A silt fence, sometimes (misleadingly) called a "filter fence," is a temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment (loose soil) in stormwater runoff. Silt fences are widely used on construction sites in North America and elsewhere, due to their low cost and simple design. However, their effectiveness in controlling sediment can be limited, due to problems with poor installation, proper placement, and/or inadequate maintenance.
Design and installation
Silt fences are often installed as perimeter controls. They are typically used in combination with sediment basins and sediment traps, as well as with erosion controls, which are designed to retain sediment in place where soil is being disturbed by construction processes (i.e., land grading and other earthworks).
A typical fence consists of a piece of synthetic filter fabric (also called a geotextile) stretched between a series of wooden or metal fence stakes along a horizontal contour level. The stakes are installed on the downhill side of the fence, and the bottom edge of the fabric can be trenched into the soil and backfilled on the uphill side, although it is quite difficult to move the trenched "spoil" from the downside to the upside of the trench. The design/placement of the silt fence should create a pooling of runoff, which then allows sedimentation to occur. Water can seep through the silt fence fabric, but the fabric often becomes "blocked off" with fine soil particles (all sediment-retention devices have this challenge, and none of them "filter" storm water for very long). A few hours after a storm event, the fabric can be "disturbed" in order to dislodge the fines, and allow clean water to flow through. Depending on the protected watershed and erosion, larger soil particles will settle out, ultimately filling the silt fence to the top of the structure; requiring another silt fence above or below it (creating a new ponding area), or for the silt fence to be removed, the sediment removed or spread out, and a new fence installed. The fence is not designed to concentrate or channel stormwater. The fence is installed on a site before soil disturbance begins, and is placed down-slope from the disturbance area.
Sediment is captured by silt fences most often through ponding of water and settling, rather than filtration by the fabric. Sand and silt tends to clog the fabric, and then the sediments settle in the temporary pond.
Super silt fence
Some government jurisdictions in the United States recommend or require the use of a reinforced fence, sometimes called a "super" silt fence or an enhanced silt fence, on some construction sites. This design uses filter fabric reinforced by a wire mesh or chain link fence. The metal backing gives the fence increased strength to resist the weight of soil and water which may be trapped by the fence in a large drainage area, and discourages construction site operators from driving vehicles over the fence. However, an improper installation of a super silt fence can create an inadvertent sediment basin when the filter fabric becomes clogged. This typically causes flooding and increased downstream pollution. Most super silt fence specifications are outdated, requiring the trenching installation method, which has been shown to be highly susceptible to "washing out" under the fabric due to improper back-filling and inadequate compaction.
Static slicing installation
Some state agencies recommend an installation technique called "static slicing" as an improved method for ensuring effectiveness and longevity of a silt fence system on a construction site. The technique involves inserting a narrow blade into the soil with a wedge-type point on its tip to slightly disrupt the soil upward, while simultaneously inserting the silt fence fabric into the slot with a moving pivot, while the machine is moving forward. This step is followed by mechanical soil compaction, setting of fence posts, and attaching the fabric.
Effectiveness
Silt fence fabrics (geotextiles) tested in laboratory settings have shown to be effective at trapping sediment particles. Although there have been few field tests of silt fences installed at construction sites, these tests have shown generally poor results. (Effectiveness testing involved measurements for both total suspended solids and turbidity.) Other studies and articles about silt fence usage and practice document problems with installation and maintenance, implying poor performance.