Hydraulic action, Cavitation, Abrasion, Attrition and Corrosion
It is the mechanical loosening and removal of the material from the rocks due to pressure exerted by the running water. The higher the velocity the greater is the pressure of the running water and hence greater is its capacity to bodily move out parts of the rock or grains of soil from the parent body occurring along its base or sides. The river water flowing with sufficient velocity often develops force strong enough to disintegrate a loose rock, displace the fragments so created and lift them up and move forward as part of bed load.
It is distinct and rare type of hydraulic action performed by running water. It is particularly observed where river water suddenly acquires exceptionally high velocity such as at the location of a waterfall. In other words there is a spontaneous change fro a liquid to vapour state and back to liquid state at that point. The phenomenon of cavitations is also observed in hydropower generation projects.
It is the principal method of stream erosion and involves wearing away of the bedrocks and rocks along the banks of a stream or river by the running water with the help of sand grain, pebbles and gravels and all such particles that are being carried by its as load. These particles grains and rock fragments moving along with river water are collectively known as tolls of erosion. The river valleys, water falls, escarpments, gorges and canyons and river terraces are some of the so well known examples developed principally by river abrasion.
This term is used for wear and tear of the load sediments being transported by a moving natural agency through the process of mutual impacts and collisions which they suffer during their transport. Every part of the sediment in load in suspension or being moved along the bed of the stream receives repeated impacts from other particles,. Due to these mutual collusions, the irregularities and angularities of the particles are worn out. These become spherical in outline
and rounded and polished at the surface. Some of the fragments any eventually get reduced to very fine particles that rea easily carried along with the running water for considerable distances.
The slow built steady chemical action of the stream water on the rocks is expresses by the term corrosion. The extent of corrosion depends such on the composition of rocks and also on
the composition of flowing water. Thus all rocks are not equally susceptible to corrosive action of stream water. Limestones, gypsum and rock salt bodies are soluble in water to varying degrees. The steam may hardly corrode sandstones, quartzites, granites and gneisses.