If the subglacial water pressure rises, effective normal pressure will fall, reducing basal friction, and consequently increasing the rate of basal sliding. Effective normal pressure determines the friction between a glacier and its bed. Subglacial water pressure is also important in determining the rate of basal sliding. Variations in subglacial water pressure and its influence on effective normal pressure and cavity formation are very important for the processes of glacial erosion. As the subglacial channel network develops during the ablation season discharge becomes more efficient and the subglacial water pressure generally falls. Early in the melt season, water pressure may be very high due to the abundance of meltwater and the relative inefficiency of the channel network ( Mair et al., 2003). Variations in the rate of water supply and the rate of meltwater discharge are responsible for much of the seasonal variation in water pressure present at some glaciers. The nature of the underlying geology: permeable bedrock will allow water to drain through it and therefore reduce subglacial water pressure. The rate of meltwater discharge: an efficient subglacial drainage system will reduce subglacial water pressure. The rate of water supply: inputs of large amounts of meltwater may increase the subglacial water pressure.
Glacier thickness: the greater the weight of the overlying ice, the greater is the subglacial water pressure. Subglacial water pressure is controlled by four variables: 1.
Theoretical calculations show that cavities can open at sliding velocities of about 9 m per year beneath a thickness of 100 m of ice, whereas velocities of 35 m per year are required with ice thickness on the order of 400 m. Cavity formation is favored by high subglacial water pressures, which reduce effective normal pressure, and by high rates of basal sliding, which give large pressure fluctuations over obstacles. The negative pressure fluctuation on the down-stream side of an obstacle may cause a cavity to form in the lee of obstacle if it exceeds the effective normal pressure at this point ( Figure 8). The pressure fluctuation caused by the flow of ice against the obstacle is, therefore, positive on the up-stream side and negative on the down-stream side. Effective normal pressure is also reduced in the lee or on the down-stream side of the obstacle ( Figure 7). As ice flows against the up-stream side on an obstacle, the effective normal pressure increases by an amount proportional to the rate of glacier flow against the obstacle. This holds only where the glacier has a flat bed but in reality effective normal pressure is modified by the flow of ice over obstacles ( Figure 7).
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