Silt fence

A silt fence, sometimes (misleadingly) called a "filter fence,"[1] 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.[2] However, their effectiveness in controlling sediment can be limited, due to problems with poor installation, proper placement, and/or inadequate maintenance.[3]

Silt fence illus EPA
Silt fence installed up-slope of a vegetated stream buffer

Design and installation

Silt fences are often perimeter controls, typically used in combination with sediment basins and sediment traps, as well as 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).[4]

Silt fence EPA
Silt fence installed on a construction site
Silt fence & chain link support
Chain link supported "super" silt fence

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.[5][6]

Sediment is captured by silt fences primarily 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.[7]:p.6–9 [8]:p.7–46

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.[9] 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 out-dated, 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 machine
Static slicing machine

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.[6][10]

Effectiveness

Silt fence installation detail EPA
Installation detail for a silt fence with specifications recommended by US EPA[11]

Silt fence fabrics (geotextiles) tested in laboratory settings have shown to be effective at trapping sediment particles.[12]:45–47 Although there have been few field tests of silt fences installed at construction sites, these tests have shown generally poor results.[12]:27–31, 53–55 (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.[13]

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.[14] 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.[7]:p.6–10 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.[12]:30–31 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.[8]:p.7–46

See also

References

  1. ^ Stevens, E., Barfield, S. Britton, AND J. Hayes (2005). "JFILTER FENCE DESIGN AID FOR SEDIMENT CONTROL AT CONSTRUCTION SITES". U.S. Environmental Protection Agency, Washington, DC. Retrieved February 9, 2018.CS1 maint: Multiple names: authors list (link)
  2. ^ Sprague, C.J. (1999). "Assuring the Effectiveness of Silt Fences and Other Sediment Barriers." Proceedings of Conference 30, International Erosion Control Association, Nashville, TN. pp. 133-154.
  3. ^ Brzozowski, Carol (Nov–Dec 2006). "Silt Fence Installation". Erosion Control. Forester Media. 13 (7).
  4. ^ "Chapter 2. Erosion and Sediment Control Principles, Practices and Costs". Virginia Erosion and Sediment Control Handbook (PDF) (Report) (3rd ed.). Richmond, VA: Virginia Department of Environmental Quality (VA DEQ). 1992.
  5. ^ "Spec. 3-05. Silt Fence". Virginia Erosion and Sediment Control Handbook (PDF) (Report) (3rd ed.). VA DEQ. 1992. p. III-19.
  6. ^ a b U.S. Environmental Protection Agency (EPA), Washington, DC (2012). "Silt Fences." Stormwater Best Management Practice. Document no. EPA 833-F-11-008.
  7. ^ a b Fifield, Jerald S. (2004). Designing for Effective Sediment and Erosion Control for Construction Sites. Santa Barbara, CA: Forester Press. ISBN 978-0-9707687-3-5.
  8. ^ a b EPA (2009). Development Document for Final Effluent Guidelines and Standards for the Construction and Development Category (Report). EPA-821-R-09-010.
  9. ^ "Section H - 26.0: Super Silt Fences". 1994 Maryland Specifications for Soil Erosion and Sediment Control (PDF) (Report). Baltimore, MD: Maryland Department of the Environment. 1994.
  10. ^ Carpenter, Thomas; Sprague, Joel (Jul–Aug 2002). "Silt Fence Installation Efficacy". Grading and Excavation Contractor. Forester Media. 4 (5).
  11. ^ EPA (2007). “Developing Your Stormwater Pollution Prevention Plan: A Guide for Construction Sites.” Document No. EPA-833-R-060-04.
  12. ^ a b c Barrett, Michael E.; Kearney, John E.; McCoy, Terry G.; Malina, Joseph F., Jr.; Charbeneau, Randall J. (March 1996). "An Evaluation of the Use and Effectiveness of Temporary Sediment Controls" (PDF) (Report). University of Texas at Austin, Center for Transportation Research. Research Report 1943-2.
  13. ^ Stevens, Ellen; Barfield, Billy J.; Britton, S.L.; Hayes, J.S. (2004). Filter Fence Design Aid for Sediment Control at Construction Sites (Report). EPA. p. 22. EPA/600/R-04/185.
  14. ^ Environmental Technology Verification Report for Installation of Silt Fence Using the Tommy® Static Slicing Method (PDF). Washington, D.C.: Environmental Technology Evaluation Center (EvTEC); Civil Engineering Research Foundation. 2001. ISBN 0-7844-0565-4. CERF Report No 40565.

Further reading

External links

Desander

Desanders and desilters are solid control equipment with a set of hydrocyclones that separate sand and silt from the drilling fluids in drilling rigs. Desanders are installed on top of the mud tank following the shale shaker and the degasser, but before the desilter. Desander removes the abrasive solids from the drilling fluids which cannot be removed by shakers. Normally the solids diameter for desander to be separated would be 45~74μm, and 15~44μm for desilter.A centrifugal pump is used to pump the drilling fluids from mud tank into the set of hydrocyclones.

Geotextile

Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain. Typically made from polypropylene or polyester, geotextile fabrics come in three basic forms: woven (resembling mail bag sacking), needle punched (resembling felt), or heat bonded (resembling ironed felt).

Geotextile composites have been introduced and products such as geogrids and meshes have been developed. Geotextiles are able to withstand many things, are durable, and are able to soften a fall if someone falls down. Overall, these materials are referred to as geosynthetics and each configuration—geonets, geosynthetic clay liners, geogrids, geotextile tubes, and others—can yield benefits in geotechnical and environmental engineering design.

Industrial stormwater

Industrial stormwater is runoff from precipitation (rain or snow) that lands on industrial sites (e.g. manufacturing facilities, mines, airports). This runoff is often polluted by materials that are handled or stored on the sites, and the facilities are subject to regulations to control the discharges.

Linear referencing

Linear referencing (also called linear reference system or linear referencing system or LRS), is a method of spatial referencing, in which the locations of features are described in terms of measurements along a linear element, from a defined starting point, for example a milestone along a road. Each feature is located by either a point (e.g. a signpost) or a line (e.g. a no-passing zone). The system is designed so that if a segment of a route is changed, only those milepoints on the changed segment need to be updated.

Linear referencing is suitable for management of data related to linear features like roads, railways, oil and gas transmission pipelines, power and data transmission lines, and rivers.

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 erosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls. Sediment controls are generally designed to be temporary measures, however, some can be used for storm water management purposes.

Silt

Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water (also known as a suspended load) and soil in a body of water such as a river. It may also exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens, exhibiting a sparkly appearance. It also can be felt by the tongue as granular when placed on the front teeth (even when mixed with clay particles).

Stormwater

Stormwater, also spelled storm water, is water that originates during precipitation events and snow/ice melt. Stormwater can soak into the soil (infiltrate), be held on the surface and evaporate, or runoff and end up in nearby streams, rivers, or other water bodies (surface water).

In natural landscapes such as forests, the soil absorbs much of the stormwater and plants help hold stormwater close to where it falls. In developed environments, unmanaged stormwater can create two major issues: one related to the volume and timing of runoff water (flooding) and the other related to potential contaminants that the water is carrying (water pollution).

Stormwater is also an important resource as the world's human population demand exceeds the availability of readily available water. Techniques of stormwater harvesting with point source water management and purification can potentially make urban environments self-sustaining in terms of water.

United States regulation of point source water pollution

Point source water pollution comes from discrete conveyances and alters the chemical, biological, and physical characteristics of water. In the United States, it is largely regulated by the Clean Water Act (CWA). Among other things, the Act requires dischargers to obtain a National Pollutant Discharge Elimination System (NPDES) permit to legally discharge pollutants into a water body. However, point source pollution remains an issue in some water bodies, due to some limitations of the Act. Consequently, other regulatory approaches have emerged, such as water quality trading and voluntary community-level efforts.

Water pollution

Water pollution is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. In turn, this can lead to public health problems for people living downstream. They may use the same polluted river water for drinking or bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases.Water pollution can be grouped into surface water pollution. Marine pollution and nutrient pollution are subsets of water pollution. Sources of water pollution are either point sources and non-point sources. Point sources have one identifiable cause of the pollution, such as a storm drain, wastewater treatment plant or stream. Non-point sources are more diffuse, such as agricultural runoff. Pollution is the result of the cumulative effect over time. All plants and organisms living in or being exposed to polluted water bodies can be impacted. The effects can damage individual species and impact the natural biological communities they are part of.

The causes of water pollution include a wide range of chemicals and pathogens as well as physical parameters. Contaminants may include organic and inorganic substances. Elevated temperatures can also lead to polluted water. A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers. Elevated water temperatures decrease oxygen levels, which can kill fish and alter food chain composition, reduce species biodiversity, and foster invasion by new thermophilic species.Water pollution is measured by analysing water samples. Physical, chemical and biological tests can be done. Control of water pollution requires appropriate infrastructure and management plans. The infrastructure may include wastewater treatment plants. Sewage treatment plants and industrial wastewater treatment plants are usually required to protect water bodies from untreated wastewater. Agricultural wastewater treatment for farms, and erosion control from construction sites can also help prevent water pollution. Nature-based solutions are another approach to prevent water pollution. Effective control of urban runoff includes reducing speed and quantity of flow. In the United States, best management practices for water pollution include approaches to reduce the quantity of water and improve water quality.

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