4.8.1 Mechanical Erosion
2. Flow properties. The rate of fluvial abrasion increases with the flow velocity. Similarly, the more turbulent the water flow the greater the rate of abrasion because sediment particles are brought into contact with the bed and channel walls more frequently than when the level of turbulence is low.
3. Properties of the channel. The roughness and orientation of facets within a channel as well as its planform all affect the rate of fluvial abrasion. Erosion is greatest where sediment-charged water impacts at a near normal angle. Consequently, obstructions within the channel, such as large boulders, will be rapidly abraded.
Fluvial abrasion may also occur without the transport of rock debris. Boulders that are too large or are wedged together may be vibrated by the passage of water. This vibration may cause abrasion or attrition of one boulder against the next or against the channel walls. Similarly boulders or stones trapped within enclosed hollows may achieve considerable amounts of abrasion as they are swirled around within the hollow.
Fluvial cavitation occurs wherever the meltwater velocity exceeds about 12 ms-1. It involves the creation of low-pressure areas within turbulent meltwater as it flows over a rough bedrock surface. These low-pressure areas form as the flow is accelerated around obstacles on the channel floor. If the pressure within the water drops sufficiently to allow the water to vaporise, bubbles of vapour (cavities) form. The cavitation bubbles grow and are moved along in the fluid until they reach a region of slightly higher local pressure where they will suddenly collapse. If cavity collapse occurs adjacent to a channel wall, localised but very high impact forces are produced against the rock. Repetition of these impact forces may lead to rock failure. In particular the shock waves are often forced into microscopic cracks within a rock or between mineral grains, causing them to loosen and allowing their removal. Once a surface has become pitted or fretted from the loss of grains or small rock fragments the shock waves tend to become concentrated or guided into the pits, thereby accelerating the process of erosion. Large bowl-shaped depressions may be produced in this way.
Glacial meltwater can also erode bedrock by the processes of chemical solution. Soluble components of rock and rock debris are dissolved and removed in solution. This process is particularly important on carbonate-rich lithologies (e.g., limestone and chalk), but is not restricted to them. Chemical denudation beneath glaciers is often neglected as a process of glacial erosion, although, in recent years its importance has been increasingly recognised. Chemical denudation is particularly effective beneath glaciers, despite the low temperatures and therefore reaction rates, for three main reasons.
1. High flushing rates. Meltwater passes through the glacial system rapidly and is rarely stored subglacially for long; its residence time is therefore short, and this ensures that it does not have time to become chemically saturated.
2. Availability of rock flour. Turbulent meltwater is able to transport large quantities of freshly ground rock particles in suspension, which provide a very high surface area, or reaction surface, over which solution can occur.
3. Enhanced solubility of carbon dioxide at low temperatures. The solution of carbon dioxide by meltwater produces a weak acid. The solubility of carbon dioxide increases at low temperatures and consequently meltwater becomes more acidic and therefore more aggressive.
Chemical denudation is restricted to warm-based ice with abundant meltwater and may be particularly important in maritime areas where high rainfall adds significantly to the volume of water passing through the glacial system.
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