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

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  2. 7. Erosion, sediment and slash control structures ›
  3. 7.6 Sediment traps and soak holes
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7.6 Sediment traps and soak holes

Sediment traps and soak holes are small excavated structures. They capture sediment-laden water, allow sediment to settle, then water to either discharge or drain. They reduce the volume of sediment that can enter sensitive sites, such as waterbodies. They should be located close to roads and landings to enable easy access for maintenance.

To be effective, sediment traps need to be large enough to slow flow, and allow some of the larger sediment particle sizes to settle, before the water is discharged. Soak holes are constructed in porous soils such as sand and pumice, allowing sediment-laden water to soak into the soil. Although sediment traps and soak holes are constructed similarly, they work differently. Some sites not suitable for either type of sediment control. These include a fill batter, where they increase the risk of bank collapse, or where the site is located within land that carries flood flows of rivers.

Construct sediment traps near culvert inlets and outlets, and immediately after water is directed, near a road, track, or landing as necessary. A recommended effective sediment trap size is 1 m deep x 1.5 m long. A good length to width ratio is 3:1, but this is not always practical at culvert inlets due to topographical constraints and safety concerns. For example, on steep terrain, adequately sized cut-outs are difficult to construct near culvert mouths as they may encroach into the roadway, creating a health and safety hazard. Multiple small traps of at least the bucket width may be an alternative option. Do not construct sediment traps or soak holes in fill where they increase the risk of bank collapse or within the floodplain of rivers.

Soak holes are constructed similarly to sediment traps, but are constructed in porous soils such as sand and pumice, allowing sediment-laden water to soak into the ground with no discharge.

Soak hole spacing guideExcavate the trap to well below the culvert inlet level, to ensure maximum sediment retention capacity for the trap. Install on hard ground rather than fill or disturbed soil. Use a rock bucket to excavate. Make them large enough so that an excavator bucket can be used during maintenance to remove the retained sediment. Keep the slope of the inlet into the soak hole reasonably flat, to avoid erosion. If the inflow or outflow is through fill, then flume into and/or out of the sediment trap. Consider sediment retention from the outflow. Additional sediment controls like slash or grassing or the use of polymers may assist, if needed.

Good example of a sediment trap at a culvert mouth
Good example of a soak hole
Making sufficiently effective sediment traps is challenging in steeper terrain. This one is a potential safety concern as it is encroaching on the roadway
Although this trap has worked, it is too small and not deep enough below the culvert mouth
A good example of a sediment trap with minimal ground disturbance and provision of an outlet
There are sediment traps at both the inlet and outlet of this culvert. Immediately below the outlet is a waterway
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