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Silt Fence

General

A silt fence is a temporary structure, constructed of woven geotextile fabric attached to posts, which minimizes the loss of sediment from a site and prevents sheet and rill erosion. These structures intercept runoff and force it to pass through the fabric, reducing its velocity and allowing suspended sediments to settle out upslope of the silt fence.

Silt fences are typically used on construction sites to trap sediment on site and around soil piles and may not be used in channels, gullies, ditches, streams, or in any other area where concentrated flow may occur. These structures, which may be prefabricated or constructed on site, should be installed prior to site disturbance. Because silt fences have a high rate of failure without proper installation and maintenance, they are best used in conjunction with other BMPs.

Silt fences must be removed and disposed of after the site has been stabilized and permanent BMPs have been established.

Advantages

  • Low cost
  • Versatile

Disadvantages

  • Ineffective for concentrated flows
  • Requires frequent maintenance
  • Maximum life span of 1 year
  • Drainage area should not exceed 0.25 acres per 100 feet of fence length
  • High rates of failure if not installed properly
  • Ineffective on slopes greater than 50%
  • Labor-intensive to install

Design

Silt fences must be designed to handle the runoff from the 10-year, 24-hour storm event, with a maximum drainage area of 0.25 acres per 100 feet of fence.

Silt fence height will vary upon the site and application of the practice, but must be between 14 and 28 inches, measured from the top of the fabric to the soil surface, and does not include any portion of the fence below ground. Silt fences should be installed in a crescent shape, parallel to the contour of the land, with the ends placed upslope of the center. This practice prevents water from escaping around the ends of the fence and forces it to pond behind it. Ponding depth must not exceed 2 feet, as greater depths greatly increase the likelihood of failure. Stabilized outlets should be placed at the ends of the fence to provide an overflow and protect the fence during larger storm events.

Silt fences are not recommended for use on slopes that exceed a 2:1 ratio. However, they may be used in series for flatter slopes if the spacing guidelines below are followed. Additional structures, however, do not increase the permissible slope length.

Silt Fence Spacing Guidelines

Percent SlopeSlope Length
0-2%100 feet
2-5%75 feet
5-10%50 feet
10-20%25 feet
20-50%15 feet

Materials

Fabric

A woven geotextile fabric should be used and must meet the following criteria:

  • Minimum grab strength of 100 pounds
  • Pore size of between 50 and 140 microns
  • Ultra-violet radiation stability of 90% (Using test method ASTM-4355)
  • The use of a top support device, such as a heavy nylon cord or equivalent

The fabric should be anchored by burying at least 8 inches in a 4x4-inch trench, with the bottom 4 inches of fabric extending upslope. Joints in the fabric should be minimized to prevent failure of the fence. Where joints are necessary, each end of the fabric should be securely fastened to a post. The posts should then be wrapped around each other to produce a stable, secure joint or may be overlapped to the next post.

Support

Silt fences may be supported by either steel or hardwood posts. The strength, dimensions, and depth of the posts will vary upon the load that they are designed to support. As a result, only the minimum dimensions are listed here. Steel posts must be at least 5 feet long with a strength of 1.33 lbs/ft. (2 kg/m) and have projections for the attachment of fasteners. Hardwood posts must be air or kiln dried and measure at least 1 1/8 inches square, with a minimum length of 3 feet for a 24-inch fence. All posts should be driven at least 20 inches below ground and should be spaced a maximum of 8 feet apart to provide proper support.

The geotextile fabric should be attached in at least 3 places to the posts on the upslope side with either ½ inch staples, 50 lb. plastic zip fasteners, or wire fasteners. To prevent damage to the fabric from fasteners, the protruding ends should be pointed away from the fabric.

For added strength and stability, silt fences may be reinforced with wire mesh. When used, the mesh should be installed behind (downslope) the geotextile fabric and in front (upslope) of the posts. The wire mesh should be attached to the fabric in at least 3 places, using wire fasteners spaced at a minimum distance of 2 feet.

Maintenance

  • Silt fences should be inspected weekly and after each rainfall for damage – all repairs should be made immediately
  • Accumulated sediment should be removed once it reaches ½ the height of the fence to ensure that a proper storage volume is preserved behind the fence
  • Silt fences should be replaced when worn out

Method To Determine Practice Efficiency

A silt fence prevents soil loss by reducing the flow velocity of runoff by forcing it through fabric. When properly installed and maintained, a silt fence with a 20-micron pore size yields an efficiency of 42%.


References