Water erosion
Raindrops can be a major problem for farmers when they strike bare soil. With an impact of up to 30 mph, rain washes out seed and splashes soil into the air. If the fields are on a slope the soil is splashed downhill which causes deterioration of soil structure. Soil that has been detached by raindrops is more easily moved than soil that has not been detached. Sheet erosion is caused by raindrops. Other types of erosion caused by rainfall include rill erosion and gullies.
Sheet erosion is defined as the uniform removal of soil in thin layers from sloping land. This, of course, is nearly impossible; in reality the loose soil merely runs off with the rain.
Rill erosion is the most common form of erosion. Although its effects can be easily removed by tillage, it is the most often overlooked. It occurs when soil is removed by water from little streamlets that run through land with poor surface draining. Rills can often be found in between crop rows.
Gullies are larger than rills and cannot be fixed by tillage. Gully erosion is an advanced stage of rill erosion, just as rills are often the result of sheet erosion.
Once rills are large enough to restrict vehicular access they are referred to as gullies or gully erosion. Major concentrations of high-velocity run-off water in these larger rills remove vast amounts of soil. This results in deeply incised gullies occurring along depressions and drainage lines.
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Wind erosion
Wind erosion is the movement and deposition of soil particles by wind.
Wind erosion occurs when soils bared of vegetation are exposed to high-velocity wind. When its velocity overcomes the gravitational and cohesive forces of the soil particles, wind will move soil and carry it away in suspension.1 Wind moves soil particles 0.1-0.5 mm in size in hopping or bouncing fashion (known as saltation) and those greater than 0.5 mm by rolling (known as soil creep). The finest particles (less than 0.1 mm) detach into suspension. 1 Wind erosion is most visible during the suspension stage, as dust storms, or subsequently as deposition along fencelines and across roads.The process sorts soil particles, removing the finer material containing the organic matter, clay and silt through suspension and leaving the coarser, less fertile material behind. In the short term this reduces the productive capacity of soil, as most of the nutrients plants need are attached to the smaller colloidal soil fraction. Over a longer period the physical nature of the soil changes as the subsoil is exposed.1 Wind erosion also causes damage to public utilities, for example soil deposition across roads, and reduces crops through sandblasting.2 It has been estimated that 700 000 ha in Victoria are affected, with another 2 800 000 ha susceptible when poor management and unfavourable weather conditions combine. The associated loss in production costs $3 million annually.
Wind erosion, unlike water, cannot be divided into such distinct types. Occurring mostly in flat, dry areas and moist sandy soils along bodies of water, wind erosion removes soil and natural vegetation, and causes dryness and deterioration of soil structure. Surface texture is the best key to wind erosion hazard potential. All mucks, sands, and loamy sands can easily be detached and blown away by the wind, and thus are rated a severe hazard. Sandy loams are also vulnerable to wind, but are not as susceptible to severe wind erosion as the previously mentioned soils. Regular loams, silt loams, and clay loams, and clays are not damaged by the wind, but on wide level plains, there may be a loss of fine silts, clays, and some organic matter.
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Gravitical erosion
In mass movement of soil - slides, slips, slumps, flows and landslides - gravity is the principal force acting to move surface materials such as soil and rock.1 When natural slope stability is disrupted, a range of complex sliding movements may occur. Detailed classification requires analysis beyond the scope of this guide. As a rule of thumb, rapid movements of soil or rock that behave separately from the underlying stationary material and involve one distinct sliding surface are termed landslides. A slower long-term deformation having a series of sliding surfaces and exhibiting viscous movement is termed 'creep'. Such movement is rarely the result of a single factor, but more often the final act in a series of processes involving slope, geology, soil type, vegetation type, water, external loads and lateral support.mass movement.
Generally mass movement occurs when the weight (shear stress) of the surface material on the slope exceeds the restraining (shear strength) ability of that material. Factors increasing shear stress include erosion or excavation undermining the foot of a slope, loads of buildings or embankments, and loss of stabilising roots through removal of vegetation. Vegetation removal and consequent lower water use may increase soil water levels, causing an increase in pore water pressure within the soil profile.2 Increased pore water pressure or greater water absorption may weaken inter-granular bonds, reducing internal friction and therefore lessening the cohesive strength of the soil and ultimately the stability of the slope.
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Frozen-melt erosion
When water freezes, it expands suddenly and with tremendous force. When water inside a crack in a rock freezes, its expansive strength may be sufficient to crack the rock and to break parts off it. Frost is tremendously active in snow-covered mountains, particularly along the snow boundary where water repeatedly thaws and freezes. It causes steep cliffs in this region.
A particularly mysterious form of frost damage is frost heave, resulting in damaged roads, buildings and cropland. It appears as if the frost heaved sections of the land upward, by as much as 20cm and usually in very irregular ways. As can be expected, frost heave works with the strength of frost.
Frost heave is not predictable but happens after a deep frost period, followed by thawing and freezing again, and a few repeats of this sequence. In permafrost soils of the arctic, it causes engineering headaches that have to be met with special solutions.
Frost heave can be understood as follows: a deep frost, or permafrost freezes the soil to a certain depth. When this frost thaws incompletely, it leaves a frozen layer behind. Underneath it, the soil may still be thawed but in permafrost places, this frozen bottom is always present. Above it, melting water collects. A repeated frost now freezes it again from the top down, forming a hard layer on top with water in between the two frozen layers. As the frost progresses deeper, the entire top layer is pushed up a few centimetres. The next thawing/freezing cycle repeats this, ratcheting the top layer higher and higher, and always with the same force. Only when the deepest layer is thawed again, will frost heaving stop.
It is not known how much erosion is caused by frost heaving, but it can damage soil structure.
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