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In Order to Answer This Question Youã¢ââ¢ll Need to Watch the Narrative Art Video on River Meanders

Ditulis Selasa, 26 April 2022 Tulis Komentar Edit

1 of a series of curves in a channel of a matured stream

A stream bed following a tilted valley. The maximum gradient is along the downwards-valley axis represented past a hypothetical directly channel. Meanders develop, which lengthen the form of the stream, decreasing the gradient.

Meanders of the Rio Cauto at Guamo Embarcadero, Republic of cuba

A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave depository financial institution (cutting depository financial institution) and deposits sediments on an inner, convex bank which is typically a signal bar. The result of this coupled erosion and sedimentation is the germination of a sinuous form as the aqueduct migrates dorsum and forth across the axis of a floodplain.[i] [two]

The zone within which a meandering stream periodically shifts its channel is known as a meander belt. Information technology typically ranges from xv to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such a short fourth dimension as to create civil technology challenges for local municipalities attempting to maintain stable roads and bridges.[1] [2]

The degree of meandering of the aqueduct of a river, stream, or other watercourse is measured by its sinuosity. The sinuosity of a watercourse is the ratio of the length of the channel to the directly line downward-valley altitude. Streams or rivers with a unmarried channel and sinuosities of ane.v or more are defined as meandering streams or rivers.[1] [3]

Origin of term [edit]

The term derives from the winding river Menderes located in Asia-Pocket-size and known to the Ancient Greeks every bit Μαίανδρος Maiandros (Latin: Maeander),[iv] [five] characterised past a very convoluted path along the lower reach. As a result, fifty-fifty in Classical Greece (and in later Greek idea) the proper noun of the river had become a common noun significant anything convoluted and winding, such as decorative patterns or speech and ideas, as well every bit the geomorphological feature.[6] Strabo said: '…its grade is and so exceedingly winding that everything winding is called meandering.'[7]

The Meander River is south of Izmir, east of the ancient Greek town of Miletus, now Milet, Turkey. It flows through serial of three graben in the Menderes Massif, only has a flood evidently much wider than the meander zone in its lower reach. Its modern Turkish name is the Büyük Menderes River.[8]

Governing physics [edit]

Straight channel culminating in a single curve

Meanders are a result of the interaction of water flowing through a curved aqueduct with the underlying river bed. This produces helicoidal flow, in which water moves from the outer to the inner bank along the river bed, then flows back to the outer bank nearly the surface of the river. This in plough increases carrying capacity for sediments on the outer bank and reduces information technology on the inner bank, so that sediments are eroded from the outer banking company and redeposited on the inner bank of the side by side downstream meander.[9]

When a fluid is introduced to an initially straight aqueduct which then bends, the sidewalls induce a pressure gradient that causes the fluid to alter grade and follow the bend. From hither, two opposing processes occur: (1) irrotational menstruum and (two) secondary catamenia. For a river to meander, secondary menstruation must dominate.

Irrotational period: From Bernoulli's equations, loftier pressure results in low velocity. Therefore, in the absenteeism of secondary flow nosotros would expect low fluid velocity at the outside bend and high fluid velocity at the inside bend. This classic fluid mechanics consequence is irrotational vortex menstruum. In the context of meandering rivers, its effects are dominated by those of secondary flow.

Secondary flow: A forcefulness residue exists between pressure forces pointing to the inside bend of the river and centrifugal forces pointing to the outside curve of the river. In the context of meandering rivers, a boundary layer exists within the thin layer of fluid that interacts with the river bed. Inside that layer and following standard boundary-layer theory, the velocity of the fluid is finer zero. Centrifugal forcefulness, which depends on velocity, is as well therefore effectively zero. Pressure strength, however, remains unaffected by the boundary layer. Therefore, within the boundary layer, force per unit area force dominates and fluid moves along the bottom of the river from the outside bend to the inside curve. This initiates helicoidal flow: Forth the river bed, fluid roughly follows the curve of the channel but is as well forced toward the inside bend; away from the river bed, fluid also roughly follows the curve of the aqueduct but is forced, to some extent, from the inside to the outside bend.

The higher velocities at the outside bend lead to higher shear stresses and therefore event in erosion. Similarly, lower velocities at the inside bend cause lower sheer stresses and deposition occurs. Thus meander bends erode at the exterior bend, causing the river to becoming increasingly sinuous (until cutoff events occur). Deposition at the inside bend occurs such that for most natural meandering rivers, the river width remains almost constant, even as the river evolves.[10]

In a oral communication before the Prussian Academy of Sciences in 1926, Albert Einstein suggested that because the Coriolis strength of the globe can cause a small-scale imbalance in velocity distribution, such that velocity on i bank is higher than on the other, it could trigger the erosion on i banking company and deposition of sediment on the other that produces meanders[eleven] However, Coriolis forces are likely insignificant compared with other forces acting to produce river meanders.[12]

Meander geometry [edit]

Uvac coulee meander, Serbia

The technical description of a meandering watercourse is termed meander geometry or meander planform geometry.[13] Information technology is characterized as an irregular waveform. Platonic waveforms, such equally a sine moving ridge, are one line thick, but in the instance of a stream the width must exist taken into consideration. The bankfull width is the distance across the bed at an average cantankerous-section at the total-stream level, typically estimated past the line of lowest vegetation.

As a waveform the meandering stream follows the down-valley axis, a straight line fitted to the curve such that the sum of all the amplitudes measured from it is null. This axis represents the overall direction of the stream.

At whatever cross-section the flow is following the sinuous axis, the centerline of the bed. Two sequent crossing points of sinuous and down-valley axes define a meander loop. The meander is two consecutive loops pointing in opposite transverse directions. The distance of ane meander along the downward-valley axis is the meander length or wavelength. The maximum distance from the down-valley axis to the sinuous centrality of a loop is the meander width or amplitude. The course at that point is the noon.

In contrast to sine waves, the loops of a meandering stream are more nearly round. The curvature varies from a maximum at the apex to nothing at a crossing indicate (straight line), also called an inflection, because the curvature changes direction in that vicinity. The radius of the loop is the direct line perpendicular to the down-valley axis intersecting the sinuous axis at the noon. As the loop is not ideal, additional information is needed to characterize information technology. The orientation angle is the angle betwixt sinuous axis and down-valley centrality at any indicate on the sinuous axis.

A loop at the apex has an outer or concave banking company and an inner or convex bank. The meander chugalug is divers by an average meander width measured from outer bank to outer bank instead of from centerline to centerline. If there is a flood plain, it extends beyond the meander belt. The meander is and so said to be costless—it tin be found anywhere in the alluvion manifestly. If there is no overflowing plain, the meanders are fixed.

Various mathematical formulae relate the variables of the meander geometry. As it turns out some numerical parameters can be established, which appear in the formulae. The waveform depends ultimately on the characteristics of the flow merely the parameters are independent of it and manifestly are caused by geologic factors. In general the meander length is 10–fourteen times, with an average 11 times, the fullbank channel width and 3 to 5 times, with an average of 4.7 times, the radius of curvature at the apex. This radius is two–3 times the channel width.[14]

A meander has a depth pattern as well. The cross-overs are marked by riffles, or shallow beds, while at the apices are pools. In a pool management of menstruation is downwards, scouring the bed fabric. The major volume, however, flows more slowly on the inside of the bend where, due to decreased velocity, information technology deposits sediment.[15]

The line of maximum depth, or channel, is the thalweg or thalweg line. It is typically designated the borderline when rivers are used equally political borders. The thalweg hugs the outer banks and returns to center over the riffles. The meander arc length is the distance along the thalweg over one meander. The river length is the length along the centerline.[xv]

Formation [edit]

Life history of a meander

In one case a channel begins to follow a sinusoidal path, the amplitude and concavity of the loops increase dramatically. This is due to the effect of helical flow which sweeps dense eroded material towards the within of the bend, and leaves the outside of the bend unprotected and vulnerable to accelerated erosion. This establishes a positive feedback loop. In the words of Elizabeth A. Woods:[16]

'…this process of making meanders seems to be a self-intensifying process…in which greater curvature results in more erosion of the bank, which results in greater curvature…'

The cross-current along the flooring of the aqueduct is role of the secondary period and sweeps dense eroded material towards the within of the bend.[17] The cross-current and so rises to the surface near the inside and flows towards the outside, forming the helical flow. The greater the curvature of the bend, and the faster the flow, the stronger is the cross-current and the sweeping.[xviii]

Due to the conservation of angular momentum the speed on the inside of the bend is faster than on the outside.[xix]

Since the flow velocity is diminished, and so is the centrifugal pressure. The pressure of the super-elevated cavalcade prevails, developing an unbalanced gradient that moves water back across the bottom from the outside to the inside. The flow is supplied by a counter-catamenia beyond the surface from the within to the outside.[20] This unabridged situation is very like to the Tea leaf paradox.[21] This secondary catamenia carries sediment from the outside of the bend to the inside making the river more meandering.[22]

Every bit to why streams of whatsoever size become sinuous in the first place, there are a number of theories, not necessarily mutually exclusive.

Stochastic theory [edit]

The stochastic theory can accept many forms but one of the most full general statements is that of Scheidegger: 'The meander train is assumed to exist the result of the stochastic fluctuations of the direction of flow due to the random presence of direction-changing obstacles in the river path.'[23] Given a apartment, smooth, tilted artificial surface, rainfall runs off it in sheets, merely even in that instance adhesion of water to the surface and cohesion of drops produce rivulets at random. Natural surfaces are rough and erodible to different degrees. The issue of all the physical factors acting at random is channels that are not straight, which then progressively become sinuous. Even channels that appear straight have a sinuous thalweg that leads eventually to a sinuous channel.

Equilibrium theory [edit]

In the equilibrium theory, meanders decrease the stream gradient until an equilibrium between the erodibility of the terrain and the send capacity of the stream is reached.[24] A mass of water descending must surrender potential energy, which, given the same velocity at the end of the drop as at the offset, is removed by interaction with the textile of the stream bed. The shortest altitude; that is, a straight channel, results in the highest energy per unit of measurement of length, disrupting the banks more, creating more sediment and aggrading the stream. The presence of meanders allows the stream to adapt the length to an equilibrium energy per unit of measurement length in which the stream carries away all the sediment that it produces.

Geomorphic and morphotectonic theory [edit]

Geomorphic refers to the surface construction of the terrain. Morphotectonic means having to do with the deeper, or tectonic (plate) structure of the rock. The features included under these categories are non random and guide streams into not-random paths. They are predictable obstacles that instigate meander formation by deflecting the stream. For case, the stream might be guided into a mistake line (morphotectonic).[25]

Associated landforms [edit]

Cut banking concern [edit]

A cutting bank is an often vertical bank or cliff that forms where the outside, concave banking company of a meander cuts into the floodplain or valley wall of a river or stream. A cutbank is also known either every bit a river-cut cliff, river cliff, or a bluff and spelled as cutbank.[1] Erosion that forms a cut banking concern occurs at the outside bank of a meander because helicoidal menses of water keeps the bank washed clean of loose sand, silt, and sediment and subjects it to constant erosion. As a upshot, the meander erodes and migrates in the direction of the outside bend, forming the cut depository financial institution.[26] [27]

Equally the cut bank is undermined past erosion, information technology usually collapses as slumps into the river aqueduct. The slumped sediment, having been broken upward by slumping, is readily eroded and carried toward the middle of the channel. The sediment eroded from a cut banking concern tends to be deposited on the point bar of the side by side downstream meander, and not on the betoken bar reverse it.[28] [26] This can be seen in areas where trees abound on the banks of rivers; on the within of meanders, trees, such equally willows, are often far from the depository financial institution, whilst on the outside of the bend, the tree roots are often exposed and undercut, eventually leading the trees to autumn into the river.[28] [29]

Meander cutoff [edit]

The Rincon on Lake Powell in southern Utah. Information technology is an incised cutoff (abased) meander.

A meander cutoff, besides known as either a cutoff meander or abased meander, is a meander that has been abased by its stream after the formation of a neck cutoff. A lake that occupies a cutoff meander is known as an oxbow lake. Cutoff meanders that have cut downward into the underlying bedrock are known in general as incised cutoff meanders.[one] Equally in the case of the Anderson Bottom Rincon, incised meanders that accept either steep-sided, ofttimes vertical walls, are often, just non always, known as rincons in the southwest U.s..[thirty] Rincon in English is a nontechnical word in the southwest U.s.a. for either a small secluded valley, an alcove or angular recess in a cliff, or a bend in a river.[31]

Incised meanders [edit]

The meanders of a stream or river that has cut its bed down into the bedrock are known as either incised, intrenched, entrenched, inclosed or ingrown meanders. Some Earth scientists recognize and use a finer subdivision of incised meanders. Thornbury[32] argues that incised or inclosed meanders are synonyms that are appropriate to describe whatever meander incised downward into bedrock and defines enclosed or entrenched meanders as a subtype of incised meanders (inclosed meanders) characterized by a symmetrical valley sides. He argues that the symmetrical valley sides are the directly result of rapid down-cutting of a watercourse into boulder.[ane] [33] In addition, equally proposed by Rich,[34] Thornbury argues that incised valleys with a pronounced asymmetry of cross section, which he chosen ingrown meanders, are the result of the lateral migration and incision of a meander during a flow of slower channel downcutting. Regardless, the germination of both entrenched meanders and ingrown meanders is thought to crave that base level falls as a upshot of either relative change in mean sea level, isostatic or tectonic uplift, the alienation of an water ice or landslide dam, or regional tilting. Classic examples of incised meanders are associated with rivers in the Colorado Plateau, the Kentucky River Palisades in cardinal Kentucky, and streams in the Ozark Plateau.[33] [35]

As noted higher up, it was initially either argued or presumed that an incised meander is feature of an antecedent stream or river that had incised its channel into underlying strata. An antecedent stream or river is one that maintains its original course and pattern during incision despite the changes in underlying rock topography and rock types.[32] [33] However, later geologists[36] argue that the shape of an incised meander is not always, if e'er, "inherited," e.g., strictly from an antecedent meandering stream where it meander pattern could freely develop on a level floodplain. Instead, they debate that as fluvial incision of bedrock proceeds, the stream grade is significantly modified past variations in rock type and fractures, faults, and other geological structures into either lithologically conditioned meanders or structurally controlled meanders.[33] [35]

Oxbow lakes [edit]

The oxbow lake, which is the most mutual type of fluvial lake, is a crescent-shaped lake that derives its proper name from its distinctive curved shape.[37] Oxbow lakes are also known as cutoff lakes.[one] Such lakes form regularly in undisturbed floodplains as a result of the normal process of fluvial meandering. Either a river or stream forms a sinuous channel as the outer side of its bends are eroded away and sediments accrue on the inner side, which forms a meandering horseshoe-shaped bend. Eventually as the result of its meandering, the fluvial channel cuts through the narrow neck of the meander and forms a cutoff meander. The terminal interruption-through of the neck, which is called a neck cutoff, often occurs during a major flood because that is when the watercourse is out of its banks and tin flow straight beyond the cervix and erode information technology with the full forcefulness of the flood.[28] [38]

Later on a cutoff meander is formed, river water flows into its end from the river builds small delta-like feature into either end of it during floods. These delta-like features block either end of the cutoff meander to form a stagnant oxbow lake that is separated from the flow of the fluvial aqueduct and independent of the river. During floods, the flood waters eolith fine-grained sediment into the oxbow lake. As a effect, oxbow lakes tend to become filled in with fine-grained, organic-rich sediments over time.[28] [38]

Bespeak bar [edit]

A betoken bar, which is as well known as a meander bar, is a fluvial bar that is formed by the slow, often episodic, addition of individual accretions of noncohesive sediment on the inside bank of a meander past the accompanying migration of the channel toward its outer bank.[1] [26] This procedure is called lateral accession. Lateral accretion occurs mostly during loftier h2o or floods when the point bar is submerged. Typically, the sediment consists of either sand, gravel, or a combination of both. The sediment comprising some point confined might grade downstream into silty sediments. Because of the decreasing velocity and strength of current from the thalweg of the channel to the upper surface of point bar when the sediment is deposited the vertical sequence of sediments comprising a point bar becomes finer upwardly inside an individual signal bar. For instance, it is typical for point bars to fine upwards from gravel at the base to fine sands at the elevation. The source of the sediment is typically upstream cut banks from which sand, rocks and debris has been eroded, swept, and rolled across the bed of the river and downstream to the inside bank of a river bend. On the within curve, this sediment and debris is eventually deposited on the slip-off slope of a point bar.[1] [26] [27]

Gyre-bars [edit]

Roll-bars are a result of continuous lateral migration of a meander loop that creates an asymmetrical ridge and swale topography[39] on the inside of the bends. The topography is generally parallel to the meander, and is related to migrating bar forms and back bar chutes,[forty] which cleave sediment from the exterior of the curve and eolith sediment in the slower flowing water on the within of the loop, in a process called lateral accretion. Coil-bar sediments are characterized by cross-bedding and a pattern of fining upward.[41] These characteristics are a result of the dynamic river system, where larger grains are transported during loftier energy flood events and and so gradually die downwardly, depositing smaller fabric with fourth dimension (Batty 2006). Deposits for meandering rivers are generally homogeneous and laterally extensive unlike the more heterogeneous braided river deposits.[42] In that location are two distinct patterns of scroll-bar depositions; the boil accretion scroll bar design and the point-bar scroll design. When looking down the river valley they tin can be distinguished considering the betoken-bar scroll patterns are convex and the boil accretion roll bar patterns are concave.[43]

Curlicue confined often await lighter at the tops of the ridges and darker in the swales. This is considering the tops tin can be shaped past current of air, either adding fine grains or by keeping the expanse unvegetated, while the darkness in the swales tin be attributed to silts and clays washing in during high h2o periods. This added sediment in addition to h2o that catches in the swales is in plow is a favorable environment for vegetation that volition besides accumulate in the swales.

Sideslip-off slope [edit]

Depending upon whether a meander is role of an entrenched river or part of a freely meandering river within a floodplain, the term slip-off slope can refer to two different fluvial landforms that comprise the inner, convex, bank of a meander loop. In case of a freely meandering river on a floodplain, a slip-off slope is the within, gently sloping bank of a meander on which sediments episodically accumulate to form a bespeak bar as a river meanders. This blazon of sideslip-off slope is located reverse the cutbank.[44] This term can also be applied to the inside, sloping banking company of a meandering tidal channel.[45]

In instance of an entrenched river, a slip-off slope is a gently sloping bedrock surface that rises from the inside, concave banking concern of an asymmetrically entrenched river. This type of sideslip-off slope is often covered by a sparse, discontinuous layer of alluvium. It is produced by the gradual outward migration of the meander as a river cuts downward into bedrock.[46] [47] A terrace on the slip-off slope of a meander spur, known equally slip-off slope terrace, can formed by a cursory halt during the irregular incision past an actively meandering river.[48]

Derived quantities [edit]

The meander ratio[49] or sinuosity alphabetize[50] is a means of quantifying how much a river or stream meanders (how much its course deviates from the shortest possible path). Information technology is calculated as the length of the stream divided by the length of the valley. A perfectly straight river would have a meander ratio of one (it would be the same length as its valley), while the higher this ratio is above 1, the more the river meanders.

Sinuosity indices are calculated from the map or from an aerial photograph measured over a distance called the achieve, which should exist at least 20 times the boilerplate fullbank aqueduct width. The length of the stream is measured by aqueduct, or thalweg, length over the achieve, while the lesser value of the ratio is the downvalley length or air altitude of the stream between two points on information technology defining the accomplish.

The sinuosity index plays a part in mathematical descriptions of streams. The index may require elaboration, considering the valley may meander as well—i.e., the downvalley length is not identical to the reach. In that example the valley alphabetize is the meander ratio of the valley while the channel index is the meander ratio of the channel. The aqueduct sinuosity index is the aqueduct length divided past the valley length and the standard sinuosity index is the channel index divided past the valley alphabetize. Distinctions may become even more than subtle.[51]

Sinuosity Index has a non-mathematical utility also. Streams tin can exist placed in categories arranged by it; for case, when the index is betwixt i and 1.5 the river is sinuous, but if between one.5 and 4, so meandering. The alphabetize is a measure too of stream velocity and sediment load, those quantities beingness maximized at an alphabetize of ane (straight).

See too [edit]

  • Baer'due south law
  • Billabong
  • Crevasse splay
  • Helicoidal period
  • Jet stream
  • Meander cutoffs in Avulsion (river)
  • Meander scar
  • Riffle-pool sequence

References and notes [edit]

  1. ^ a b c d e f 1000 h i Neuendorf, Grand.Thou.E., J.P. Mehl Jr., and J.A. Jackson, J.A., eds. (2005) Glossary of Geology (fifth ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN 0-922152-76-4
  2. ^ a b Charlton, R., 2007. Fundamentals of fluvial geomorphology. Routledge, New York, New York. 234 pp. ISBN 0-415-33453-5
  3. ^ Leopold, L.B., Wolman, M.One thousand., Wolman, M.G. and Wolman, M.G., 1957. River Channel Patterns: Braided, Meandering, and Directly. United States Geological Survey Professional person Paper no. 282B, Usa Government Printing Office, Washington DC., 47 pp.
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  7. ^ Strabo, Geography, Book 12 Affiliate viii Section 15.
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  10. ^ Weiss, Samantha Freeman. (April 2016). Meandering River Dynamics (Doctoral dissertation). Retrieved from Ideals. https://www.ethics.illinois.edu/bitstream/handle/2142/92706/WEISS-DISSERTATION-2016.pdf?sequence=1&isAllowed=y
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  18. ^ Hickin 2003, p. 434.
  19. ^ Hickin 2003, p. 432. "In the absence of secondary flow, bend catamenia seeks to conserve athwart momentum and then that information technology tends to conform to that of a gratis vortex with high velocity at the smaller radius of the inner bank and lower velocity at the outer bank where radial acceleration is lower."
  20. ^ Hickin 2003, p. 432. "Near the bed, where velocity and thus the centrifugal effects are lowest, the residuum of forces is dominated by the inward hydraulic gradient of the super-elevated water surface and secondary flow moves toward the inner bank."
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  23. ^ Scheidegger, Adrien Due east. (2004). Morphotectonics. Berlin, New York: Springer. p. 113. ISBN3-540-20017-vii.
  24. ^ Riley, Ann 50. (1998). Restoring Streams in Cities: A Guide for Planners, Policymakers and Citizens. Washington DC: Island Press. p. 137. ISBN1-55963-042-6.
  25. ^ D'Alessandro, Leandro; Miccadei, Enrico; Piacentini, Tommaso (November 2008). "Morphotectonic study of the lower Sangro River valley (Abruzzi, Primal Italy)". Geomorphology. 102 (1): 145–158. Bibcode:2008Geomo.102..145D. doi:x.1016/j.geomorph.2007.06.019.
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  34. ^ Rich, J.L., 1914. Certain types of stream valleys and their meaning. The Journal of Geology, 22(5), pp. 469–497.
  35. ^ a b Barbour, J.R., 2008. The origin and significance of sinuosity along incising boulder rivers. Doctoral dissertation, Columbia University, New York, New York, 172 pp.
  36. ^ Hack, J.T., and Young, R.S., 1959. Intrenched meanders of the North Fork of the Shenandoah River, Virginia. United States Geological Survey Professional Paper 354-A, 10 pp.
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Bibliography [edit]

  • Hickin, Edward J. (2003). "Meandering Channels". In Middleton, Gerard 5. (ed.). Encyclopedia of Sediments and Sedimentary Rocks. Kluwer Academic Encyclopedia of Earth Sciences. Dordrecht; Boston: Kluwer Academic Publishers. pp. 430–434. ISBNone-4020-0872-four.
  • Leopold, Luna B.; Langbein, W.B. (June 1966). "River Meanders". Scientific American. 214 (six): threescore. Bibcode:1966SciAm.214f..60L. doi:10.1038/scientificamerican0666-threescore. Virtual Luna Leopold
  • Thonemann, P., The Maeander Valley: A historical geography from Artifact to Byzantium (Cambridge, 2011) (Greek Culture in the Roman Earth Series).

External links [edit]

  • Movshovitz-Hadar, Nitsa; Alla Shmuklar (2006-01-01). "River Meandering and a Mathematical Model of this Phenomenon". Physicalplus. Israel Concrete Social club (IPS) (7).

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Source: https://en.wikipedia.org/wiki/Meander

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