7.2 What are the causes of soil erosion?

There are 3 main physical causes of erosion
  • Sheet erosion
  • Gulley erosion
  • Wind erosion
Plus human induced or accelerated soil erosion

Sheet Erosion

Where there is sufficient rainfall, exposed soil will be moved downhill as amass movement – sheet erosion
Raindrop impact is the major cause of soil particle detachment which can result in the particles moving down slope. This can happen in 2 main ways:
as sheet erosion during a rainfall event. Sheet erosion is the removal of fairly uniform layer of surface material from the land surface by continuous sheets of runoff water rather than concentrated into channels.7.2A_Sheet_erosion.png

Heavy rain that leads to a sheet of water removing a more or less uniform layer of fine particles from the entire surface of an area is sheet erosion. It often includes the best soil particles along with much of the organic matter. While it causes severe erosion, it is very difficult to see, as the amount removed is often slight from any particular spot. Notice how these ploughed areas in Romania have been covered by the sheet erosion.

Gulley Erosion

More frequently, the water gathers together and quickly erodes a channel. This is called gulley erosion.
7.2B_Durham_Canyon.pngThe example below can be seen in it all its glory in the blog (http://lindym.wordpress.com/2009/07/22/the-power-of-water/ ). It was named locally as the Durham Great Canyon and appeared literally over night in a cornfield.
You may also see mention of rill 7.2C_Wye_Rill.pngerosion which is a diminutive example of something similar.

.This is an example of a rill forming during one particularly heavy rainstorm in Autumn 2008 in the field behind our house.

Wind erosion

7.2D_Wind_diag.pngSoil erosion by wind may occur wherever dry, sandy or dusty surfaces, inadequately protected by vegetation, are exposed to strong winds. Erosion involves the pickin g up and blowing away of loose fine grained material within the soil.
Dust storms are very disagreeable and also the land is robbed of its long-term productivity7.2E_Wind_storm.png (humus (vegetable matter) is lighter and likely to be removed first). Crop damage, especially of young crops, can be serious. Either the roots are exposed as the wind blows away the top soil or else wind blown soil from elsewhere cover the seeding up – either way the crop will be lost.

Long term damage is even greater. Finer soil fractions (silt, clay, and organic matter) are removed and carried away by the wind, leaving the coarser 7.2F_Wind_erosion_eg.pngfractions behind. This sorting action not only removes the most important material from the standpoint of productivity and water retention, but leaves a more sandy, and thus an even more erodible, soil than the original.

The Impacts of humans on soil erosion

The most common human impact is due to population growth. This leads to increased pressure on the land and its resources. Overgrazing is a major problem. This causes vegetation loss and makes the soil much more vulnerable to erosion without the protective net of roots to withstand the pressures of water and wind. Intensive cultivation can cause loss of nutrients and soil exhaustion. This may lead to deforestation. Another major cause of deforestation is the cutting down of trees for fuel wood or clearing it for agriculture.
In practice the casues of soil eroiosn are usually a combination of physical and human causes, as you see below.

An example: The impacts of human activity on National Parks

Most types of vegetation can withstand some disturbance and will recover naturally in time. The level of recreational disturbance which an area can tolerate without damage is described as its recreational carrying capacity.
The effects of recreation are not evenly spread. They are mainly found around places such as car parks, pony trekking routes, river crossing points, riverside picnic sites and on popular walking routes such as from a car park to a tor.
Erosion is often caused by a combination of factors. Livestock grazing, farmers’ vehicles, hikers’ boots, horse riding and mountain bikes can all play a part in damaging the vegetation. These factors, combined with natural forces, determine the extent of erosion and the speed at which it occurs.
Erosion Problems

1. Trampling by walkers, climbers and livestock has exposed the soil around the base of the rocks.
2. People have driven vehicles onto the open land causing damage around the car park and roadsides.
3. Popular routes used by thousands of visitors which created erosion gullies and muddy impassable ground
4. Paths suffer from both narrowing by gorse encroachment and increased erosion from water runoff.

Techniques Used to improve the situation
1. Low grassed banks have been created beside the roads and in car parks
2. Access using boulders can block off grass parking areas in the winter when the ground is soft and more vulnerable to damage from tyres. This both protects the grass and reduces the number of people using the area.
3 These gullies have been filled in and the turf restored on the steeper parts of the path.
4 Gorse clearance (burning and mowing) has been undertaken so as to widen or increase the number of paths and so spread the load of walkers walking between the two points.
5. Grassed over, open drainage gullies have been created to divert rainwater away from the well used paths and reduce the possible gully erosion.
6 Granite paving slabs and rocks have been used to create a solid base on a wet, boggy part of the path
7 Education by way of information sheets and notices are used to ask people to behave in a way that protects the environment.

A neat but simple summary: