Silt fence, silt sock or silt curtain: which and where

SSI Environmental · Spill, Sediment & Site Environmental · Ireland

Silt Fence, Silt Sock or Silt Curtain: Which and Where

A silt fence is a trenched-in geotextile barrier for intercepting sediment-laden sheet flow across dry land, the standard perimeter control on a construction site. A silt sock is a flexible tubular filter (a mesh sock filled with media) that's laid on the ground to filter low flows and is especially useful around drains, along kerbs and where a trenched fence isn't practical, conforming to hard and uneven surfaces. A silt curtain is a floating barrier deployed in water, a river, lake or harbour, to contain suspended sediment stirred up by in-water works and stop it spreading. In short: silt fence for dry-land perimeters, silt sock for drains and flexible low-flow situations, silt curtain for sediment in water.

Silt fences, silt socks and silt curtains all control sediment, but they work in different places on dry ground, at drains, and in water, and using the wrong one in the wrong place is a common site mistake. Knowing which barrier suits which location is the key to specifying sediment control that actually holds. Here's how they compare.

Silt Fence - Dry-Land Perimeter

The silt fence is the workhorse for dry ground. Trenched in along the contour, it intercepts sheet flow running off disturbed areas and settles the sediment out before the water passes through, protecting the site perimeter and what's downhill. It needs to be installed properly to work (see silt fence), and it's for diffuse overland flow, not concentrated channels or drains, and not for use in water.

Silt Sock - Flexible and for Drains

A silt sock is a flexible filter tube, a mesh sock filled with a filter medium, that sits on the ground and filters water passing through it. Its strength is flexibility: it conforms to hard, uneven or paved surfaces where you can't dig a trench, so it's ideal around gully and drain inlets, along kerb lines, and as a quick, movable low-flow control. It's often used to protect drains from sediment (see protecting site drains), and it can be repositioned as work moves.

Silt Curtain - Sediment in Water

• 1
  A floating barrier deployed in water - rivers, lakes, harbours, ponds.
• 2
  It hangs down from floats to contain suspended sediment stirred up by in-water works.
• 3
  It stops that sediment plume spreading through the watercourse.
• 4
  Used for dredging, piling, bank works and any activity disturbing sediment in water.

A silt curtain is a completely different tool for a different medium, it contains sediment already suspended in water, rather than filtering runoff on land.

Matching Barrier to Location

Choose by where the sediment is moving: sheet flow across dry ground calls for a silt fence; water passing over hard surfaces or into drains calls for a silt sock; sediment suspended in a watercourse calls for a silt curtain. Many sites use all three in different places. The wider dewatering and discharge question is separate - see dewatering: silt bags and discharge to watercourse. SSI Environmental supplies silt fences, socks and curtains across Ireland, and can advise on the right barrier for each part of a site.

Frequently Asked Questions

• What is the difference between a silt fence and a silt sock?

  A silt fence is a trenched-in geotextile barrier for intercepting sheet flow across dry ground. A silt sock is a flexible filter tube laid on the surface, which conforms to hard and uneven ground where you can't trench, making it ideal around drains and kerbs and as a movable low-flow control.
• When do you use a silt curtain?

  A silt curtain is used in water - rivers, lakes, harbours, to contain suspended sediment stirred up by in-water works such as dredging, piling or bank works, stopping the sediment plume spreading through the watercourse. It's a floating barrier for sediment already in water, not a runoff filter for land.
• Which sediment barrier do I need for my site?

  It depends where sediment is moving. Use a silt fence for sheet flow across dry ground, a silt sock for water over hard surfaces or into drains and where you can't trench, and a silt curtain for sediment suspended in a watercourse. Many sites use all three in different locations.