Erosion Due to Water

Water erosion occurs when the soil is moved or removed by the action of water. It is caused by both liquid water (flowing water, such as rivers and rainfall runoff) and solid water (glaciers). Below are the different forms, mechanisms, and factors affecting water erosion in more detail.

Forms of Water Erosion

1. Splash Erosion: Splash erosion happens when raindrops hit the soil surface with force, detaching soil particles. This is particularly pronounced on bare soil, where no protective vegetation is present to absorb the impact.
2. Sheet Erosion: Sheet erosion is the uniform removal of a thin layer of soil from the surface due to the flow of water. This type of erosion is difficult to observe but can lead to significant soil loss, especially on large flat areas.
3. Rill Erosion: Rill erosion occurs when runoff water concentrates in small channels or rills. These are shallow, narrow channels that form when water flows over the surface and removes soil from small areas. These rills can eventually evolve into larger channels.
4. Gully Erosion: Gully erosion is more severe than rill erosion and involves the removal of large amounts of soil, forming deep, wide gullies. This type of erosion typically occurs in areas with high runoff and steep slopes.
5. Bank Erosion: Bank erosion occurs in areas where water bodies like rivers, lakes, or reservoirs erode the sides or banks due to the action of water waves. Over time, this can lead to the widening of the watercourse.
6. Pipe (Tunnel) Erosion: Pipe or tunnel erosion happens when water runs beneath the soil surface, forming hollow channels or pipes. This often occurs in areas with loose, permeable soils. When these tunnels collapse, they cause subsurface erosion, creating cavities or depressions on the surface.
7. Glacial Erosion: Glacial erosion occurs due to the movement of glaciers over the land, scraping and carrying away soil and rock. In India, this type of erosion is primarily observed in the Himalayan regions where glaciers are present.

Mechanisms of Water Erosion

Water erosion occurs through various physical and chemical processes. These processes cause the detachment, transport, and eventual deposition of soil particles.

Hydraulic Action

• Hydraulic action refers to the pressure exerted by water as it flows over the soil surface. When water flows across the land, it compresses the soil, and the air trapped in the soil voids exerts pressure on the soil particles, leading to detachment. These particles are then carried away by the flowing water. The effectiveness of hydraulic action is higher in loose or sandy soils compared to compacted soils.

Abrasion

• Abrasion is caused when soil particles, mixed with water, act as an abrasive tool. As the particles move with the water, they scrape and wear down other soil particles. The process helps to erode larger particles from the surface and increases the capacity of water to remove soil from the land.

Attrition

• Attrition is the process of mechanical breakdown of soil particles due to collisions with other moving particles. As these particles rub against each other, they break into smaller fragments. These smaller particles are then transported by the flowing water, and this cycle continues, causing further erosion of the land.

Solution

• Solution erosion is a chemical process where the water dissolves soluble materials in the soil or underlying rocks. This occurs in areas where rocks or soils are easily soluble in water, like limestone or gypsum. The dissolved materials are then carried away by the water, contributing to erosion.

Types of Water Erosion

Water erosion is a significant process that shapes the landscape, and it can be categorized into various types based on the severity and nature of the erosion. These include:

1. Splash Erosion:

• Also known as raindrop erosion, this type occurs when raindrops hit the exposed soil surface. The impact creates craters, and soil particles are splashed into the air.
• Process:
  • Raindrops hit the soil, causing soil particles to be lifted and moved.
  • In flat areas, splashed particles can move horizontally up to 1.5 meters, while on sloping land, most particles move downhill with runoff.
  • The height of splashing depends on the size of the raindrops, and the soil can be lifted as high as 50 to 75 cm

2. Sheet Erosion:

• Sheet erosion refers to the removal of a thin layer of soil over a broad area due to flowing water. This is a subtle form of erosion, often going unnoticed, but over time, it can lead to significant soil loss.
• Process:
  • Raindrop impact detaches soil particles, which are then transported by surface runoff.
  • Sheet flow detaches soil particles and carries them across the surface. The extent of erosion depends on water depth and velocity.
  • This type of erosion becomes more prominent when rainfall exceeds the soil’s infiltration rate, causing excess water to flow over the surface and strip away soil.

3. Rill Erosion:

• Rill erosion occurs when water accumulates in small depressions of the land surface, forming small channels (called rills).
• Process:
  • Water concentrates in depressions and starts to erode the soil, forming narrow channels.
  • As water moves, it carries away soil particles, which accumulate in suspension, contributing to further erosion.
  • The channels are small and can be easily corrected by tillage.

4. Gully Erosion:

• Gully erosion is an advanced stage of rill erosion where rills grow into larger channels or gullies that are too large to be fixed by normal tillage.
• Process:
  • Gullies form when rills are not managed and continue to grow over time, becoming deeper and wider.
  • Causes include steep slopes, poor soil texture, high rainfall intensity, and improper land use.
  • Large gullies, known as ravines, can form extensive networks that disrupt the landscape.

5. Stream Bank Erosion:

• Stream bank erosion occurs when water erodes the soil along the banks of rivers and streams.
• Process:
  • This type of erosion happens due to the flow of water that undermines the bank, causing soil to slough off or get scoured.
  • Factors like vegetation removal, overgrazing, and flooding can exacerbate stream bank erosion.
  • Sloughing occurs when underground water movement pressures the stream bank, causing parts of it to slide off.

6. Sea-shore/Coastal Erosion:

• Coastal erosion, also called sea-shore erosion, involves the removal of sediment and rocks from the coastline due to wave action, tidal currents, and storms.
• Process:
  • The movement of water, especially during storms or by human activities, leads to the erosion of beaches and dunes.
  • This can happen through processes like hydraulic action, where water impacts the shore, abrasion, and corrosion, where waves wear away the coastline over time.
  • Coastal erosion can result in long-term sediment loss or temporary redistribution.

7. Landslide Erosion:

• Landslide erosion occurs when large amounts of soil and rock move downhill, typically triggered by heavy rainfall or seismic activity.
• Process: During intense rainfall or after earthquakes, the soil on slopes can become saturated, causing it to slip or slide. The movement of soil can expose the underlying layers and may lead to significant erosion, especially in areas with loose or unstable soils.

Each of these types of water erosion can have devastating effects on soil fertility, agricultural productivity, and the landscape, requiring effective soil and water conservation practices to mitigate their impact.

Factors Affecting Water Erosion

Water erosion is influenced by several natural and environmental factors that affect the soil’s susceptibility to erosion and the water’s capacity to transport the eroded material.

Climatic Factors

Climate plays a key role in water erosion, with elements like rainfall, temperature, and wind influencing the intensity and frequency of erosion.

• Rainfall: Frequency, Intensity, and Duration: These aspects of rainfall influence how much water flows across the land. Heavy rainfall can cause significant runoff and increase erosion potential. Short, intense storms are more erosive than light, steady rainfall because they cause more surface runoff.
• Temperature: Frozen Soils: Frozen soils are more resistant to erosion as the water cannot penetrate the surface. However, when frozen soils thaw, they can become highly erodible, especially if the thaw is accompanied by rainfall.
• Wind: Wind Erosion: Wind can dry out the soil, making it more vulnerable to erosion. Wind also exacerbates the need for additional irrigation and soil conservation practices to prevent erosion.

Soil Characteristics

Soil properties such as texture, structure, organic matter content, and permeability significantly influence the rate and extent of erosion.

• Texture:
  Soils that are rich in sand and silt are more prone to erosion because they lack cohesion. Clay-rich soils are more resistant to erosion but can be easily transported by water once detached.
• Organic Matter: Soils high in organic matter are less prone to erosion as organic material binds the soil particles together, increasing soil cohesion and reducing the impact of raindrops.
• Permeability: Soils with high permeability (e.g., sandy soils) allow water to seep in quickly, preventing surface runoff and reducing erosion. Conversely, low permeability soils lead to increased surface runoff and more erosion.

Vegetative Cover

Vegetation plays a crucial role in preventing erosion:

• Raindrop Impact Absorption: Vegetation reduces the impact of raindrops on the soil surface, thus preventing soil detachment.
• Slowing Down Runoff: Vegetation helps slow down surface water flow, reducing the erosive power of water.
• Binding Soil: Roots from plants help bind the soil, making it more resistant to erosion.
• Evapotranspiration: Plants also remove excess moisture from the soil through evapotranspiration, which reduces surface runoff during rainfall events.

Topographic Factors

The shape and slope of the land significantly influence the potential for erosion:

• Slope: Steeper slopes increase runoff velocity and water’s erosive power. On steeper terrain, water has more momentum and is more likely to detach and transport soil particles.
• Slope Length: Longer slopes allow water to accumulate more runoff, increasing the potential for erosion at the base of the slope.
• Slope Shape:
  • Convex Slopes: These slopes are more prone to erosion at the bottom as the slope increases in steepness.
  • Concave Slopes: These slopes tend to accumulate water and reduce the rate of erosion at the base.