AS- Birth, development , migration and death of a meander. Click on this link to download this post.
Meanders start when friction with the channel bed and banks causes turbulence in the water flow. This results in a spiralling flow of water called helicoidal flow- a corkscrew like movement of the water as it spirals downstream from bank to bank -.
Figure 1 Helicoidal flow and the formation of pools and riffles
This causes erosion in some areas of the banks where velocity is high and deposition in other places where velocity is reduced. A sequence of deep sections (pools) and shallow sections (riffles) develops at equal intervals along a stretch of the river. The spacing between pools and riffles is usually very regular, being five to six times that of the bed width. Energy increases within a pool area due to less friction and is then lost as the water flows over the shallower riffle where friction is greater.
Flow over pools and riffles become uneven and results in the maximum flow being towards one side downstream, producing erosion of the material of the concave bank, forming a river cliff. The erosion of the outside bank of the meaner is accompanied by deposition on the convex bank (the inside of the meander), forming a sandy or gravelly area called a point bar or slip off slope. This is due to helicoidal flow which transports sediment eroded from the concave bank downstream to the next convex bank.
Thus, the meanders have a distinct asymmetrical profile.
Figure 2 Cross-section of a meander
The greater erosion of the concave bank occurs just downstream of the axis of the meander bend, because the course of the maximum velocity zone in the channel does not reflect the meander shape. This causes meander to migrate down the valley. The lateral erosion of the meanders and their migration widen the flood plain.
Figure 3: How meanders migrate
The width of the channel remains relatively constant, while the whole channel migrate laterally and become more sinuous. Sinuosity is described as the actual channel length divided by the straight-line distance.
If sinuosity becomes too great, the river in flood may be able to break through the meander neck, thereby straightening its channel. The abandoned channel, a cut-off, may be used during flood, but frequently the ends nearest the new course become blocked with alluvium and the channel becomes a small lake or oxbow lake.
Figure 4 Meanders and ox-bow lakes