5. Processes of Soil Erosion

Erosion consists of two distinct scales which for this comparison can be described as micro and macro. The effect on individual grains of rain or wind is at the micro scale whereas the cumulation of many micro events forms the macro events of visible erosion.

 
5.1 Micro Scale - Erosion at Grain Size

Whether water or wind erosion, both are subject to the same laws. If a fluid, in this case water or air, passes over a soil particle, then fluid forces act in two ways, as shown in Figure 3. Firstly, the fluid has to increase velocity to pass over the grain due to an increase in travel distance at that point. This lowers the pressure above the grain and provides a lifting force FL which, if greater than the force due to gravity, FG, will cause the particle to be dislodged.

Figure 3 - Forces on soil grains (A) and streamlines (B)

(After Boggs)
Once the grains are lifted above the soil surface, the pressure differential between the top and bottom of the grain approaches zero and they fall again, accelerating until they reach terminal fall velocity, a function of fluid viscosity and particle diameter. When the particle hits the ground again, its momentum can be dispersed by either rebounding as saltation, displacing other particles into the flow or breaking up the soil surface. If terminal fall velocity is less than mean upward velocity caused by eddies, then the grain will remain suspended. Certain of these processes, such as saltation, where grains bounce along the soil surface, depend on the depth and velocity of flow and therefore are more likely in deep fluid flows found during heavy rainstorms or large wind movements. In the case of rain impact, the dislodging of soil is caused by momentum in the raindrop transferring to soil particles in a splash.
Erosion in a splash, as shown on Figure 4, becomes more efficient as slopes become steeper (Ellison, 1944) and when in the presence of wind. This is an important process but only moves soil relatively short distances. It does however provide loose material for sheet or rill flow.
(a) Splash on a horizontal surface therefore gravity has no along slope component. Adjacent spashes give net result of no soil movement. (b)Splash on an inclined surface - Effect of gravity causes the down slope path to be extended therefore soil will tend to migrate downslope.

Figure 4 - Effects of water splash impact on soil particles

Duley (1939), Ellison and Slater (1945) found that raindrops containing particles can seal surfaces forming a thin, ~ 1mm, impermeable crust or seal layer and below this, a thicker layer, ~ 5mm, containing inwashed fine soil. Infiltration can be as much as 2000 times less through the surface seal than the soils below (McIntyre, 1958).
 
5.2 Macro Scale - Erosion at Field Size

Sheet wash occurs when rainfall exceeds infiltration. As water is not confined in channels, it is likely to be a laminar flow and therefore less erosive per unit area (Emmett, 1970). An exception to this is on sandy soils where sheet flow is a more effective soil eroder than rills (Morgan, 1977). Rills are continuous narrow channels a few centimetres deep. They form mainly on recently cultivated soils where water is easily able to form a path between loose soil clumps. Gullies, the natural development of rills, can be between centimetres and over 30m deep, are distinguishable in that they cannot be ploughed out. When broad rills merge, they form sheet erosion. As rills and gullies are constricted at the sides and base they are more likely to carry more erosive turbulent flow.
6. Amelioration of soil erosion on arable land in Northwest England

As can be seen on Figure 5, Northwest England does not suffer from excessive wind erosion and for this reason, the first step must be to identify those areas of the field affected by runoff. These may be lower down slopes such as valley bottoms and adjacent to farm roads or hard standing. Increase in soil erosion in the UK over the last 20 years is due to rise in area of arable land use and the associated eroding causes. Although runoff ultimately is the eroder, the causes of runoff and their remedies can be placed into one of 3 groups; cultivation, grazing and general management.

6.1 Cultivation

To avoid bare soil, such as while slow growing winter crops are still young, grass buffer strips between crop rows should be planted. Larger bare areas should be protected with ground cover plants such as winter rye, clover or mustard which are ploughed in as green manure prior to sowing a new crop. Mulches such as organic manure, sugar beet waste or, subject to Environment Agency limits, sewage sludge should be applied to seed beds.

Figure 5 - Areas at most risk from erosion

(MAFF)
Irrigation water should be applied in drop sizes and flow volumes low enough to prevent erosion. Generally, the soil structure can be improved by addition of marl to increase the clay content or by the use of synthetic stabilisers such as polyvinylacetate or polyacrylamides. The use of phosphogypsum reduces runoff by a factor of 6 times. In order to reduce disturbance of the soil, deep ploughing should be avoided and ploughing with controlled follow up compaction should stiffen the soil surface without increasing runoff. Ploughing on hill slopes should be along the contour, subject to angle of the tractor not being at risk from overturning. If ploughing along contours, spoil from the plough should be overthrown uphill to discourage downward soil drift such as creep. Fine seed beds should be avoided. Courser media aids infiltration and with careful choice of crop varieties and herbicides, a similar result may be obtained. Tramlines parallel to crop beds used for chemical spraying should not be formed until the crops are well established, to discourage rill formation and in the case of ridge crops such as potatoes, soil walls should be placed between ridges to discourage rill formation.
6.2 Grazing

Soil is especially at risk from compaction when wet and therefore livestock should not graze during this period and be fed with silage. Better still, livestock should be fed on silage all year, although a good compromise is only to graze young cattle and sheep on prone areas. Plate 5 illustrates the effects of uncontrolled livestock grazing at a river bank.

Plate 5 - Livestock contribute towards bank erosion

(MAFF)
6.3 General Management

Trees and hedges should be planted to control runoff. Hedges will also reduce wind erosion. Drainage can be used in a variety of ways. Ditches intercept runoff and redirect the water for outfall to a soakaway or watercourse, although the latter may need approval from the Environment Agency. Farm roads and other hard standing areas should be correctly drained to prevent runoff onto fields. As with livestock, use of machines on wet ground should be avoided and where they must be used, shallow tines behind the wheels help break up the surface lightly after compaction. If none of the erosion control techniques mentioned here successfully address the problem, land re-profiling as practised in the US could be used, but its scale and cost are likely to be excessive. A measure more likely in the UK is to return the land to permanent grass or woodland. Government aid may be available for this under such schemes as Set Aside.
7. Conclusion

As has been inferred above, a temperate climate as in the UK does not suffer as badly from soil erosion as a semi-arid one does. Frost and Speirs (1984) concluded that 25t/ha/200 years had to be experienced before a significant drop in crop yield would occur. However, this figure, that is 0.125t/ha/yr, has been exceeded in the UK. Boardman (1990) monitored soil erosion in the South Downs between 1982 and 1988 and noted figures between medians of 0.6m3/ha/yr and 5.0m3/ha/yr. Assuming a bulk density of 1.5t/m3, this gives figures of between 0.9 and 7.5t/ha/yr. This suggests that erosion in the UK could become a major problem if left unchecked. Generally, the land management techniques of semi-arid countries is far below that required to successfully expedite the soil erosion problem and it is only by better land management that the problems will be reduced.

 

References
 
Binns T, 1990, Is Desertification a Myth?, Geography 75(2)
Boardman J, 1990, Soil Erosion in Britain:costs, attitudes & policies, Education Network for Environment & Development
Duley F L, 1939, Surface factors affecting the rate of intake of water by soils, Proc Soil Sci Soc Am 4
Ellison W D, 1944,Studies of Raindrop Erosion, Agric Engng 17
Ellison W D, Slater C S, 1945, Factors that affect surface sealing and infiltration of exposed soil surfaces, Agric Engng 26
Emmett W W, 1970, The hydraulics of overland flow on hill slopes, US Geol Survey Prof Paper 662-A
Frost C A, Speirs R B, 1984, Water Erosion in Soils of Southeast Scotland - a case study, Research & Development in Agriculture 1(3)
Lal R, 1993, Soil Erosion & Conservation in West Africa (World Soil Erosion & Conservation, Pimental D), Cambridge University Press
McIntyre D S, 1958, Permeability measurements of soil crusts formed by raindrop impact, Soil Sci 85
Morgan R P C, 1977, Soil Erosion in the UK: Field studies in the Silsoe area 1973-75
Nat College of Agric Engng, Occ Paper 4
 
Bibliography
 
Bayliss-Smith T, Owens S, 1990, Britains Changing Environment from the Air, Cambridge University Press
Boardman J, 1990, Soil Erosion on Agricultural Land, Wiley
Boggs S Jr, 1995, Principles of Sedimentology and Stratigraphy, Prentice Hall
Bradshaw M, Weaver R, 1993, Physical Geography, an Introduction to Earth's Environments, Mosby
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Kirkby M J, Morgan R P C, 1980, Soil Erosion, Wiley
Landon J R, 1991, Booker Tropical Soil Manual, Longman
MAFF, 1998, Code of Good Agricultural Practice for the Protection of Soil, MAFF Publications
MAFF, 1999, Controlling Soil Erosion, MAFF Publications
Pickering K T, Owen L A, 1997, An Introduction to Global Environmental Issues, Routledge
Pimental D, 1993, World Soil Erosion and Conservation, Cambridge University Press
Press F, Siever R, 1998, Understanding Earth, Freeman
Stanners D, Bourdeau P, 1995, Europe's Environment: The Dobris Assessment, EEA

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