Editing Soil erosion (section)

==Physical processes==

===Rainfall and surface runoff===
[[File:Water and soil splashed by the impact of a single raindrop.jpg|thumb|right|[[Soil]] and water being [[Splash (fluid mechanics)|splashed]] by the impact of a single [[raindrop]]]]
[[Rainfall]], and the [[surface runoff]] which may result from rainfall, produces four main types of soil erosion: ''splash erosion'', ''sheet erosion'', ''rill erosion'', and ''gully erosion''. Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by [[sheet erosion]], then rill erosion and finally gully erosion (the most severe of the four).<ref>{{cite book|author=Toy, Terrence J.|title=Soil Erosion: Processes, Prediction, Measurement, & Control|publisher=John Wiley & Sons|year=2002|isbn=978-0-471-38369-7|pages=60–61|url=https://books.google.com/books?id=7YBaKZ-28j0C&pg=PA60|display-authors=etal}}</ref><ref>{{cite book|author= Zachar, Dušan|chapter=Classification of soil erosion|title=Soil Erosion|volume=10|publisher=Elsevier|year=1982|isbn=978-0-444-99725-8|page=48|chapter-url=https://books.google.com/books?id=o8ny2dUkpM8C&pg=PA48}}</ref>

In ''splash erosion'', the [[Rainfall#Raindrop impacts|impact of a falling raindrop]] creates a small crater in the soil,<ref name="Fig. 4">See figure 4 in {{cite journal|last1=Obreschkow|title=Confined Shocks inside Isolated Liquid Volumes – A New Path of Erosion?|year=2011|arxiv=1109.3175 |journal=Physics of Fluids|bibcode=2011PhFl...23j1702O|doi=10.1063/1.3647583|volume=23|issue=10|page=101702|s2cid=59437729}}</ref> ejecting soil particles.<ref>{{cite journal | author = Cheraghi M., Jomaa S., Sander G. C., Barry D. A. | year = 2016 | title = Hysteretic sediment fluxes in rainfall-driven soil erosion: Particle size effects | url =https://repository.lboro.ac.uk/articles/Hysteretic_sediment_fluxes_in_rainfall-driven_soil_erosion_particle_size_effects/9439820 | journal = Water Resour. Res. | volume = 52 | issue = 11| pages = 8613–8629 | doi = 10.1002/2016WR019314 | bibcode = 2016WRR....52.8613C| s2cid = 13077807 }}</ref> The distance these soil particles travel can be as much as 0.6&nbsp;m (two feet) vertically and 1.5&nbsp;m (five feet) horizontally on level ground.

If [[Surface runoff#Saturation excess overland flow|the soil is saturated]], or if the rainfall rate is [[Surface runoff#Infiltration excess overland flow|greater than the rate at which water can infiltrate]] into the soil, surface runoff occurs. If the runoff has sufficient [[Fluid dynamics|flow energy]], it will [[Sediment transport|transport]] loosened soil particles ([[sediment]]) down the slope.<ref name="FAO-1965-pp23-25">{{cite book|author=Food and Agriculture Organization|chapter=Types of erosion damage|title=Soil Erosion by Water: Some Measures for Its Control on Cultivated Lands|publisher=United Nations|year=1965|isbn=978-92-5-100474-6|pages=23–25|chapter-url=https://books.google.com/books?id=6KeL3ix6ZqQC&pg=PA23}}</ref> ''[[Sheet erosion]]'' is the transport of loosened soil particles by overland flow.<ref name="FAO-1965-pp23-25" />
[[File:Rummu aherainemägi2.jpg|thumb|A [[spoil tip]] covered in rills and gullies due to erosion processes caused by rainfall: [[Rummu]], [[Estonia]]]]
''[[Rill]] erosion'' refers to the development of small, [[ephemeral]] concentrated flow paths which function as both sediment source and [[sediment]] delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, [[rill]]s are active. Flow depths in rills are typically of the order of a few centimeters (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit [[hydraulic]] physics very different from water flowing through the deeper wider channels of streams and rivers.<ref>{{cite journal | last1 = Nearing | first1 = M.A. | last2 = Norton | first2 = L.D. | last3 = Bulgakov | first3 = D.A. | last4 = Larionov | first4 = G.A. | last5 = West | first5 = L.T. | last6 = Dontsova | first6 = K.M. | year = 1997 | title = Hydraulics and erosion in eroding rills | journal = Water Resources Research | volume = 33 | issue = 4 | pages = 865–876 | doi = 10.1029/97wr00013 | bibcode = 1997WRR....33..865N | doi-access = free }}</ref>

''[[Gully erosion]]'' occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to a considerable depth.<ref>{{cite book|author=Poesen, Jean|chapter=Gully erosion in Europe|editor=Boardman, John|editor2=Poesen, Jean|title=Soil Erosion in Europe|publisher=John Wiley & Sons|year=2007|isbn=978-0-470-85911-7|pages=516–519|chapter-url=https://books.google.com/books?id=vvOFRskFunwC&pg=PA516|display-authors=etal}}</ref><ref>{{cite book|author=Poesen, Jean|chapter=Gully erosion in dryland environments|editor=Bull, Louise J.|editor2=Kirby, M.J.|title=Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels|publisher=John Wiley & Sons|year=2002|isbn=978-0-471-49123-1|chapter-url=https://books.google.com/books?id=qjHoYZXQee0C&pg=PA229|display-authors=etal}}</ref><ref>{{cite book|author=Borah, Deva K.|chapter=Watershed sediment yield|editor=Garcia, Marcelo H.|title=Sedimentation Engineering: Processes, Measurements, Modeling, and Practice|publisher=ASCE Publishing|year=2008|isbn=978-0-7844-0814-8|page=828|chapter-url=https://books.google.com/books?id=1AsypwBUa_wC&pg=PA828|display-authors=etal}}</ref> Another cause of [[gully]] erosion is grazing, which often results in ground compaction. Because the soil is exposed, it loses the ability to absorb excess water, and erosion can develop in susceptible areas.<ref>{{cite web | url=https://agriculture.vic.gov.au/farm-management/soil/erosion/gully-erosion | title=Gully erosion - Agriculture | date=4 June 2020 }}</ref>

===Rivers and streams{{Anchor|gully erosion|ephemeral gully erosion}}===

{{further|topic=water's erosive ability|Hydraulic action}}
[[File:Dobbingstone Burn - geograph.org.uk - 1291882.jpg|thumb|Dobbingstone Burn, Scotland—This photo illustrates two different types of erosion affecting the same place. Valley erosion is occurring due to the flow of the stream, and the boulders and stones (and much of the soil) that are lying on the edges are [[glacial till]] that was left behind as ice age glaciers flowed over the terrain.]]
''Valley'' or ''stream erosion'' occurs with continued water flow along a linear feature. The erosion is both [[Downcutting|downward]], deepening the valley, and [[headward erosion|headward]], extending the valley into the hillside, creating [[Head cut (stream geomorphology)|head cuts]] and steep banks. 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 [[meander]]s across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, [[pebble]]s and [[boulder]]s can also act erosively as they traverse a [[Erosion surface|surface]], in a process known as ''traction''.<ref>Ritter, Michael E. (2006) [http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/geologic_work_of_streams.html "Geologic Work of Streams"] {{webarchive|url=https://web.archive.org/web/20120506040721/http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/fluvial_systems/geologic_work_of_streams.html |date=2012-05-06 }} ''The Physical Environment: an Introduction to Physical Geography'' University of Wisconsin, {{OCLC|79006225}}</ref>

''Bank erosion'' is the wearing away of the banks of a [[stream]] or [[river]]. This is distinguished from changes on the bed of the watercourse, which is referred to as ''scour''. Erosion and [[River bank failure|changes in the form of river banks]] may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times.<ref>{{Cite book |chapter-url=https://books.google.com/books?id=_PJHw-hSKGgC&pg=PA113 |title=Stream hydrology: an introduction for ecologists |author=Nancy D. Gordon |chapter=Erosion and Scour |isbn=978-0-470-84357-4 |date=2004-06-01 |publisher=John Wiley and Sons }}</ref>

''Thermal erosion'' is the result of melting and weakening [[permafrost]] due to moving water.<ref name="nsidc_thermal">{{cite web|url=http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion|title=Thermal Erosion|work=NSIDC Glossary|publisher=[[National Snow and Ice Data Center]]|access-date=21 December 2009|archive-url=https://web.archive.org/web/20101218124656/http://nsidc.org/cgi-bin/words/word.pl?thermal%20erosion|archive-date=2010-12-18|url-status=live}}</ref> It can occur both along rivers and at the coast. Rapid [[river channel migration]] observed in the [[Lena River]] of [[Siberia]] is due to [[Fluvio-thermal erosion|thermal erosion]], as these portions of the banks are composed of permafrost-cemented non-cohesive materials.<ref name="lena">{{cite journal|doi=10.1002/esp.592|title=Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)|year=2003|last1=Costard|first1=F.|last2=Dupeyrat|first2=L.|last3=Gautier|first3=E.|last4=Carey-Gailhardis|first4=E.|journal=[[Earth Surface Processes and Landforms]]|volume=28|pages=1349–1359|bibcode = 2003ESPL...28.1349C|issue=12 |s2cid=131318239}}</ref> Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the [[Arctic]] coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along the shoreline and cause them to fail. Annual erosion rates along a {{convert|100|km|mi|abbr=off|adj=on}} segment of the Beaufort Sea shoreline averaged {{convert|5.6|m|ft|abbr=off}} per year from 1955 to 2002.<ref name="jones_arctic">{{cite journal|last=Jones|first=B.M.|author2=Hinkel, K.M.|author3=Arp, C.D.|author4=Eisner, W.R.|year=2008|title=Modern Erosion Rates and Loss of Coastal Features and Sites, Beaufort Sea Coastline, Alaska|journal=Arctic|volume=61|issue=4|pages=361–372|url=http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|doi=10.14430/arctic44|url-status=dead|archive-url=https://web.archive.org/web/20130517101602/http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/44/115|archive-date=2013-05-17|hdl=10535/5534|hdl-access=free}}</ref>

===Floods===

At extremely high flows, [[Kolk (vortex)|kolk]]s, or [[vortex|vortices]] are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called [[rock-cut basin]]s. Examples can be seen in the flood regions result from glacial [[Lake Missoula]], which created the [[channeled scablands]] in the [[Columbia River drainage basin|Columbia Basin]] region of eastern [[Washington (state)|Washington]].<ref>See, for example: {{cite book|author=Alt, David|title=Glacial Lake Missoula & its Humongous Floods
|publisher=Mountain Press|year=2001|isbn=978-0-87842-415-3|url=https://books.google.com/books?id=s4y3c8fxeEwC}}</ref>

===Wind erosion===
[[File:Im Salar de Uyuni.jpg|thumb|[[Árbol de Piedra]], a rock formation in the [[Altiplano]], [[Bolivia]], sculpted by wind erosion]]
{{main|Aeolian processes}}
Wind erosion is a major [[geomorphological]] force, especially in [[arid]] and [[Semi-arid climate|semi-arid]] regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as [[deforestation]], [[urbanization]], and [[agriculture]].<ref>{{Cite book|author=Zheng, Xiaojing|author2=Huang, Ning|name-list-style=amp|title=Mechanics of Wind-Blown Sand Movements|publisher=Springer|year=2009|isbn=978-3-540-88253-4|pages=7–8|url=https://books.google.com/books?id=R6kYrbA3XSAC&pg=PA7}}</ref><ref>{{cite book|author=Cornelis, Wim S.|chapter=Hydroclimatology of wind erosion in arid and semi-arid environments|editor=D'Odorico, Paolo|editor2=Porporato, Amilcare|title=Dryland Ecohydrology|publisher=Springer|year=2006|isbn=978-1-4020-4261-4|page=141|chapter-url=https://books.google.com/books?id=rUsUPZbFHK8C&pg=PA141}}</ref>

Wind erosion is of two primary varieties: ''[[Aeolian processes#Wind erosion|deflation]]'', where the wind picks up and carries away loose particles; and ''[[Abrasion (geology)|abrasion]]'', where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) ''[[Downhill creep|surface creep]]'', where larger, heavier particles slide or roll along the ground; (2) ''[[Saltation (geology)|saltation]]'', where particles are lifted a short height into the air, and bounce and saltate across the surface of the soil; and (3) ''[[Suspension (chemistry)|suspension]]'', where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%).<ref>{{cite book|author=Blanco, Humberto|author2=Lal, Rattan|name-list-style=amp|chapter=Wind erosion|title=Principles of Soil Conservation and Management|publisher=Springer|year=2010|isbn=978-90-481-8529-0|pages=56–57|chapter-url=https://books.google.com/books?id=Wj3690PbDY0C&pg=PA57}}</ref><ref>{{Cite book|author=Balba, A. Monem|chapter=Desertification: Wind erosion|title=Management of Problem Soils in Arid Ecosystems|publisher=CRC Press|year=1995|isbn=978-0-87371-811-0|page=214|chapter-url=https://books.google.com/books?id=uS62XNzDZDsC&pg=PA214}}</ref> Silty soils tend to be the most affected by wind erosion; silt particles are relatively easily detached and carried away.<ref>Jefferson, I.F., Smalley>I.J. 1999. Saltating sand erodes metastable loess ground: events in the impact zone. https://infosys.ars.usda.gov/WindErosion/Symposium/proceedings/jefferso.pdf {{Webarchive|url=https://web.archive.org/web/20170211122659/https://infosys.ars.usda.gov/WindErosion/symposium/proceedings/jefferso.pdf |date=2017-02-11 }}</ref>

Wind erosion is much more severe in arid areas and during times of drought. For example, in the [[Great Plains]], it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.<ref>{{Cite book|author=Wiggs, Giles F.S.|chapter=Geomorphological hazards in drylands|editor=Thomas, David S.G.|title=Arid Zone Geomorphology: Process, Form and Change in Drylands|publisher=John Wiley & Sons|year=2011|isbn=978-0-470-71076-0|page=588|chapter-url=https://books.google.com/books?id=swz4rh4KaLYC&pg=PA588}}</ref>

===Mass movement===
[[File:NegevWadi2009.JPG|thumb|Wadi in Makhtesh Ramon, Israel, showing gravity collapse erosion on its banks]]
''[[Mass wasting|Mass movement]]'' is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of [[gravity]].<ref>{{cite book|author=Van Beek, Rens|chapter=Hillside processes: mass wasting, slope stability, and erosion|editor=Norris, Joanne E.|display-editors=etal|title=Slope Stability and Erosion Control: Ecotechnological Solutions|publisher=Springer|year=2008|isbn=978-1-4020-6675-7|chapter-url=https://books.google.com/books?id=YWPcffxM_A0C&pg=PA17}}</ref><ref>{{cite book|author=Gray, Donald H.|author2=Sotir, Robbin B.|name-list-style=amp|chapter=Surficial erosion and mass movement|title=Biotechnical and Soil Bioengineering Slope Stabilization: A Practical Guide for Erosion Control|publisher=John Wiley & Sons|year=1996|isbn=978-0-471-04978-4|page=20|chapter-url=https://books.google.com/books?id=kCbp6IvFHrAC&pg=20}}</ref>

Mass movement is an important part of the erosional process, and is often the first stage in the breakdown and transport of weathered materials in mountainous areas.<ref>{{cite book|author=Nichols, Gary|title=Sedimentology and Stratigraphy|publisher=John Wiley & Sons|year=2009|isbn=978-1-4051-9379-5|page=93|url=https://books.google.com/books?id=zl4L7WqXvogC&pg=PA93}}</ref> It moves material from higher elevations to lower elevations where other eroding agents such as streams and [[glacier]]s can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; 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 form of such activity is a [[scree]] slope.<ref>{{Cite journal|last1=Sun|first1=Wenyi|last2=Shao|first2=Quanqin|last3=Liu|first3=Jiyuan|last4=Zhai|first4=Jun|date=2014-10-01|title=Assessing the effects of land use and topography on soil erosion on the Loess Plateau in China|url=http://www.sciencedirect.com/science/article/pii/S0341816214001362|journal=CATENA|language=en|volume=121|pages=151–163|doi=10.1016/j.catena.2014.05.009|bibcode=2014Caten.121..151S |issn=0341-8162}}</ref>

''[[Slump (geology)|Slumping]]'' happens on steep hillsides, occurring along distinct fracture zones, often within materials like [[clay]] that, once released, may move quite rapidly downhill. They will often show a spoon-shaped [[isostatic depression]], in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along [[highway]]s where it is a regular occurrence.<ref>{{Cite journal|last1=van den Berg|first1=J.|last2=van de Wal|first2=R. S. W.|last3=Milne|first3=G. A.|last4=Oerlemans|first4=J.|date=2008-05-31|title=Effect of isostasy on dynamical ice sheet modeling: A case study for Eurasia|journal=Journal of Geophysical Research|language=en|volume=113|issue=B5|pages=B05412|doi=10.1029/2007JB004994|bibcode=2008JGRB..113.5412V|issn=0148-0227|doi-access=free}}</ref>

''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 {{convert|0.5|to|1.0|mm|abbr=on|2}} in diameter by wind along the soil surface.<ref>{{Cite book|title=Soils and Their Environment|last=Hassett|first=John|publisher=Prentice Hall|year=1992|isbn=9780134840499|pages=377|url=https://books.google.com/books?id=RbkdAQAAMAAJ}}</ref>

=== Tillage erosion ===
{{excerpt|Tillage erosion}}