Special Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.) 3. Different types of anti-erosion ditches (introduction...) 3.1 Silt catchment pits 3.2 Forest tiering 3.3 Forest terracing 3.4 Cropping terraces

Special Public Works Programmes - SPWP - Anti-Erosion Ditches - Training Element and Technical Guide for SPWP Workers, Booklet No. 1 (ILO - UNDP, 84 p.)

3. Different types of anti-erosion ditches

There are many types of anti-erosion ditches and ridges. The engineer will decide which type is most suitable for the land in question. In deciding, he will take into account:

- rainfall intensity;
- the nature of the soil;
- the slope of the land.

The choice of ditch is not necessarily final and can be modified to take local experience and opinion into consideration.

Without going into detail on the choice of structure, a few basic considerations must be observed.

The choice of structure depends above all on:

- the slope of the land;
- the permeability of the soil;
- the use to which the land is put (type of crop).

Steeply sloping land (greater than 25%)

This type of land is primarily pastureland or forest. On land such as this, the purpose of anti-erosion structures is to protect the cultivated land which lies below. The appropriate structures here are ditches or terraces, in conjunction with tree plantations or silt catchment pits.

3.1 Silt catchment pits

The purpose of this type of ditch is to stop the flow of overland run-off water and allow it to be absorbed by the soil; in addition, the sediment carried by the water is collected by the pit.

These are protective structures which are located uphill from cultivated land.

They are rectangular sections, 40 cm wide, 20 cm deep and between 3 to 25 m in length and are dug along the contour line with 2 m of levelling between each ditch.

The ditches are blocked off in a zigzag pattern by a mound of earth 50-100 cm wide every 3 to 25 m to provide a path for men and animals.

SILT CATCHMENT PITS

notes

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3.2 Forest tiering

This kind of tiering is used on steep ground to allow plantations to be established.

Figure

The ditch, which is dug uphill from the retaining ridge, allows the water to be absorbed by the soil.

3.3 Forest terracing

This system is used on slopes of between 15% and 20%. Fruit trees may be used for the plantation at the base of the ridge.

Figure

3.4 Cropping terraces

These should be built on slopes of less than 15%. The gentleness of the slope they form (flattened cross-section) means that farm machinery has easy access to the land.

There are two main types of cropping terrace:

Channel-type terraces where the ditch is deeper that the hump is high and is used to divert the water.

Crest-type terraces where the hump is raised and the ditch shallow so that the water is retained and absorbed.

CHANNEL-TYPE TERRACE

CREST-TYPE TERRACE

ABSORPTION NETWORK

DIVERSION NETWORK

CHOICE OF STRUCTURE

NATURE OF THE SOIL	SLOPE
	GENTLE	MEDIUM	STEEP
SAND	Crest-type terrace	Forest tiering	Silt catchment ditch
		Normal-type terrace	Forest tiering
CLAY	Channel-type terrace	Forest tiering	Silt catchment ditch
		Normal-type terrace	Forest tiering

SOME RULES FOR THE DESIGN OF DITCHES

Length of terraces: 200-400 m
Longitudinal slope: 2/1000 maximum
Difference in level between two ditches: 0.60-2.00 m

notes

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