Types of subglacial drainage system

Heretofore in our discussion of subglacial drainage, we have been dealing, implicitly if not explicitly, with systems composed of relatively straight channels cut upward into the ice and resting on hard beds. Such channels are commonly calledRothlisberger channels, or simply R channels. Nye (1973b) suggested, alternatively, that channels might be incised into the bed of a glacier, and such channels, frequently called Nye or N channels, have been described (Walder and Hallet, 1979; Hallet and Anderson, 1980). They are typically0.1-0.2mdeepand0.2-0.5mwide, although some reach widths and depths of several meters. Nye channels are not common, perhaps because changes in ice surface profile, movement of the ice, and melting of the conduit walls can all displace the flow laterally, so streams do not stay in one place long enough.

Two other types of drainage system on hard beds have also been suggested: the linked-cavity system and the multi-branched arborescent system. In addition, there are drainage systems on soft beds. These are described in the next three sections.

The linked-cavity system

In some experiments on Variegated Glacier, Alaska, it was found that despite a water discharge, Q, of 5 m3 s—1, dye moved through the subglacial drainage system with a speed, v, of only 0.025 m s—1. Because Q = vA, where A is the cross-sectional area of the conduit, A must have been ~200 m2. If the flow were in a single conduit, this would present a problem because, for any reasonable conduit roughness, n', Equation (8.13) would then predict velocities that were one to two orders of magnitude higher than those observed.

Kamb (1987) suggested that the flow, rather than being in a single conduit, was in a network of linked cavities (Figure 8.14a). The cavities are believed to form in the lee of steps in the bed (Figure 8.14b), and indeed precipitates and the lack of striations in such locations on deglaciated bedrock surfaces argue strongly for their existence. The cavities are linked together by orifices that are much smaller in cross-sectional area than the cavities (Figure 8.14b). The cavities provide the large A required, and the orifices throttle the flow, reducing the velocity.

A cavity or orifice formed in the lee of a step is shown in Figure 8.15. Panel (a) shows the geometry under certain basal-water-pressure and sliding-velocity conditions. Panel (b) illustrates the geometry when heat released by viscous dissipation in the flowing water enlarges the orifice by melting its roof. Note that the cavity or orifice becomes both longer and more arched in this case. In the case of an orifice, Kamb assumes that all of the heat is used to melt ice in the orifice in which the heat is produced. As some of the heat will be advected into the next cavity, this will overestimate the orifice size. He also assumes that deformation of ice can be represented by a Newtonian flow law (n = 1)

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