Subglacial hydrology is investigated for an ice sheet where the substrate consists of a deformable aquifer resting on an aquitard. If sliding velocities are low or absent, subglacial melt-water drainage is dominated by drainage through the aquifer to water channels. Drainage along the bed is negligible. Efficient melt-water drainage requires that a system of subglacial water channels exists; otherwise, pore-water pressures will exceed the overburden pressure. In general, aquifer deformation near (away from) the terminus is most likely to occur during the winter (summer). The effect of short-term high channel pressures is, in general, not critical to aquifer deformation because the pressure pulse does not propagate far into the aquifer. (For aquifers of high permeability, short periods of high channel pressures constitute the most critical condition.) Aquifer deformation at the terminus is very likely to occur if the terminus ice slope exceeds tanϕ, whereϕis the Coulomb friction angle of the aquifer material. Upwelling of basal melt water near the terminus will normally cause soil dilation if the aquifer has a low permeability (e.g. till). Maximal profiles are computed corresponding to various aquifer materials using channel spacings which provide efficient drainage. (A maximal profile is the highest ice profile which the aquifer can sustain without deformation.) In general, maximal profiles lie well above observed profiles (such ash(x) = 3x1/2(m)) except near the terminus. However, if channel spacings are sufficiently large, pore-water pressures are increased and maximal profiles can lie well belowh(x) = 3x1/2.
The role of subglacial water storage beneath continental ice sheets is investigated, primarily for a deformable bed. Subglacial ponding is shown to occur under most regions of warm-based ice sheets, and large subglacial lakes can become established, for example in the Hudson Bay basin. The formation of large lakes depends upon the fact that the ice-surface gradient is reduced once subglacial ponding occurs and upon the feedback between the reduced ice-surface gradient and increased subglacial ponding. Subglacial ponding likely played a large role in determining the ice-sheet topography during late deglaciation and in speeding up the deglaciation process.
Field evidence and a theoretical model support the concept that during Wisconsinan glaciation subglacial water sheet outburst floods issued from a large subglacial lake located in the Hudson Bay basin. The lake was fed by supraglacier meltwater that was trapped in a depressed ice lid over the lake. Water may have also fed the lake by reversed outburst floods from proglacial lakes, particularly after 9000 BP, when a very low ice elevation over Hudson Bay is calculated. Deglaciation was accelerated by surges associated with the lift-off of ice by the sheet floods; ice lobe extensions the order of 100 km are possible. The model supports the concept of a multidomed Laurentide Ice Sheet in the form of an annular dome around Hudson Bay.
The likely genesis of low-relief ice-sheet lobes is episodic subglacial floods which occur in the form of water sheets hundreds of kilometers wide. During the few weeks prior to termination, when the flood discharge is large, the average thickness of a water sheet beneath the Lake Michigan lobe likely exceeded 1 m and reached thicknesses in excess of 6 m. The release of a part of the basal shear stress during this period produced an elongation of the lobe of tens of kilometers.
Subglacial hydrology is investigated for an ice sheet where the substrate consists of a deformable aquifer resting on an aquitard. If sliding velocities are low or absent, subglacial melt-water drainage is dominated by drainage through the aquifer to water channels. Drainage along the bed is negligible. Efficient melt-water drainage requires that a system of subglacial water channels exists; otherwise, pore-water pressures will exceed the overburden pressure. In general, aquifer deformation near (away from) the terminus is most likely to occur during the winter (summer). The effect of short-term high channel pressures is, in general, not critical to aquifer deformation because the pressure pulse does not propagate far into the aquifer. (For aquifers of high permeability, short periods of high channel pressures constitute the most critical condition.) Aquifer deformation at the terminus is very likely to occur if the terminus ice slope exceeds tan ϕ, where ϕ is the Coulomb friction angle of the aquifer material. Upwelling of basal melt water near the terminus will normally cause soil dilation if the aquifer has a low permeability (e.g. till). Maximal profiles are computed corresponding to various aquifer materials using channel spacings which provide efficient drainage. (A maximal profile is the highest ice profile which the aquifer can sustain without deformation.) In general, maximal profiles lie well above observed profiles (such as h(x) = 3x1/2 (m)) except near the terminus. However, if channel spacings are sufficiently large, pore-water pressures are increased and maximal profiles can lie well below h(x) = 3x1/2.
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