A silt fence, sometimes (misleadingly) called a filter fence, is a temporary

sediment control A sediment control is a practice or device designed to keep eroded soil on a construction site, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or sea. Sediment controls are usually employed together with e ...

device used on

construction Construction are processes involved in delivering buildings, infrastructure, industrial facilities, and associated activities through to the end of their life. It typically starts with planning, financing, and design that continues until the a ...

sites to protect

water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...

in nearby streams, rivers, lakes and seas from sediment (loose

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

) in

stormwater Stormwater, also written storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed lan ...

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 basin A sediment basin is a temporary pond built on a construction site to capture eroded or disturbed soil that is washed off during rain storms, and protect the water quality of a nearby stream, river, lake, or bay. The sediment-laden soil settles i ...

s and

sediment trap Sediment traps are instruments used in oceanography and limnology to measure the quantity of sinking particulate Organic material, organic (and inorganic) material in aquatic ecosystem, aquatic systems, usually oceans, Lake, lakes, or Reservoir, ...

s, as well as with

erosion control Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, coast, coastal areas, Bank (geography), river banks and construction. Effective erosion controls handle surface runoff and are ...

s, which are designed to retain sediment in place where soil is being disturbed by construction processes (i.e.,

land grading Grading in civil engineering and landscape architectural construction is the work of ensuring a level base, or one with a specified slope, for a construction work such as a foundation, the base course for a road or a railway, or landscape and ...

and other earthworks). A typical fence consists of a piece of

synthetic Synthetic may refer to: Science * Synthetic biology * Synthetic chemical or compound, produced by the process of chemical synthesis * Synthetic elements, chemical elements that are not naturally found on Earth and therefore have to be created in ...

filter fabric (also called a

geotextile Geotextiles are versatile permeable fabrics that, when used in conjunction with soil, can effectively perform multiple functions, including separation, filtration, reinforcement, protection, and drainage. Typically crafted from polypropylene or ...

) 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 The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

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 Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

. 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 State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...

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 In geotechnical engineering, soil compaction is the process in which stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water (or other ...

, 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 Total suspended solids (TSS) is the dry-weight of suspended particles, that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus known as sintered glass crucible. TSS is a water qu ...

and

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wa ...

.) Other studies and articles about silt fence usage and practice document problems with installation and maintenance, implying poor performance. Since 1998, static slicing the material into the ground has proven to be the most efficient and most effective installation method because slicing maintains the soil on both sides of the fence, and is conducive to proper compaction—which is critical to performance, as well. In 2000 the U.S. Environmental Protection Agency (EPA) co-sponsored silt fence efficacy field research through its Environmental Technology Verification Program, and in general, the report found the static slicing method to be highly effective, and efficient. CERF Report No 40565. Silt fence effectiveness is best determined by how many hundreds of pounds of sediment are contained behind a given silt fence after a storm event, and not turbidity, etc. as sediment-retention is the end goal, and not a water-quality measurement used in erosion control, for instance. Silt fences may perform poorly for a variety of reasons, including improper location (e.g. placing fence where it will not pond runoff water), improper installation (e.g. failure to adequately embed and backfill the lower edge of fabric in the soil) and lack of maintenance—fabric falling off of the posts, or posts knocked down. A silt fence top-full of sediment may need maintenance/replacement, but it is a huge success. The fabric may become damaged with holes and tears if construction materials are stored next to or on top of the fence. During various phases of construction at a site, a silt fence may be removed relocated and reinstalled multiple times. It may be difficult to maintain effectiveness of a silt fence under such operating conditions. Location of fences in areas with high flows may lead to fence failures when the installation is not adequately back-filled and properly compacted, and/or the post-spacing is inadequate.

References

External links

International Erosion Control Association
��Professional Association, Publications, Training
WatchYourDirt.com
��An Erosion and Sediment Control Video Education Resource Earthworks (engineering) Environmental soil science Water pollution Water treatment