Glacier surge mechanism based on linked cavity configuration of the basal water conduit system

Abstract

Based on observations of the 1982-1983 surge of Variegated Glacier, Alaska, a model of the surge mechanism is developed in terms of a transition from the normal tunnel configuration of the basal water conduit system to a linked cavity configuration that tends to restrict the flow of water, resulting in increased basal water pressures that cause rapid basal sliding. The linked cavity system consists of basal cavities formed by ice-bedrock separation (cavitation), ~1 m high and ~10 m in horizontal dimensions, widely scattered over the glacier bed, and hydraulically linked by narrow connections where separation is minimal (separation gap •< 0.1 m). The narrow connections, called orifices, control the water flow through the conduit system; by throttling the flow through the large cavities, the orifices keep the water flux transmitted by the basal water system at normal levels even though the total cavity cross-sectional area (-200 m2) is much larger than that of a tunnel system (-10 m2). A physical model of the linked cavity system is formulated in terms of the dimensions of the "typical" cavity and orifice and the numbers of these across the glacier width. The model concentrates on the detailed configuration of the typical orifice and its response to basal water pressure and basal sliding, which determines the water flux carried by the system under given conditions. Configurations are worked out for two idealized orifice types, step orifices that form in the lee of downglacier-facing bedrock steps, and wave orifices that form on the lee slopes of quasisinusoidal bedrock waves and are similar to transverse "N channels." The orifice configurations are obtained from the results of solutions of the basal-sliding-with-separation problem for an ice mass constituting a near half-space of linear rheology, with nonlinearity introduced by making the viscosity stress-dependent on an intuitive basis. Modification of the orifice shapes by melting of the ice roof due to viscous heat dissipation in the flow of water through the orifices is treated in detail under the assumption of local heat transfer, which guarantees that the heating effects are not underestimated. This treatment brings to light a meltingstability parameter E that provides a measure of the influence of viscous heating on orifice cavitation, similar but distinct for step and wave orifices. Orifice shapes and the amounts of roof meltback are determined by E. When E •> 1, so that the system is "viscous-heating-dominated," the orifices are unstable against rapid growth in response to a modest increase in water pressure or in orifice size over their steady state values. This growth instability is somewhat similar to the j6kulhlaup-type instability of tunnels, which are likewise heating-dominated. When E <• 1, the orifices are stable against perturbations of modest to even large size. Stabilization is promoted by high sliding velocity v, expressed in terms of a v 4/2 and v-• dependence of E for step and wave cavities. The relationships between basal water pressure and...

Surge dynamics in the Nathorstbreen glacier system, Svalbard

“…In previous studies the existence of turbid water has been related to a collapse of a linked-cavity drainage system (Kamb, 1987) and hence an abrupt end of the surge (Kamb et al, 1985). By 2012 the surge had been active for at least 10 years before flow rates decreased substantially and NGS reached shallow sea bed.…”

“…The effect of the frictional heat will be most important when sliding and basal drag are intermediate (Raymond, 2000). A blockage of the subglacial drainage system and change to a distributed drainage system (Robin and Weertman, 1973;Raymond and Harrison, 1985;Kamb, 1987;Eisen et al, 2005) caused by compression between the activated and not-yet-activated ice masses enables the growth of the surge nucleus. Superimposed on this are the seasonal variations where the summer drainage system contracts in autumn and traps water which is eventually released (e.g.…”

“…(Meier and Post, 1969;Dolgoushin and Osipova, 1975;Clarke et al, 1984;Cuffey and Patterson, 2010). Based on studies from the temperate Variegated Glacier, Alaska, a hydraulic mechanism was proposed where surges are explained by switches between an efficient tunnel-based basal drainage system and slow linked-cavity system (Kamb et al, 1985;Kamb, 1987). Some surges display an abrupt speed-up and slow-down (days to weeks), while other glaciers go through slower (years-long) accelerations and decelerations.…”

“…Differences in observed surge characteristics and dynamics have also led to a suggestion of at least two separate surge mechanisms between polythermal and temperate glaciers (Murray et al, 2003a). Surges in polythermal glaciers were explained by changes in basal thermal regime that are controlled by a thermal mechanism (Murray et al, 2000;Fowler et al, 2001), while the temperate surges were explained by a hydraulic mechanism (Kamb, 1987). Detailed spatial measurements of surge onset are rare due to the difficulty in predicting them.…”

Spatially distributed runoff at the grounding line of a large Greenlandic tidewater glacier inferred from plume modelling

“…Fowler, 1987;Kamb, 1987;Schoof, 2010) for the prevalence of this style of drainage when ice slides rapidly over its bed. Such work was initially motivated by the study of surging glaciers, but is equally applicable to the fast flowing termini of calving glaciers.…”