Technologies

Silt fences to trap sediment in areas affected by gully erosion [South Africa]

Silt fences

technologies_6174 - South Africa

Completeness: 90%

1. General information

1.2 Contact details of resource persons and institutions involved in the assessment and documentation of the Technology

Key resource person(s)

SLM specialist:

Van Heerden Heinrich

Eco Rhythm Management

South Africa

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1.3 Conditions regarding the use of data documented through WOCAT

The compiler and key resource person(s) accept the conditions regarding the use of data documented through WOCAT:

Yes

1.4 Declaration on sustainability of the described Technology

Is the Technology described here problematic with regard to land degradation, so that it cannot be declared a sustainable land management technology?

No

1.5 Reference to Questionnaire(s) on SLM Approaches (documented using WOCAT)

Working for Water
approaches

Working for Water [South Africa]

Government funded restoration/rehabilitation initiative as part of Working for Water project. Aim was to eradicate alien invasive.

  • Compiler: Klaus Kellner

2. Description of the SLM Technology

2.1 Short description of the Technology

Definition of the Technology:

Silt fences together with brush packing were introduced to reduce sediment transport and restoration of gully erosion in the Potlake nature reserve, Limpopo Province, South Africa.

2.2 Detailed description of the Technology

Description:

Silt fences, together with brush packing were introduced in the Potlake Game Reserve (2817 ha) in the Limpopo Province of South Africa to rehabilitate areas affected by serious rill and gully erosion. Due to overgrazing on highly erodible soils, gully headcuts are actively migrating upstream. Implementation of the technology leads to enhanced vegetation cover and reduced sediment transport in the gullies. Silt fences are temporary structures and are therefore placed closer together than permanent structures. The placing is such that the silt from the downstream structure builds up against the upstream structure to delay the perishing of the restoration materials. Vegetation must be established in the gully (donga) as soon as possible so that, when the materials decay after about ten years, the area will be restored. Altitudes vary from 1 174 m to 780 m above sea level. The reserve is located in a summer rainfall region and receives an average annual rainfall of 438 mm per year. Clayey duplex soils are prone to both natural and man-induced erosion. The vegetation comprises Central Bushveld vegetation units of the Savanna Biome. Savanna is characterized by a herbaceous layer (usually dominated by grasses) with a woody component. In Southern Africa, Bushveld is an apt description of the vegetation structure, as the vegetation most often does not comprise distinct shrub and tree layers. Instead, the shrubs and trees occur in a matrix with a grass-dominated herb layer. A number of large gullies exist along drainage lines on highly erodible soils. Some of the gullies are up to 1.5 m deep and 5 m wide. A total of around 200 m of silt fences were erected at selected sites in the reserve to retain sediment and to establish vegetation. The following steps were followed in the establishment of the silt fences:
1. Identification of priority sites to erect the silt fences - using Google Earth imagery and field surveys.
2. Acquisition of restoration material and training of local community members on the implementation of the technology (training by Mr. Buckle at that stage from the Department).
3. Levelling of the area where fences would be erected.
4. Construction of fences.
5. Brush packing (upstream of fence) stone packing (downstream in the gully).

2.3 Photos of the Technology

2.5 Country/ region/ locations where the Technology has been applied and which are covered by this assessment

Country:

South Africa

Region/ State/ Province:

Limpopo Province

Further specification of location:

Potlake nature reserve

Specify the spread of the Technology:
  • evenly spread over an area
If precise area is not known, indicate approximate area covered:
  • 1-10 km2
Is/are the technology site(s) located in a permanently protected area?

Yes

If yes, specify:

Located in the Potlake nature reserve

2.6 Date of implementation

If precise year is not known, indicate approximate date:
  • less than 10 years ago (recently)

2.7 Introduction of the Technology

Specify how the Technology was introduced:
  • through projects/ external interventions

3. Classification of the SLM Technology

3.1 Main purpose(s) of the Technology

  • reduce, prevent, restore land degradation
  • conserve ecosystem
  • protect a watershed/ downstream areas – in combination with other Technologies

3.2 Current land use type(s) where the Technology is applied

Land use mixed within the same land unit:

No


Other

Other

Specify:

Protected area

Remarks:

Potlake nature reserve

3.3 Has land use changed due to the implementation of the Technology?

Has land use changed due to the implementation of the Technology?
  • No (Continue with question 3.4)
Land use mixed within the same land unit:

No

3.4 Water supply

Water supply for the land on which the Technology is applied:
  • rainfed

3.5 SLM group to which the Technology belongs

  • improved ground/ vegetation cover
  • cross-slope measure

3.6 SLM measures comprising the Technology

structural measures

structural measures

  • S6: Walls, barriers, palisades, fences

3.7 Main types of land degradation addressed by the Technology

soil erosion by water

soil erosion by water

  • Wt: loss of topsoil/ surface erosion
  • Wg: gully erosion/ gullying

3.8 Prevention, reduction, or restoration of land degradation

Specify the goal of the Technology with regard to land degradation:
  • reduce land degradation
  • restore/ rehabilitate severely degraded land

4. Technical specifications, implementation activities, inputs, and costs

4.1 Technical drawing of the Technology

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Technical specifications (related to technical drawing):

Silt fences are cheap and easy to erect, but the overflow depth must be limited. Silt fences are therefore erected in the wider section of a gully.

Author:

Jan van Heerden

Date:

01/01/2012

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Technical specifications (related to technical drawing):

Silt fences are temporary structures and are therefore placed closer together than permanent structures. The placing is such that the silt from the downstream structure builds up against the upstream structure to delay the perishing of the restoration materials. Vegetation must be established in the gully (donga) as soon as possible so that, when the materials decay after about ten years, the area will be restored.

Author:

Jan van Heerden

Date:

01/01/2012

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Technical specifications (related to technical drawing):

1. Dig a pit trench 300 mm × 300 mm from peg to peg over the entire width of the gully.
2. Drive iron standards (iron pole) in on the sides of the spillway length followed by more standards around 1.5 meters apart - ensure equal spacing between standards (standards must be driven in at least 500 mm deep - ensure that eyes (holes in standards) point upstream and that poles are in line with the others. Place apron on the ground from the excavation downstream. The width of the material must be three times the height of the overflow height and it must be at least 500 mm longer on both sides than the spillway length.
3. Span a 4 mm wire between the spillway poles on the total height of the structure. Tighten only manually and fasten on both sides of the two standards.
4. Drive in two anchor poles in line with the structure on both sides of the donga wall.
5. Saw off or cut heads of standards on the height of the structure’s height. Place anchor wires between two outer poles and anchor poles and tighten.
6. Thread 4 mm wire through on spillway height from side to side and bind to the two poles anchored to the anchor poles.
7. Also place wire between the spillway width poles and the end pole on the structure height and tighten.
8. Span wires from the spillway height wire on 100 mm to 150 mm distances apart, downwards until the last wire is at least 100 mm below the surface.
9. Place netting material (course plastic grid placed behind the filter material) on upstream side of the poles into the excavated pit trench and cut according to shape of the structure.
10. Place filter material flat in the pit trench and in front of the netting material on the upstream side of the poles and cut according to the shape of the structure. Tie the material to the structure with binding wire and fill the pit trench with soil.
11. Place netting material on the apron and tie it to the structure with binding wire and drive in T pegs at strategic places on downstream side.
12. The construction of the drop inlet is built in the same way as a weir structure as close as possible to the head of the donga. The gap between the structure and the donga head is filled with soil and must be filled again after the first rains. The spillway is made ±100 mm higher than the normal ground surface. The structure can be built to a maximum of 1 000 mm from the gully floor to the ground level.
13. Brush packing in front of the silt fence and stone packing downstream of the apron in the gully.
14. In order for silt fences to function more efficiently, it is recommended that that portion of the veld is fenced and withdrawn from grazing. Sowing of grasses or planting of common reed or vetiver grass can speed up the repair process considerably.

Author:

Jan van Heerden

Date:

01/01/2012

4.2 General information regarding the calculation of inputs and costs

Specify how costs and inputs were calculated:
  • per Technology unit
Specify unit:

silt fence

Specify dimensions of unit (if relevant):

6 to 8 meters

other/ national currency (specify):

Rand

If relevant, indicate exchange rate from USD to local currency (e.g. 1 USD = 79.9 Brazilian Real): 1 USD =:

16.0

Indicate average wage cost of hired labour per day:

R200

4.3 Establishment activities

Activity Timing (season)
1. Preparation of the site to be restored Winter (outside raining season)
2. Installation of silt fences followed by brush and stone packing Winter

4.4 Costs and inputs needed for establishment

Specify input Unit Quantity Costs per Unit Total costs per input % of costs borne by land users
Labour Unskilled labour (including transport) Per day 6.0 280.0 1680.0
Equipment Power tools - electric power generator, angle grinder, hammer drill, Per day 3.0 500.0 1500.0
Equipment Measuring tape, wire tensioning tool, pole driver, heavy and light hammers, pick, spade, pliers and scissors Per day 10.0 20.0 200.0
Construction material 4mm wire Per meter 100.0 15.0 1500.0
Construction material Standards Per piece 6.0 90.0 540.0
Construction material Netting Per meter 15.0 90.0 1350.0
Construction material Anchor poles Per piece 6.0 10.0 60.0
Construction material T pegs Per piece 20.0 20.0 400.0
Construction material Binding wire Per kg 5.0 40.0 200.0
Construction material Filter material (UV treated shade cloth - 80%) Per meter 86.0 20.0 1720.0
Total costs for establishment of the Technology 9150.0
Total costs for establishment of the Technology in USD 571.88
If land user bore less than 100% of costs, indicate who covered the remaining costs:

Department of Forestry, Fisheries and the Environment - South Africa

4.5 Maintenance/ recurrent activities

Activity Timing/ frequency
1. Repair of fences after floods After flooding event

4.6 Costs and inputs needed for maintenance/ recurrent activities (per year)

Specify input Unit Quantity Costs per Unit Total costs per input % of costs borne by land users
Labour Repair of fences Per day 2.0 280.0 560.0
Equipment Pliers, hammers, wire tensioning tool, pick, spade, pliers and scissors Per day 5.0 20.0 100.0
Construction material Binding wire and filter material Per meter 4.0 150.0 600.0
Total costs for maintenance of the Technology 1260.0
Total costs for maintenance of the Technology in USD 78.75
If land user bore less than 100% of costs, indicate who covered the remaining costs:

Department of Forestry, Fisheries and the Environment

4.7 Most important factors affecting the costs

Describe the most determinate factors affecting the costs:

1. Timing - during the raining season these areas could be inaccessible due to clayey soils and water in gullies.
2, Availability of labour.

5. Natural and human environment

5.1 Climate

Annual rainfall
  • < 250 mm
  • 251-500 mm
  • 501-750 mm
  • 751-1,000 mm
  • 1,001-1,500 mm
  • 1,501-2,000 mm
  • 2,001-3,000 mm
  • 3,001-4,000 mm
  • > 4,000 mm
Specify average annual rainfall (if known), in mm:

438.00

Agro-climatic zone
  • semi-arid

5.2 Topography

Slopes on average:
  • flat (0-2%)
  • gentle (3-5%)
  • moderate (6-10%)
  • rolling (11-15%)
  • hilly (16-30%)
  • steep (31-60%)
  • very steep (>60%)
Landforms:
  • plateau/plains
  • ridges
  • mountain slopes
  • hill slopes
  • footslopes
  • valley floors
Altitudinal zone:
  • 0-100 m a.s.l.
  • 101-500 m a.s.l.
  • 501-1,000 m a.s.l.
  • 1,001-1,500 m a.s.l.
  • 1,501-2,000 m a.s.l.
  • 2,001-2,500 m a.s.l.
  • 2,501-3,000 m a.s.l.
  • 3,001-4,000 m a.s.l.
  • > 4,000 m a.s.l.
Indicate if the Technology is specifically applied in:
  • concave situations

5.3 Soils

Soil depth on average:
  • very shallow (0-20 cm)
  • shallow (21-50 cm)
  • moderately deep (51-80 cm)
  • deep (81-120 cm)
  • very deep (> 120 cm)
Soil texture (topsoil):
  • medium (loamy, silty)
Soil texture (> 20 cm below surface):
  • fine/ heavy (clay)
Topsoil organic matter:
  • medium (1-3%)
If available, attach full soil description or specify the available information, e.g. soil type, soil PH/ acidity, Cation Exchange Capacity, nitrogen, salinity etc.

A variety of soil forms occur in the reserve and can generally be grouped according to the vegetation unit it supports. Sekhukhune Mountain Bushveld soils are predominantly shallow, rocky and clayey, with lime soils of the Glenrosa and Mispah soil forms often occurring in low-lying areas. Steep slopes commonly have rocky areas with no soil. The Dwars River valley is characterised by prismacutanic horizons with melanic structured diagnostic horizons.

Sekhukhune Plains Bushveld mainly occurs on red apedal clayey soils rich in metals. Soils on the plains are characterised by deep loamy Valsrivier soils. Shallow Glenrosa soils are found on the low-lying, rocky hills. Small mountains commonly have erodible black, melanic structured horizons.

5.4 Water availability and quality

Ground water table:

5-50 m

Availability of surface water:

medium

Water quality (untreated):

good drinking water

Water quality refers to:

ground water

Is water salinity a problem?

No

Is flooding of the area occurring?

Yes

5.5 Biodiversity

Species diversity:
  • high
Habitat diversity:
  • high
Comments and further specifications on biodiversity:

Although the reserve has a very high biodiversity it also includes a potential 16 plant species of conservation concern, of which one is critically endangered and two are endangered.
The reserve has a fair representation of game for its size, but it should be noted that the area is very prone to erosion if its ecological capacity is exceeded. During an aerial census conducted in the reserve a total of 582 head of game was recorded. There are currently 78 verified avian species occurring at the reserve, of which 10 occur on the IUCN Red List of Threatened Species. A total of 33 reptile species has been recorded and a possible 32 amphibian species. No data is currently available for the fish species occurring at the reserve.

5.6 Characteristics of land users applying the Technology

other (specify):

Tourists

Market orientation of production system:
  • commercial/ market
Off-farm income:
  • 10-50% of all income
Relative level of wealth:
  • poor
Individuals or groups:
  • employee (company, government)
Level of mechanization:
  • mechanized/ motorized
Gender:
  • women
  • men
Age of land users:
  • youth
  • middle-aged
  • elderly
Indicate other relevant characteristics of the land users:

Nature reserve - tourism

5.7 Average area of land used by land users applying the Technology

  • < 0.5 ha
  • 0.5-1 ha
  • 1-2 ha
  • 2-5 ha
  • 5-15 ha
  • 15-50 ha
  • 50-100 ha
  • 100-500 ha
  • 500-1,000 ha
  • 1,000-10,000 ha
  • > 10,000 ha
Is this considered small-, medium- or large-scale (referring to local context)?
  • small-scale

5.8 Land ownership, land use rights, and water use rights

Land ownership:
  • state
  • State land
  • State land
Are land use rights based on a traditional legal system?

No

5.9 Access to services and infrastructure

health:
  • poor
  • moderate
  • good
education:
  • poor
  • moderate
  • good
technical assistance:
  • poor
  • moderate
  • good
employment (e.g. off-farm):
  • poor
  • moderate
  • good
markets:
  • poor
  • moderate
  • good
energy:
  • poor
  • moderate
  • good
roads and transport:
  • poor
  • moderate
  • good
drinking water and sanitation:
  • poor
  • moderate
  • good
financial services:
  • poor
  • moderate
  • good

6. Impacts and concluding statements

6.1 On-site impacts the Technology has shown

Socio-economic impacts

Other socio-economic impacts

Job creation

Ecological impacts

Soil

soil moisture

decreased
increased

soil cover

reduced
improved

soil loss

increased
decreased

soil accumulation

decreased
increased
Specify assessment of on-site impacts (measurements):

The on-site impact assessment mainly includes a visual assessment of the amount of sediment trapped behind the silt fences and the establishment of vegetation.

6.2 Off-site impacts the Technology has shown

downstream siltation

increased
decreased

6.3 Exposure and sensitivity of the Technology to gradual climate change and climate-related extremes/ disasters (as perceived by land users)

Climate-related extremes (disasters)

Meteorological disasters
How does the Technology cope with it?
local thunderstorm moderately
Hydrological disasters
How does the Technology cope with it?
flash flood moderately

6.4 Cost-benefit analysis

How do the benefits compare with the establishment costs (from land users’ perspective)?
Short-term returns:

positive

Long-term returns:

positive

How do the benefits compare with the maintenance/ recurrent costs (from land users' perspective)?
Short-term returns:

positive

Long-term returns:

positive

6.5 Adoption of the Technology

  • 11-50%
Of all those who have adopted the Technology, how many did so spontaneously, i.e. without receiving any material incentives/ payments?
  • 0-10%

6.6 Adaptation

Has the Technology been modified recently to adapt to changing conditions?

No

6.7 Strengths/ advantages/ opportunities of the Technology

Strengths/ advantages/ opportunities in the land user’s view
Very important to stabilise landscapes.
Helps to improve the habitat for wildlife, biodiversity in the protected area.
Strengths/ advantages/ opportunities in the compiler’s or other key resource person’s view
Helps to improve the habitat for wildlife, biodiversity in the protected area.

6.8 Weaknesses/ disadvantages/ risks of the Technology and ways of overcoming them

Weaknesses/ disadvantages/ risks in the land user’s view How can they be overcome?
Sensitive to floods Timing of construction crucial - ready before the floods occur
Weaknesses/ disadvantages/ risks in the compiler’s or other key resource person’s view How can they be overcome?
Same as above

7. References and links

7.1 Methods/ sources of information

  • field visits, field surveys

Revisited the restoration sites by Mr. Van Heerden

  • interviews with SLM specialists/ experts

Various discussions with Mr. Van Heerden and Mr. Buckle

  • compilation from reports and other existing documentation

Silt fence report by Mr. Jan van Heerden (Agricultural Research Council) and the Strategic plan for the Potlake Nature Reserve (Limpopo Department of Economic Development, Environment and Tourism - LEDET)

When were the data compiled (in the field)?

09/09/2021

Comments:

Meetings commenced with Mr. Buckle

7.2 References to available publications

Title, author, year, ISBN:

Combating erosion with silt fences, Jan van Heerden, 2006, ISBN 1-919849-76-9

Available from where? Costs?

https://www.arc.agric.za/arc-iae/Documents/Publication%20List%20and%20Orderform.pdf

Title, author, year, ISBN:

Five-year strategic plan for the Potlake Nature Reserve, Limpopo Province, South Africa

Available from where? Costs?

LEDET - no cost

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