Basal Water and High-Pressure Basal Ice

Abstract

AbstractExpressions are found for the water pressure in channels and cavities under a temperate glacier in a steady state. By steady state is meant that not only is the sliding velocity constant but also the basal water flow as well as the basal water production. The calculations are based on a simplified bimodal obstacle bed. One-size obstacles are of the magnitude of the conventional “controlling” obstacles size. The other obstacle size is several orders of magnitude larger than the controlling size. The large obstacles are introduced in order to investigate the storage of large amounts of water at the bed. The effect of the missing obstacles is mimicked by adding a friction stress whose values decrease linearly with ice–rock separation. It is found that water flow is primarily in a water film if the water flux is below a critical limit. The physical reason for this result is that a minimum water flow must be exceeded in order to keep a water channel open in high ice-pressure regions in front of obstacles. The new ingredient in our analysis of water flow is the explicit account taken of the effect of excess ice pressure (at the up-stream side of obstacles) on the closing of water channels. The excess ice-pressure regions play a critical role in the development of water films. The recognition of the possible importance of high-pressure basal ice regions to the regulation of water flow is the primary result of this paper. It is found that, in steady state, the water pressure is close to the overburden ice pressure provided that water transported through Nye channels is only a minor part of the total. The water flows primarily in a water film. If a large flux of upper-surface melt water descends to the bed, this water could cause a reduction of the water pressure and a channelization of the water flow.