Fluvial works
Erosional work of river :
The word erosion has been derived from the Greek word 'erodere' which means to gnaw. Fluvial erosion is a dynamic process which involves the removal of Geo material from the rocks. The factors controlling the erosional work of river include channel gradient and volume of water, velocity of water, river load etc. The quantity and angularity of erosion tools largely control nature and magnitude of fluvial erosion.
The following relationship may be identified between the rate of fluvial erosion and river load.
1. Erosion becomes minimum in the absence of required amount of river load.
2. Erosion also becomes minimum when the river has maximum load.
3. Erosion becomes minimum when the river carries load according to its transporting capacity.
Besides river load velocity and channel gradient are also important factors which effectively control fluvial erosion. The velocity of water flow depends on the channel gradient and volume of water. The relationship between the velocity and a capacity of erosion can be expressed-
Capacity of erosion = (Velocity of water)²
It means that if the velocity of water doubled, erosion becomes four fold and when velocity of water increases fivefold erosion also increases 25 time.
The types of Fluvial erosion:
The erosional work of river is performed in two ways (a) chemical erosion, and (b) mechanical erosion. Chemical erosion involves solution and carbonation. Again mechanical erosion involves abrasion, attrition, and Hydraulic action.
1. Solution :
Solution involves dissolution of soluble materials through the process of disintegration and decomposition of carbonate rocks (eg: limestone, dolomite).
2. Abrasion or Corrosion :
Abrasion involves the removal of loosened materials of the rocks with the help of erosional tools carried by the rivers. (Erosional tools are like boulders, pebbles, cabbles, etc.).
3. Attrition:
Attrition involves mechanical tear and wear of erosional tools in themselves. The boulders, cables, pebbles etc. while moving with water, collied against each other and are fragmented into smaller and finner pieces in the transit.
4. Hydraulic Action:
Hydraulic action involves breaking down of rocks by water currents. it is the breaking down of of rocks by water alone.
Erosional landforms produced by rivers:
1. River valleys : Valleys formed in the youthful stage of fluvial cycle of erosion and in the initial stage of Valley development is V shaped having steep convex Valley side slopes. The valley is very deep and a narrow. It is the result of accelerated rate of vertical erosion.
2. Waterfalls : when the water of a river suddenly falls down a steep or vertical slop waterfalls are formed. Fairbridge define waterfall as a vertical drop of water magnitude both in volume and height.
3. River Terraces : The narrow flat surfaces on either side of the valley floor are called river terraces which represent the level of former valley floors and remants former flood plains.
4. Ox-bow lake : the lakes formed due to impounding of water in the abandoned meander loops are called ox-bow or horse-show lakes.
Depositional landforms produced by water :
Rivers deposit sediments in different parts of their caurses and thus forms various types of landforms which are called constructional or depositional landforms. For example Alluvial fans, Alluvial cones, natural levees,floodplains etc.
1. Natural levees: the narrow belt of ridges of low height built by the deposition of sediments by the spill water of the stream on its either bank is called natural Levee or neutral embankment.
2. Delta: the depositional feature of almost triangular shape at the mouth of the river debouching either in a lake or sea is called Delta. The word Delta was first used by Greek historian Herodotus for the triangle and depositional feature at the mouth of Nile river.
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