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Silt Sock

General

Silt sock is a temporary erosion control practice constructed by filling a fabric tube with permeable media. They are primarily used as perimeter control to minimize the loss of sediment from a site or stockpile, but can also be used as a slope break to reduce sheet and rill erosion. By intercepting runoff and forcing it to pass through (or over) the silt sock, the runoff's velocity is reduced, which allows suspended sediments to settle out. Silt sock should be installed prior to site disturbance and must maintain ground contact at all times to be effective.

They may not be used in channels, gullies, ditches, streams, or in any other area where concentrated flow may occur. Because silt sock has a high rate of failure without proper installation and maintenance, they are best used with other BMPs.

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

Advantages

  • Low cost
  • Versatile
  • Easy to install

Disadvantages

  • Ineffective for concentrated flows
  • Requires frequent maintenance
  • Maximum life span of 1 year
  • High rate of failure if not installed properly
  • Ineffective on slopes greater than 50%
  • More difficult to transport and move than silt fence

Design

Silt sock diameter can vary upon the site and application of the practice, but must be at least 8 inches tall during use. They should be installed in a crescent shape, parallel to the contour of the land, with the ends placed upslope of the center to prevent water from escaping around the ends of the practice.

They are not recommended for use on slopes that exceed a 2:1 ratio, but may be used in series on flatter slopes if the spacing guidelines below are followed.

Silt Sock Spacing Guidelines

Percent SlopeMax. Spacing for 8" SockMax Spacing for 12" SockMax Spacing for 18" sock
0-2%30 feet55 feet75 feet
3-5%25 feet40 feet55 feet
6-10%15 feet30 feet40 feet
11-33%10 feet15 feet20 feet
34%+5 feet10 feet15 feet

Installation

Silt sock must be installed on smooth ground to ensure good ground contact and sections must be overlapped by 24 inches. Staking is required if the dry weight of the silt sock is less than 6 pounds per foot. Multiple silt socks can be stacked on top of each in a pyramid to achieve a desired height.

Maintenance

  • Silt sock should be inspected weekly and after each rainfall for damage, with all repairs being made immediately
  • Accumulated sediment should be removed once it reaches ½ the height of the silt sock to ensure that a proper storage volume is preserved
  • Sections of damaged or compacted silt sock should be replaced

Method To Determine Practice Efficiency

Silt sock prevents soil loss by reducing the flow velocity of runoff by forcing it through or over the practice. When properly installed and maintained, silt sock yields an efficiency of 42%.


References