Difference between revisions of "Erosion" - New World Encyclopedia

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*[http://eusoils.jrc.it/ESDB_Archive/pesera/pesera_cd/index.htm Pan European Soil Erosion Assessment]
 
*[http://eusoils.jrc.it/ESDB_Archive/pesera/pesera_cd/index.htm Pan European Soil Erosion Assessment]
 
* Belwood, D. R. (1995). "Direct estimate of bioerosion by two parrotfish species, ''Chlorurus gibbus'' and ''C. sordidus'', on the Great Barrier Reef, Australia." ''Marine Biology'', 121(3), 419-429. [http://www.springerlink.com/app/home/contribution.asp?wasp=062adee92a994ec496adcb3289502841&referrer=parent&backto=issue,3,19;journal,24,460;linkingpublicationresults,1:400441,1]
 
* Belwood, D. R. (1995). "Direct estimate of bioerosion by two parrotfish species, ''Chlorurus gibbus'' and ''C. sordidus'', on the Great Barrier Reef, Australia." ''Marine Biology'', 121(3), 419-429. [http://www.springerlink.com/app/home/contribution.asp?wasp=062adee92a994ec496adcb3289502841&referrer=parent&backto=issue,3,19;journal,24,460;linkingpublicationresults,1:400441,1]
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* Soil Erosion and Its Control. Van Nostrand Reinhold Science Series. Edited by R.P.C. Morgan. Van Nostrand Reinhold Company: New York, 1986. ISBN: 0442264410
  
 
== External links ==
 
== External links ==

Revision as of 22:30, 27 November 2006

Severe soil erosion in a wheat field near Washington State University, USA.

Erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water, or ice, by downward or down-slope movement in response to gravity, or by living organisms (in the case of bioerosion). Erosion is distinguished from weathering, or the decomposition of rock and particles through processes in which no movement is involved, although the two processes may be concurrent.

Erosion is an intrinsic natural process but in many places it is increased by human land use. Poor land use practices include deforestation, overgrazing, unmanaged construction activity and road or trail building. However, improved land use practices can limit erosion, using techniques like terrace-building, or the leveling of the land interrupting steep slopes <"Terrace." The Free Dicitionary by Farlex. http://www.thefreedictionary.com/terrace>, and tree planting.

A certain amount of erosion is natural and, in fact, healthy for the ecosystem. For example, gravels continually move downstream along watercourses. Excessive erosion, however, can cause problems, such as increasing water sedimentation, ecosystem damage and the outright loss of soil.

Causes

What causes erosion to be more severe in some areas and less in others is a combination of many factors, including the amount and intensity of precipitation, the texture of the soil, the gradient of the slope, ground cover (from vegetation, rocks, etc.) and land use. Though rain is the first and major agent of erosion, the degree of erosion is governed by the other factors listed.

The first three factors mentioned above can remain fairly constant over time. In general, given the same kind of vegetative cover, you expect areas with high-intensity precipitation, sandy or silty soils, and steep slopes to be the most erosive. Soils with a greater proportion of clay that receive less intense precipitation and are situated along lower slope gradients tend to erode less.

Among the factors listed above, the one most subject to change over time is the amount and type of ground cover. When fires burn an area or when vegetation is removed as part of the process of timber operations or building a house or a road, a soil's susceptibility to erosion is greatly increased. The construction of roads are likely to cause increased rates of erosion because, in addition to removing vegetative ground cover, they can significantly alter drainage patterns. However, a road that has a lot of rock and one that is "hydrologically invisible" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion.

Bank erosion started by four-wheeler, all-terrain vehicles in Yauhanna, South Carolina.

Changes in the kind of vegetation in an area can also effect erosion rates. Different kinds of vegetation effect the infiltration rates of rain into the soil. Forested areas are able to take up water at higher rates, so precipitation there results in less surface runoff, and subsequently, less erosion of surface materials. In these areas, much of the water goes into subsurface flows, which are generally less erosive. Leaf litter and low shrubs also contribute to the high infiltration rates of forested systems, and removing them can lead to an increase in erosion rates. Leaf litter also shelters the soil from the impact of falling raindrops, which is a significant agent of erosion. Vegetation can also change the speed of surface runoff flows, so the presence of grasses and shrubs play an important role in this respect as well.

Many human activities, like logging and heavy grazing by livestock can reduce vegetation from an area, making the soil more susceptible to increased erosion. One of the main causes of erosive soil loss in the year 2006 is the result of slash and burn treatment of tropical forests. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive. For example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows scattered along the landscape (typically larger than 50 meters deep and one kilometer wide). Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world.

When land is overused by animal activities (including humans), there can be mechanical erosion as well as removal of vegetation, leading to erosion. In the case of the animal kingdom, this effect materializes primarily with very large animal herd stampedes, such as the Blue Wildebeest on the Serengeti plain. Even in this case there are broader material benefits to the ecosystem, such as continuing the survival of grasslands, that are indigenous to this region. This effect may be viewed as anomalous or a problem only when there is a significant imbalance or overpopulation of one species.

In the case of human use, the effects are also generally linked to overpopulation. For example, when large numbers of hikers use trails or extensive off roading by vehicles occurs, erosive effects often follow, arising from vegetation removal and furrowing of foot traffic and off road vehicle tires. These effects can also accumulate from a variety of outdoor human activities, again arising from too many people using a finite land resource.

One of the most serious and long-running water erosion problems worldwide is in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow into the ocean each year. The sediment originates primarily from water erosion in the Loess Plateau region of northwest China.

Erosion processes

A heavily eroded roadside near Ciudad Colon, Costa Rica.

Gravity erosion

Mass wasting is the down-slope movement of rock and sediments, mainly due to the force of gravity. Mass wasting is an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are occurring continuously on all slopes. Some mass-wasting processes act very slowly, while others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow type of a landslide is a scree slope.

Slumping is a form of mass-wasting in which consolidated materials or rock layers move together a short distance down a slope. Slumping happens on steep hillsides, and often with materials like clay that, once released, may move quite rapidly downhill. The area left behind by the moving material will often show a spoon-shaped depression. In some cases, the slump is caused by water beneath the slope, weakening layers of rock above it. In many cases it is simply the result of poor engineering along highways, where it is a regular occurrence. <parts adapted from 'slump' in Wiki>

Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm in diameter by wind along the soil surface.

Water erosion

Nearly perfect sphere in granite, Trégastel, Brittany.

Splash erosion is the detachment and airborne movement of small soil particles upon the impact of raindrops on falling on the soil. Sheet erosion is the result of heavy rain on bare soil, where water flows as a sheet down a gradient, carrying soil particles. In any area where precipitation rates exceed the absorbtion and infiltration rates into the soil, runoff occurs. Surface runoff turbulence can often cause more erosion than the initial raindrop impact. Gully erosion results where water flows along a linear depression, eroding a trench or gully, a furrow in the soil, of more than one meter in depth <Rapp, Anders. "Soil Erosion and Sedimentation in Tanzania and Lesotho." Royal Swedish Academy of Sciences, 1975>

Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep. When some base level is reached the erosive activity switches to lateral erosion which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when greater amounts and faster-moving water is available to carry a larger sediment load. In such processes, it is not only water that erodes, suspended abrasive particles, like pebbles and boulders, can also act erosively as they traverse a surface.

At extremely high flows, kolks, or underwater vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features. Examples can be seen in the flood regions having resulted from glacial activity Lake Missoula, which created the channeled scablands in the Columbia Basin region of eastern Washington.[1]

Wave cut platform caused by erosion of cliffs by the sea, at Southerndown in South Wales

Shoreline erosion

File:Coastal Erosion.jpg
Coastal erosion at Happisburgh, Norfolk, England.

Shoreline erosion, on both exposed and sheltered coasts, primarily occurs through the action of currents and waves, but sea level (tidal) change can also play a role. Hydraulic action takes place when air in a joint is suddenly compressed by a wave closing the entrance of it. This then cracks it. Wave pounding is when the sheer energy of a wave hitting a cliff or rock breaks pieces off. Abrasion or Corrasion is caused by waves launching seaload at the cliff. It is the most effective and rapid form of shoreline erosion (not to be confused with Corrosion). Corrosion is the dissolving of rock by carbonic acid in sea water. Limestone cliffs are particularly vulnerable to this kind of erosion due to their porosity. Finally, Attrition is where particles/seaload carried by the waves are worn down as they hit each other and rock surfaces like cliffs. This then makes the material easier to wash away.

Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent amount of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline. Where there is a bend in the coastline, quite often a build up of eroded material occurs, forming a long narrow bank (a spit). Underwater sandbanks offshore may also protect parts of a coastline from erosion. Over the years, as the sandbanks gradually shift, the erosion may be redirected to impact different parts of the shore.

Ice erosion

Ice erosion is caused by movement of ice, typically as glaciers. Glaciers can scrape down a slope and break up rock and then transport it, leaving moraines, drumlins and glacial erratics in their wake, typically at the terminus or during glacier retreat. Ice wedging is the weathering process in which water trapped in tiny cracks of rock freezes and expands, breaking the rock. This can lead to gravity erosion on steep slopes. The scree which forms at the bottom of a steep mountainside is mostly formed from pieces of rock broken away by this means. It is a common engineering problem, wherever rock cliffs are alongside roads, because morning thaws can drop hazardous rock pieces onto the road. In some places that are cold enough , water seeps into rocks during the daytime, then freezes at night. Ice expands, thus, creating a wedge in the rock. Over time, the repetition in the forming and melting of the ice causes fissures, which eventually breaks the rock down.

Wind erosion

Wind erosion, also known as eolian erosion, is the movement of rock and sediment by the wind. The wind causes dust particles to be lifted up, where thay can be moved to other regions. Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation. An example is the formation of sand dunes, on a beach or in a desert. Windbreaks are often planted by farmers to reduce wind erosion. This includes the planting of trees, shrubs, or other vegetation, usually perpendicular or nearly so to the principal wind direction.

Bioerosion

Bioerosion describes the erosion of hard ocean substrates by living organisms by a number of mechanisms. Bioerosion can be caused by mollusks, polychaete worms, sponges, crustaceans, echinoids, and fish. It can occur on coastlines, on coral reefs, and on ships. Mechanisms of bioerosion include biotic boring, drilling, rasping, and scraping.

Bioerosion of coral reefs generates the fine and white coral sand characteristic of tropical islands. The coral is converted to sand by internal bioeroders such as algae, fungi, bacteria (microborers) and sponges (Clionidae), bivalves (Lithophaga), sipunculans (Aspidosiphon), polychaetes (Eunicidae), generating extremely fine sediment of 10 to 100 micrometres. External bioeroders include urchins (Diadema) and chitons (Acanthopleura). The combination of these internal and external bioeroders result in a great deal of erosion. Sea urchin erosion of calcium carbonate (CaCO3) has been reported in some reefs at annual rates exceeding 20 kg/m².

Fish also erode coral while eating algae. Parrotfish cause a great deal of bioerosion, due to their well developed jaw muscle and tooth armature, and a pharyngeal mill, which grinds up ingested material into sand-sized particles. Bioerosion of reef calcium carbonate by parrotfish can range from 1017.7±186.3 kg yr-¹ (0.41±0.07 m³ yr-¹) for Chlorurus gibbus and 23.6±3.4 kg yr-¹ (9.7 10-³±1.3 10-³ m²yr-¹) for Chlorurus sordidus (Bellwood, 1995).

Tectonic effects of erosion

The removal by erosion of large amounts of rock, from a particular region and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region. Research undertaken since the early 1990´s suggests that the spatial distribution of erosion at the surface of an orogen can exert a key influence on its growth and its final internal structure.

Erosion control

Terraces, conservation tillage, and conservation buffers save soil and improve water quality on this Iowa farm. [Photo by Lynn Betts, 1999, USDA Natural Resources Conservation Service.]

Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, and construction. This usually involves the creation of some sort of physical barrier, such as vegetation or rock, to absorb some of the energy of the wind or water that is causing the erosion.

Examples of some erosion control methods include:

  • conservation tillage is used as a way to conserve soil and water resources as well as mitigate the problems of erosion by tilling strips of soil for planting new crops and maintaining untilled portions of the land to serve as anchors for the soil <"Sustainable Practices For Vegetable Production in the South: Conservation Tillage." NY State University, October 2001. http://www.cals.ncsu.edu/sustainable/peet/tillage/tillage.html>
  • contour plowing is a farming technique where plowing is patterned according to the contours of a sloped region to prevent run off and allow water retension in the land <copied from Wiki Contour plowing>
  • terracing is a technique used in agriculture to aid in planting along sleep slopes breaking the slopes down into a series of slopes that are shorter and less steep, slowing down runoff and preventing water erosion <"Backyard Conservation Tip Sheet: Terracing." NRCS (Natural Resources Conservation Service. http://www.nrcs.usda.gov/Feature/backyard/terrac.html>
  • cover crops are any surface crops that are planted annually, biennially, or perennially, in conjunction with other crops or alone, and last during a part or over the course of an entire year, preventing the erosion of the soil by wind or water. They are used as a means of sustainable cropping, prevent weeds, and reduce the leaching of nutrients from the soil <Sullivan, Preston. "Overview of Cover Crops and Green Manures: Fundamentals of Sustainable Agriculture." National Sustainable Agriculture Information Service. ATTRA, 2003. http://attra.ncat.org/attra-pub/covercrop.html>
  • gabions prevent shoreline erosion by creating a strong barrier of rock between fast flowing waves and the shore, resisting the impact of the water on the land and dissipating the energy of the waves against the shore <from wiki Gabions and riprap articles>
  • riprap is rock or other material that works in the reduction of shoreline erosion, and are constructed into gabion barriers. The shape of the rocks make an impact on their overall effectiveness against wave attack on them <copied from wiki riprap>
  • hydroseeding, also referred to as hydromulching, is used as a quick, economical, and effective means of ground cover by planting grass in large areas of exposed soil. The seeds are mixed together in a slurry of mulch and other essential fertilizers and sprayed uniformly across the soil, creating a favorable, nutrient rich environment for the germination and establishment of grass seedlings into the soil <"Hydromulching and hydroseeding defined: a grass planting process." Turfmaker, 2000. http://www.turfmaker.com/Information_Questions_and_Feed/Request_Process/The_Processes_/the_processes_.html> <Copied also from Wiki Hydroseeding>
  • mulching is an agricultural process that covers the soil and serves to protect it from water erosion as well as maintain moisture in the soil <copied from Wiki mulch>
  • reforestation is instrumental in mitigating soil erosion through the restoration of forest and woodland areas that were previously deforested, or stripped of vegetation <copied from Wiki reforestation>
  • riparian strips are regions along a river bank that serve as natural controls against runoff and soil erosion by stabilizing the soil. They are characterized by the vegetation that inhabit these marginal areas <copied from Wiki riparian zones>
  • strip farming is used in steep sloping areas to prevent soil erosion. In strip farming, alternating row and more tightly grown crops are sown beside one another, to stabilize the soil and enhance water retention and percolation <copied from Wiki Strip farming>
  • vegetated waterways are channels developed to stabilize and direct runoff in such a way as to reduce erosion "Vegetated Waterways." Resource Conservation District of Monterey County. RCDMC, 2005 http://www.rcdmonterey.org/Growers_Ranchers_Landowners/Planning_Design/vegetated_waterway.html>
  • wattle (construction)is an organization of interwoven plant materials used as a fence, preventing sedimentation by runoff and erosion <From Wiki Wattle and daub>
  • windbreaks, also known as shelterbelts, involve the planting of trees or shrubs along distinct parameters or in an arranged pattern to reduce erosion by wind <From Wiki Windbreak>

Terminology

Origin of term

The first known occurrence of the term "erosion" was in the 1541 translation by Robert Copland of Guy de Chauliac's medical text The Questyonary of Cyrurygens. Copland used erosion to describe how ulcers developed in the mouth. By 1774, 'erosion' was used outside medical subjects. Oliver Goldsmith employed the term in the more contemporary geological context, in his book Natural History, with the quote

"Bounds are thus put to the erosion of the earth by water."

Usage in materials science

In materials science, erosion is the recession of surfaces by repeated localized mechanical trauma as, for example, by suspended abrasive particles within a moving fluid. Erosion can also occur from non-abrasive fluid mixtures. Cavitation is one example.

Figurative use

The concept of erosion is commonly employed by analogy to various forms of perceived or real breakdown, removal (such as "erosion of boundaries"), "leveling out", or even the decline of anything from morals to indigenous cultures. It is a common trope of the English language to describe erosion as the gradual, organic mutation of something thought of as distinct, more complex, harder to pronounce or more refined into something indistinct, less complex, easier to pronounce, or (disparagingly) less refined.

See also

  • Badland
  • Biorhexistasy
  • Groundwater sapping
  • Erosion prediction
  • Riparian strips
  • Surface runoff
  • Weathering

Footnotes

  1. Template:Cite book''

References
ISBN links support NWE through referral fees

  • World Bank, 2001: China: Air, Land, and Water.
  • Pan European Soil Erosion Assessment
  • Belwood, D. R. (1995). "Direct estimate of bioerosion by two parrotfish species, Chlorurus gibbus and C. sordidus, on the Great Barrier Reef, Australia." Marine Biology, 121(3), 419-429. [1]
  • Soil Erosion and Its Control. Van Nostrand Reinhold Science Series. Edited by R.P.C. Morgan. Van Nostrand Reinhold Company: New York, 1986. ISBN: 0442264410

External links

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