Floating silt curtains
Floating silt curtains Applications for aquatic pollution control
This fact sheet is still under construction and its contents may change or be inaccurate.
Floating silt curtains, also known as turbidity barriers or floatation containment booms, are a tool used to prevent the migration of pollutants in aquatic environments. They function by creating a quiescent zone in which sediment can settle to the bottom and floating pollutants are trapped by the top of the barrier. While they have multiple uses in construction, they are not meant to be used alone and instead work in conjunction with other sediment control measures to reduce the area affected by construction sediment and other pollutants.
This image shows a floating silt curtain being used to protect a nearby water body. Note the redundant perimeter controls, as well as the floating debris held back by the curtain. Photo courtesy of MPCA (Flickr) The construction stormwater permit specifically notes that floating silt curtain placed in the water is not a best management practice (BMP) for sediment control and does not satisfy perimeter control requirements except work on a shoreline and below the waterline. If both these conditions are met, a silt curtain can be used as perimeter control if installed as close to the shore as possible. Once shoreline construction is complete, perimeter control must be installed upgradient if exposed soils still drain to the surface water. For work beneath the ordinary high-water line, consult the DNR for additional restrictions and requirements, and follow their best practices guidance for in-water construction work (https://files.dnr.state.mn.us/waters/watermgmt_section/pwpermits/gp_2004_0001_chapter3.pdf). Structure A floating silt curtain consists primarily of a geotextile curtain that spans the length from the surface of the water to the bottom of the water body. The length of the curtain varies, so it is necessary to choose a length that spans the entire depth of the water body. The curtain is anchored on the bottom by a cable or chain along with anchors and kept floating with a floatation carrier in the upper part of the curtain. This floatation carrier should consist of high-quality polyethylene foam and have a cable in it to ensure the curtain can carry loads.
Drawing courtesy of MPCA (https://www.pca.state.mn.us/sites/default/files/swm-ch6.pdf)
There are two main varieties of floating silt curtains: one designed for still waters such as lakes and ponds, and a sturdier moving type for rivers and streams.
Still water type (left) versus moving water type (right). Photos courtesy of Dwayne Stenlund (MnDOT). The still water type has an open joint that causes leaks, while the moving type has an interlocking joint. Because of this, the still water type should be installed with a second curtain to accompany it, with the joints spaced as far apart diagonally as possible to increase the flow path. Implementation and Maintenance To prevent water quality violations from sediment transport, floating silt curtains should be maintained until the construction area is stable and will not cause turbidity effects above background level. In addition, one of the most important steps is to remove the curtain when it is no longer needed. Limiting use to only times of in-water disturbance reduces degradation of the material and increases reusability. When removing the silt curtain, be sure to follow invasive species requirements, such as leaving the curtain out to dry. Be sure to remove the curtain before freezing or thawing occurs, as ice can damage the material.
Proper installation is also important. It’s necessary to keep a 5-foot gap between each curtain (if more than one is in use), as a quiescent zone can’t be established between curtains if they are touching each other. It’s also important to isolate the flowing zone of a river from the working zone to ensure a quiescent zone for settling. To limit the amount of the water body impacted by sediment, the curtain should be installed as close to the shore as possible. The end of the curtain should also be anchored on land or adjacent perimeter control. It is critical to use sediment control and erosion prevention BMPs on the land nearby if construction allows it, as this provides redundancy and reduces the amount of sediment that will end up in the water body. These devices are only effective for minimizing amount of area that is impacted and limiting migration of sediment. Therefore, they should only be used as a last resort to isolate a construction area, not as a standalone perimeter control.
Sediment-laden water being pumped and held back by a silt curtain. Photo courtesy of MPCA construction stormwater. Restrictions on use While floating silt curtains have many uses, they have significant limitations. Most importantly, they do not function as a sediment control or erosion prevention best management practice (BMP). They cannot be used in situations of high flow. Silt curtains should never be used to dam waters or to block large culverts, as they are not sturdy and will be moved by the flow. If a more robust barrier is needed, sheet piling, cofferdams, and other in-water construction methods should be used.
Sheet piling used for in-water construction. Photo courtesy of MN DNR. Other uses Floating silt curtains have uses outside of the most common ones. Due to their flexibility and ability to prevent the movement of pollutants, they can be used as floating baffles in stormwater ponds to increase flow path length and prevent short circuiting. Specifically, they can be used to improve wet pond performance during construction by dividing the pond into four cells, with decreasing turbidity in each subsequent cell. Instead of the entire pond sediment being resuspended in a storm with high flow conditions, only one cell is disturbed.
An example of use as floating baffles in a stormwater pond. Photo courtesy of MnDOT. They can also be used as a floating containment boom around barges and construction sites to keep oil or hydraulic fluid leaks contained, as well as prevent the migration of trash. If a spill occurs that will cause pollution, such as sediment or dangerous chemicals, the state duty officer should be called immediately at 651-649-5451 or 800-422-0798. Oil spills smaller than 5 gallons do not need to be reported. Alternatives Floating silt curtains may not always be the best fit for a given situation. Other best management practices can be used to perform the same function. These include sand totes for shallow water, completion of a project within 24 hours, temporary fill, stream diversions, and more. One recent technology called a “bubble curtain” also provides a similar protection from silt and garbage as a silt curtain.
Temporary causeway built for demolition and construction. Image courtesy of MN DNR. Examples of use
There is heavy erosion on this slope, indicating the need for stabilization, and the silt fence rebar (use for stability) needs maintenance. However, the positioning of the silt curtain is good.
This site shows good stabilization with hydraulically-applied mulch as well as good perimeter control and double silt curtains for redundancy.
--more images at X:\Programs\Stormwater\Construction_Program\CSW Staff Photos\CSW photo library\Working In Water (stream crossings, diversions, etc.)
This page was last edited on 29 August 2023, at 20:23.