Arbeitsbericht NAB 12-48
Glacial OverdeepeningResults of a workshop held in Zürich, Switzerland, 20-21 April 2012
Glacially-overdeepened valleys and basins are common features in high mountain belts and their forelands. These overdeepenings are of practical and scientific interest, representing sites of potential lake formation, influencing the dynamics and hydrology of ice masses, presenting opportunities for glacial and climate reconstruction, and raising management issues related to aggregate, groundwater and hydrocarbon resources and radioactive waste disposal in deep geological repositories. Although the basic principles of glacial erosion are generally known, the formation of overdeepened valleys beneath glaciers and ice sheets remains incompletely understood.
A workshop organized by the National Cooperative for the Disposal of Radioactive Waste (Nagra) in collaboration with the University of Zürich and held in Zürich, Switzerland, in April 2012 was aimed at assessing the questions of why, how, where and when future glaciations can lead to glacial overdeepening in the foreland of the Alps. This is relevant for the siting and long-term safety of radioactive waste repositories in northern Switzerland. An international group of leading experts was invited to contribute to a discussion of the feedbacks associated with overdeepening among such factors as ice flow, ice mass geometry, bed topography, heat transport, basal thermal regime, subglacial drainage, sediment transfer and basal sliding.
The workshop featured presentations on field observations and measurements in overdeepenings beneath modern glaciers and on modelling studies as a means for developing an improved quantitative understanding of glacial erosion processes. During discussion periods, participants probed questions geared toward exploring the origin and evolution of glacial overdeepening, the role of subglacial drainage in overdeepening formation, and the significance and implications of glaciohydraulic supercooling.
While overdeepening of glacier beds is expected at confluences and in trunk valleys where ice flux and, hence, basal sliding and bedrock abrasion are large, observations of terminal overdeepening in areas of divergent ice flow highlight the limitations of the simple relationship of erosion to glacier basal motion commonly used in current glacial erosion models. Participants suggested that localized bedrock quarrying may play a significant role in glacial overdeepening, whereby the formation of crevasses above initial perturbations caused by bed irregularities focuses the delivery of surface melt and, hence, water pressure fluctuations, to the head of incipient overdeepenings.
According to participants, subglacial drainage along the adverse slope of an overdeepening is perhaps the most critical part of the water system, affecting not only the morphology and transmissivity of drainage in the overdeepened section but also the dynamics of ice flow and the mechanisms and patterns of sediment entrainment, transport, evacuation and deposition. Issues discussed during the workshop included the effect of bedrock lithology on subglacial hydrology and its influence on the efficiency of erosion and sediment evacuation, and the importance of overdeepening formation by the direct action of subglacial meltwater.
The general consensus among participants was that the depth of overdeepening is likely to depend on the extent to which processes and feedbacks are able to focus erosion and maintain the evacuation of water and sediment. However, many of the processes are difficult to observe in nature. Participants pointed to the need for detailed, quantitative measurements of water drainage in overdeepenings, accurate descriptions of the bedrock morphology of existing overdeepenings, systematic analyses of glaciohydraulic supercooling, and the development of numerical models that incorporate coupled ice-water-sediment-erosion processes.