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Processes and Mechanics

Process Basics

Soil erosion is a two-phase process consisting of the detachment of individual soil particles from the soil mass and their transport by erosive agents such as running water, wind or mechanical impact (e.g. tillage). When sufficient energy is no longer available to transport the particles, a third phase, deposition, occurs.

Detaching agents loosen the soil so that it is easily removed by agents of transport.

Detaching Agents

Detaching agents are

• Rainsplash:

Kinetic energy of Raindrops striking bare soil surface detaches and breaks up soil aggregates. Continuous exposure to intense rainstorms considerably weakens the soil by surface sealing and crusting processes.

• Weathering processes:

Both, mechanical, by alternate wetting and drying, freezing and thawing and frost action, as well as biochemical weathering breaks up the soil.

• Soil utilization:

Soil ist disturbed by tillage operations and by the trampling of people and livestock.

• Running water and wind:

Shear stress affecting the surface soil by running water and wind detaches soil particles.

Transporting Agents

Transporting agents comprise those that act alreally and contribute to the removeal of a uniform thickness of soil (interrill erosion), and those that concentrate their action in channels (rill erosion).

Transporting agents in interrill zones are

• Rainsplash:

Particles may be thrown through the air over distances of several centimeters. Soil surface slope effects a dominance of downslope transport.

• Overland flow:

Surface runoff in the form of shallow flow of infinite width moves soil particles towards rills.

• Wind:

Shear, the magnitude of which depends on wind velocity, moves particles of different sizes according to their critical shear velocity.

Transporting agents in rills are

• Rill runoff:

Transport by concentrated water runoff in small rills, which can be obliterated by weathering and tillage operations.

• Gully runoff:

Concentrated runoff in larger more permanent channel features with very specific formation threshold conditions and mechanisms.

• Brook/river runoff

The severity of erosion depends upon the quality of material supplied by detachment over time and the capacity of eroding agents to transport it. Where the agents have the capacity to transport more material than is supplied by detachment, the erosion is described as detachment-limited. Where more material is supplied than can be transported, the erosion is transport-limited.

Available Energy

The energy available for erosion takes two forms: potential and kinetic.

Potential Energy $PE$ results from the difference in height of one body (e.g. particle) with respect to another (e.g. earth). It is the product of mass $m$, height difference $h$ and acceleration due to gravity $g$:

$PE=mgh$

The potential energy of erosion is converted into kinetic energy $KE$, the energy of motion. This is related to the mass $m$ and velocity $v$ of the eroding agent:

$PE={1 \over 2}mv^{2}$

Most of this energy is dissipated in friction with the surface over which the agent moves. Only 3-4 % of the energy of running water, and 0.2 % of that of falling raindrops is expeded in erosion.