Fencing against Soil Erosion

Slope: gentle slopes (2-5%)

A silt fence is a temporary barrier made from interwoven wire and cloth that acts as a filter for sediment-rich runoff, allowing water to pass through while silt and other deposits build up. Usually supported by wooden stakes hammered into the ground, silt fences are employed in areas where soil is disturbed or affected by construction, fires, earthquakes, floods, or areas that have experienced significant loss of vegetation in a relatively short period of time as these environments tend to have lost the physical structures that usually hold the soil in place.
Silt fences are usually temporary structures due to their ability to allow sediment buildup that in turn becomes a natural barrier. This does mean that silt fences should be placed closer together than permanent structures such as the interventions below. Silt Fences should be maintained until the disturbed land has reached the desired level of regeneration and build-up that is sufficient to maintain healthy soils. Progress can be observed in soil restoration when grasses and other plants begin to grow on the inner side of fencing.

METHOD OF APPLICATION:

Materials:

• Digging Equipment (mallet, axe, spade)
• Wood stakes (for fence posts)
• Fabric/Interwoven wire/geotextile cloth
• Nails/staples

Silt fence method:

1. Dig a trench 15-20cm deep and 10-15cm wide, and place fill on the downside.
2. Lay fabric along the bottom and on the uphill side of the trench.
3. Compact soil back into the trench to hold the fabric down.
4. Drive wooden stakes 1-2 m apart and 15 cm below the fabric trench.
5. Attach the fabric onto the stakes to form a storage area to catch sediment, and staple to each stake.

CONSIDERATIONS FOR IMPLEMENTATION:

Detention pond: When inputting silt fences in stormy environments or seasons, landowners should always consider diverting sediment-laden stormwater to a detention pond in conjunction with the use of silt fences.

Slope: On moderate slopes (8-30%)

Synonyms: bans ko atta, Wattle fences, Manra bandhi

Dry, hillside croplands in semi-arid regions are subjected to intense soil erosion from sudden rainfall. Woven wooden fences are an effective and relatively cheap way of land stabilization and reducing soil erosion caused by water flows. Similar designs to Gully Rehabilitation with Stem Cuttings, Woven fences help decrease the overland flow while increasing crop yields by encouraging better water infiltration into the soil.

METHOD OF APPLICATION:

MATERIALS:

• Digging Equipment (mallet, axe, spade)
• Wood stakes (for fence posts)
• Branches for weaving between branches
• Nails/staples
• Preferred plants for planting

1. Identify and mark:
   Draw contour lines along where the fences will be built (maintain a distance of 30m between each fence line for a slope of 10%).
2. Build catchment pit:
   Build a shallow ditch of 5x15cm on the inner side of where the fence will stand.
3. Build the fence:
   Place posts close enough to each other that the fence forms an effective barrier against eroded soil upslope, e.g. 0.5-1m apart. Posts are typically around 100-150 cm long and may be planted up to 30-50cm deep to ensure stability. Then fill the space between the posts by interweaving branches between. Suitable species for this include alder (Alnus) and bamboo (Dendrocalamus). This will in turn trap eroded soil on the upslope side and create an earth bank.
4. Planting:
   Plant trees and shrubs of preference to plant in or between the ditch and fence to increase soil catchment.
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CONSIDERATIONS FOR IMPLEMENTATION:

Tillage: Implementing this intervention will require that contour tillage takes place instead of tilling downslope. Changing from tilling downslope to contour tilling is estimated to increase tillage costs by around 10%.

Slope: steep slopes (30-60%):

Synonyms: Wooden fencing, Sediment traps

Landslides and extreme erosion occur in gullies and channels on steep hillsides, leading to a decrease in the agricultural  area and significant surface runoff. ‘Biotrampas’ are hard, infrastructural wooden interventions that combine reforestation with the establishment of sediment traps (biotraps) to consolidate and stabilize the slopes and stop the enlargement of existing gullies. Biotrampas tend to be shorter in length and more plentiful than silt and woven fences in landscapes, however are more durable.

METHOD OF APPLICATION:

MATERIALS:

• Digging Equipment (mallet, axe, spade)
• Wood stakes (for fence posts and logs)
• Preferred trees/plants for planting
• Nails

1. Build catchment trench:
   On a 30% slope, dig a rectangular pit of 0.4m.
2. Build log barrier:
   Wooden stakes with a diameter of 15-20 cm and 1.5m long are planted into the soil and then logs of approximately 2.5m in length should be nailed to these posts, as can be seen in the diagram.
3. Planting:
   Plant your tree or shrub of preference, e.g. alder and poplar, in the middle of the pit created.

For more information, visit here.

CONSIDERATIONS FOR IMPLEMENTATION:

• Cost & establishment: Establishing fencing interventions does not require high investments as the fences can be constructed using local wood. However, transportation will likely be required for carrying sourced wooden stakes and poles to the implementation area as the eroded sloped landscapes at-topic tend to have little available large vegetation. It will also be significantly harder to implement fencing on the steeper slopes over 30% due to uneven, loose soils and rocks. Making labor, implementation and maintenance more challenging.
• Maintenance: These fencing interventions do not require high levels of technical knowledge, and maintenance costs are relatively cheap. However the interventions do require regular monitoring, such as pruning the planted plants and trees and replacing damaged fences. Fences should also be investigated for any damage after extreme rainfall events and repaired immediately, adding wood to the fence where needed.

This intervention contributes to:

Estimated and benefits of implementing this intervention:

Silt fences - Potlake, South Africa

Silt fences were introduced in the Potlake Game Reserve in the Limpopo Province of South Africa to rehabilitate areas affected by serious rill and gully erosion. Some of the gullies are now up to 1.5 m deep and 5 m wide travelling further uphill due to overgrazing on highly erodible soils. Implementation of silt fences in turn has led to increased vegetation cover and reduced sediment transport in the gullies.

Bamboo fences - Gagalphedi, Nepal

Woven or wattle fences made from bamboo are also used for eroded gullies in more humid climates. These structures are complemented by vegetation on their inner side, which establish quickly in degraded sites and control erosion by stabilizing the land and serving as fodder, fuelwood and timber

Biotrampas - Cercado, Bolivia

In the Cercado District a degraded catchment led to loss of cropland as well as serious downstream damage to the city of Cochabamba. Simple practices, such as the inputting stone and wooden check dams and slope reforestation in biotrampas were undertaken, leading to an increased vegetation cover stabilized the soil and safer discharge of runoff through the main gullies into the valley.

Silt Fences

Woven (living) Fences