Techniques for Successful Sediment Control
Siltation in water containment structures is problematic, resulting in the loss of space in tanks and other structures such as reservoirs, ponds, and catch basins. It also increases the cost of treating drinking water, with high loads interfering with coagulation, filtration, and disinfection.
While there are generally three ways silt can get into a water containment area, there are corresponding solutions available to mitigate the issue, notes Byant Jefferson, technical projects manager for Colorado Lining International, a geosynthetic company that offers fabrication, installation, and design and build services.
Silt can be blown in through dust and debris carried by the wind into the water containment structure. To prevent this, a cover—such as a structural roof or a floating roof—is used over the containment area, he says.
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Silt also can come by way of erosion through water draining into the containment from an open source that can carry it in with it, Jefferson adds. To prevent the source of water from entering directly into the containment, Jefferson points out that creating a settling tank would allow the silt to settle out of the water.
Silt also can be entrained, in which the silt is already present in the water that is put in containment. That calls for the use of filtration or settling tanks prior to entry into containment, Jefferson says. The most common types of silt in open containments run the gamut from blown leaves to sand and dust, says Jefferson.
“Closed containments typically will see sand if water is being pumped from a well and finer silt if raw water is from a river,” says Jefferson. “You also can see a lot of calcium buildup—scaling—in drinking water storage tanks if the mineral is present in the water supply.”
Mitigation methods are typically driven by financial considerations, notes Jefferson.
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“Filtration is expensive and settling requires time and structures to do so,” he says. “Some water districts will simply let it build up and send divers in to vacuum it out or periodically drain the reservoirs to clean them.”
BTL Liners utilizes double-scrim reinforced polyethylene (RPE) material in its products, designed to be strong and thick. The company’s products are LEED certified.
One use is the floating turbidity curtain or barrier, which consists of a top flotation boom, an impervious fabric curtain extending down under water, and a galvanized steel chain sealed in a pocket or sleeve provide weight to keep the curtain vertical in the water. The ends of the sleeve have a connecting rope to attach to additional lengths of curtain.
Type 2 turbidity curtains have 20% of the curtain or skirt replaced with a filter fabric that allows moving water penetration, says Michael Baron, president of BTL Liners.
Silt curtains can be made with different material that corresponds to the requirements of the actual water flow and is usually a woven geotextile. BTL Liners will sometimes engineer mesh windows into the product to allow a greater flow to come through, says Jared Santoro, BTL Liners’ general manager and vice president.
Water security is one of the benefits of BTL Liners’ floating covers.
“Some of the ones we make are because of construction or a company is doing some sort of repair or maintenance by a canal or existing stream and they don’t want to let all of those fines go downstream,” says Santoro.
“A lot of times they’ll string this from the shore down the river a ways and back over to the shore and the water can pass through but bigger debris items cannot. It’ll keep it wrangled up to where if it’s wood or something, it’s going to float on the water and when they are done with their construction, they can bring some sort of netting in there to scoop and clean that stuff out.”
The top of a geomembrane is made of a variety of materials, typically an 18- or 22-ounce yellow or orange vinyl, says Santoro.
“We have a foam log rolling machine so we can make different diameters of floats. There are different foam ones rolled into a round log. Sometimes they’re square. They might be six inches, eight inches, or 12 inches, and then those are welded completely into the vinyl. The first foot of it is a bright shiny vinyl,” he adds.
A woven material that allows water to pass through it is used in conjunction with a ballast to hold the material, “otherwise it would just float around in the water,” notes Baron.
“You can only make it long in one piece before it gets too big, so if it’s a very long area, you need to make multiple sections to connect with one another,” says Baron, adding that the connection is accomplished with a universal aluminum Z plate, attached in a tongue and groove fashion.
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One of the most common uses of the technology is in offshore oil spill clean-up, Baron points out, adding it can be made of more fuel-resistant materials and shaped to function as an oil containment boom. The curtain is impermeable.
Silt curtains are used in a variety of applications to control sediment in water operations, from dredging, to mining, to controlling toxic algae in an effort to prevent environmental and financial consequences, says Baron.
“Floating silt curtains can help lower Total Suspended Solids and keep work sites in compliance,” he says. “Silt curtains have many benefits in mining applications when sediment control is necessary.”
While silt curtains aren’t designed to dam turbid water, they do assist in the dispersion of sediment-rich water and allow for settlement, says Baron.
“In mining, tailing dams and settling ponds pose a potential risk for silt and sediment to migrate to environmentally sensitive areas,” he says. “Silt curtains play a large role in preventing this from happening and causing problems throughout the operation. In mining, silt curtains are used to screen mine tailing overflow and sediment control and runoff.”
In designing an appropriate silt curtain for an application, it’s important to know what kinds of materials and sediment will be suspended during mining operations,
“By identifying the types of sediment and materials that will be released during the mining process, you can identify potentially toxic materials,” he says. “Both heavy metals and nutrients can be released in the turbidity cloud during mining operations and could be released into the water column.”
Silt curtains help knock sediment to the ground floor during mining operations, Baron points out.
“The turbidity cloud leaves lingering sediment in tailing dams,” he says. “Mining tailings occur after the separation of valuable material from uneconomic material during the mining process. These curtains are used to reduce the amount of suspended sediment in tailing ponds and dams.”
BTL Liners’ products were featured on the Discovery Channel’s Gold Rush show to control the silt during the washing process. “They recirculate and pump out of the wash plant; we put in curtains so they could pump from the far end of the pond from where it came in and have that material settle down,” says Santoro.
For the Gold Rush project, the company manufactured a settling pond liner that measured approximately 85,000 square feet made from BTL-30 for a pond measuring 160 feet by 525 feet. Additionally, BTL provided 300 linear feet of turbidity curtain. Eight staff members from the company delivered the materials and assisted in the installation.
The wash plant is placed on an elevated setting above one end of the settling and retention pond. The materials are washed; the dirty water goes into a smaller settling pond. The water then flows into the pond lined by BTL.
The return pump is on the other end of the pond lined by BTL, causing the water to flow from the end that has the wash plant to the end that has the return line. The turbidity curtains help knock debris down so as to not clog the return pump and the water is then sent back to the wash plant, says Baron.
While open water containment structures can be prone to siltation, “if you have a potable water tank, you should be designing a system that has no chance of infiltration from
surface water or groundwater. That would be absolutely critical. There should never be any silt in the tank,” points out Peter Young, vice president of Flowtite Water Products, which manufactures underground fiberglass water storage tanks that can be used in many LEED applications.
“The only way it would happen if there was some kind of leak in the piping or in the access cover that somebody would have to get into the tank,” he says, adding that would be a heads-up to a problem needing to be addressed immediately.
“The reason you’re using a tank as opposed to a pond is because the piping is tight,” he says. “It could be tested and you should never get any infiltrating water or silt in the tank.”
Some people don’t realize the resin in a potable water tank is slightly different than the resin used for a stormwater tank, says Young.
“It’s important that the tank resin is NSF/ANSI 61 for drinking water components,” he says. “If a utility is pumping surface water out of a pond, they have to filter it. Let’s say they were to put a liner in the pond because they don’t want it to soak into the ground. That liner should be NSF/ANSI 61-approved material also.”
Young points out that NSF/ANSI 61 standards are becoming an increasingly desired component of rainwater harvesting tanks to where NSF/ANSI 61 is becoming a factor in roofing materials.
“The water that’s coming off of the roof, which is not potable water, needs to be treated,” he points out. “There are people who are starting to look at this and say they want the pipes and roofing material to be NSF/ANSI 61 approved.”
While the storage of water is one factor, the delivery of it is another, notes Young.
“There is still infiltration into the piping system,” he says. “The education part here is that every component of the system should use NSF/ANSI 6- approved material.”