Guidelines for practical soil erosion control in rural Africa

Wells for Zoë – Enyazini, Mzimba District, Malawi

ReStory narrator: Ken Coetzee (Conservation Management Services; www.conservationmanagementservices.co.za)

Erosion type: head cut

Examples

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Action: Re-slope the vertical edges (see Figure 1) to create a favourable slope for plant re-establishment, reduce the water speed by constructing brush fences in and outside the treated area and then brush pack to exclude animals, to create a micro-climate and to put back organic material (compost).

Control steps: (see Plates 1-6 below)

• Remove all vegetation around the immediate edge of the head cut, up to 2,5m from the edge, and stock pile for later use. Where possible, do so with roots and soil to retain soil mycorrhiza.
• Slope the vertical edges down to a gentle slope (approx. 35° angle) and smooth out the soil. See Figure 2A & B.
• Install brush fences above and within the sloped area, as per design illustrated in Figure 2C. Ensure that the fences are constructed perpendicular to the water flow direction (i.e., 90° angle).
• Make sure to compact the soil directly in front and behind the brush fences.
• Do brush packing on all areas that are sloped, in between brush fences and any bare areas surrounding the treated head cut.
• If large branches are used, care should be taken to cut the branches in to 500mm section so as to allow the branches to lay flat on the ground, ensuring contact with ground.
• It is important to harvest brush in such a way that the surrounding area is not depleted of plants, this will necessitate moving some distance away from the site when harvesting. Where possible, select re-sprouting plants and never remove more than 50% of a given plant.
• If brush is limited then any type of organic material can be utilized, especially maize crop residue or unused or old thatching grass. Alternatively, tall grasses can be harvested and used to cover the bare ground. However, this approach will require a few heavier branches laid down on top of the lighter grass to prevent it blowing away.

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Erosion type: gully

Examples

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Action: Starting at the top of the gully, construct brush fences in the gully to reduce the erosive force of the water, to trap silt and to capture seeds, as well as organic materials, which will facilitate plants to grow and stabilize the gully. The steeper the slope, the closer the spacing of the structures.

Control steps: (see Plates 7-11 below)

• Install brush fences in the gully to trap silt and reduce the speed and erosive force of the water.
• Always ensure that the fences are constructed perpendicular to the water flow direction (i.e., 90° angle).
• Start by digging the foundation trench, make sure that the trench is adequately cut into the side walls and bottom of the gully. See Figure 3A.
• Hammer in the support poles, which should be strong enough to handle the pressure of both the water pressing against the structure and silt which will build-up in front of the fence. Figure 3B.
• Weave thin, supple branches in between the support poles, making sure to push down to create a dense barrier and also fill in the end gaps to properly seal the structure. Figure 4C.
• Pack a thick, dense layer of leafy branches in front of the structure and cover it with a compacted layer of soil to enhance the effectiveness of the fence. Figure 4D.
• If available in abundance, then brush can be packed in the gully, in between the structures.  Figure 5.
• It is important to harvest brush in such a way that the surrounding area is not depleted of plants, this will necessitate moving some distance away from the site when harvesting. Where possible, select re-sprouting plants and never remove more than 50% of a given plant.

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Erosion type: steep slope

Examples

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Action: Construct and install grass-fence structures along the contour lines of the eroded slope to reduce the runoff speed of the water, allowing water infiltration, as well as to holding back topsoil and capturing seed.

Control steps: (see Plates 12-17)

• Construct a suitable amount of grass-fence sections/panels prior to starting with the erosion control work on the slope. Follow the design guidelines in Figure 6.
• Lay-out and mark, as accurately as possible, the contour lines along which to construct the grass-fence structures. The objective is to ensure that the construction or contour line remain as close to level as possible.
• Dig the foundation trench along the demarcated line, 300mm-400mm deep and 200mm wide.
• The pre-fabricated grass-fence panels are then fitted into the trench, overlapping slightly (150mm). See Figure 7.
• Hammer in the support poles approximately 1,2m apart (or at each joint) and secure the grass-fence panels to the support poles using sisal string or wire.  The 1,2m spacing used for the support poles will allow for a 150mm overlap on each side of the adjoining 1,5m grass-fence panels. See Figure 7.
• Using the previously excavated soil, fill-in and compact the trench on both sides of the structure.
• Brush pack the open areas in between the structures, especially the bare areas created in front and behind the structure.
• Again, it is important to harvest brush in such a way that the surrounding area is not depleted of plants, this will necessitate moving some distance away from the site when harvesting. Where possible, select re-sprouting plants and never remove more than 50% of a given plant.
• Lastly, the steeper the slope the closer the spacing of the grass-fence structures. On steep slopes the spacing in between structurers should preferably not exceed 5m.  This will ensure that run-off water does not generate enough momentum to cause accelerated soil erosion.

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Erosion type: flat bare areas (sheet)

Example

Action: Where bare (denuded) areas, with capped (hard) soil surfaces exist, hollows are constructed to allow water to infiltrate and rehydrate the soil, to reduce the amount of water run-off and to facilitate the establishment of grasses and/or protective ground cover.

Control steps: (see Plates 18-21)

• Taking care not to disturb any rooted plants, mark out the outline of the hollow to be dug. Use the recommended ‘horse shoe’ design and spacing as illustrated in Figure 8.
• It is critically important that each and every hollow is orientated to ensure that the maximum amount of water is captured and retained, i.e. the opening should face towards the flow of the water.
• Dig the bowl of the hollow by either using a pick & shovel or the local hoe and place the excavated soil along the rim of the bowl to create the soil berm.
• The berm needs to be compacted continuously during the construction process, layer for layer, which will ensure a strong ‘retaining wall’.
• The hollows need not be larger than 800mm wide x 500mm broad and no deeper than 300mm.
• Brush or mulch (can also be maize crop residue) should be placed in every hollow to create a micro-climate, protect seedlings and to provide organic material (compost).
• Lastly, a mixture of local grass seed can also be sown into each hollow to re-establish the protective plant cover.

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