Flow, Dynamics And Hydrology Notes
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Explain the mechanisms by which glaciers flow. In what ways do the different flow mechanisms lead to variations in glacier velocity at different timescales?
The movement of glaciers was first recognized in the 19th century by Franz Josef Hugi, who observed the motion of a boulder on a glacier in Switzerland (Benn &
Evans 1998). Today it is recognized that glaciers flow due to a variety of mechanisms and at a range of velocities. In general, glaciers will adopt a configuration in which the forces driving and resisting motion are suitably balanced to generate the appropriate velocity to balance the mass budget - hence glaciers tend toward equilibrium in geometry and velocity. However, flow velocities can often vary independently of mass balance at timescales from hours to decades. This essay will first give an overview of the mechanisms of glacier flow, and then explain how these may lead to variations in velocity at different timescales. The three main mechanisms of glacier flow are ice deformation, bed deformation and ice sliding. Ice deformation is the most basic and earliest-recognized of these. It occurs principally through the process of creep, a form of viscous/plastic deformation between ice crystals. The level of stress depends on the weight and slope of upstream ice. The amount of strain this causes is governed by Glen's Flow Law (ε=Aτn), where ε is the strain rate, A is a temperature-dependent constant, τ is stress and n is another constant usually of around 3. Creep is therefore enhanced by higher temperatures and increases exponentially with additional stress (leading to the plastic ability of ice to exhibit a sudden increase in deformation rates). Another form of ice deformation is the brittle failure of fracturing; this occurs principally in the form of lateral cavities like crevasses. However these are often superficial surface features (Benn & Evans 1998). As well as deforming in place, ice may slide over the bed of the glacier (ice sliding). This was the second mechanism of movement to be recognized and was described theoretically by Weertman (1957) and later Kamb (1970). It is controlled by factors including the level of adhesion to the bed (influenced by the glacier temperature and ice overburden stress, and often concentrated in 'sticky places'); the amount of friction presented by debris or till at the base; and the deceleration caused by bed roughness which is overcome by regelation or advanced creep over bumps (Benn & Evans 1998). However the biggest and fastest effect on ice sliding velocity comes from the presence and distribution of water at the bed. Water can accelerate ice sliding through a number of processes which will be examined later, principally through inhibiting basal friction. The third form of movement, bed deformation, is also influenced by glacier hydrology and closely related to ice sliding. Bed deformation consists of movement of the loose sediment (till) at the bottom of a glacier in response to shear stress. Till
susceptibility to deformation is dependent on the till lithology and size, which vary at long timescales. However an influential (and dangerous) study by Boulton &
Hindmarsh (1987), in which they tunnelled underneath a the Breidarmurkurjoküll Glacier in Iceland, showed how deformation was also enhanced by increases in water pressure which saturate the bed, lowering frictional resistance within the till and leading to deformation. Eventually the glacier may become 'decoupled' from the bed, reducing bed deformation, but this is then offset by increased ice sliding (Benn &
Evans 1998). The latter two mechanisms illustrate the key effect of hydrology on velocity variations. There are three main ways in which basal water contributes to velocity. The first (as mentioned above) is the effect on bed deformation. The second is through the contribution to sliding: water in a thin film at the base, or more significantly in linked cavities, reduces basal contact thus lubricating the bed against friction and submerging small bumps which would otherwise need to be overcome by regelation or enhanced ice sliding. The removal of glacial contact with the bed also concentrates shear stress on areas without water-filled cavities, contributing to sliding there and creating a possible feedback process by lowering the pressure melting point and creating more liquid water. Finally, there is the lesser effect of hydraulic 'jacking' where water exerts pressure on upstream-facing glacier slopes, contributing to downhill movement (Benn & Evans 1998). Accordingly Willis et. al. (1995) noted hydrology as the most important control on short-term fluctuations as well as periodic surges. Water may be introduced to the bed either by basal melting due to friction, or by drainage systems from the surface introducing rainwater or meltwater in the spring and summer (Fountain &
Walder 1998). Its effect is mediated by the form of the subglacial drainage system - whether distributed in till porewater or a thin basal film, a linked cavity system, or in a more hydrologically efficient channelized system (including Röthlisberger or Nye channels) with less effect on sliding and bed deformation (Fowler 1997). The former are common early in the melt season but tend to evolve into more efficient channelized networks over time (Willis 2008). As a result, increases in glacial velocity from water inputs are accentuated early in the melt season and may take the form of regular 'Spring Events' lasting several days, when melt water is introduced into underevolved drainage systems (Mair et. al. 2003). Rain events may cause sharp increases in input lasting as little as a few hours. The effect of inputs may be magnified significantly by positive feedbacks. At a larger timescale, hydrologically-influenced velocity increases may occur cyclically at intervals of several decades, and with episodes lasting several years. This is known as surging, and is a characteristic of particular glaciers which are concentrated in regional clusters including Svalbard, Iceland and North-Eastern North
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