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close this bookSpecial Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.)
View the document(introduction...)
View the document1. What is erosion?
View the document2. How to recognise erosion
Open this folder and view contents3. Different types of anti-erosion ditches
Open this folder and view contents4. Worksite organisation
Open this folder and view contents5. Procedure
View the document6. Maintenance of the retaining banks

2. How to recognise erosion

A farmer is often unaware that his land is suffering from the effects of erosion. By the time he is forced to abandon a field through poor yields, and a great deal of arable soil has disappeared, it is often too late to restore what has already been lost.

With this in mind, warning signs must be recognised early on in order for the control of erosion to be effective. These signs are:

- poorer crop yields;
- channels becoming visible on the soil's surface;
- the formation of small gullies;
- the build-up of sediment in the thalwegs.

HOW CAN EROSION BE STOPPED?

We have seen how erosion is due to the force of water flowing over the soil's surface. The rate of erosion becomes greater as the volume and speed of the water flow increase.

For erosion to be controlled, therefore, three principles must be applied:

REDUCTION OF THE SPEED AT WHICH THE WATER FLOWS
ABSORPTION OF THE WATER
DIVERSION OF EXCESS WATER

To REDUCE THE RATE OF RUN-OFF, obstacles must be created along the surface of the soil to stop the flow temporarily.

The strongest rate of run-off is to be found on steep bare land where there is no vegetation.

The speed of overland run-off is slowed down by vegetation and by appropriate structural improvements, such as contour ploughing.

ABSORPTION OF THE WATER

Structures which reduce the speed of run-off water also encourage more water to be absorbed by the soil. With an increased volume of water infiltration, the quantity of run-off water is reduced and, consequently, also the rate of its flow.

On land which is very permeable, where water is easily absorbed, RETAINING RIDGES are constructed to allow total infiltration of the water.

Where land is much less permeable, the same structures are used to retain the water for as long as possible but with the addition of a waterway to divert part of the surplus water.


Figure

The earth at the base of the ditch must first be loosened with a pick-axe to facilitate the infiltration of the water.

DIVERSION OF EXCESS WATER

When rainfall is heavy, the continuous flow of run-off water cannot be totally absorbed, but must not be allowed to overflow the ditches as this would cause damage to the structures (holes, ravines, destruction of ridges, crop damage).

Precautionary measures must, therefore, be taken in the form of outlet channels where the surplus water can flow away. These are known as WATERWAYS. Generally, natural waterways are used - low-lying areas (thalwegs) where water naturally flows. These waterways must first be improved, however, otherwise the increased volume of water they have to carry may give rise to erosion.


Figure

ANTI-EROSION MEASURES

Ploughing and crop-tilling following the contour line of the slope reduce the flow of water and encourage its infiltration. Ditches and ridges built along the contours stop the flow of water and allow it to be absorbed. Waterways and channels divert excess water.


STRONG FLOW


REDUCED FLOW - By contour-ploughing


REDUCED FLOW - By retaining ridges


REDUCED FLOW - By vegetation


REDUCED FLOW - By reducing the slope (terracing)

notes

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