A point bar is a deposition feature made of alluvium that accumulates inside the bend of rivers and streams below the slip-off slopes. Bar dots are found in abundance in adult or winding streams. They are crescent-shaped and located on the inside of the bend of the river, very similar to, though often smaller than, towheads, or river islands.
The point bar consists of well sorted sediments and usually reflects the overall capacity of the flow. They also have a very soft slope and their altitude is very close to the water level. Due to the lowlands, they are often overtaken by floods and can accumulate driftwood and other debris during high water periods. Due to their near flat topography and the fact that slow water speeds in the shallow end of the bar their point is a popular rest spot for sailors and rafters. However, camping at the bar point can be dangerous because flash floods that raise flow rates by as little as a few inches (centimeters) can flood the campsite in a moment.
Point bar is the deposition area whereas the cut bank is the erosion area.
The point bars are formed as a secondary stream from the sweeping stream and rolls the sand, gravel and small rocks laterally across the river bed and up to the shallow sloping floor of the dot bar.
Video Point bar
Formation
Any liquid, including water in the flow, can only flow around the bend in the vortex flow. In the vortex flows the fastest liquid velocity in which the radius of the stream is the smallest, and the slowest in which the radius is the largest. (Tropical cyclones, tornadoes, and the rotating motion of water as it passes through the waterways are all visible examples of vortex flow.) In the case of water flowing around the bend in the secondary stream flow at the boundary layer along the river floor does not flow parallel to the riverbank but flows partly across the bottom of the stream toward the inner side of the river (where the radius of curvature is smallest). This boundary movement is capable of sweeping and rolling loose particles including sand, gravel, small stones and other submerged objects along the base of the stream to the point of the stem.
This can be shown at home. Some fill a round bowl or cup with water and sprinkle a little sand, rice or sugar into the water. Arrange water in a circular motion by hand or spoon. The secondary flow will quickly sweep the solid particles into a neat pile in the middle of a bowl or cup. The primary stream (vortex) may be expected to sweep the solid particles to the perimeter of the bowl or cup, but instead the secondary stream along the floor of the bowl or cup sweeps the particles toward the center.
If flow follows a straight line, the slower boundary layer along the bottom of the stream also follows the same straight path. It sweeps and rolls sand, gravel and smooth rocks downstream, along the river bed. However, since the flow enters the flow of bends and the vortex begins as the main stream, the secondary flow also begins and flows partly across the base of the stream toward the convection bank (bank with a smaller radius). Sand, pebbles, and polished stones that have traveled by the river for a great distance where the stream follows a straight path can finally come to rest at the first river bend point bar.
Due to the circular path from the stream around the bend, the water level is slightly higher near the sunken bank (bank with a larger radius) than near the convex bank. The small slopes on the surface of the river water lead to slightly larger water pressures at the bottom of the river near the concave bank than near the convex bank. This pressure gradient pushes the slower boundary layer across the bottom of the flow toward the convex bank. The pressure gradient is capable of moving the boundary layer to the shallow beveled floor of the bar point, causing sand, gravel and polishing stones to be swept and rolled up the hill.
Concave banks are often bank cutters and erosion areas. The eroded material is swept and overthrown at the base of the stream by the secondary stream and can be deposited at the point of the bar just a small distance downstream from its original location in the sunken bank.
Point bars usually have sloping floors with shallow water. Shallow water is largely an accumulated boundary layer and does not have fast speed. However, in the deepest part of the stream where the flow flows freely, the vortex flow is in effect and the flow flows at the most rapidly where the bend radius is the smallest, and the slowest where the largest radius is. Shallow water around the stem point can become dangerous when the flow rises. As the water depth rises above the shallow end of the point rod, the vortex flow may extend closer to the convex bank and the water velocity at any point can increase dramatically in response to only a small increase in water depth.
Maps Point bar
Error related to point formatting
An old error exists concerning the formation of the dot and elbow lake which indicates that they are formed by the deposition of a water-stuck burden that claims the velocity and energy of the flow decreases towards the inside of the bend. This error depends on the false assumption that the momentum of water is "always" the slowest in the inside of the bend (where the smallest radius) and fastest outside the bend (where the largest radius), which ignores the increase in angular momentum.
Suspended solids mass deposition is rare in one bank deposited in the tidal estuary; on the contrary, a faster vortex flow inside the bank compensates for higher heights and hence the mass of water flowing downstream along the concave bank, and the shallow, shallow beds typically provide per liter of water over more agitation to retain each particle which is suspended. Any open flow of a relatively stable gradient does not meet with complex interactions with opposing currents, such as currents, or major obstacles, flowing around the bend in a simple vortex flow model, with relatively few variables and coefficients.
All point stems usually have a sloping floor with shallow water. Clearly higher water proportions in very shallow waters are much more likely to work on friction above and below (especially in winds that blow) that lower their speed, see Bernouilli's principle. It may be this close observation that led early geographers to believe in sedimentation by suspended sedimentation rather than near-bed secondary currents.
In the stable gradient part of the water flow, sedimentation can occur where saturated water and shallow banks have high flow resistance but do not interfere with suspension. Similarly, the error has little explanation as to why the deposition occurs at the bend of the river, and little or nothing happens where the flow follows a straight direction, with the exception of a steep slope (river gradient) where the stream has formed a natural cut or waterfall and can then deposit part of the cargo at the point of meeting the less steep section eg tortuous large.
In the low gradient section of the winding water course, slow water velocity, low turbulence, and water are unable to withstand gritty sand and gravel in suspension. In contrast, the point rod consists of coarse sand, gravel, polishing stones, and other submerged objects. These materials have not been brought in suspension and then dropped at the bar point - they have been swept and rolled into place by secondary streams on the floor/bed around the river bend, which will be intensified if there is a reflection especially from an irregular counter bank and swept away.
See also
- Bank erosion
- Bar (river morphology)
- Cut banks
- Fluvial
- Helicoidal stream
- Oxbow Lake
- River pocket
- Secondary stream in bowl or cup
- Secondary stream at the bend of the river
- Vortex
Note
References
- Tarbuck, E. J. and F. K. Lutgens. Earth , Issue 7. Prentice Hall: Upper Saddle River, New Jersey, 2002. pp. 277, 279.
Source of the article : Wikipedia
0 Comments