Contour bunds

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Description

INTRODUCTION

Contour bunds are a form of microcatchment technique and are a very simple and cheap form of water control. The bunds are created thanks to several dikes, closely spaced and placed along the contour lines. There are also small earth ties, perpendicular to the bunds, that subdivide the system into microcatchments. Contour bunds are very similar to Negarim in that it aims to slow down runoff and improve water infiltration in the soil. For this reason, contour bunds are often associated with the cultivation of crops, fodder or trees between the bunds. Contour bunds also help to control soil erosion. Contour bunds for tree planting is suitable for arid and semi-arid areas with rainfall rates between 200 and 750 mm. It can be applied on slopes up to 5% but it requires even terrains, without the presence of gullies or rills. The soil should  preferably be 1.5 or 2 m deep in order to ensure proper root development and water storage.

MECHANISM

Looking at a contour bunds field from above, it consists in a sequence of parallel earth bunds running along the contour lines. Their spacing depends on the slope and decreases with an increasing slope: the steeper the slope, the closer the contour bunds will be. It is recommended to grow vegetation along the bunds to improve their stability.

Contour bunds are especially used in the valley bottom, since they are often slightly concave. The bunds will be built with a parabolic shape in order to allow water to spread to the sides.

The soil excavated from the adjacent ditch is relocated upslope and will form the bunds. Bunds height is usually around 20-40 cm, but can vary depending on the slope and their base should be at least 75 cm wide.  There are also small earth ridges perpendicular to the bunds, and located on the upslope side, that divide the system into microcatchments. These cross-ties shouldn’t be less than 2 m long and are formed by the soil obtained from the excavation of the infiltration pits. The layout of this intervention also implies infiltration pits (usually 80x80x40 cm) that are excavated at the intersection between bunds and ties.

Contour bunds might also include  the presence of trees. Planting sites are expected to be between the infiltration pit and the cross-tie. It is recommended to plant seedlings that are at least 30 cm high, and the best planting period is right after the first runoff has been collected. If needed, it is also possible to add manure or compost to the planting pit to increase fertility and its water holding capacity.

Lastly, where needed, in the layout there might also be a diversion ditch, to protect the entire system from flooding during heavy rainfalls.

Like Negarim, some maintenance interventions might be required to repair damaged bunds early in the first season. Such damages are frequently caused by animals invading the areas.

OTHER

It is interesting to notice that contour bunds can be also mechanized, therefore this technique can be scaled up to large areas. One of the main advantages of contour bunds, indeed, is that, even though trees are in place, cultivation using oxen or machineries can still take place between the bunds and fodder or crops can be grown before trees become productive. On the other hand, this would imply a reduction in the amount of runoff that can reach the trees.

Another big advantage of this type of microcatchment intervention, compared for instance to Negarim, is its cost efficiency. Whether mechanized or not, the realization of this technique is relatively cheap especially on a large scale and on even lands because less earth is moved. Nonetheless, contour bunds technique is not yet as common as Negarim. Examples of contour bunds can be found in Baringo district in Kenya and in Kongwa district in Tanzania.

Additional information

Stones

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Slope

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