Soil erosion is a form of soil degradation where the upper layer of the soil is displaced. This can be caused by all sorts of erosive agents such as wind, water, ice, snow, humans, animals, and plants. Soil erosion can be a slow process that sometimes goes unnoticed, but it can occur at alarming rates in some cases. The loss of the soil can hinder crop production, damage drainage networks, and lower the quality of water.
Human activity has increased the rate of erosion by 10-15 times around the world. Erosion has been accelerated both on-site and off-site. On-site erosion can affect ecological and agricultural collapse. This is due to the loss of the nutrient-rich upper soil layers. Off-site erosion can cause water bodies and waterways to develop sediment or damage. This can lead to a lot of damage to the land and organisms in the area.
Wind and water erosion cause around 84% of all land degradation but human activity has continued to expose the land to them. Deforestation, unsustainable agricultural practices, roads, climate change, and urban sprawl have led to a significant increase in soil erosion. This is why it is important to study all the different types of soil erosion carefully so that we can work together to combat the adverse effects of our human activity. Due to so many different erosive agents, soil erosion is categorized between water, glacial, snow, wind, zoogenic, and anthropogenic erosion.
Table of Contents
Surface Runoff and Rainfall Erosion
Surface runoff from rainfall and rainfall itself can result in large amounts of soil erosion. There are four main types of soil erosion that take place: splash, sheet, rill, and gully.
The first stage of the erosion process is splash erosion. This occurs when raindrops fall on top of the soil. The soil is usually aggregated together but begins to break up into individual particles. These particles can rise by 60cm above the soil surface. They detach from the other soil aggregates and tend to create a crust on top of the soil, which can easily runoff when water or wind is introduced.
Sheet erosion is the second stage of the erosion where a thin layer of the soil is removed. This can occur when heavy rain occurs, or the shallow surface begins to runoff from water flow. This results in the loss of the organic matter and nutrients since the top layer of the finest soil particles are lost. This type of soil loss is very gradual and can often go unnoticed until huge amounts of soil have been lost.
When the soil has faced overgrazing or has a lack of vegetation cover, it can lead to a higher rate of erosion. It is important to learn more about the signs of erosion so that it can be prevented. These signs are:
Bare areas in the soil
Puddles of water emerging as soon as rain occurs
Visible roots of trees and grass
In such cases, a vegetation cover can help to protect the particles of soil and even increase the aggregation of soil.
Rill Erosion is the third stage of soil erosion. Rills are the shallow drainage lines that are around 30cm deep into the ground. The surface water begins to collect on the soil in the form of depressions and begins to erode the soil. It forms these rills that can make hills and the land more prone to erosion.
Rill erosion is common where there is loose soil, overgrazing, and bare soil on agricultural land. People can reduce the chances of rill erosion through grassed waterways, contour drains, and mulching. If they don’t, it can lead to the most extreme stage of erosion –gully.
Gully erosion persists more than 30cm into the soil. It is the worst type of erosion because soil conservation cannot take place through normal cultivation practices. It can occur when runoff begins to flow strong and concentrates in one location. The water flow can be strong enough to detach and move complete soil particles along with it.
For example, a spring or small waterfall may form on the soil. As the runoff flows down the land, it begins to pick up energy as it plunges over the gully head. The splashes from the water can also erode the subsoil, and the gully can eat away at the topsoil on the slope as well. This type of erosion is very extensive and visible. It can have drastic effects on soil productivity and restrict land use. Gullies can be at least 2m deep but may go down to depths of 10-15m.
Gullies are caused by many different agents, such as:
Increased runoff due to changes in land such as clearing of trees
An increased concentration of runoff from watercourses
Improper construction, design, or maintenance of waterways
Poor vegetation cover
Seepage flows over a long period
Diversion of drainage lines
Continued water flow along a linear pathway can also cause erosion. The erosion can deepen valleys or extend the valley into the hillside. It can create steep banks or head cuts as it flows over the ground.
When it is just starting out, the erosion is mostly vertical in nature, which is why valleys usually have a V-shape. In such cases, the stream gradient can become really steep as the flowing water continues to affect the land surface. When the running water has reached the base level, the erosion can switch to lateral erosion. This can end up widening the valley floor and cause the floodplain to shrink.
When the stream gradient continues to become flat, the deposition of sediments can increase. This is due to the fact that the water begins to meander against the floor of the valley. During flooding, this can increase as the water is moving faster and is able to carry an even larger chunk of sediment. Pebbles, boulders, and other suspended abrasive particles can also begin to erode away at the surface; this is known as traction.
Tunnel erosion causes the loss of soil from the subsoil layer. This happens when water runs through small cracks or hole where roots have decayed. The soil begins to disperse from the inside and is carried away with the water flow. This leaves a small tunnel in the soil.
The soil structure on the surface tends to remain intact initially, but as the water continues to flow, the tunnel tends to become larger. Eventually, the soil can collapse from the top and create a gully. The whole process begins to speed up when the inner layer gets even more exposed to the free flow of water. The mixture of silt, clay, and sand can start to move, resulting in a loss of sediment.
Bank erosion occurs when the edges of a river or stream begin to wear away. This is different from normal changes in the watercourse bed, which is usually called scour. Erosion is measured by the inserting metal rods into the bank and checking how far the bank surface pushes back at different moments. Flooding can make it worse and cause complete removal of soil along the stream bank.
One of the biggest causes of stream bank erosion is the vegetation destruction along the edges of the river. This increases the removal of gravel and sand from the stream bed. It can also be caused by:
- Human or vehicle traffic up and down the bank
- Lowering of streams or infill
- Inundation of soils along the bank
- Saturation of the banks due to off-stream causes
- Acceleration of water flow
- Changes in soil characteristics
- Wave action from boats or wind
- Intense rainfall events such as dust storms or cyclones
Riverbank erosion can be extremely critical in some regions and may even result in human displacement.
Thermal erosion can occur when the permafrost begins to weaken due to the movement of water or simply due to it melting away. It can occur along the coast or rivers. It may also be a result of near-shore temperatures and wave action from boats or wind that cut through the permafrost. There are three main types of glacial erosion: plucking, abrasion, and freeze-thaw.
Plucking is when the melted water from the glacier begins to freeze around broken or cracked rock pieces. The rocks begin to get plucked from the back wall when the ice moves downward, taking huge chunks of soil with it.
Abrasions occur when frozen rocks at the back of the glacier scrape against the surface of the soil or rock bed. It begins to scrape through off the surface of the ground.
Freeze-thawing occurs when the rain or melted water seeps through the cracks in the soil or rocks. When it gets colder, such as at night, the water begins to freeze and expand. This can cause large cracks to develop, resulting in the rock or soil breaking away completely. Glacial erosion can sculpt down complete mountains and carry huge pieces of land along with them. They can have adverse effects on the land, resulting in a lot of havoc.
Rapid amounts of rushing water in large volumes can cause vortices and kolks in the soil bed. Kolks can pluck bedrock, create huge rock-cut basins, and cause local erosion. Some of the best agricultural lands are sometimes on floodplains due to the high soil fertility and availability of irrigation water; it can all get eroded away due to high-velocity flooding if there isn’t enough surface cover.
This flooding can essentially remove the entire topsoil of cultivation and expose the sub-soils to more erosion. Erosive flooding can result in the stripping off of 0.1 to 0.15m of topsoil. An increased risk of floodplain erosion is dependent on:
Flood velocities – a higher velocity of water results in bigger floods
The amount of protective cover and orientation of the crop rows at the time of flooding
Slow-moving water at the end of flooding over-saturated soil types
Floodplain erosion can result in a lot of sediment flowing into the riverbank, as well as the loss of fertile agricultural land.
Coastal or shoreline erosion occurs on sheltered or exposed coasts. The primary cause of coastal erosion is due to the waves and currents, and secondary causes such as sea-level changes.
The wave can sometimes push the air against a crack or joint in the soil surface, resulting in hydraulic action. This can crack the surface, and wave pounding can result in pieces of rock or parts of a cliff breaking off from the sheer force and weakened surface. The most rapid and effective form of shoreline erosion is caused by waves launching the sea load against the surface, resulting in ‘corrasion’ or abrasion.
Corrosion can also occur when the rock begins to dissolve due to the carbonic acid present in seawater. This is especially harmful to limestone cliffs. Sea load or particles carried by the waves can also hit against the cliff, resulting in attrition. This can weaken the surface and make it easier for the water to carry the material away. The material is eroded into sand or shingle.
Sea waves may even transport sediment along the coast. On some occasions, the amount of up-current sediment can be more than the amount being carried back into the sea. This can result in erosion since gravel or sandbanks can form along the coast. These banks begin to migrate slowly along the coast, creating a long-shore drift. It can expose some parts of the coastline while protecting others. This can result in bends forming in the coastline, which can result in even more buildup of materials.
It is important to create armor for the beaches so that the coastline doesn’t begin to erode away.
The wind is a major geomorphological force. Wind erosion plays a major role in arid or semi-arid regions. This can result in evaporation, desertification, increase in airborne dust, crop damage, and land degradation. Recently, the effects of wind on soil erosion have increased due to human activity such as urbanization, deforestation, and agriculture.
There are two main varieties of wind erosion: deflation and abrasion. Deflation occurs when the wind carries away loose particles by picking them up. Deflation occurs in three main ways:
Surface creep occurs when heavy, large particles roll or slide along the ground.
Saltation is where soil particles are lifted into the air at a short height, bounce back onto the surface and saltate across the surface.
Suspension is when light or small particles are lifted by the wind into the air and carried for long distances.
On the other hand, abrasion occurs when the surface of the soil is worn down when it is struck by airborne particles carried by the wind.
Soils with high amounts of silt tend to the most affected by wind erosion. Silt particles aren’t aggregated tightly and can be easily carried away. Wind erosion is also more severe in places of drought when the soil has become dry or arid. It is estimated that the Great Plains face 6100 times more erosion in drought years than wet years. Grazing can also significantly increase the chance of wind erosion.
In areas where cropping is normal, wind erosion may not be a huge issue. This is because the soil is often too coarse and aggregated to be carried by the wind. Vegetation covers and forests are important protection that humans should install over bare soil. This can prevent excessive loss of soil and result in better soil management.
A mass movement can cause erosion due to the movement of rocks or sediment against a sloped surface. This is also known as gravity erosion in some regions.
A mass movement is actually a major erosional process which occurs in stages. First, the transport or breakdown of worn-down materials in a mountainous area takes place. This results in the movement of material from higher elevations to lower ones. Lower elevations often have other eroding agents like glaciers and streams that pick up the materials and carry them to even lower levels.
Mass movement erosion usually occurs continuously on the slopes. It can be extremely slow in some cases and occur suddenly in others, leading to extremely disastrous results. Any movement of sudden rock or sediment down the slope is referred to as landslides.
Slumping can also occur against steep hillsides with materials like clay that can fall quite rapidly. They mostly occur across fracture zones that have already been weakened. In some cases, the downward movement of the slope can also cause a depression in the surface. This slump can further weaken the soil underneath it since it is now exposed to wind or water.
Soil erosion by mass movement can be controlled with the help of some measures such as vegetative covers, barriers, permanent plant structures, breaking up channels, check dams, and reduced water flow.
Scalding occurs when water or wind erosion removes the topsoil layer. This exposes the sodic or saline soils in the subsoil layers. Raindrop impacts can also result in the soil being moved since the internal layer is exposed to erosion. However, wind and water are the biggest contributors to removing even more soil. This can result in sheet, tunnel, rill, and gully erosion.
This type of erosion will also move smaller, lighter soil particles away from the surface first such as silt and clay. This leaves behind dry, coarse, fine soils which cannot be used for agriculture. The soil can harden and seal off due to the mixture of small amounts of clay with large amounts of fine sandy soils. This can prevent water from infiltrating through the soil, causing it to become dry.
This can result in a few square meters being made infertile. In severe situations, it can destroy land up to hundreds of hectares at a time. It can be very hard to revive the soil since there is low permeability, high salinity, and a lack of topsoil.
Chemical erosion results in the loss of materials in the soil in the form of solutes. Solutes from the landscape begin to run into the streams, which can help us measure the amount of landscape destruction taking place. Releasing toxic waste and chemicals can increase the chance of erosion of the banks and the river bed. It makes it easier for the affected water to eat away at the soil layers.