What is Silt Curtain
A silt curtain, also commonly referred to as a turbidity curtain, turbidity barrier or a silt barrier is used to contain turbidity (sediment and silt) that is stirred up by construction activities taking place in or near bodies of water, dredging operations and rainwater runoff. Floating turbidity curtains have a series of continuous flotation elements along its top and a fabric skirt hanging below the floats.
The skirt is typically long enough to be within one foot of the bottom and can be used to contain various turbidity. Turbidity curtains & silt curtains are usually made in lengths of 25, 50 or 100 ft and those sections are joined to make one continuous containment barrier that encloses a worksite. These silt curtains also come in three different types, which are all dependent on different applications and environmental variables.
Silt Curtain Components
FREEBOARD
The portion of the silt curtain that sits above the water line.
DRAFT
The submerged portion of the silt curtain.
FLOTATION
Flotation consists of high density closed cell, polyethylene foam. These floats are crumble resistant and oil resistant, ensuring continued
flotation. We offer various sizes and configurations.
SKIRT
The material used will depend on the conditions in which the curtain will be installed. Our most commonly used option is a 270gsm non woven
geotextile fabric that stops anything larger than 90 microns.
BALLAST
The curtain is maintained in position by applying a ballast of galvanized chain sewn into a chain pocket at the base of the curtain. This
ballast extends consistently for the full length of the curtain allowing for continuous tension.
STRENGTH WEBBING
Seat belt webbing will be installed along each section of curtain. One on top of the float, one directly below the float and, for large curtains,
one between the ballast and skirt. The webbing will assist in supporting horizontal forces placed on the curtain.
CONNECTORS
The curtains shall be connected using specially moulded ASTM 962 connectors to attach the freeboard section of the curtain. These connectors provide strength and durability in the water. For offshore conditions opt for heavy duty moulded connectors. Heavy duty marine zipper is utilised to connect the lengths of skirt which allows for identical sections of curtain to be replaced if necessary. Further, a selection of bow shackles will be used to ensure the connection of the curtains.
ANCHOR POINTS
Attachment points are present on all ASTM 962 connectors via stainless steel eye nuts and furnished with a galvanized steel chain, attached with a bow shackle on one side of the skirt and floating buoys.
Applications
Silt curtains have features for various applications including lakes, ponds and other bodies of water. Our materials are of the highest quality and feature reinforcement of all highly stressed areas. We also have furling systems that assist with easy deployment and retrieval. Our specialists will help you select the appropriate curtain grade as well as the optimum installation design for your project, using the following criteria:
- Site conditions and hydrodynamic regime:
- wind
- waves
- current
- water depth
- tides or other changes in water level
- bottom geometry
- bottom composition
- Project duration
- Type of material in suspension
Construction Use
Silt curtains are the ideal solution for any type of construction project, including:
- Dredging
- Bridge construction
- Dam removal and restoration
- Marina/harbor projects (bulkheads, piers, retaining walls)
- Port and terminal projects (groins, dikes, breakwaters)
- Ferry landings
- Pipeline crossings
- Coastal and shoreline restoration and rehabilitation
- Intake construction
Uses of Silt Curtain and How They Work
A turbidity curtain sits in the water and traps various sediments from continuing past the barrier. There is also an option to use filtered fabric that lets water pass through, but not the sediment that is contained. Filtering fabrics are the same quality as general turbidity curtains, but they allow for the exchange of water between each side of the barrier, helping to maintain oxygen levels and preventing the water inside the turbidity curtain from becoming stagnant. Common uses include:
- Construction Sites – Silt and dirt sediments from construction move through the air and eventually land in the surrounding water areas. Turbidity curtains sit in the water and stop silt, dirt, or debris from continuing past the barrier, eventually causing the sediment to settle to the bottom.
- Environmental Remediation – Turbidity curtains can also be used in environmental remediation to prevent toxic chemicals and heavy metals from leaving the worksite.
- Marine Life Protection – Turbidity barriers work to control the release of organics in the water column, which at concentrated levels can be toxic to marine life.
Oil Containment
Construction sites can also create hydrocarbons due to dredging operations. In this case, turbidity curtains should be lined with oil sorbent booms parallel to the curtain. This can be anchored directly to the turbidity curtain and will absorb the hydrocarbons before they can reach the curtain.
If there is a possibility of a great amount of hydrocarbons that could be released, it would be beneficial to have a line of oil containment booms inside the turbidity curtain enclosure. Oil containment booms paired with turbidity curtains creates the optimal situation for your construction site.
How to Choose Your Curtain
Silt curtain effectiveness is considered as the degree of turbidity reduction achieved within the controlled area relative to the turbidity levels outside of the area. Factors which affect this effectiveness are:
- The quantity and type of material in suspension
- The characteristics, design and construction of the silt curtain
- The mooring and square meter area of the silt curtain deployed
- The hydrodynamic conditions experienced such as tidal movement, wind velocity and wave
- height.
In the instance of typical construction projects and pipeline disposals where suspended solid concentrations are high, a vast majority of the silt will drop to the bottom while only about 5% of the sediment remains suspended in the water column. The silt curtain is not designed to dam the turbid water but instead provides a control for the dispersion of the sediment laden water and allowing it to settle.
How deep should the curtain be?
As a rule of thumb, turbidity is most active in the top two metres of the water column. Since the purpose of a silt curtain is to disrupt the water flow and allow the suspended solids to settle, your curtain should be deep enough to:
- Provide sufficient disruption to the water flow (current),
- Remain clear from the sea bed (or river bed) at low tide, and
- Take into consideration any EPA or other environmental requirements.
Unless required by regulatory or project requirements, a silt curtain does not need to go down to the sea or river bed to be effective. Allow a minimum half metre gap between the curtain and the sea bed at low tide. If the silt curtain is too deep, slack can be generated in the curtain skirt at low tide. This can create issues during periods of high wind as the curtain slack will billow and cause considerable forces against the curtain and mooring systems. Examples of airborne silt curtains have been cited due to incorrect skirt depths in wind prone areas. Some other issues arising with silt curtains that incorporate full depth skirts are:
- In calm water, sediment could build up over the ballast chain and start to drag the curtain down. This is also known as ‘making sand’ as the curtain moves back and forth over the bottom.
- In moving water, the curtain needs to be able to move freely allowing the forces of the water to pass through and under the curtain.
- A totally contained area through total depth silt curtains may have an adverse affect on marine fauna.