Silt Fence

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Design & Site Selection for the Silt Fencing

Maintenance and other Silt Fence Facts

See Other Barriers for Controlling Silt

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Learn More about the Silt Fence

The silt fence is typically manufactured from a woven geotextile fabric that is then attached to wooden stakes. These stakes should be places a regular intervals along the site perimeter for the most effective use. When installed correctly and inspected frequently, these fences can be an economical effective barrier to sediment leaving the site.

Silt Fencing Design Considerations:

• Fabrics: Fabrics used for silt fences should be a pervious sheets of geotextile material such as polypropylene, nylon or polyester. These are chosen based on minimum synthetic fabric requirements, as shown below.

Site Selection and Design use

• Places the Fence is Used: Typically this silt control fence is used on construction sites with relatively small drainage areas. Runoff can be a low-level shallow flow, but should not exceed 0.5 cfs.

• Drainage Area: This area should not exceed 1/4 acre per 100 ft. of fence length. Slope length above this fence should also not exceed 100 ft. (NAHB, 1995) as this can have a negative impact on performance.

• Life Expectancy: The life expectancy of this fence is approximately 6 months, depending on your local conditions.
  • The Burlap Fence is usually only 2 months.
  • Fabric can be reinforced with wire mesh for increased life.

• Barrier Placement:
  • Wooden stakes: If oak is used, these stakes should be at least 5 feet long and have a minimum diameter of 2 inches. Softer woods such as pine should be at least 4 inches in diameter.
  • Metal Post: When using metal post in place of wooden stakes, they should have a minimum weight of 1.00 to 1.33 lb/linear foot. These metal post require attachment points such as hog rings for fastening the filter fabric using wire ties.

• Height of Fence Post: The height should be between 16 and 34 inches above grade.
  • If standard strength fabric is used in combination with wire mesh, the posts should be spaced no more than 10 feet apart.
  • If extra-strength fabric is used without wire mesh reinforcement, the support posts should be spaced no more than 6 feet apart which is typical on most D.O.T. projects(VDCR, 1995).

• Fence Placement: A silt fence should be erected to eliminate unwanted gaps in the fence. If a continuous roll of fabric is not available, the fabric should overlap from both directions only at stakes or posts with a minimum overlap of 6 inches. A trench should be excavated to backfill the bottom of the fabric fence at least 6 inches below the ground surface. This minimizes entrainment of stormwater runoff.

The design should be able withstand the runoff from a 10-year peak storm event, and once installed should remain in place until all areas up-slope have been permanently stabilized.

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Maintenance & Considerations when using Silt Fencing

Barriers like these have limitations and should not be installed along areas where rocks or other hard surfaces will prevent uniform anchoring of fence posts and entrenching of the fence fabric.

Staked silt barriers are not suitable for areas where large amounts of concentrated runoff are likely. If this may occure consider the use of other BMP's in conjunction with silt fence in order to direct water to settlings ponds. High winds may present a maintenance challenge, as high winds may damage the filter fabric.

When the pores of the fence filter fabric become clogged with sediment, pools of water are likely to form on the uphill side of fence. Siting and design of the silt fence should take this into consideration and care should be taken to avoid further erosion damage.


Maintenance

Our fences should be inspected frequently, as well as after each rainfall event, to ensure that they are intact. If gaps or tears are found, they should be repaired or the fabric should be replaced immediately.

Accumulated sediments should be removed from the fence base when the sediment reaches one-third to one-half the height of the fence to avoid failure. Sediment removal should occur more frequently if accumulated sediment is creating noticeable strain on the fabric. When the silt fence is removed, the accumulated sediment also should be removed.

Effectiveness

USEPA (1993) reports the following effectiveness ranges for silt fences constructed of filter fabric that are properly installed and well maintained:

• Average Total Suspended Solids Removal: 70 percent
• Sand Removal: 80 to 90 percent
• Silt-loam Removal: 50 to 80 percent,
• Silt-Clay-Loam Removal: 0 to 20 percent.

Removal rates are highly dependent on local conditions and installation.

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Other Turbidity Control Options

Other Turbidity Barriers

• Type I (Economy & DOT)
• Type II (Medium & Heavy Duty)
• Type III (Medium & Heavy Duty)
• Dredging Barrier

Other Filtering Devices

• Coir Logs
• Geotextiles
• Staked Silt Barrier (Standard & DOT)

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