Construction activities can generate 400 times more erosion than takes place on undisturbed land, according to the Planning and Zoning Division in Jefferson County, CO.
Sediment, the most abundant form of water pollution, generally contains a mixture of clay, silt, and sand, with particles ranging in size from 0.004 millimeter (0.00016 inch) to 2 millimeters (0.08 inch) in diameter. More ominously, contaminants such as cadmium, mercury, lead, PCBs, PAHs, and many pesticides may accompany or sorb to soil particles.
Sediment reaches waterways either directly or indirectly-for example, through storm drains-and settles once it reaches still water. People notice that the water is murky. Such recreational activities as swimming and boating are impaired, and the shallow lakes and bays fill in faster.
Sediment can clog or damage the gills of fish, weakening their resistance to disease. It decreases the amount of light underwater and can stunt the growth of aquatic plants that fish depend on for food and shelter. Once it settles to the bottom, sediment can suffocate newly hatched larvae and bottom-dwelling organisms, prevent proper egg development, and fill in cracks and crannies that could have been used for habitat.
The Clean Water Act of 1972 originated to protect fish, shellfish, and wildlife from pollutants including excess sedimentation, as well as to provide for recreation in and on the water. The act’s National Pollutant Discharge Elimination System (NPDES) regulates stormwater discharge from certain construction activities. Before discharging any stormwater, the operators must obtain an NPDES permit. The condition of the erosion and sediment control BMPs must be documented, usually every 14 days or within 48 hours of a stormwater runoff event.
Operators often use a turbidimeter, or nephelometer, to estimate the amount of sediment, which is measured in nephelometric turbidity units (NTUs) in the runoff. This instrument relies on the scattering of a light beam when focused on sediment particles.
The meter focuses a beam of light on the particles, and a detector beside the beam estimates the amount of scattered light. More light reaches the detector if there are many small particles than if there are few; the more sediment in the water, the higher the NTUs.
There are many temporary methods of controlling sediment at construction sites, landfills, mining operations, and similar sites. One preventive measure is to disturb as little of the land surface as possible at a time to preserve existing vegetation.
Most structural BMPs are passive, and often erosion and sediment control methods are combined. Erosion control BMPs include blankets and mats of both natural fibers and engineered fabrics, and use of hydraulic mulch or hydroseeding.
There is a wide variety of structural temporary BMPs for sediment control. The choice may depend on the season and the expected weather conditions or the soil type or topography of a site. These BMPs include sediment ponds, silt fence, and sediment control logs or wattles.
When there’s a need to clarify the water further, active treatment systems come into play. These systems use chemistry to help the finest clay particles settle out of the water.
Particles with like charges repel each other. Clay is a colloid, a negatively charged coagulant particle. In suspension, particles of clay will not stick together and settle out.
Polymers such as anionic polyacrylamide and flocculants such as chitosan are positively charged coagulant particles, which attract the negatively charged clay particles. These positively charged and negatively charged particles stick together in the process of flocculation and form a larger, neutrally charged aggregate called a floc. Because the floc is neutrally charged, it can combine with other floc particles.
As long as the polymer or the flocculent is thoroughly mixed with the stormwater, the flocs form larger and larger masses. Eventually, they’re too heavy to be carried in the runoff that flows to the waterway, and they settle out of the water stream.
Below are examples of two very different but very effective wattles and an example of an active treatment system.
Maricopa County, AZ
Stopping sediment in a desert can be more of a challenge than one might think. Maricopa County in the Sonoran Desert in central Arizona, for example, is known for its extremely hot summers and mild winters. It averages only 7.6 inches of rain per year.
But flash floods can occasionally shut down freeways and roads, not to mention the Phoenix Sky Harbor International Airport, and can prompt states of emergency, according to the Flood Control District of Maricopa County. In 2000, 1 inch of rain fell in 15 minutes in one part of the county. In 2010, from 3 to 5 inches fell in three days.
Flash floods can cause such havoc that the state has enacted a “Stupid Motorist Law,” which fines motorists who have to be rescued when they get stranded on a road that’s been barricaded due to flood hazards.
The developed areas of Maricopa County are generally flat, and the soil type varies widely, says Carl Bloomfield, P.E., owner of 6K SWPPP LCC, in Mesa, AZ. There’s more sand and silt closer to the hills and clays on the flat land. Still, a “soil erosion by water” map developed by the Natural Resources Conservation Service (NRCS) shows that at least 50% of the county has either a severe or a moderate erosion hazard.
“The water drains to MS4 [municipal separate storm sewer system] storm drains and desert washes,” Bloomfield explains. “And any time a desert wash runs, it can be 40 to 50% sediment. It’s a little too thick to drink and a little too thin to plow, as John Wesley Powell said about the Colorado River.”
6K SWPPP is a small engineering company that focuses on land development, from consulting to installation to inspections during construction. The company uses Gator Guard, made in Boise, ID, on many of its sites, Bloomfield says. “It’s a good blend between silt fence and straw wattle.”
6K often works in subdivisions where the lots have been laid out but no improvements, such as water lines, have been installed. Gator Guard is very lightweight, so it’s easy to pull up along the front of the lots and manually put in place without creating dust, he says.
6K uses it in other situations as well, he says. For example, at the CantaMia master planned retirement community at the foot of the Sierra Estrella Mountains just outside Phoenix, utilities has been installed and the lots were ready for homes to be built. “We placed Gator Guard along the streets to prevent sediment on the lots from going onto the streets. And in a rain event, there’s potential for a lot of runoff. Gator Guard reduces maintenance costs and maintains the appearance at the same time,” says Bloomfield.
Gator Guard has a durable, all-round filter fabric, Bloomfield says, and is filled with recycled scrap foam. 6K uses 6- to 9-inch wattles. A 25-foot length weighs only 8 pounds.
Gator Guard usually has to be installed on a firm base such as undisturbed soil. In disturbed areas, the loose soil usually has to be removed or compacted. 6K lays down the wattle with the filter apron facing inward, pins the apron to the soil approximately every 24 inches, and covers it with soil. A 6-inch wattle has 5 inches of freeboard.
On the right, the socks and the
netting before the vegetation has
grown through. Elsewhere, vegetation
grows through the netting.
“One project in downtown Phoenix was unique,” he says. “LA Fitness in the area is one of our main clients. On one of the projects, the site of a demolished Kmart, LA Fitness kept the asphalt parking lot. There was no available dirt, and we didn’t want to haul in gravel, so we put down Gator Guard, nailed roofing nails through the apron, and sealed it off with caulking.”
During a rain, runoff pushes against the wattle, which helps hold it down and keep it upright. As the water slows, much of the sediment drops out. As the water filters through the apron and the wattle, it loses more sediment. When Bloomfield expects higher flows than Gator Guard can handle, he puts up silt fence.
On active construction sites, Gator Guard can get run over a few times, but the company doesn’t recommend it. 6K makes small repairs by caulking the damaged edges. If a wattle is badly damaged, workers can splice two ends together. And because Gator Guard lasts two to five years, when a project is over, crews can pick it up and recycle it, which helps earn LEED points.
“The initial price of Gator Guard is a little on the high side,” Bloomfield says. “But in the long term, it’s less expensive. Installation costs are less because of its light weight, repair costs are less, and I can reuse it. For my business, Gator Guard helps me be successful.”
Charlotte-Douglas International Airport Expansion
The Charlotte-Douglas International Airport in North Carolina is expanding, and with it are mountains of dirt from the excavation.
Charlotte is a national financial center and a highway, rail, and air transportation hub. Some 39 million passengers passed through the airport last year. The expansion, which will cost $1 billion and take seven years to complete, includes a new control tower, an expanded main terminal, a freestanding international terminal, 12 gates, a baggage system with 2 miles of automated conveyor belts, an eight-lane entrance road, a fifth runway, and an intermodal rail yard for transferring freight between trucks, trains, and airplanes.
“They’re moving millions of cubic yards of dirt out of there,” says Jeff Johnson of Power Mulch Systems in Smithfield, NC. “In some places, they’re digging out dirt 15 feet deep or more.”
Power Mulch follows up with sediment
control. Crews position the blower
truck, which carries up to 40 cubic
yards of mulch or compost, and stretch
the truck’s 100-foot-long hose to the
sock. The truck blows compost through
the hose and into the socks.
Most of the slopes of red clay on the once-flat land rise approximately 50 feet, but some are higher than 100 feet, says Johnson, who supervised the project to control sediment on the site. The majority of the slopes are 3:1; some are steeper. When two slopes meet, rock fills the ditch between them. Stormwater runoff flows to a wetland, a sewer system, or a retention pond.
To prevent erosion, the contractor is grading the slopes and stabilizing them with hydroseeded summer grass seed, predominantly millet. He’s covering the seed with coconut netting. Power Mulch, which uses pneumatic blower trucks for sediment and erosion control, follows behind.
The company is laying approximately 165,000 linear feet of FilterSoxx from Filtrexx International LLC in Grafton, OH, across the slopes to filter sediment in the runoff. It’s one of biggest installations of its kind in North Carolina, says Filtrexx owner Rod Tyler.
FilterSoxx were chosen for the project because of the site’s watershed and the amount of rain it would be getting, notes Johnson. A compost filter sock, the FilterSoxx consists of a mesh tube filled with compost.
During a rain, runoff pools temporarily behind the sock and drops the sediment. As the runoff filters through, additional sediment is trapped inside the sock. This site was newly logged, so there is no need to filter pollutants.
“This is the first phase,” Johnson says. “We started in May 2012, and we’ve done about 3 miles so far. The whole project is about 25 to 30 miles. We’re thinking it’ll take about a year.”
There’s the frequent occurrence of fine rain, which makes staying on schedule a challenge. Charlotte is in the path of subtropical moisture that flows from the Gulf of Mexico up the eastern seaboard, bringing showers and thunderstorms. On average, the city receives 43.5 inches of precipitation annually.
The high, steep slopes present another challenge. Crews are placing the rows more closely together than usual, approximately every 10 to 15 feet. The higher, steeper slopes have eight or nine rows.
“We’ve never done this many rows before,” Johnson says. “It’s been a learning experience because of the amount of sock we’re using. We’re getting more used to it, but we’re still learning as we go along.”
Power Mulch is prestaking the 15-foot-long socks, because once they’re filled, they’re very heavy and they have a tendency to slip down the steeper slopes, he says. Crews pound in the downhill stakes approximately every 10 feet, place the sock against them, and then pound in another set of stakes on the uphill side to keep the socks in position. They overlap the socks at the ends.
Three to four men work on this job instead of the usual two or three. They position the blower truck, which carries up to 40 cubic yards of mulch or compost, either at the bottom of a slope or, if there’s a road on top, at the top. They stretch the truck’s 100-foot-long, 4- to 5-inch-diameter hose to the sock and put the sock over the hose. The truck blows compost through the tube and into the socks.
“The socks are very effective,” Johnson says, “and they can be used for a number of situations.”
Water leaving the sediment bag is
cleaned further as it flows downhill
through a jute matting and from there
through a filter cloth made of a nonwoven
geotextile.
The weight of the compost gives very good ground contact, and although the sock doesn’t decompose, the compost does. The FilterSoxx can be left in place for several years, or, once the ground is stabilized, usually in approximately three to five months, the filter fabric can be slit and removed, leaving the compost in place.
Crews can put a fine grade of compost and seeds inside the sock or flatten the sock and spread seeds on top. They can even put irrigation hoses inside. They’ve had a lot of luck growing vegetables and strawberries, Johnson says.
The heavier grades of sock are very resilient, he notes. If a portion of one gets damaged, replacement sections can be added.
“Filtersoxx has done well for us,” he says. “It seems that every year it gets better.”
Preston Gateway Corporate Park
Imagine being able to clarify and empty a 250,000-gallon, sediment-laden pond well below 125 NTUs in approximately one day, or a 1-million-gallon pond in three to four days.
That’s the rate the Glen Arm Building Co. is reaching with the HaloKlear SockMaster Manifold Kit at its Preston Gateway Corporate Park project, says Harold Krebs, supervisor of the project.
The company is clearing and grading the land as well as building the infrastructure for the project, approximately a million square feet of warehouse space. The buildings will range in size from 42,000 square feet to 181,000 square feet, says Ken Kruger, the project manager.
The entire site, approximately 86 acres, is adjacent to the Baltimore Washington International Airport and surrounded by environmentally sensitive wetlands. Sixty-four acres are
A murky sediment pond
appears in the background.
The sediment bag is 10
feet long and 10 feet wide,
expanding up to 2 to 3 feet
when full.
buildable, and 22 acres are wetlands and forest conservation areas. Glen Arm has cleared the first sequence, approximately 40 acres.
Twelve sediment ponds onsite collect stormwater runoff. Their tendency to draw migratory birds creates an attractive nuisance, so the ponds have to be pumped out within 24 hours of a rainfall to be in accordance with state regulations. Runoff flows to the wetlands.
The soils are a sand-clay mix, Kruger says. “The most problematic are the clay soils. They get broken down by the rain and flow into the catch basins. The water stands there, a chocolate milk color.”
According to information from supplier Hanes Geo Components, headquartered in Winston-Salem, NC, the ponds were at levels as high as 1,712 NTUs before the system was installed.
The HaloKlear kit, from HaloSource Inc. of Bothell, WA, was on the approved plans for the project. It consists of a dual polymer system (DPS) inside a manifold made of 6-inch white PVC pipe, which is reusable. The DPS consists of a set of at least two socks containing materials to clarify the runoff, depending on the turbidity of the water and the desired level of clarity.
Glen Arm first tests the water with a turbidity meter.
“One of the challenges is that it’s a new product and somewhat new technology,” Kruger says. “At the federal level, the maximum is 250 NTUs, but at the local level, people like to say it’s 125. My goal is to get below 125. That’s fairly easy to do.”
Hanes Geo helped put the system together, which was very straightforward, Krebs says. The operation is straightforward as well. A pump sends approximately 100 gallons of water per minute from the pond into the pipe and through the socks. The first sock contains HaloKlear, a flocculent. The second contains chitosan, or ground crab shells. The negatively charged clay particles and the positively charged materials begin to coagulate.
The water flows from the pipe through a hose, which has to be at least 75 feet long for the flocculant to work completely. From there, it goes into a 10-foot-long by 10-foot-wide sediment bag, which sits on a wooden pallet to keep it above the ground. Most of the sediment is captured there. When the bag is full, it expands up to 2 to 3 feet. At this point, Glen Arm takes another reading.
Because some of the smallest clay particles get through the sediment bag, the water is cleaned further. It flows downhill through a jute matting and from there through a 12-foot by 12-foot, 8-ounce filter cloth made of a nonwoven geotextile on a bale of hay.
Glen Arm takes another reading, and only then does the runoff discharge into the forest conservation area.
“The system has worked very well,” Krebs says. “The biggest challenge has been that it works so well that the filter bags fill up really quickly. Depending on how dirty the water is, sediment can fill one bag every day.”
Krebs has been experimenting with ways to lower the NTUs even further.
“I was starting at 300 to 800 NTUs,” he says. “Sometimes I got it down to 6 to 50 NTUs.” Sometimes he uses one sock in first pipe and two socks in the second.
“With someone who takes pride in the job, HaloKlear can be very successful,” says Keith Martin, regional manager with Hanes Geo. “Glen Arm did one of the better jobs we’ve ever seen implementing it.”