Keeping Sediment in Check

May 1, 2009

Sediment can trip up even the easiest projects. Various government regulations to preserve natural waterways must be observed, meaning excess sediment has to be kept out of freshwater resources. Storm sewer lines must be kept clear; sediment buildup requires costly mucking out, and, of course, many storm drains eventually end up in natural waterways. To meet government regulations, as well as to save money and time, most contractors install sediment controls before doing any dirt moving.

Silt Doesn’t Go Where It Wilt
Michael Radford, president of Top-Notch Erosion and Stormwater Control Inc. in Benton, AR, says his state’s topography often determines what type of sediment control he uses onsite. “In central Arkansas, where we concentrate our work, there are lots of rolling hills, so on many jobs we have a lot of slope work. I can think of two locations where we had some problems.”

He often uses products from Silt-Saver. “I use the SS-700 Belted Silt Retention Fence [BSRF] and storm sewer inlet covers a great deal. We work on residential and commercial new construction sites and use a combination of different tools and items. For example, on a slope, we might use riprap in front of the silt fence to slow runoff down. We also use mulch and hydroseeding to fight runoff. We try to stay with Silt-Saver products as much as we can.”

Silt-Saver, located in Conyers, GA, produces various storm drain and curb inlet filters, as well as the BSRF.

“On one project in Little Rock, just the way the site was designed, there was so much slope work-and our BMPs were constantly being knocked out or over,” Radford recalls. “We ended up digging a small retention ditch to catch all the runoff, rather than put in all sorts of other measures that just get washed out. This was in an area where they hadn’t started working yet, but the way the site was sloped, sediment just kept flowing that way. We put in riprap, silt fence, and other measures, but none of those would hold it. So we made ditch with a French drain in it, and it held sediment well. We put a silt fence further down the slope to catch any other sediment.”

A Hot Springs site with steep slopes got a “green treatment.”

“We stabilized the banks by hydroseeding, but we first germinated the seeds early, by letting the solution soak for a while before it was applied, so the plants started right away.”

Radford uses a variety of methods on his work sites. “There’s no single answer out there; every site uses many techniques-silt fence, blankets, inlet covers.” His company remains on a site as long as needed. “If customers just want a silt fence, we can be in and out in a day, maybe two or three days, depending on the length of the fence,” he says. “If we do a complete package for clients, with silt fence monitoring and maintenance, we can be there as long as construction lasts-six months to a year.”

Radford’s experience with Silt-Saver has been positive. “For the past eight or nine months, we’ve been using Silt-Saver products; we’re very, very pleased. Its silt fence is the best product out there-it withstands high-velocity runoff where other fences would fall over. Clients don’t have to worry about Silt-Saver’s fence leaking. The old-type silt fence is photodegradable and it tears easily, but once this fence is in the ground, it stays there. Some we’ve had in the ground up to eight months, where the other stuff would biodegrade into nothing.”

Skimming for Clean Streams
Faircloth Skimmer surface drains are nothing new to senior project engineer Ray Smalling of Kirkland, WA’s Otak Inc. “I had been using skimmers in different projects in Kansas City five years ago. I’ve since moved to the Seattle area, and am specifying them here.”

Produced by J. W. Faircloth & Son of Hillsborough, NC, the skimmers float on the surface of a basin, releasing the cleanest water, instead of draining from the bottom as conventional outlets do. The product is available in eight sizes that are designed to handle specific flow rates and amounts of water.

Smalling describes Barbee Mill, a 22-acre, 115-lot residential subdivision adjacent to Lake Washington in Renton, WA. “We created dewatering sediment basins during construction and installed skimmers. Since there were two ponds, we put one in each pond. In Kansas City, there were sites where we made skimmers a permanent addition.

“The Skimmers dewater the permanent sediment basins near this lake community because there were agricultural fields upstream that also drained to the basins,” he explains. “The site is nearing completion now. Ultimately, Washington’s Department of Ecology requires permanent ponds, which will have a different dewatering system, with a similar philosophy to the skimmer, but contained in a concrete structure that will maintain a lined, wet pond and an aesthetic pond adjacent to the 100-square-mile lake.”

Is the Skimmer new to Washington? “I specified them on this site, in my SWPPP [stormwater pollution prevention plan], but on other projects, people were fashioning their own “˜constant head weirs,’ which is the generic term for what Faircloth makes. People would take 5-gallon buckets and attach them to PVC pipe, or lash pieces of PVC as a flotation tube to another pipe to create a similar function. I suppose that works, but the skimmer gives you measured performance. It’s a beautiful thing-the skimmer provides a constant flow rate. Not only does it take cleanest water off the top and discharge it first, but it also does so at a constant flow rate, which allows for other treatment options.

“The old standard method of perforated drain pipe discharges the dirtiest water first, so this is a vast improvement. The skimmer is articulated; it adjusts to water levels and it keeps floating debris away from the intake device. And it’s durable; I have seen people ride it like a hobby horse-what teenage boys will do on a hot summer’s day is amazing-yet it still performs.”

The Barbee Mill development required other erosion and sediment control methods, as well. “We also had to use silt fence. Washington requires you to reduce sediment loads. We had to meet 25 NTUs [nephelometric turbidity units], which is not based on volume, but based on being able to see through it. [Water with] 25 NTUs is nearly as clear as drinking water. To achieve this standard, we used a variety of methods: silt fences, ditch checks, temporary seeding, level spreaders, native vegetation retention, and grassy filter strips,” Smalling explains. “Runoff was discharged by the skimmer via a pump, which, in turn, was discharged to a level PVC spreader upstream of native vegetation. Runoff runs through that vegetation before it gets to May Creek, which feeds the lake. Fifty feet of native vegetation adjacent to the creek was left as a buffer just for this purpose, which is a standard practice in Washington.

“Faircloth Skimmers can greatly improve the performance of a retention basin,” he concludes, “But they have to be cleaned out every so often. You take out the silt and distribute it back along the site-no soil lost. Catching the silt there-it’s more accessible to do it that way, rather than having to dredge someone’s lake downstream. The skimmer catches fine sands and silts and preserves the lifespan of downstream lakes. Streams run clear out here, and that’s how we like it.”

Little Rain Can Cause Big Problems
Arid climates pose different challenges for sediment control. “We have a lot of different soil types out here. Our biggest challenge is UV radiation, and making sure the product works when it does rain,” reports Kevin Stumpff, president of Phoenix, AZ’s Windswept Organix. “We’re pleased with how well Filtrexx Siltsoxx stand up to UV radiation.”

Established in 2003, Windswept Organix, which performs services for both commercial and residential projects, is Filtrexx’s licensed representative for Arizona, Utah, Nevada, and New Mexico.

Filtrexx International LLC of Grafton, OH, produces a line of netting products that can be used for erosion and sediment control, including Filtrexx SiltSoxx, a silt fence alternative; InletSoxx; and Ditch Chexx, a straw bale alternative.

The hot desert sun beating down causes other challenges, as well. “We have a lack of vegetation to keep soil in place,” says Stumpff. “We have a lot of soil migrating when it does rain. We may get only seven inches of rain a year-but three of those inches may fall at one time.” His company also applies polymers and tackifiers to keep soil down in the wind. “For Department of Transportation and utility jobs, we try to put in vegetation, because we have to meet 14-day stabilization rules. If you’re not going to touch or work a piece of exposed land in 21 days, you have to have it stabilized in 14-for water erosion and dust as well. We have another set of dust control laws here that overlap with the NPDES [National Pollutant Discharge Elimination System] laws.”

On one project, he recalls, “Filtrexx Siltsoxx were onsite for 20 months at an Arizona [department of transportation] DOT site. I could not pull the netting apart-it was as strong as the day we installed it. It’s a durable product. It gives us all the filtration capabilities, while it also withstands desert-strength UV. I’ve seen BMPs [best management practices] that photodegrade in a matter of weeks, then have to be torn out and reinstalled. Filtrexx Siltsoxx will last the life of the project, even those that take up to two years.”

When telling new clients about BMPs, Stumpff points out the concept of TCO: total cost ownership. “Filtrexx Siltsoxx are competitively priced up front, and redo costs, if any, are much lower. Many people just look at installation costs; they forget that there can be risks and fines if the items they’ve purchased fail. With their efficiency and long life, Filtrexx Siltsoxx are a big win-win.”

Other tools are also used as needed-hydroseeding, drill seeding, silt fences, wattles, geotextiles-“You name it, we do it,” Stumpff says. “We do kind of hang our hat on Filtrexx because of its durability, and its proven filtration performance-but you can’t just build a house with a hammer.”

Because of the area’s low annual rainfall, erosion control costs “are less here than, say, in the Midwest,” he goes on. “There’s a lot less chance of rain to fall and soil to move. However, due to large growth in this area, we still do a lot of business. Sometimes I see folks skirt compliance because we have less rain. If they’re getting rain every day I’m sure they do more.” Yet Stumpff knows that the seemingly waterless desert is sometimes anything but: “Standing in the desert the other day, we saw a white waterfowl-a crane of some sort. You never know what you will see.”

Windswept Organix performs many other chores in its average year. “We do everything from mine reclamation to sweeping streets in residential areas. Sweeping streets is much more important than people might realize. It’s not so much that we need to pick up the chemicals that cars leave behind, but those chemicals bind with soil, and it would be difficult filtering all the pollution that attaches to the sediment.”

His company is going to great heights to reduce runoff: “We’re working on a green roof project in Scottsdale. Soil is moved through pipes, up to roofs, and then we plant the roofs. Some buildings are seven stories tall, so they have strong roofs to hold the weight of the soil and plants. Green roofs reduce runoff better than impervious surfaces, such as commercial roofs, because plants take the water up. In addition, green roofs reduce the amount of energy needed to power the building’s air conditioning. Cities suffer the “˜heat island effect’ because impervious surfaces, such as rooftops, parking lots, streets, and sidewalks, retain heat. Rooftop plants help cool down the air.”

Caught in a Trap
Sediment Trapps are recent extensions of the Filtrexx “toolbox,” and Wrightsville, PA’s River Valley Organics finds that they work well just about anywhere, as well as in difficult site conditions, such as uneven terrain or delicate wetlands.

“The Trapp is a berm, built from Filtrexx Siltsoxx, which varies in size from eight to 24 inches in diameter,” explains River Valley Organics’ Jonathan Hunt. “The berm is placed around a low, shallow detention pond-which can be to five feet in depth, or limited to 5,000 cubic feet in Pennsylvania-so you’re building almost a small dam. You need to lay this over virgin ground; you wouldn’t like to have loose fill underneath, because that’s nothing but sediment.

“The Filtrexx Siltsoxx are filled with composted, shredded hardwood mulch that’s been run through a two-inch sieve,” he explains. “At that size and composition, the mulch interlocks and provides structural integrity. Using these to trap sediment is mimicking the natural process-using natural materials as water filters. The compost eventually breaks down in the soil.”

Hunt describes a recent installation: “The site contained a natural swale, which was a good place to put in a sediment trap. We installed a single 24-inch Filtrexx Siltsoxx as a natural trap. A 100-foot berm was made, and earth disturbance was virtually nil. We used safety Filtrexx Siltsoxx, which have a lifetime of five years. If you get a gash in the netting, you haven’t destroyed the integrity of the berm, because they are three-dimensional-they have more depth and width than a silt fence. If need be, you can throw another short segment over it and stake it in.”

River Valley Organics often leaves Filtrexx Siltsoxx segments on a site for such a purpose-or for other needs the contractor might have. “The contractor might want to make a filter ring. He can take a 50-foot section, put some geotextile under it, and have an immediate concrete washout area. Concrete waste has a high pH, which is bad for aquatic life, so it has to be contained, but you can use that soil for construction.”

Often, the berms are left on site. “We can plant around it. It filters stormwater, removing 98% of hydrocarbons-we used diesel fuel to test this. Filtrexx Siltsoxx also removes 25% of nitrogen, and trap phosphorus, too.”

Sometimes Hunt’s clients need to learn the hard way of the benefits of using the right BMPs for the job. “We’ve had clients who’d put in silt fence, and then it failed. Rather than get a fine, they spent more money to put in Filtrexx Siltsoxx. Those are much cheaper than the fines-and, of course, then they knew just to start with the product.”

About the Author

Janis Keating

Janis Keating is a frequent contributor to Forester Media, Inc. publications.