Glacier hydrology

Meltwater is an important component in glacier systems, and glacial meltwater exerts a strong influence on the hydrology of proglacial areas. Within and underneath glaciers and ice sheets, water affects glacier behaviour, controlling rates of glacier flow and influencing processes and rates of erosion and deposition. Surface and basal melting of glacier ice can produce large volumes of meltwater. In regions with low summer precipitation, but with glaciers in their catchment, glacial meltwater is an important source of water in the plant growing season, allowing cultivation of fields otherwise too arid for agriculture. Catastrophic floods from glaciers are, however, a recurrent threat in the Himalayas and in Iceland. In Scandinavia and in the European Alps, glacier meltwater has also been used for hydroelectric power production. The movement of glaciers and ice sheets is also strongly influenced by pressure and distribution of meltwater.

Water in a glacier drainage system may originate from melting of ice and snow, rainfall, runoff, or sudden release of stored water

(Fig. 4.47). The runoff from glacier catchments varies significantly both annually and seasonally. Surface melting increases with rising air temperature, solar radiation, and rainfall when air temperatures are relatively high. Englacial and subglacial melting can also take place as a result of frictional heat caused by deforming and/or sliding ice, geothermal heat, and pressure melting when the glacier moves over topographic irregularities.

Water drainage through a glacier is influenced by the permeability of the ice. In the sense of glacier hydrology, we talk about primary permeability (permeability of intact ice and snow) and secondary permeability (related to the size and distribution of tunnels and crevasses). Primary permeability is commonly high for snow and firn where the air spaces between the snow crystals are linked. In solid ice, on the other hand, the primary permeability is low, because the air bubbles are more or less isolated from each other. However, in ice at the pressure melting point, water can penetrate through systems of interconnected lenses and veins between the ice crystals (Benn and Evans, 1998, and references therein). Below the pressure melting point, intact ice is impermeable. Most of the water draining through a glacier is related to secondary permeability, the water flowing through a system of conduits of varying length and diameter. Englacial conduits in temperate glaciers are formed and maintained by melting from circulating air and flowing/standing water. Polar glaciers do not normally have englacial conduit systems, so most of the surface water drains supraglacially. Crevasses may, however, take meltwater down to the basal ice layers which can be at the pressure melting point.

The water flow is determined by variations in the hydraulic potential, which is a measure of the available energy at a particular time and place. For water flowing on the surface, the hydraulic potential depends on the elevation only, and the water flows downslope. The flow of englacial and subglacial meltwater is, however, more complicated because the hydraulic potential depends on altitudinal differences and water pressure. The hydraulic

STORAGE

STORAGE

LIQUID Groundwater, lakes and cavities

LIQUID Groundwater, lakes and cavities

GLACIAL DRAINAGE

Superglacial

Engiacial

Subglacial

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t t

SUB AERIAL RUNOFF

Figure 4.47 Water sources and pathways in glaciated catchments. (Adapted from Benn and Evans, 1998)

potential in an englacial or subglacial conduit is dependent on the shape and size of the conduit, elevation and water pressure. The water pressure may vary between atmospheric pressure (pressure of the open air) and cryostatic pressure (pressure of the weight of the overlying ice). The difference between the water pressure and the ice pressure is termed the effective pressure, an important property for sub- and englacial drainage.

The flow direction in an englacial and subglacial drainage network is mainly controlled by variations in the hydraulic potential; the water follows the hydraulic gradient, flowing from areas of high potential towards areas of low potential. The gradient depends on the surface slope, and to a minor extent, the slope of the water-filled conduit. The equipotential surfaces (planes connecting points with the same potential) therefore rise downglacier with a gradient of about ten times that of the ice surface. Water draining freely through a glacier will flow at right angles to the equipotential surfaces.

Hodson et al. (1997) compared estimates of suspended-sediment yield and discharge from two glacier basins in Svalbard. Austre Brogger-breen (12 km2) is almost entirely cold-based, whereas Finsterwalderbreen (44 km2) is dominated by basal ice at the pressure melting point. Specific suspended-sediment yields from Finsterwalderbreen of 710-29001 km-2 a 1 were

^ Equipotential lines Streamlines

Figure 4.48 Subglacial water body formation, (a) A deep depression beneath an ice dome; (b) a shallow depression beneath an ice dome; (c) a subglacial cupola formed beneath a depression on the ice surface. (Adapted from Nye, 1976)

^ Equipotential lines Streamlines

Figure 4.48 Subglacial water body formation, (a) A deep depression beneath an ice dome; (b) a shallow depression beneath an ice dome; (c) a subglacial cupola formed beneath a depression on the ice surface. (Adapted from Nye, 1976)

more than an order of magnitude larger than at Austre Broggerbreen (81-1101 km a-1). The difference is explained by the influence of the thermal regime of the meltwater drainage system and the sources of suspended sediments.

If water flow is prevented, water may be stored in subglacial, supraglacial, englacial or ice-dammed lakes. Such water bodies are commonly temporary, expanding and contracting in response to glacier fluctuations, glacier dynamics or volcanic activity. Subglacial water bodies vary significantly in size; the largest modern reservoirs known from radio echo-sounding under the Antarctic ice sheet are up to 8000 km2 in area. In areas with low hydrological potential surrounded by areas where the hydrological potential is high, water may accumulate in subglacial ponds (Ridley et al., 1993). Hydrological gradients will in this case cause the water to drain towards the water reservoir. Figure 4.48 shows situations where subglacial ponding can occur. In case (a), water ponds within a bed depression beneath an ice dispersal centre. When the bedrock depression is shallower than the shape of the equipotential contours (b), ponding will normally not occur. In the case of (c), the glacier bed is nearly horizontal and there is a central ice depression. In this case, the hydrological potential beneath the depression is lower than the surrounding areas. Water will commonly flow towards the bed beneath the depression and form a dome-shaped upper surface along an equipotential surface.

An effective mechanism for the production of subglacial lakes is the melting caused by subglacial volcanic activity. The western part of Vatnajokull in Iceland is located above an active part of the mid-Atlantic spreading ridge. Glacier ice resting on the Grimsvotn caldera melts and form a supraglacial depression. The lake system at Grimsvotn empties about every six years, in most cases catastro-phically. Such an event normally involves the release of up to 4.5 km3 of water, with a maximum discharge of ca. 50,000 m3 s-1. A large subglacial eruption in the Grimsvdtn caldera in the autumn of 1996 caused an enormous jokulhlaup. The Icelandic term jokulhlaup is used for periodic or occasional release of large amounts of stored water in catastrophic floods. Jokulhlaups may be caused by sudden drainage of ice-dammed lakes, overflow of lake water, or the growth and collapse of subglacial reservoirs. The best-documented jokulhlaups are reported from Iceland. During these flooding events, the large sandur plains of Skeiderarsandur and Myrdalssandur are totally flooded.

Where glaciers or ice sheets form a barrier to meltwater drainage, water will be stored to form an ice-dammed lake. At valley glaciers, ice-dammed lakes may form in ice-free tributary valleys blocked by the glacier in the main valley. In other cases, tributary valley glaciers may block water drainage. A third case may be that ice-dammed lakes form at the junction between two valley glaciers. One of the largest ice-dammed lakes was Lake Agassiz (2 million km2), which formed during the d├ęglaciation of the Wisconsin event (e.g. Teller, 1995). In Siberia, northwards draining rivers were dammed by the Eurasian ice sheet(s), forming huge ice-dammed lakes.

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