Silt fences Print

​​​​​​​1.3.1 Design


Definition

A silt fence is a temporary barrier of woven geotextile fabric that is used to capture mainly coarse sediments carried in sheet flow (refer Figures 80 to 83). Silt fences temporarily impound sediment-laden runoff, slowing down the flow rate and allowing sediment to settle out of the water.

Purpose

The purpose of a silt fence is to detain runoff flows so that deposition of transported sediment can occur through settlement. They are not used to filter sediment out of runoff.

Conditions where practice applies

Use silt fences:

  • Where there is a need to control sediment by intercepting sheet flow
  • Where a site is low gradient, or is confined with a small contributing catchment, such as short batter fills and around watercourses
  • To delineate the limit of disturbance on an earthworks site, such as riparian areas or bush reserves
  • Where the installation of an earth or topsoil bund would destroy sensitive areas, such as bush and wetlands.

Do not install silt fences across watercourses or in areas of concentrated flows. Avoid trench excavations within the root zones of protected trees and trees that are to be retained.

Where there is a change in slope, no section of the fence should exceed a grade of 5% for a distance of more than 15 m.

Limitations

The following limitations apply to silt fences:

  • Silt fences do not capture many soil particles finer than 0.02 mm in diameter (for example fine silts and clays) due to the short detention time of water behind the silt fence and relatively large pore size of most fabrics
  • The pores in the silt fence fabric become clogged relatively quickly with fine textured sediments, which the result that the fabric becomes impermeable. Consequently, additional reinforcing (such as chain link fence – super silt fence) might be required (refer Section F3.0 for super silt fences)
  • Silt fences should only be used for sheet flows, not concentrated flows. Do not use silt fences as checks dams in channels (to reduce velocities) or place them where they will intercept concentrated flows
  • Silt fences are best used a part of a treatment train approach.
Key design criteria

Key design criteria for silt fences are outlined below:

  • Ensure silt fence height is 600 mm above ground level and 200 mm below ground level
  • Maximum slope lengths, spacing of returns and angles for silt fences are shown in Table 12
  • Locate supporting posts/waratahs for silt fences 2-4 m apart with support provided by a tensioned wire (2.5 mm HT) along the top of the silt fence
  • Where a strong woven fabric is used in conjunction with a wire support, the distance between posts can be up to 4 m. Double the silt fence fabric over and fasten to the wire with silt fence clips at 500 mm spacings
  • Ensure supporting posts/waratahs are embedded a minimum of 400 mm into the ground
  • Always install silt fences along the contour (at a break in slope). Where this is not possible, or  where there are long sections of silt fence, install short silt fence returns (refer Figure 82) projecting up-slope from the silt fence to minimise the concentration of flows. Silt fence returns should be a minimum 2 m in length, and can incorporate a tie back. They are generally constructed by continuing the silt fence around the return and doubling back, eliminating joins
  • Join lengths of silt fence by doubling over fabric ends around a waratah or by stapling the fabric ends to a batten and butting the two battens together as shown in Figure 82
  • Install silt fence returns at either end of the silt fence, projecting up-slope to a sufficient height to prevent outflanking
  • In catchments of more than 0.3 ha, use of silt fences requires careful consideration of specific site measures, and other control measures may be better, such as a super silt fence (refer Section F1.4).

Table 12:  Silt fence design criteria​

Slope steepness %

Slope length (m) (maximum)

Spacing of returns (m)

Silt fence length (m) (maximum)

Flatter than 2%

Unlimited

N/A

Unlimited

2 – 10%

40

60

300

10 – 20%

30

50

230

20 – 33%

20

40

150

33 – 50%

15

30

75

> 50%

6

20

40​





  • Where water may pond regularly behind the silt fence, provide extra support for the silt fence with tie backs from the silt fence to a central stable point on the upward side. Extra support can also be provided by stringing wire between support stakes and connecting the filter fabric to this wire.
  • As a minimum, the silt fence cloth must meet the following criteria for geotextile fabric:
    • ​G​rab tensile strength: >440N (ASTM D4632)
    • Tensile modulus: 0.140 pa (minimum)
    • Apparent opening Size: 0.1 – 0.5 mm (ASTM D4751).​
​​​

1.3.2 Construction, operation and maintenance


Construction and operation

For constructing and/or operating silt fences, follow the following steps and refer Figure 84 below:

​​
  • Use silt fence material appropriate to the site conditions and in accordance with the manufacturer's specifications
  • Always install silt fences along the contour (refer Figure 85)
  • Excavate a trench a minimum of 100 mm wide and 200 mm deep along the proposed line of the silt fence
  • Use waratahs at least 1.5 m in length
  • Install the support waratahs on the down-slope edge of the trench and silt fence fabric on the up-slope side of the support waratahs to the full depth of the trench, then backfill the trench with compacted soil
  • Install the waratahs so that they are as flat as possible against the silt fence. If the waratah edge is against the silt fence, it will rub and eventually rip against the waratah
  • Use correct silt fence clips (refer Figure 86) to secure the silt fence material to the top wire.
  • Wire ties and staples rip the silt fence material when the weight of the impounded water pushes against the silt fence and are not to be used
  • Reinforce the top of the silt fence fabric with a support made of high tensile 2.5 mm diameter galvanised wire. Tension the wire using permanent wire strainers attached to angled waratahs at the end of the silt fence
  • Where ends of silt fence fabric come together, ensure they are overlapped, folded and stapled/ screwed to prevent sediment bypass.​
​​​Maintenance

To maintain silt fences:

  • Inspect silt fences at least once a week and after each rainfall
  • Check for damage including rips, tears, bulges in the fabric, broken support wires, loose waratahs, overtopping, outflanking, undercutting, and leaking joins in fabric
  • Make any necessary repairs as soon as identified
  • As the geotextile material becomes clogged with sediments, this will result in increased duration of ponding. Therefore, careful cleaning of the silt fence geotextile with a light broom or brush may be appropriate
  • Remove sediment when bulges occur or when sediment accumulation reaches 20% of the fabric height
  • Remove sediment deposits as necessary (prior to 20% of fabric height) to continue to allow for adequate sediment storage and reduce pressure on the silt fence
  • Dispose of sediment to a secure area to ensure that it does not discharge to the receiving environment.​​


1.3.3 Decommissioning

When decommissioning a silt fence:

  • Do not remove silt fence and accumulated sediment until the catchment area has been appropriately stabilised
  • Remove and correctly dispose of accumulated sediment
  • Backfill trench, re-grade and stabilise the disturbed area.​
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