Mechanics of Water Erosion

 There are three steps for accelerated erosion by water:

i) Detachment or loosening of soil particles caused by flowing water, freezing and thawing of the topsoil, and/or the impact of falling raindrops. Rain splash is the most important detaching agent.

ii) Transportation of soil particles by floating, rolling, dragging, and/or splashing and

iii) Deposition of transported particles at some places of lower elevation.

Types of Water Erosion

Water erosion can be classified as splash erosion, sheet erosion, rill erosion, gully erosion, stream bank erosion, sea-shore erosion, and landslide erosion. They are discussed as follows.

1. Splash Erosion: A falling raindrop is capable of generating a force equals to almost 14 times its weight.

  • It is also known as raindrop erosion because it is caused by the impact of raindrops on the exposed soil surface.
  • Most important detaching agent.
  • A raindrop produces three important effects during splash erosion, i) beating action breaks down soil aggregates ii) it detach soil particles and iii) its splash after touching the ground causes displacement of soil.

2. Sheet Erosion: Sheet erosion may be defined as more or less uniform removal of soil in the form of a thin layer or in “sheet” form by the flowing water from a given width of sloping land.

  • It is an inconspicuous type of soil erosion because the total amount of soil removed during any storm is usually small. Soil matrix is lost but remains undetected for a long period and a thin veneer of soil is removed from large areas uniformly during every rain.

3. Rill Erosion: It is the removal of soil by water from small channels which develop due to the concentration of surface flow along the slope.

  • This type of water erosion is formed in the bare, unprotected, and cultivated fields where the land surface is almost irregular.
  • It is the most common form of soil erosion by water.
  • It is an intermediate stage between sheet and gully erosion. It can be removed by normal tillage operation

4. Gully Erosion: It is the advanced stage of rill erosion that cannot be obliterated by normal tillage operation.

  • Rills with more than 30 cm depths are generally called gullies.
  • They cut up large fields into small fragments and in course of time make them unfit for cultivation.
  • Large gullies and their network are called ravines. It is the advanced and last stage of water erosion.

5. Stream Bank Erosion: It is caused by streams and torrents, also called chos, cutting their banks and adjoining fields.

  • Streambank erosion is mainly due to runoff flowing over the side of the stream banks or by scouring and undercutting below the surface water.
  • Scour erosion is influence by the velocity of water flow, depth, and width of channels.

6. Sea-shore Erosion: It is also called coastal erosion.

  • The strong waves of sea strike against the seashore causing large-scale soil erosion.
  • It is the combined effect of wind and water.

7. Mass movement/Landslide Erosion: During monsoon season or heavy downpour the hills get saturated with water resulting in instability of the landmass. With gravity, the complete surface portion of the soil may slip down from its actual position resulting landslide.

  • Common in hills with undulating topography.

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