Fencing against Soil Erosion

Slope: gentle slopes (2-5%)

1.1 DESCRIPTION

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.

1.2 METHOD OF APPLICATION:

Materials required

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

Steps of implementation

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

2.1 DESCRIPTION

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.

2.2 METHOD OF APPLICATION

Materials required

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

Steps of implementation

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.

For more information, visit here.

Slope: steep slopes (30-60%):

Synonyms: Log check dams, Wooden fencing, Sediment traps

3.1 DESCRIPTION

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.

3.2 METHOD OF APPLICATION

Materials required

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

Steps of implementation

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.

This intervention contributes to:

Estimated and benefits of implementing this intervention:

(Source)

1. Silt fences in Potlake, South Africa

Description
Silt fences, along with brush packing, have been introduced in the Potlake Nature Reserve, Limpopo Province, South Africa, to address sediment transport and restore areas suffering from gully erosion. The purpose of this technology is to rehabilitate land affected by rill and gully erosion, which is a result of overgrazing on highly erodible soils. The method involves the use of temporary silt fences placed closely together to trap sediment and reduce erosion. These fences are designed to last about ten years, during which time vegetation is expected to establish and stabilize the soil. Additional support is provided by brush packing upstream and stone packing downstream. The Potlake Nature Reserve spans altitudes from 1,174 to 780 meters above sea level and is located in a summer rainfall region with an annual average rainfall of 438 mm. The soil in the reserve is clayey and highly prone to erosion. The vegetation consists of Central Bushveld units within the Savanna Biome, characterized by a mix of grasses, shrubs, and trees. The reserve contains several large gullies, some up to 1.5 meters deep and 5 meters wide, which are targeted by erosion control measures. Approximately 200 meters of silt fences have been erected at selected sites within the reserve to retain sediment and establish vegetation. The project covers an area of about 1-10 km² and is evenly spread across the targeted sites. The Potlake Nature Reserve, where the technology has been implemented, is a permanently protected area. This erosion control technology was introduced less than ten years ago through external projects and interventions.

2. Bamboo fences in Gagalphedi, Nepal 

Description

In Nepal's middle hills, a combination of vegetative and structural measures is used to stabilize landslips, stream banks, and gullies, protecting agricultural land and preventing downstream problems. This approach involves local families working together, using bamboo wattle fences and planting species like Nepalese alder, bamboo, cardamom, and broom grass. These plants quickly establish themselves, control erosion, and provide economic benefits such as cash crops and resources like fodder and timber. The technology is suitable for steep slopes at altitudes of 1,000-1,500 meters and has minimal maintenance costs. It enhances biodiversity and protects natural springs. The project covers 0.14 km² in the Kathmandu/Bagmati watershed and was introduced through external projects with significant local involvement. This method has been used for over 50 years and continues to benefit the community and environment.

3. Biotrampas is Cercade, Bolivia (in Spanish)

Description

Biotrampas is a technology used in Bolivia to stabilize slopes by combining reforestation with sediment traps. This method is applied along stream and riverbanks where erosion and landslides threaten agricultural land and cause flooding. The technology works by stabilizing slopes through the planting of native species, which helps prevent further erosion and the enlargement of gullies. It involves using local materials such as trees, shrubs, and wood, requiring minimal investment and technical knowledge. Maintenance is simple and inexpensive, involving regular tasks like pruning trees and replacing plants. Biotrampas are most effective when used alongside other technologies, and in this context, they cover the entire watershed area, influencing around 6 km². This technology has been implemented in the Cercado/Cochabamba region of Bolivia for over 50 years, introduced through various projects and external interventions. It has been tested and proven effective in Suchu Mayq'o (Sacaba) and a neighboring watershed (Taquiña), demonstrating significant success in stabilizing slopes and preventing soil loss. For more information please click here.

External sources

• For more information on silt fences, please click here.
• For more information on woven fences, please click here.