In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.[1]
Avulsions are common in river deltas, where sedimentdeposits as the river enters the ocean and channel gradients are typically very small.[2] This process is also known as delta switching.
Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta of the Mississippi River, pictured at right with its sediment plumes. As the deltaic lobe advances, the slope of the river channel becomes lower, as the river channel is longer but has the same change in elevation. As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course downslope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[1] Second, as its slope is reduced, the amount of shear stress on the bed will decrease, resulting in deposition of more sediment within the channel and thus raising of the channel bed relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope.
When this avulsion occurs, the new channel carries sediment out to the ocean, building a new deltaic lobe.[3][4] The abandoned delta eventually subsides.[5]
This process is also related to the distributary network of river channels that can be observed within a river delta. When the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events happen repeatedly over time, a mature delta will gain a distributary network.[6]
Subsidence of the delta and/or sea-level rise can further cause backwater and deposition in the delta. This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basins. On average, an avulsion will occur every time the bed of a river channel aggrades enough that the river channel is superelevated above the floodplain by one channel-depth. In this situation, enough hydraulic head is available that any breach of the natural levees will result in an avulsion.[7][8]
Erosional avulsions
Rivers can also avulse due to the erosion of a new channel that creates a straighter path through the landscape. This can happen during large floods in situations in which the slope of the new channel is significantly greater than that of the old channel. Where the new channel's slope is about the same as the old channel's slope, a partial avulsion will occur in which both channels are occupied by flow.[9] An example of an erosional avulsion is the 2006 avulsion of the Suncook River in New Hampshire, in which heavy rains caused flow levels to rise. The river level backed up behind an old mill dam, which produced a shallowly-sloping pool that overtopped a sand and gravel quarry, connected with a downstream section of channel, and cut a new shorter channel at 25–50 meters per hour.[10] Sediment mobilised by this erosional avulsion produced a depositionally-forced meander cutoff further downstream by superelevating the bed around the meander bend to nearly the level of the floodplain.[11]
Meander cutoffs
An example of a minor avulsion is known as a meander cutoff, where the high-sinuositymeander bend is abandoned in favour of the high-slope (i.e. Large bending meander has river cut through a straighter course, and the meander has water drain away) This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than about 1/5.[1]
Occurrence
Avulsion typically occurs during large floods which carry the power necessary to rapidly change the landscape. Dam removal could also lead to avulsion.
Avulsions usually occur as a downstream to upstream process via head cutting erosion. If a bank of a current stream is breached a new trench will be cut into the existing floodplain. It either cuts through floodplain deposits or reoccupies an old channel.[12]
Avulsions have been investigated in deltas or coastal plain channels as a result of obstructions such as log-jams and possible tectonic influences.[13]
See also
Aggradation – Increase in land elevation due of the deposition of sediment
^ abcSlingerland, Rudy; Smith, Norman D. (1998). "Necessary conditions for a meandering-river avulsion". Geology. 26 (5): 435–438. Bibcode:1998Geo....26..435S. doi:10.1130/0091-7613(1998)026<0435:NCFAMR>2.3.CO;2.
^Marshak, Stephen (2001), Earth: Portrait of a Planet, New York: W.W. Norton & Company, ISBN0-393-97423-5 pp. 528–9
^Stanley, Steven M. (1999) Earth System History. New York: W.H. Freeman and Company, ISBN0-7167-2882-6 p. 136
^Easterbrook, Don J.Surface Processes and Landforms Second EditionPrentice Hall, New Jersey: 1999.
^Bryant, M.; Falk, P.; Paola, C. (1995). "Experimental study of avulsion frequency and rate of deposition". Geology. 23 (4): 365–368. Bibcode:1995Geo....23..365B. doi:10.1130/0091-7613(1995)023<0365:ESOAFA>2.3.CO;2.
^Mohrig, D.; Heller, P. L.; Paola, C.; Lyons, W. J. (2000). "Interpreting avulsion process from ancient alluvial sequences: Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado)". Geological Society of America Bulletin. 112 (12): 1787–1803. Bibcode:2000GSAB..112.1787M. doi:10.1130/0016-7606(2000)112<1787:IAPFAA>2.0.CO;2.
^Slingerland, Rudy; Smith, Norman D. (2004). "River Avulsions and Their Deposits". Annual Review of Earth and Planetary Sciences. 32: 257–285. Bibcode:2004AREPS..32..257S. doi:10.1146/annurev.earth.32.101802.120201.
^Perignon, M. C. (2007). Mechanisms governing avulsions in transient landscapes: Analysis of the May 2006 Suncook River Avulsion in Epsom, New Hampshire (S.B. Thesis). Massachusetts Institute of Technology.
^Perignon, M. C. (2008). Sediment wave-induced channel evolution following the 2006 avulsion of the Suncook River in Epsom, New Hampshire (Thesis). Massachusetts Institute of Technology. hdl:1721.1/45792.
^Nanson, G.C.; Knighton, A.D. (1996). "Anabranching rivers: Their cause, character, and classification". Earth Surface Processes and Landforms. 21 (3): 217–39. Bibcode:1996ESPL...21..217N. doi:10.1002/(SICI)1096-9837(199603)21:3<217::AID-ESP611>3.0.CO;2-U.
^Phillips, J. D. (2012). "Log-jams and avulsions in the San Antonio River Delta, Texas". Earth Surface Processes and Landforms. 37 (9): 936–950. Bibcode:2012ESPL...37..936P. doi:10.1002/esp.3209. S2CID 128660882.
April 30, 2023
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In sedimentary geology and fluvial geomorphology avulsion is the rapid abandonment of a river channel and the formation of a new river channel Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course 1 Plumes of sediment enter the ocean from several mouths of the Mississippi River bird s foot delta This sediment is responsible for building the delta and allowing it to advance into the sea As it extends further offshore the channel slope will decrease and its bed will aggrade promoting an avulsion Contents 1 Deltaic and net depositional settings 2 Erosional avulsions 3 Meander cutoffs 4 Occurrence 5 See also 6 ReferencesDeltaic and net depositional settings EditAvulsions are common in river deltas where sediment deposits as the river enters the ocean and channel gradients are typically very small 2 This process is also known as delta switching Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea An example of a deltaic lobe is the bird s foot delta of the Mississippi River pictured at right with its sediment plumes As the deltaic lobe advances the slope of the river channel becomes lower as the river channel is longer but has the same change in elevation As the slope of the river channel decreases it becomes unstable for two reasons First water under the force of gravity will tend to flow in the most direct course downslope If the river could breach its natural levees i e during a flood it would spill out onto a new course with a shorter route to the ocean thereby obtaining a more stable steeper slope 1 Second as its slope is reduced the amount of shear stress on the bed will decrease resulting in deposition of more sediment within the channel and thus raising of the channel bed relative to the floodplain This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope When this avulsion occurs the new channel carries sediment out to the ocean building a new deltaic lobe 3 4 The abandoned delta eventually subsides 5 This process is also related to the distributary network of river channels that can be observed within a river delta When the channel does this some of its flow can remain in the abandoned channel When these channel switching events happen repeatedly over time a mature delta will gain a distributary network 6 Subsidence of the delta and or sea level rise can further cause backwater and deposition in the delta This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basins On average an avulsion will occur every time the bed of a river channel aggrades enough that the river channel is superelevated above the floodplain by one channel depth In this situation enough hydraulic head is available that any breach of the natural levees will result in an avulsion 7 8 Erosional avulsions EditRivers can also avulse due to the erosion of a new channel that creates a straighter path through the landscape This can happen during large floods in situations in which the slope of the new channel is significantly greater than that of the old channel Where the new channel s slope is about the same as the old channel s slope a partial avulsion will occur in which both channels are occupied by flow 9 An example of an erosional avulsion is the 2006 avulsion of the Suncook River in New Hampshire in which heavy rains caused flow levels to rise The river level backed up behind an old mill dam which produced a shallowly sloping pool that overtopped a sand and gravel quarry connected with a downstream section of channel and cut a new shorter channel at 25 50 meters per hour 10 Sediment mobilised by this erosional avulsion produced a depositionally forced meander cutoff further downstream by superelevating the bed around the meander bend to nearly the level of the floodplain 11 Meander cutoffs EditAn example of a minor avulsion is known as a meander cutoff where the high sinuosity meander bend is abandoned in favour of the high slope i e Large bending meander has river cut through a straighter course and the meander has water drain away This occurs when the ratio between the channel slope and the potential slope after an avulsion is less than about 1 5 1 Occurrence EditAvulsion typically occurs during large floods which carry the power necessary to rapidly change the landscape Dam removal could also lead to avulsion Avulsions usually occur as a downstream to upstream process via head cutting erosion If a bank of a current stream is breached a new trench will be cut into the existing floodplain It either cuts through floodplain deposits or reoccupies an old channel 12 Avulsions have been investigated in deltas or coastal plain channels as a result of obstructions such as log jams and possible tectonic influences 13 See also EditAggradation Increase in land elevation due of the deposition of sediment Alluvial fan Fan shaped deposit of sediment Lake Ragunda Former whitewater rapid on the river Indalsalven Sweden Meander One of a series of curves in a channel of a matured stream Megafan Large fan shaped deposit River delta Silt deposition landform at the mouth of a river River plume Mix of fresh river water and seawater Stream capture Geomorphological phenomenonReferences Edit a b c Slingerland Rudy Smith Norman D 1998 Necessary conditions for a meandering river avulsion Geology 26 5 435 438 Bibcode 1998Geo 26 435S doi 10 1130 0091 7613 1998 026 lt 0435 NCFAMR gt 2 3 CO 2 Marshak Stephen 2001 Earth Portrait of a Planet New York W W Norton amp Company ISBN 0 393 97423 5 pp 528 9 Stanley Steven M 1999 Earth System History New York W H Freeman and Company ISBN 0 7167 2882 6 p 136 Marshak pp 528 9 Stanley p 136 Easterbrook Don J Surface Processes and Landforms Second EditionPrentice Hall New Jersey 1999 Bryant M Falk P Paola C 1995 Experimental study of avulsion frequency and rate of deposition Geology 23 4 365 368 Bibcode 1995Geo 23 365B doi 10 1130 0091 7613 1995 023 lt 0365 ESOAFA gt 2 3 CO 2 Mohrig D Heller P L Paola C Lyons W J 2000 Interpreting avulsion process from ancient alluvial sequences Guadalope Matarranya system northern Spain and Wasatch Formation western Colorado Geological Society of America Bulletin 112 12 1787 1803 Bibcode 2000GSAB 112 1787M doi 10 1130 0016 7606 2000 112 lt 1787 IAPFAA gt 2 0 CO 2 Slingerland Rudy Smith Norman D 2004 River Avulsions and Their Deposits Annual Review of Earth and Planetary Sciences 32 257 285 Bibcode 2004AREPS 32 257S doi 10 1146 annurev earth 32 101802 120201 Perignon M C 2007 Mechanisms governing avulsions in transient landscapes Analysis of the May 2006 Suncook River Avulsion in Epsom New Hampshire S B Thesis Massachusetts Institute of Technology Perignon M C 2008 Sediment wave induced channel evolution following the 2006 avulsion of the Suncook River in Epsom New Hampshire Thesis Massachusetts Institute of Technology hdl 1721 1 45792 Nanson G C Knighton A D 1996 Anabranching rivers Their cause character and classification Earth Surface Processes and Landforms 21 3 217 39 Bibcode 1996ESPL 21 217N doi 10 1002 SICI 1096 9837 199603 21 3 lt 217 AID ESP611 gt 3 0 CO 2 U Phillips J D 2012 Log jams and avulsions in the San Antonio River Delta Texas Earth Surface Processes and Landforms 37 9 936 950 Bibcode 2012ESPL 37 936P doi 10 1002 esp 3209 S2CID 128660882 Retrieved from https en wikipedia org w index php title Avulsion river amp oldid 1149742540, wikipedia, wiki, book, books, library,
