (Lake Oeschinen, Switzerland)

“…Climate warming forces us to face a continuous environmental change in high-mountain landscape settings. The accelerating melting of glaciers (e.g., Paul et al, 2004;Schiefer et al, 2007) will form an increasing number of proglacial lakes (Haeberli et al, 2016). In combination with enhanced failures from recently deglaciated rock slopes (Leith et al, 2014), moraine walls, and proglacial sediments, this landscape change will lead to new hazardous situations (Huggel et al, 2013;Paul & Haeberli, 2008).…”

Assessment of evolution and risks of glacier lake outbursts in the Djungarskiy Alatau, Central Asia, using Landsat imagery and glacier bed topography modelling

“…The difference grid between the filled DEM and the initial DEM without glaciers resulted in a bathymetry raster of the overdeepenings. The following morphometric characteristics of overdeepenings were derived: area, maximum and mean depth, volume, maximum length, maximum width (perpendicular to the longest line), and elongation (width-to-length ratio) following Linsbauer et al (2016) and Haeberli et al (2016). Overdeepenings with individual areas larger than 11 000 m 2 , corresponding to the area of approximately two grid cells in the glacier bed topography, were considered to exclude potential model artefacts in line with previous studies (Linsbauer et al, 2012;Haeberli et al, 2016).…”