Box 45 Links Between Subglacial Hydrology And Glacier Velocity

Many valley glaciers experience an early-melt-season high-velocity event (the so-called 'spring event'), where glacier velocities increase suddenly. Working on Haut Glacier d'Arolla, Switzerland, Mair et al. (2003) investigated the relationship between the spring event and the nature of the subglacial hydro-logical drainage system. They collected data concerning the spatial patterns of ice-surface velocity, internal ice deformation rates, the spatial extent of high subglacial water pressures and rates of subglacial sediment deformation around the time of three spring events. During two of the three events, their data suggest widespread ice-bed decoupling, particularly along the axis of a known subglacial drainage channel, which explains the observed increase in glacier velocity. The other event was marked by less extensive ice-bed decoupling and sliding along the drainage axis, suggesting subglacial sediment deformation may have been important. This link between subglacial hydrology and glacier velocity has also been established for the polythermal John Evans Glacier, Ellesmere Island, Canada (Copland et al., 2003). These spring events demonstrate that changes in subglacial hydrology are intimately linked to changes in glacier dynamics. The overall conclusion of these studies is that there is a close association between: (i) the timing and spatial distribution of the temporal pattern of surface water input to a glacier; (ii) the formation, seasonal evolution and distribution of subglacial drainage pathways; and (iii) horizontal and vertical glacier velocities. Consequently increases in glacier velocity can be directly attributed to changes in glacier hydrology.

Sources: Copland, L., Sharp, M.J. and Nienow, P.W. (2003). Links between short-term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier. Journal of Glaciology, 49, 337-48. Mair, D., Willis, I., Fischer, U.H., et al. (2003). Hydrological controls on patterns of surface, internal and basal motion during three 'spring events': Haut Glacier d'Arolla, Switzerland. Journal of Glaciology, 49, 555-67.

5. Autumn. Cessation of melting on the glacier causes a dramatic drop in discharge. The internal network of conduits and tunnels within the glacier begins to collapse as the water flowing within them declines and they close due to ice deformation. Only a few major drainage arteries contain sufficient flow to be maintained.

6. Winter. The degree to which the drainage network shuts down depends on the climate and severity of the winter. Most meltwater discharge if there is any will be derived from internal melting.

The degree to which the internal drainage network of conduits and tunnels within a glacier collapses each year is probably highly variable. Some shut down will occur in all cases, but in large ice sheets the major discharge arteries are likely to be maintained because the rate of flow, due to melt generated by internal deformation and geothermal heat, is likely to be much larger. The smaller and thinner the glacier, the more likely it is that the drainage system evolves each year.

On some glaciers this pattern of diurnal and seasonal discharge fluctuation are interrupted by catastrophic subglacial floods known as jokulhlaups. These are high-magnitude events, often several orders of magnitude greater than normal peak flows. They are distinct from glacial lake outburst floods (GLOFs; see

Box 2.3) in that they are more restricted in scope being wholly ice derived, however in practice the distinction is rather artificial. Jokulhlaups may occur in one of two ways: (i) through subglacial volcanic activity; and (ii) through the drainage of ice-dammed lakes. Volcanic activity beneath glaciers is common today in Iceland. A spectacular example is Grimsvotn, beneath the Vatnajokull ice cap. Here water builds up subglacially above a volcano, which then drains catastrophically in the space of a few hours, through a 50 km long subglacial tunnel. The average volume of water discharged from Grimsvotn is between 3 and 3.5 km3. The volcano has erupted, causing major jokulhlaups, in 1960,1965,1972, 1976,1982,1983,1986,1991,1996,1998 and 2004. During the last major eruption in 1996, the jokulhlaup discharged approximately 3 km3 of water with a peak flow rate of 45 000 m3 s-1. This is more than the discharge of the Mississippi River.

Jokulhlaups due to the sudden drainage of ice-dammed lakes are much more widespread. Ice-dammed lakes occur wherever a glacier blocks the down-valley flow of water (Figure 4.11). When it occurs, the outburst from such lakes is typically catastrophic in nature. Lakes can drain catastrophically for a number of reasons: (i) vertical release of the ice dam by flotation caused by rising water levels in the lake; (ii) overflowing of an ice dam, causing rapid melting of the dam due to friction from the water flow; (iii) destruction or fissuring of an ice dam by earthquakes; and (iv) enlargement of pre-existing tunnels beneath the dam by increased water flow and melt-enlargement due to frictional heating. In many situations the maximum water

Figure 4.11 Photograph of a partially drained ice-dammed lake in South West Greenland. Note the stranded icebergs that indicate the lake level has dropped during a drainage event.

[Photograph: N. F. Glasser]

Figure 4.11 Photograph of a partially drained ice-dammed lake in South West Greenland. Note the stranded icebergs that indicate the lake level has dropped during a drainage event.

[Photograph: N. F. Glasser]

level in a lake is limited by a spillway or channel via which the lake can drain. In these cases some other mechanism is required to trigger catastrophic lake drainage.

The shape of thejokulhlaup hydrograph will be determined by the nature of the trigger and the volume of water that is drained (Figure 4.12). Jokulhlaups induced by volcanic activity tend to produce high-magnitude, short-duration floods, whereas jokulhlaups caused by the drainage of ice-dammed lakes may produce floods with a greater duration and lower magnitude (Figure 4.12). As we might expect, the size of the flood peak from a jokulhlaup caused by the drainage of ice-dammed lakes is proportional to the volume of water within the lake (Box 4.6). The high-magnitude nature of jokulhlaups makes them of considerable geomor-phological significance (see Section 8.2).

TYPE I 'Normal' glacial discharge sequence

A: Water discharge (Q)

TYPE II Sudden drainage from ice-dammed lake a

TYPE III Jokulhlaup triggered by volcanic eruption a

B: Sediment discharge (Qs)

Time (days)

Time (days) Flood hydrographs a

Time (days)

Time (days)

Figure 4.12 Flood hydrographs illustrating the different types of flow recorded in a 'normal' glacial river, during drainage of an ice-dammed lake and during a jokulhlaup. [Modified from: Maizels and Russell (1992) Quaternary Proceedings, 2, figure 7, p. 142]

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