An oxbow is a U -shaped metal pole (or larger wooden frame) that fits the underside and the sides of the neck of an ox or bullock . A bow pin holds it in place.
87-497: The term "oxbow" is widely used to refer to a U-shaped meander in a river, sometimes cut off from the modern course of the river that formed it, creating an oxbow lake . Its upper ends pass through a purpose-drilled hole through the bar of the yoke that is held in place into the yoke with a metal screw or key, called a bow pin . Where wood is used it is most often hardwood steamed into shape, especially elm , hickory or willow . A ring, enabling left/right movement controlled from
174-452: A maximum at the apex to zero at a crossing point (straight line), also called an inflection, because the curvature changes direction in that vicinity. The radius of the loop is the straight line perpendicular to the down-valley axis intersecting the sinuous axis at the apex. As the loop is not ideal, additional information is needed to characterize it. The orientation angle is the angle between sinuous axis and down-valley axis at any point on
261-405: A meander because helicoidal flow of water keeps the bank washed clean of loose sand, silt, and sediment and subjects it to constant erosion. As a result, the meander erodes and migrates in the direction of the outside bend, forming the cut bank. As the cut bank is undermined by erosion, it commonly collapses as slumps into the river channel. The slumped sediment, having been broken up by slumping,
348-411: A meander is part 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 inside, gently sloping bank of a meander on which sediments episodically accumulate to form a point bar as
435-431: A meandering watercourse is termed meander geometry or meander planform geometry. It is characterized as an irregular waveform . Ideal waveforms, such as a sine wave , are one line thick, but in the case of a stream the width must be taken into consideration. The bankfull width is the distance across the bed at an average cross-section at the full-stream level, typically estimated by the line of lowest vegetation. As
522-440: A non-mathematical utility as well. Streams can be placed in categories arranged by it; for example, when the index is between 1 and 1.5 the river is sinuous, but if between 1.5 and 4, then meandering. The index is a measure also of stream velocity and sediment load, those quantities being maximized at an index of 1 (straight). Secondary flow In fluid dynamics , flow can be decomposed into primary flow plus secondary flow ,
609-444: A relatively weaker flow pattern superimposed on the stronger primary flow pattern. The primary flow is often chosen to be an exact solution to simplified or approximated governing equations, such as potential flow around a wing or geostrophic current or wind on the rotating Earth. In that case, the secondary flow usefully spotlights the effects of complicated real-world terms neglected in those approximated equations. For instance,
696-428: A river meanders. This type of slip-off slope is located opposite the cutbank. This term can also be applied to the inside, sloping bank of a meandering tidal channel. In case of an entrenched river, a slip-off slope is a gently sloping bedrock surface that rises from the inside, concave bank of an asymmetrically entrenched river. This type of slip-off slope is often covered by a thin, discontinuous layer of alluvium. It
783-528: A river to meander, secondary flow must dominate. Irrotational flow : From Bernoulli's equations, high pressure results in low velocity. Therefore, in the absence of secondary flow we would expect low fluid velocity at the outside bend and high fluid velocity at the inside bend. This classic fluid mechanics result is irrotational vortex flow. In the context of meandering rivers, its effects are dominated by those of secondary flow. Secondary flow : A force balance exists between pressure forces pointing to
870-413: 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 floor of the channel is part of the secondary flow and sweeps dense eroded material towards the inside of the bend. The cross-current then rises to the surface near the inside and flows towards the outside, forming the helical flow . The greater
957-509: A short time as to create civil engineering challenges for local municipalities attempting to maintain stable roads and bridges. The degree of meandering of the channel 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 straight line down-valley distance. Streams or rivers with a single channel and sinuosities of 1.5 or more are defined as meandering streams or rivers. The term derives from
SECTION 10
#17327720978521044-434: A symmetrical valley sides. He argues that the symmetrical valley sides are the direct result of rapid down-cutting of a watercourse into bedrock. In addition, as proposed by Rich, Thornbury argues that incised valleys with a pronounced asymmetry of cross section, which he called ingrown meanders , are the result of the lateral migration and incision of a meander during a period of slower channel downcutting . Regardless,
1131-413: 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 zero. This axis represents the overall direction of the stream. At any cross-section the flow is following the sinuous axis, the centerline of the bed. Two consecutive crossing points of sinuous and down-valley axes define a meander loop. The meander
1218-411: Is a pressure gradient from the perimeter of the bowl or cup toward the center. This pressure gradient provides the centripetal force necessary for the circular motion of each parcel of water. The pressure gradient also accounts for a secondary flow of the boundary layer in the water flowing across the floor of the bowl or cup. The slower speed of the water in the boundary layer is unable to balance
1305-405: Is also a loss to the power-producing capability of the engine. Thrust-producing flow which passes through an engines thermal cycle is called primary airflow. Using only cycle flow was relatively short-lived as the turbojet engine. Airflow through a propeller or a turbomachine fan is called secondary flow and is not part of the thermal cycle. This use of secondary flow reduces losses and increases
1392-419: Is called lateral accretion. Lateral accretion occurs mostly during high water 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 bars 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
1479-403: Is partly across the isobars rather than parallel to them. This flow of air across the isobars is a secondary flow ., a difference from the primary flow which is parallel to the isobars. Interference by surface roughness elements such as terrain, waves, trees and buildings cause drag on the wind and prevent the air from accelerating to the speed necessary to achieve balanced flow. As a result,
1566-424: Is produced by the gradual outward migration of the meander as a river cuts downward into bedrock. A terrace on the slip-off slope of a meander spur, known as slip-off slope terrace , can be formed by a brief halt during the irregular incision by an actively meandering river. The meander ratio or sinuosity index is a means of quantifying how much a river or stream meanders (how much its course deviates from
1653-458: Is readily eroded and carried toward the middle of the channel. The sediment eroded from a cut bank tends to be deposited on the point bar of the next downstream meander, and not on the point bar opposite it. This can be seen in areas where trees grow on the banks of rivers; on the inside of meanders, trees, such as willows, are often far from the bank, whilst on the outside of the bend, the tree roots are often exposed and undercut, eventually leading
1740-467: Is similar to tropical cyclones but on a much smaller scale so that the Coriolis effect is not significant. The primary flow is circular around the vertical axis of the tornado or dust devil. As with all vortex flow, the speed of the flow is fastest at the core of the vortex. In accordance with Bernoulli's principle where the wind speed is fastest the air pressure is lowest; and where the wind speed
1827-434: Is slightly higher near the concave bank than near the convex bank. A pressure gradient results from the concave bank toward the other bank. Centripetal forces are necessary for the curved path of each parcel of water, which is provided by the pressure gradient. The primary flow around the bend approximates a free vortex – fastest speed where the radius of curvature of the stream itself is smallest and slowest speed where
SECTION 20
#17327720978521914-407: Is slowest the air pressure is highest. Consequently, near the center of the tornado or dust devil the air pressure is low. There is a pressure gradient toward the center of the vortex. This gradient, coupled with the slower speed of the air near the earth's surface, causes a secondary flow toward the center of the tornado or dust devil, rather than in a purely circular pattern. The slower speed of
2001-484: Is the Büyük Menderes River . Meanders are a result of the interaction of water flowing through a curved channel 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 near the surface of the river. This in turn increases carrying capacity for sediments on the outer bank and reduces it on
2088-491: Is the distance to the wall. As the vorticity of each blade onto each other will be of opposite directions, a secondary vorticity will be generated. If the deflection angle, e, between the guide vanes is small, the magnitude of the secondary vorticity is represented as w s = − 2 e ( d c 1 d z ) {\displaystyle w_{s}=-2e\left({\frac {dc_{1}}{dz}}\right)} This secondary flow will be
2175-478: Is the length along the centerline. Once 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 inside 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. Wood: "...this process of making meanders seems to be
2262-405: Is the most common type of fluvial lake, is a crescent-shaped lake that derives its name from its distinctive curved shape. Oxbow lakes are also known as cutoff lakes . 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 accumulate on
2349-430: Is two consecutive loops pointing in opposite transverse directions. The distance of one meander along the down-valley axis is the meander length or wavelength . The maximum distance from the down-valley axis to the sinuous axis of a loop is the meander width or amplitude . The course at that point is the apex. In contrast to sine waves, the loops of a meandering stream are more nearly circular. The curvature varies from
2436-425: Is typically a point bar . The result of this coupled erosion and sedimentation is the formation of a sinuous course as the channel migrates back and forth across the axis of a floodplain . The zone within which a meandering stream periodically shifts its channel is known as a meander belt . It typically ranges from 15 to 18 times the width of the channel. Over time, meanders migrate downstream, sometimes in such
2523-458: The Coriolis effect define an approximate geostrophic wind or gradient wind , balanced flows that are parallel to the isobars . Measurements of wind speed and direction at heights well above ground level confirm that wind matches these approximations quite well. However, nearer the Earth's surface, the wind speed is less than predicted by the barometric pressure gradient, and the wind direction
2610-483: The Ozark Plateau . As noted above, it was initially either argued or presumed that an incised meander is characteristic 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. However, later geologists argue that
2697-449: 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 argues that incised or inclosed meanders are synonyms that are appropriate to describe any meander incised downward into bedrock and defines enclosed or entrenched meanders as a subtype of incised meanders (inclosed meanders) characterized by
Oxbow - Misplaced Pages Continue
2784-469: The geomorphological feature. Strabo said: ‘...its course is so exceedingly winding that everything winding is called meandering.’ The Meander River is south of Izmir, east of the ancient Greek town of Miletus , now Milet, Turkey. It flows through series of three graben in the Menderes Massif, but has a flood plain much wider than the meander zone in its lower reach. Its modern Turkish name
2871-402: The Earth's surface converges toward the center of the cyclone, ascending in the eyewall to satisfy mass continuity . As the secondary flow is drawn upward the air cools as its pressure falls, causing extremely heavy rainfall and releasing latent heat which is an important driver of the storm's energy budget. Tornadoes and dust devils display localised vortex flow. Their fluid motion
2958-417: The air at the surface prevents the air pressure from falling as low as would normally be expected from the air pressure at greater heights. This is compatible with Bernoulli's principle. The secondary flow is toward the center of the tornado or dust devil, and is then drawn upward by the significantly lower pressure several thousands of feet above the surface in the case of a tornado, or several hundred feet in
3045-411: The average fullbank channel width. The length of the stream is measured by channel, or thalweg, length over the reach, while the bottom value of the ratio is the downvalley length or air distance of the stream between two points on it defining the reach. The sinuosity index plays a part in mathematical descriptions of streams. The index may require elaboration, because the valley may meander as well—i.e.,
3132-554: The bottom from the outside to the inside. The flow is supplied by a counter-flow across the surface from the inside to the outside. This entire situation is very similar to the Tea leaf paradox . This secondary flow carries sediment from the outside of the bend to the inside making the river more meandering. As to why streams of any size become sinuous in the first place, there are a number of theories, not necessarily mutually exclusive. The stochastic theory can take many forms but one of
3219-429: The boundary layer. Therefore, within the boundary layer, pressure force dominates and fluid moves along the bottom of the river from the outside bend to the inside bend. This initiates helicoidal flow: Along the river bed, fluid roughly follows the curve of the channel but is also forced toward the inside bend; away from the river bed, fluid also roughly follows the curve of the channel but is forced, to some extent, from
3306-432: The case of a dust devil. Tornadoes can be very destructive and the secondary flow can cause debris to be swept into a central location and carried to low altitudes. Dust devils can be seen by the dust stirred up at ground level, swept up by the secondary flow and concentrated in a central location. The accumulation of dust then accompanies the secondary flow upward into the region of intense low pressure that exists outside
3393-559: The case of the Anderson Bottom Rincon, incised meanders that have either steep-sided, often vertical walls, are often, but not always, known as rincons in the southwest United States . Rincon in English is a nontechnical word in the southwest United States for either a small secluded valley, an alcove or angular recess in a cliff, or a bend in a river. The meanders of a stream or river that has cut its bed down into
3480-417: The center as a result of the secondary flow along the floor. Water flowing through a bend in a river must follow curved streamlines to remain within the banks of the river. The water surface is slightly higher near the concave bank than near the convex bank. (The "concave bank" has the greater radius. The "convex bank" has the smaller radius.) As a result, at any elevation within the river, water pressure
3567-476: The centre, is attached by a plate to the centre underside of a wooden yoke to enable a pair of bullocks/oxen to be chained to any other pairs in a team and to be hitched to the load behind the animal team. The load is a plough or any other dragged, non-motorised, field agricultural machinery . Wooden staves can be used instead with a yoke, which is then termed a withers yoke , named after animals with high backs (withers) (e.g. zebu cattle ) which pull mostly on
Oxbow - Misplaced Pages Continue
3654-436: The characteristic depression toward the axis of the spinning fluid. At any elevation within the water the pressure is a little greater near the perimeter of the bowl or cup where the water is a little deeper, than near the center. The water pressure is a little greater where the water speed is a little slower, and the pressure is a little less where the speed is faster, and this is consistent with Bernoulli's principle . There
3741-406: The concave (outer) bank tends to be steep and elevated due to heavy erosion. Different definitions have been put forward for secondary flow in turbomachinery, such as "Secondary flow in broad terms means flow at right angles to intended primary flow". Secondary flows occur in the main, or primary, flowpath in turbomachinery compressors and turbines (see also unrelated use of term for flow in
3828-545: The consequences of viscosity are spotlighted by secondary flow in the viscous boundary layer , resolving the tea leaf paradox . As another example, if the primary flow is taken to be a balanced flow approximation with net force equated to zero, then the secondary circulation helps spotlight acceleration due to the mild imbalance of forces. A smallness assumption about secondary flow also facilitates linearization . In engineering , secondary flow also identifies an additional flow path. The basic principles of physics and
3915-408: The curvature of the bend, and the faster the flow, the stronger is the cross-current and the sweeping. Due to the conservation of angular momentum the speed on the inside of the bend is faster than on the outside. Since the flow velocity is diminished, so is the centrifugal pressure. The pressure of the super-elevated column prevails, developing an unbalanced gradient that moves water back across
4002-621: The design condition, and secondary vorticity. The following, from Dixon, shows the secondary flow generated by flow turning in an axial compressor blade or stator passage. Consider flow with an approach velocity c1. The velocity profile will be non-uniform due to friction between the annulus wall and the fluid. The vorticity of this boundary layer is normal to the approach velocity c 1 {\displaystyle c_{1}} and of magnitude w 1 = d c 1 d z , {\displaystyle w_{1}={\frac {dc_{1}}{dz}},} where z
4089-405: The downvalley length is not identical to the reach. In that case the valley index is the meander ratio of the valley while the channel index is the meander ratio of the channel. The channel sinuosity index is the channel length divided by the valley length and the standard sinuosity index is the channel index divided by the valley index. Distinctions may become even more subtle. Sinuosity Index has
4176-565: The formation of both entrenched meanders and ingrown meanders is thought to require that base level falls as a result of either relative change in mean sea level , isostatic or tectonic uplift, the breach of an 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 central Kentucky , and streams in
4263-428: The full force of the flood. After 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 channel and independent of the river. During floods, the flood waters deposit fine-grained sediment into
4350-404: 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 2–3 times the channel width. 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 direction of flow is downward, scouring the bed material. The major volume, however, flows more slowly on
4437-513: The impeller in a centrifugal compressor but are less marked in axial compressors due to shorter passage lengths. Flow turning is low in axial compressors but boundary layers are thick on the annulus walls which gives significant secondary flows. Flow turning in turbine blading and vanes is high and generates strong secondary flow. Secondary flows also occur in pumps for liquids and include inlet prerotation, or intake vorticity, tip clearance flow (tip leakage), flow separation when operating away from
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#17327720978524524-403: The influence of the ground. When water in a circular bowl or cup is moving in circular motion the water displays free-vortex flow – the water at the center of the bowl or cup spins at relatively high speed, and the water at the perimeter spins more slowly. The water is a little deeper at the perimeter and a little more shallow at the center, and the surface of the water is not flat but displays
4611-407: The inner bank, so that sediments are eroded from the outer bank and redeposited on the inner bank of the next downstream meander. When a fluid is introduced to an initially straight channel which then bends, the sidewalls induce a pressure gradient that causes the fluid to alter course and follow the bend. From here, two opposing processes occur: (1) irrotational flow and (2) secondary flow . For
4698-416: 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 final break-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 can flow directly across the neck and erode it with
4785-443: The inside bank of a river bend. On the inside bend, this sediment and debris is eventually deposited on the slip-off slope of a point bar. Scroll-bars are a result of continuous lateral migration of a meander loop that creates an asymmetrical ridge and swale topography 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, which carve sediment from
4872-467: The inside bend of the river and centrifugal forces pointing to the outside bend 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 effectively zero. Centrifugal force, which depends on velocity, is also therefore effectively zero. Pressure force, however, remains unaffected by
4959-406: The inside of the bend where, due to decreased velocity, it deposits sediment. The line of maximum depth, or channel, is the thalweg or thalweg line. It is typically designated the borderline when rivers are used as 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
5046-450: The inside to the outside bend. The higher velocities at the outside bend lead to higher shear stresses and therefore result in erosion. Similarly, lower velocities at the inside bend cause lower shear stresses and deposition occurs. Thus meander bends erode at the outside 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,
5133-452: The integrated effect of the distribution of secondary vorticity along the blade length. Gas turbine engines have a power-producing primary airflow passing through the compressor. They also have a substantial (25% of core flow in a Pratt & Whitney PW2000 ) secondary flow obtained from the primary flow and which is pumped from the compressor and used by the secondary air system. Like the secondary flow in turbomachinery this secondary flow
5220-487: The isobars. This descent causes a reduction in relative humidity and explains why regions of high pressure usually experience cloud-free skies for many days. The flow around a tropical cyclone is often well approximated as parallel to circular isobars , such as in a vortex . A strong pressure gradient draws air toward the center of the cyclone, a centripetal force nearly balanced by Coriolis and centrifugal forces in gradient wind balance. The viscous secondary flow near
5307-420: 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 but the parameters are independent of it and apparently are caused by geologic factors. In general the meander length is 10–14 times, with an average 11 times,
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#17327720978525394-418: The more heterogeneous braided river deposits. There are two distinct patterns of scroll-bar depositions; the eddy accretion scroll bar pattern and the point-bar scroll pattern. When looking down the river valley they can be distinguished because the point-bar scroll patterns are convex and the eddy accretion scroll bar patterns are concave. Scroll bars often look lighter at the tops of the ridges and darker in
5481-504: The most general statements is that of Scheidegger: "The meander train is assumed to be the result of the stochastic fluctuations of the direction of flow due to the random presence of direction-changing obstacles in the river path." Given a flat, smooth, tilted artificial surface, rainfall runs off it in sheets, but even in that case adhesion of water to the surface and cohesion of drops produce rivulets at random. Natural surfaces are rough and erodible to different degrees. The result of all
5568-529: The outside of the curve and deposit sediment in the slower flowing water on the inside of the loop, in a process called lateral accretion. Scroll-bar sediments are characterized by cross-bedding and a pattern of fining upward. These characteristics are a result of the dynamic river system, where larger grains are transported during high energy flood events and then gradually die down, depositing smaller material with time (Batty 2006). Deposits for meandering rivers are generally homogeneous and laterally extensive unlike
5655-418: The overall efficiency of the propulsion system. The secondary flow may be many times that through the engine. During the 1960s cruising at speeds between Mach 2 to 3 was pursued for commercial and military aircraft. Concorde , North American XB-70 and Lockheed SR-71 used ejector-type supersonic nozzles which had a secondary flow obtained from the inlet upstream of the engine compressor. The secondary flow
5742-415: The oxbow lake. As a result, oxbow lakes tend to become filled in with fine-grained, organic-rich sediments over time. A point bar , which is also 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 by the accompanying migration of the channel toward its outer bank. This process
5829-475: 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. In the equilibrium theory, meanders decrease the stream gradient until an equilibrium between the erodibility of the terrain and the transport capacity of the stream is reached. A mass of water descending must give up potential energy , which, given
5916-399: The pressure gradient and so its path is partly downstream and partly across the stream from the concave bank toward the convex bank, driven by the pressure gradient. The secondary flow is then upward toward the surface where it mixes with the primary flow or moves slowly across the surface, back toward the concave bank. This motion is called helicoidal flow . On the floor of the river bed
6003-455: The pressure gradient. The boundary layer spirals inward toward the axis of circulation of the water. On reaching the center the secondary flow is then upward toward the surface, progressively mixing with the primary flow. Near the surface there may also be a slow secondary flow outward toward the perimeter. The secondary flow along the floor of the bowl or cup can be seen by sprinkling heavy particles such as sugar, sand, rice or tea leaves into
6090-399: The radius is largest. The higher pressure near the concave (outer) bank is accompanied by slower water speed, and the lower pressure near the convex bank is accompanied by faster water speed, and all this is consistent with Bernoulli's principle . A secondary flow is produced in the boundary layer along the floor of the river bed. The boundary layer is not moving fast enough to balance
6177-636: The river width remains nearly constant, even as the river evolves. In a speech before the Prussian Academy of Sciences in 1926, Albert Einstein suggested that because the Coriolis force of the earth can cause a small imbalance in velocity distribution, such that velocity on one bank is higher than on the other, it could trigger the erosion on one bank and deposition of sediment on the other that produces meanders However, Coriolis forces are likely insignificant compared with other forces acting to produce river meanders. The technical description of
6264-416: The same velocity at the end of the drop as at the beginning, is removed by interaction with the material of the stream bed. The shortest distance; that is, a straight channel, results in the highest energy per unit of length, disrupting the banks more, creating more sediment and aggrading the stream. The presence of meanders allows the stream to adjust the length to an equilibrium energy per unit length in which
6351-401: The secondary air system of a gas turbine engine). They are always present when a wall boundary layer is turned through an angle by a curved surface. They are a source of total pressure loss and limit the efficiency that can be achieved for the compressor or turbine. Modelling the flow enables blade, vane and end-wall surfaces to be shaped to reduce the losses. Secondary flows occur throughout
6438-507: The secondary flow sweeps sand, silt and gravel across the river and deposits the solids near the convex bank, in similar fashion to sugar or tea leaves being swept toward the center of a bowl or cup as described above. This process can lead to accentuation or creation of D-shaped islands, meanders through creation of cut banks and opposing point bars which in turn may result in an oxbow lake . The convex (inner) bank of river bends tends to be shallow and made up of sand, silt and fine gravel;
6525-476: The secondary flow toward the center of a region of low pressure is also drawn upward by the significantly lower pressure at mid altitudes. This slow, widespread ascent of the air in a region of low pressure can cause widespread cloud and rain if the air is of sufficiently high relative humidity . In a region of high pressure (an anticyclone ) the secondary flow includes a slow, widespread descent of air from mid altitudes toward ground level, and then outward across
6612-417: The sediment is deposited the vertical sequence of sediments comprising a point bar becomes finer upward within an individual point bar. For example, it is typical for point bars to fine upward from gravel at the base to fine sands at the top. 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
6699-505: The shape of an incised meander is not always, if ever, "inherited", e.g., strictly from an antecedent meandering stream where its meander pattern could freely develop on a level floodplain. Instead, they argue that as fluvial incision of bedrock proceeds, the stream course is significantly modified by variations in rock type and fractures , faults , and other geological structures into either lithologically conditioned meanders or structurally controlled meanders . The oxbow lake , which
6786-445: The shortest possible path). It 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 1 (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 reach , which should be at least 20 times
6873-411: The sinuous axis. A loop at the apex has an outer or concave bank and an inner or convex bank. The meander belt is defined 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 then said to be free—it can be found anywhere in the flood plain. If there is no flood plain,
6960-419: The stream carries away all the sediment that it produces. Geomorphic refers to the surface structure 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 not random and guide streams into non-random paths. They are predictable obstacles that instigate meander formation by deflecting the stream. For example,
7047-403: The stream might be guided into a fault line (morphotectonic). A cut bank is an often vertical bank or cliff that forms where the outside, concave bank of a meander cuts into the floodplain or valley wall of a river or stream. A cutbank is also known either as a river-cut cliff , river cliff , or a bluff and spelled as cutbank . Erosion that forms a cut bank occurs at the outside bank of
7134-416: The swales. This is because the tops can be shaped by wind, either adding fine grains or by keeping the area unvegetated, while the darkness in the swales can be attributed to silts and clays washing in during high water periods. This added sediment in addition to water that catches in the swales is in turn is a favorable environment for vegetation that will also accumulate in the swales. Depending upon whether
7221-406: The trees to fall into the river. A meander cutoff , also known as either a cutoff meander or abandoned meander , is a meander that has been abandoned 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 . As in
7308-411: The water and then setting the water in circular motion by stirring with a hand or spoon. The boundary layer spirals inward and sweeps the heavier solids into a neat pile in the center of the bowl or cup. With water circulating in a bowl or cup, the primary flow is purely circular and might be expected to fling heavy particles outward to the perimeter. Instead, heavy particles can be seen to congregate in
7395-446: The wind direction near ground level is partly parallel to the isobars in the region, and partly across the isobars in the direction from higher pressure to lower pressure. As a result of the slower wind speed at the earth's surface, in a region of low pressure the barometric pressure is usually significantly higher at the surface than would be expected, given the barometric pressure at mid altitudes, due to Bernoulli's principle . Hence,
7482-603: The winding river Menderes located in Asia-Minor and known to the Ancient Greeks as Μαίανδρος Maiandros ( Latin : Maeander ), characterised by a very convoluted path along the lower reach. As a result, even in Classical Greece (and in later Greek thought) the name of the river had become a common noun meaning anything convoluted and winding, such as decorative patterns or speech and ideas, as well as
7569-418: The yoke part of the equipment, not as greatly on the bow shape borne by the stronger front quarters of oxen and bullocks. Meander 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 bank ( cut bank or river cliff ) and deposits sediments on an inner, convex bank which
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