Contents

  • Cover
  • Front matter
  • Preface
  • Scope, purpose and use
  • 1. Terminology, economic analysis, risk management
    • 1.1 Terminology
    • 1.2 Economic analysis
    • 1.3 Understanding risk
  • 2. Regulations, consents and approvals
    • 2.1 National Environmental Standards for Plantation Forestry (NES-PF)
    • 2.2 Heritage New Zealand Pouhere Taonga
    • 2.3 The Health and Safety at Work Act
    • 2.4 NZ Transport Agency approval for access onto state highways
    • 2.5 District council approval for access onto council roads
  • 3. Planning for roads
    • 3.1 Road classes
    • 3.2 Arterial roads
    • 3.3 Secondary roads
    • 3.4 Spur roads
    • 3.5 Establishment tracks
    • 3.6 Spatial information
    • 3.7 Initial field work
    • 3.8 Manual design method: Stepping out a roadline on a topo
    • 3.9 Running a grade line in the field
    • 3.10 Full road design
    • 3.11 Working with road survey data
    • 3.12 Geometric road design
    • 3.13 Curve widening
    • 3.14 Horizontal alignment
    • 3.15 Vertical alignment
    • 3.16 Calculating the safe stopping distance
    • 3.17 Setting out the roadline
  • 4. Planning for landings
    • 4.1 Common landing layouts
    • 4.2 Landing planning considerations
  • 5. Road and landing construction
    • 5.1 Soil and rock properties
    • 5.2 Managing adverse environmental effects
    • 5.3 Marking clearing widths
    • 5.4 Roadline salvage
    • 5.5 Daylighting
    • 5.6 Road formation
    • 5.7 Drainage control during earthwork construction
    • 5.8 Earthwork machinery
    • 5.9 Estimating machinery production
    • 5.10 Stabilising cut and fill slopes during construction
  • 6. Pavement design, subgrade preparation, pavement construction
    • 6.1 Traffic loading
    • 6.2 Evaluating subgrade properties
    • 6.3 Determining pavement depth
    • 6.4 Pavement material properties
    • 6.5 Compaction of subgrade and pavement
    • 6.6 Compaction equipment
    • 6.7 Pavement construction
    • 6.8 Weak subgrades
    • 6.9 Chemical stabilisation of pavement or subgrade
  • 7. Erosion, sediment and slash control structures
    • 7.1 Ditches
    • 7.2 Cut-outs
    • 7.3 Berms
    • 7.4 Drainage culverts
    • 7.5 Flumes
    • 7.6 Sediment traps and soak holes
    • 7.7 Silt fences
    • 7.8 Sediment retention ponds
    • 7.9 Debris traps
  • 8. River crossings
    • 8.1 Fish passage
    • 8.2 Selecting the location and crossing type
    • 8.3 Fords
    • 8.4 Temporary river crossings
    • 8.5 Single culvert river crossings
    • 8.6 Battery culvert river crossings
    • 8.7 Drift deck river crossings
    • 8.8 Single span bridge river crossings
    • 8.9 Prediction of flood flows, and sizing culverts
  • 9. Road maintenance, repairs and upgrades
    • 9.1 Maintenance programme
    • 9.2 Economic evaluation of road maintenance projects
    • 9.3 Managing maintenance requirements
    • 9.4 Commonly used maintenance machinery
    • 9.5 Road surface maintenance
    • 9.6 Road foundation maintenance
    • 9.7 Landing rehabilitation and decommissioning
    • 9.8 Roadside vegetation maintenance
    • 9.9 Erosion and sediment control structure maintenance
    • 9.10 River crossing maintenance
  • Forest road engineering terminology
  • References
  • Websites, resources, databases

NZ Forest Road Engineering Manual

  1.  ›
  2. 7. Erosion, sediment and slash control structures ›
  3. 7.7 Silt fences
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7.7 Silt fences

A well-constructed application for a sediment fenceA well-constructed application for a sediment fenceSilt fences are designed to intercept sheet flow sediment-laden stormwater runoff, and filter out larger, and some of the smaller, sediment particles, but not dissolved clays. Silt fences, and the larger ‘super’ silt fences, are a short-term solution to reduce sediment movement until the site stabilises, and vegetation re-establishes. Silt fences are usually made from geotechnical fabric, but at times shade cloth can also be used to allow water to pass through it while filtering larger particle sizes. Silt fences can be used in conjunction with other sediment treatment measures, such as sediment traps or ponds.

Silt fences need to be carefully located. Only use silt fences to intercept sheet flow water on low gradient sites, or in confined areas where the contributing area is small, less than 0.5 ha. Plan the location of silt fences to be constructed where they will not be overwhelmed by large flows. Use longer ‘super’ silt fences for larger areas, where the catchments are greater than 0.5 ha. Do not use them in larger areas, or in concentrated flow paths to capture sediment like rivers, gullies or in ditches. They are not designed to reduce water flow velocity. Fence fabric is only partially porous, so too much flow may cause them to fill too rapidly and either fail or be bypassed.

Silt fences also need to be correctly constructed. Make sure the correct fabric for the silt fence is selected. Silt fence fabric is a close weave and intended to capture fine sediment. Shade cloth and open weave fabrics will trap larger sediment grain sizes, but not fine sediment. It is also important to install the fence along the contour. If this is not possible, or where there are long sections of silt fence, install short silt fence returns projecting upslope from the silt fence, to minimise concentration of flow. Also, construct silt fence wings at either end to contain sediment where there is a risk of bypass.

Silt fence design criteriaThe following is an overview of the installation process:

  1. Silt fence returns should be a minimum of 2 m in length, and can incorporate a tie-back if required. Continue the silt fence around the return and double back to eliminate joins
  2. Use support posts or Y-post steel standards (waratahs) at a maximum 2 m apart, unless tensioned wire (2.5 mm HT along the top of the silt fence) is used between posts, top and bottom. If tensioned, the distance can be widened to 4 m
  3. Double the silt fence fabric over and fasten to the wire and posts with wire ties or cloth fastening clips at 150 mm spacing
  4. Join lengths of fabric by doubling over fabric ends around a wooden post or batten, or by stapling the fabric ends to a batten and butting the battens together
  5. Recommended maximum slope lengths, spacing of returns and angles for silt fences are given in the adjacent table
  6. Excavate a trench at least 100 mm wide and 200 mm deep along the proposed line of the silt fence
  7. Install the support posts on the downslope edge of the trench. These should be tanalised timber of a minimum of 50 mm square, or waratahs at least 1.5 m in length. Drive in until solid, at least 400 mm deep
  8. Tie silt fence fabric on the upslope side of the support posts to the full depth of the trench
  9. Silt fence height should not exceed 300-400 mm above ground level
  10. Backfill the trench with compacted fill
  11. Use angled waratahs at the end of the silt fence to tension wires
  12. Reinforce and tension the top of the silt fence with a 2.5 mm support wire (refer back to point 2)
  13. Where ends of silt fence fabric come together, ensure they are overlapped, folded and stapled to prevent sediment bypass
  14. Construct extra tie-backs, on the upward side, where water may pond behind the silt fence.
Well-functioning fences. They have captured most of the larger sediment and have not failed, despite containing a significant weight of material
Maintenance is required for the shade cloth fence
Sediment fences are not designed for watercourses. They fill rapidly, divert flow, or fail
This fence is inadequate. The lower edge has not been buried properly, so has blown out. The fence is not wide enough, and there are no wings so stormwater flow would have bypassed it. Also, it is too close to the waterway immediately behind it
This large silt fence looks well constructed, however the fabric has been installed on the wrong side. It should be on the uphill, so the wire will help support the fabric and stop it from blowing out. The lower edge of the fabric has not been buried because you can see where the sediment has flowed underneath it
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