- METHODS OF EROSION CONTROL
Knowledge of the factors which effect soil erosion is essential in designing the control measures. Certain of these factors such as rainfall, the erosiveness of the rainfall and the soil erodibility cannot be changed by man; but he can take precautions to minimize their effects. For instance, the soil erodibility factor is made up partly of the physical properties of the soil e.g.: the texture, and partly the treatment of the soil by the farmer and the soil structure. Because the farmer can do so much to control his soil structure, he can do much to control the erodibility of his soil by the management of his crops and stock.
Slope and slope length are physical factors of the topography of a farm. They cannot be altered and so the farmer must take into account the slope when designing soil conservation systems. Slope length can be altered by the farmer when designing mechanical conservation layouts, but this requires an understanding of the exact effect it has on erosion.
In practice, all the factors of rainfall, soil, slope, vegetation cover, and management interact.
SOIL LOSS/SLOPE CURVES
Figures 1 and 2: The following two diagrams show the relationship between soil loss by erosion, the slope of the ground and the crop cover on the soil.
ACCEPTABLE LIMITS OF EROSION
As erosion occurs naturally and is increased to dangerous levels only by man, it is necessary to determine what an acceptable rate of soil loss is. The acceptable limit of soil loss is that point at which the rate of the soil loss is equal to the rate of soil formation. This rate of soil formation cannot be easily measured, but for Central and Southern Africa conditions it is estimated to be:
- For sandveld: 9 tons per hectare per year
- For heavier soils: 6,7 tons per hectare per year
On the basis of these figures, three important systems for soil erosion control have been designed. These are:
- The Land Capability Classification System.
- The Recommended Intensity of Use of Arable Lands, which deals with Rotations.
- The Layout of Mechanical Conservation Works.
For example, the recommended rotations of crops on sandveld will be such that the average soil loss from a land will not be greater than 9,0 tones per year.
The Land Capability Classification system makes it possible to decide the most suitable use for a land and to ensure that lands unsuitable to cropping are not cropped.
In designing the layout of mechanical conservation works, the slope, soil type, land use etc will be considered in order to reduce soil loss to the above limits.
- BIOLOGICAL AND MECHANICAL CONTROL
Biological control measures are the factors which reduce erosion by the management of Crops and animals. Mechanical control measures are the earth moving and soil shaping measures such as the building of contours and storm drains.
Both kinds of control measures should be used by the farmer, as they are complementary with one assisting the other. Mechanical conservation is “the first line of defense” and is particularly necessary early in the season, when the land is bare. Biological measures are much more desirable than mechanical measures because they are productive and part of good crop or stock management. They are also simpler and cheaper and do not reduce the size of the arable area. Mechanical conservation works can reduce the area of arable land crop by as much as 30%, depending on the land slope and the type of works constructed.
LIMITATIONS OF MECHANICAL WORKS
All mechanical works concentrate water in an artificial channel (especially at their outlet). This in itself is contrary to the principles of good soil erosion control. There is the danger of conservation works failing which could cause much more damage than would have occurred if no mechanical works had been built.
Drains or channels built on too steep a gradient will scour and form a gulley. Those built on too shallow a gradient will deposit silt, dam up, and burst. All works must be made large enough to cope with the heaviest intensity storms at the most vulnerable time of year or they will be a liability rather than an asset:
It follows therefore that any mechanical conservation works must be:
- Adequately designed
- Adequately constructed (includes pegging)
- Adequately maintained.
The principal cause of failure of works is poor maintenance. Ploughing systems should be designed to build up contour ridges each year. Storm water drains must be cleaned out when necessary and the gradients of works should be checked every few years. Waterways must be protected and grass cover encouraged. If maintenance is not carried out, works will fail and cause untold damage.
PURPOSE OF MECHANICAL CONSERVATION WORKS
Although biological control measures are probably more important in reducing soil loss (and maintaining fertility), mechanical conservation measures are nevertheless essential in Central and Southern Africa where rainfall intensities are high and rainfall is confined to less than half of the year.
The primary purpose of mechanical works is to gather water flowing in a sheet over the surface of the land before it can cause serious damage. It is also to conduct this water at a safe velocity off the land to an area where it can do no harm. However it should be remembered that, once water has started running over a land some erosion will have already started, (especially while the land is bare and rain drop splash can have its full effect) and contours, etc, can only minimize the soil loss.
TYPES OF MECHANICAL PROTECTION WORKS:
Works most commonly used in arable lands are:
- Storm drains (storm water diversion drains)
This structure is placed above the land and serves to divert the stormwater which would otherwise flow onto the land from higher ground. This is vital to the protection system since it protects all the structures lower down which will be designed on the assumption that they will only have to carry water falling onto the arable area itself.
- Contour ridges (channel terraces, contour terraces)
These are situated within the arable land and serve to collect surface run‐off water, and to discharge this water safely into a stabilized (artificial or natural) waterway. Each “contour” is designed to carry only the water running off the area of land immediately above it. Various types of contour ridges exist (e.g. broad and narrow based ridges) and different layouts of contour ridges are used for different cropping systems and/or soil types.
- Waterway (grass waterways, meadow strips)
These are the depressions into which the contour ridges (and/or storm drains) usually discharge. They must have a stable vegetative cover, but may be either natural depressions or artificially constructed depressions.
OTHER TYPES OF MECHANICAL CONSERVATION WORKS
These are used mainly in other countries and are:
- Grass strips
Serve the same function as contour ridges in that they slow down the rate of flow of surface run‐off within a land. They have little use here as water tends to break through the strip at low points, but are employed on certain large blocks of fairly flat land. They may also cause increased infiltration.
- Bench terraces
Where a steep slope is converted to a series of steps with horizontal (or nearly so) ledges. To hold up the vertical face some structural wall is necessary, usually of stone. Variations are the terraces which are sloping slightly in towards the hill. There may be scope for the building up of terraces between contour ridges by ploughing over the years, especially for irrigation lands.
- Tier ridges
Which are placed throughout the land and serve to hold the water where it falls thus aiding infiltration on soils with poor infiltration (“t” factor). The ridges should be on a slight gradient to carry excess water off the land after very heavy rain. A back‐up system of contour ridges should be used.
- Contour cultivation
All tillage operations should be carried out on the contour where possible. On gentle slopes this practice may be sufficient to slow down run‐off to a minimum, especially if combined with a system of minimum tillage. Also on the contour (i‐e: leaving mulch or large clods between the rows of a crop).
- Mechanical conservation
Works are sometimes practiced in non‐arable areas, i.e. grazing areas. However, it is generally better to improve grass cover of the veld by good grazing management and hence to improve infiltration and reduce run‐off. Storm drains are often used to protect a building, or a road, or to cut off water from the head of a gulley.
There are occasions where mechanical works can be used effectively in the veld, but this must be together with grazing management improvement. Arable lands are usually much more vulnerable to erosion than veld, and the expense is more justified on arable areas.