One critical step in controlling erosion at landfills is understanding which of the many available products and practices are best for your projects. Here are some ideas to get you started.

By Greg Northcutt

Like other sites where construction activities disturb 1 acre of land or more, landfill owners must have a US Environmental Protection Agency National Pollutant Discharge Elimination System (NPDES) permit to discharge stormwater offsite. One of the steps required to obtain that permit is to develop a Storm Water Pollution Prevention Plan (SWPPP). Among other provisions, this plan must describe practices that will be used to stabilize the soil and manage stormwater to control erosion and offsite transport of sediment.

The key to cost-effective compliance with this part of the permit, say the experts, is controlling erosion. Do that and you limit the time, money, and effort spent to control sediment and the harm it causes to air and water quality, fish and wildlife habitat, and other environmental features. It’s important to understand the differences between these two concepts. Erosion is the process in which wind or water dislodges soil particles. Sediment is the material displaced by the erosion process that is suspended in stormwater runoff or in the air. The better you control erosion, the better you control sediment. However, no amount of sediment control measures will limit erosion.

At the same time, erosion control professionals point out, short of covering a disturbed site completely with an impervious material and containing every drop of stormwater runoff or windblown dust onsite, there is no 100% effective method of preventing erosion or offsite transport of sediment. The most effective erosion control measures, or best management practices (BMPs), can only minimize as much erosion as possible using the best available technology.

Planning Considerations
Eric Conrad of E.R. Conrad & Associates LLC, located in Enola, PA, has been involved with permitting and inspecting environmental protection practices at landfills for 25 years. He lists four key areas to consider in planning and implementing an erosion control program.

Construction Of A New Landfill
Make sure you have a good erosion and sediment control plan in place prior to beginning any earthmoving activities and follow it throughout the construction process, he advises. During construction, a variety of temporary erosion control practices can be used to stabilize disturbed soil.

Once construction activities are completed, permanent erosion control measures can be installed. Except in the cases of heavy, concentrated flows of storm water runoff, terracing along with grasses and other vegetation is usually the most effective permanent method of controlling erosion. The leaves of the vegetation protect soil against the impact of raindrops, while the roots help hold the soil in place. Grasses can also filter harmful chemicals out of contaminated water.

“It’s important to establish vegetation as quickly as possible,” Conrad says. “That means making sure you provide the proper nutrients so that the seed will grow and thrive once it germinates.” Soil fertilization is especially critical in landfill construction where slopes may be composed of poor-quality subsoils.

Typically, terracing and side slope swales are used to reduce slope length and the amount and velocity of stormwater runoff flowing down a slope and to direct this collect runoff to chutes conveying runoff to the bottom of the slope.

Because the velocity and duration of water flows in these channels often exceed the capacity of mature vegetation to resist these forces, these channels can be armored with various natural and synthetic materials to control erosion. Often, long, steep slopes also require hardening to limit erosion.

Expanding The Footprint Of An Existing Landfill
This is probably the most difficult phase for controlling erosion at a landfill, Conrad notes. The idea is to prevent increasing any erosion.

“When tying a new area into the existing footprint, the goal should be to produce a seamless transition between the two areas in terms of controlling erosion and sediment and managing stormwater runoff,” he says. “Watch the weather so that you don’t begin connecting the two areas together in the middle of a rainy or windy period. Ensure that there are no open areas lacking appropriate erosion and sediment control measures. That includes protecting any borrow areas so that you’re not increasing erosion and sediment transport.”

Closing the Landfill
Over time, Conrad says, waste buried in a landfill settles as it decomposes, creating depressions in the surface. If not properly controlled, stormwater runoff can collect in these pockets, creating the potential for increased infiltration and leachate and for the formation of gullies that can threaten the integrity of the final cap.

“To prevent this, it’s important to establish a positive sheet flow of stormwater runoff and to fill in any depressions and revegetate these areas to protect the cap,” he says. “The basic infiltration rates have been calculated as part of the final design for the closure cap and you don’t want to exceed them.”

Good Communications
Effective erosion control at any stage of landfill development hinges on good communications.

“It’s important that everyone involved in the erosion control plan—the design engineer, the contractor and the bulldozer operator—understand what they are doing, why they are doing it and how to phase in the various practices at the right time,” Conrad says. “That can prevent such mistakes as redirecting stormwater runoff away from a channel or ripping out a grade control structure too soon. Finally, communicate often with the regulatory inspector. By working with this person, corrective actions can be taken quickly and effectively and fines and penalties can be avoided.”

Protecting the Slopes
As federal, state, and local authorities have tightened the requirements controlling erosion at landfills and other construction sites, manufacturers have responded by developing a wide array of products to protect soil over a wide range of site conditions and time frames. Made from natural, synthetic, or a combination of materials, the choices include those designed for short-term or permanent applications on slopes as well as those that armor channels with soft or hard materials.

Vegetation is ideal for erosion control. It’s relatively inexpensive to install and maintain, aesthetically pleasing and environmentally friendly. A diverse selection of degradable materials and products is available to protect seed and soil until vegetation can become established.

Conventional Mulches
Applied over a bare, seeded site, biodegradable mulches reduce the erosive force of wind and rain on the soil and shield the seed from heat stress, while capturing and holding moisture to support seed germination and growth. Loose straw, applied after seeding, has long been a popular choice of mulch for short-term applications on flat to gently sloping terrain. Hydraulically applied mulches made of paper or wood fibers have also been available for a number of years. They offer more protection over a longer period of time and are effective on steeper slopes than loose straw. These mulches can be applied with seed, fertilizer, and tackifier in a one-step application. Tackifiers, made of dry vegetable gums, like guar, or co-polymers, can be used to help hold the mulches in place as slope angles increase.

Al Schrand, the eastern United States sales manager for Canfor, recommends including wetting agents and biostimulants when applying hydraulic mulches in landfill projects and other excavated sites. “Excavating activities strip away the soils containing bacteria needed to breakdown fertilizers into a form that plants can use,” he says. “Biostimulants provide nutrients and plant hormones that are critical to germination and seed growth. Meanwhile, the wetting agents help hold moisture on the soil surfaces and pull in moisture from humid air to further support seed growth.”

Newer Mulches
In the past few years, several new mulch materials have been introduced as alternatives to conventional wood fiber and paper mulches.

Straw Products
One of these is Strawnet, a mulch made by compressing straw into a small pellet. Produced by Pelletized Straw LLC, it has a bulk density eight times greater than straw, says Ed Lee, the company’s general manager. As a result, it doesn’t wash or blow away like loose straw. Also, it can hold up to four times its weight in moisture vital for seed establishment, for increasing water infiltration, and for reducing water runoff. High temperatures in the palletizing process kill weeds that may be present in conventional loose straw, he adds.

“The product, which can be applied with a three-point hitch spreader, swells and ruptures with rain or irrigation to form a thin, but dense ground cover,” Lee says. “Special binding agents help the straw fibers net together with themselves and the soil to create a moisture-trapping layer of straw fiber.”

The company also manufactures HydroStraw, a hydraulically applied mulch product made from organic fibers, which includes a tackifier and other additives. In tests, it proved more effective than either hydraulic wood fiber or paper mulch products in controlling erosion, Lee reports. Also the product results in a better carbon to nitrogen ratio, to enhance seed establishment.

Because it can be applied with less water than conventional wood or paper mulch, Lee notes, 50% more of the product can be added to the hydroseeding tank to treat 50% more area per tank load.

More information is available at www.strawnet.com and www.hydrostraw.com.

Cotton-Based Mulch
The erosion control industry’s first cotton and straw hydromulch, GeoSkin, was developed in 2004 through joint research by Cotton Incorporated, the US Department of Agriculture, and Mulch and Seed Innovations LLC. It combines cotton end product and straw with a blend of additives and tackifiers to form a fiber matrix (patent pending), reports Wae Ellis, vice president of sales for Mulch and Seed Innovations, which manufacturers the product.

“Cotton’s natural strength, absorbency, and porosity combine with the strength of straw to produce a biodegradable honeycomb seal that fuses with the soil to provide superior erosion control and optimum seed-soil contact while retaining moisture,” he says.

In USDA tests, GeoSkin has consistently provided better erosion control and retained more moisture than competing wood and paper products, Ellis reports. Also it provides an excellent carbon-nitrogen ratio for sound seed establishment.

More information is available at www.mulchandseed.com.

Compost
Another alternative to conventional mulches that’s attracting the attention of erosion control professionals is compost. It’s made by mixing organic wastes, such as grass, leaves, tree trimmings, food waste, biosolids, and manure. This material is then subjected to a controlled heat process that breaks down organic materials and destroys pathogens and weed seeds. Once cured, the result is a high-quality, biologically stable material that can be applied pneumatically as a blanket to control erosion of bare soils. It can also be applied in the form of a berm or roll across slopes to shorten slope length and slow runoff velocities as another type of erosion control. When placed at the bottom of a slope, compost berms or socks can be used to control the transport of sediment from the site.

In addition to protecting soil and seed from wind and rain, compost improves soil structure, increasing water infiltration and water-holding capacity, and buffers soil pH to improve establishment of vegetation and growth.

More information on the use of compost for controlling erosion and application equipment is available at www.expressblower.com.

“Compost is a very effective mulch for controlling erosion,” says John McCullah, a certified professional in erosion and sediment control (CPESC). He’s president of Salix Applied Earthcare, an erosion control software and consulting firm based in Redding, CA.

“It’s a win-win BMP that’s especially appropriate for controlling erosion at landfills,” he says. “By recycling waste it reduces the demand for landfill capacity and it adds organic life and soil-building benefits to bare soil.”

McCullah speaks from personal experience. In 2004, he studied the effectiveness of greenwaste compost in stabilizing and revegetating fragile soils to repair a landslide on the steep slopes of a burn dump in northern California that had exposed fly ash, trash, and toxic materials. Using a blower truck, his two-person crew applied 715 cubic yards of the compost, along with native grass, clover seed, an organic biosolids fertilizer, and tackifier in a 2-inch blanket covering 5.4 acres of 1.5:1 (H:V) slopes. They also installed compost socks, made of high-density polyethylene knitted mesh netting, and berms, formed by using a special hose attachment, at the toe of the 31-ft. long slopes to control sediment and promote vegetation establishment along the base.

Immediately after installation, the site received about 4.75 inches of rain in a 24-hour period. “There was no soil movement on the steep slopes treated with the compost,” McCullah says. “The slopes have remained stable over various large storm events since then. This project also demonstrated the ability of compost to improve soil health for establishing vegetation on steep erodible soils that lacked topsoil.”

Fiber Matrix Products
In the early 1990s, wood fiber hydraulic manufacturers began developing a new class of spray-on products called a bonded fiber matrix (BFM). Made of long-strand wood fibers and special bonding agents, they typically require from about 24 to 48 hours to cure. Once dry they form a porous, water insoluble mat that bonds firmly with the soil to control erosion under more demanding conditions than hydraulic mulches. They combine the low cost and ease of application of hydroseeding, the advantages of a mulch and are promoted as a more effective alternative to some types of erosion control blankets. Depending on climate, some BFM products may remain effective for as long as about 18 months before they biodegrade.

More recent variations on the bonded matrix theme include two new classes of hydraulically applied products—a stabilized fiber matrix and a flexible growth medium. Stabilized fiber matrices consist of long-strand wood fibers and bonding agents/flocculants added at a lower ratio than BFMs. Stabilized fiber matrices outperform hydraulic mulches but do not match the performance of a bonded fiber matrix or an erosion control blanket. They provide three to six months of erosion protection and generally are used to stabilize slopes and building pads. They can be applied as a stand-alone product or combined with seed to establish vegetation.

A flexible growth medium (FGM), on the other hand, represents the ultimate in erosion and sediment control for a hydraulically applied product, outperforming a BFM or an erosion control blanket. Made of long-strand wood fibers, man-made fibers and bonding agents that form a porous, water-insoluble interlocking matrix, it can control erosion immediately on application even as it dries and cures. FGMs are used on severe slopes and do not require the smooth grading that rolled erosion control blankets require to ensure soil contact.

In deciding which of the various hydraulically applied mulch and matrix products to select for a given project, Schrand suggests considering four key factors: the degree and length of slope, the susceptibility of the soil to erosion, the expected weather conditions, and how quickly the seed is likely to germinate.
In selecting among Canfor’s eight different hydraulically applied products, Schrand offers these recommendations.

For slopes ranging from flat to 4:1 (H:V) with good germination conditions, use EcoFibre wood fiber mulch.

Where slopes from vary flat to 3:1 and heavy rains or winds are likely, use EcoFibre+Tac, which includes a tackifier.

EcoMatrix stabilized fiber matrix, which lasts up to six months, is designed for normal to moderate slopes.

On 1:1 or steeper slopes, or any slope is likely to experience extreme winds, rainfall, or soil erosion—or if it’s late in the season and vegetation is not expected to becomes established until the following growing season—use EcoAegis bonded fiber matrix, which can remain effective for up to 18 months.

EcoFlex Plus flexible growth medium is designed to control erosion under severe slope conditions for up to 18 months,
More information is available at www.canforpfd.com.

A flexible growth medium can also be combined with other types of erosion control materials to fit a wide range of conditions. For example, Flexterra FGM is made by Profile Products LLC. “It can significantly improve the performance of complementary products such as straw blankets and turf reinforcement mats,” reports Marc S. Theisen, CPESC, the company’s director of business development. “The FGM is applied at a reduced rate before the blankets or mats are installed,” he says.

To control soil erosion, Flexterra has also been used on landfills in conjunction with fiber filtration tubes made by Profile Products to control sediment in storm water runoff. In this case, Terra-Tubes, an engineered composite of wood-fibers, man-made fibers and polymers encased in heavy-duty, knitted cylindrical tubes, are installed across slopes at parallel intervals of 25 ft. to 50 ft. As slope interruption devices, these tubes slow run off to trap sediment in water flowing down the slope. After installation of these fiber filtration tubes, the FGM is sprayed on the slopes to protect seed and soil.

Choices for Slopes and Channels
The selection of erosion control products for landfills also includes those that can be used to protect both slopes and some types of channels.

Erosion Control Blankets
Erosion control blankets are designed to provide immediate protection of the soil while enhancing establishment of vegetation on sites that are beyond the performance limits of mulches. Commonly made of materials such as straw, jute, wood excelsior, or coconut fiber (coir) stitched to a single or double-layer netting, erosion control blankets are available for a broad range of both short-term and long-term applications—from gradual to steep slopes and low to moderate flow channels. These products are held in place by staples.

As an example of the broad selection of these products, North American Green offers six types of short-term and four types of long-term erosion control blankets. They include a blanket made of 100% straw stitched to a single photodegradable polypropylene netting designed for up to 12 months of use on roadside slopes, reclaimed mined lands and other low maintenance areas. At the other end of the performance and durability spectrum is one made of a 100% coconut fiber matrix stitched between two natural fiber nets for controlling erosion and establishing vegetation on slopes steeper than 1:1 and in high-flow channels.

More information is available at www.nagreen.com.

Turf Reinforcement Mats
Nondegradable turf reinforcement mats (TRMs) are used in vegetated channels where flow conditions exceed the limits of mature vegetation. This soft armor offers a green alternative to riprap and concrete in certain high-flow applications. Turf reinforcement mats feature a matrix made of 100% synthetic components, such as polypropylene fibers, or a composite of synthetic fibers and durable natural materials, such as coconut fiber (coir), providing immediate erosion control protection and a three-dimensional netting that permanently anchors and reinforces plant roots and stems.

Rolanka International Inc. offers three permanent TRMs:

• A synthetic product, constructed of a layer of polyolefin fibers sewn between to polypropylene grids and reinforced with a polypropylene net
• A composite TRM made of coir fiber with the same reinforcement that is designed for flow velocities of 20 feet per second and a shear stress of 8 lbs per square foot
• A woven coir mat with a synthetic net structure within the mat

The last item features an open-weave structure, which facilitates grass growth, as well as a heavier weight to hold the mat to the soil better and moveable, wildlife-friendly intersecting reinforced coir twines.

More information is available at www.rolanka.com.

Protection for High-Flow Channels
Several different types of systems offer alternatives to the installation expenses and maintenance and durability concerns of using riprap or rigid concrete structures for controlling erosion or protecting chutes and run-on diversion and run-off interceptor channels around the perimeter of landfills.

Cellular Confinement Systems
Cellular confinement systems are expandable, honeycomblike structures that can dramatically improve the performance of various infill materials, like sand, soil, rock, or concrete. Made of geosynthetic materials, such as polyethylene, in heights of about 3 to 8 in., these versatile engineered systems can be used in single or multiple-layers in load support, earth retention, slope protection, and channel protection applications.

In addition to increasing the bearing capacity of weak subgrades on haul roads, they can provide a vegetated or non-vegetated solution to slope erosion problems. They can be filled with topsoil and seeded or filled with onsite materials and left unvegetated to protect slopes from erosion or to build an earth retention structure, such as a retaining wall or a mechanically stabilized earth wall.

“Cellular confinement systems are very unique in that they can solve veneer stability problems on steep slopes where you can’t use other materials,” says John Henry of ACF Environmental, a Richmond, VA-based erosion control products distributor. “It’s very versatile. Often the engineering design doesn’t exactly fit the specific project. If there’s a design glitch with a cellular confinement system project, we can usually make it work.

As Henry points out, cellular confinement systems can also be filled with concrete to protect chutes, including steep ones, from erosion. “The lightweight cellular panels are much easier to install than a thick layer of rock or other hard armor systems,” he says. “And, you don’t have the time and expense of building forms for pouring the concrete, because the cells are the forms.”

Henry has been involved with a number of land projects where Presto Products Co.’s Geoweb cellular confinement system has been installed to control erosion. One was installed eight years ago at the Empire Landfill, near Scranton, PA. There, a 4-inch high concrete-filled Geoweb system was used to line interceptor channels that follow the toe of the landfill, with gradients ranging from 2% to 16%. This system was selected as the more time- and cost-efficient alternative to rock-filled baskets or gabions for protecting the trapezoidal shape of the channels from the erosive forces of a 100-year, 24-hour storm.

The cellular confinement system was installed across the 8-foot-wide bottom and 4-feet up each 2:1 side slope. This lining system was designed to control erosion on a maximum bed slope of 16.2% with a flow volume of 210 square feet per second and a flow velocity of 23.7 feet per second, Henry reports.

More information on cellular confinement systems is available at www.prestogeo.com.

Gabions
Gabions—rock-filled cages of steel wire—are another versatile erosion control tool. In addition to building gravity retaining structures and stabilizing slopes, they can be used to control erosion of chutes and perimeter ditches by improving the shear resistance of channel surfaces exposed to erosion or by reducing the kinetic energy of the stormwater flow. What’s more, gabions lend themselves to soil bioengineering techniques that provide a more environmentally friendly solution to erosion problems than traditional hard-armor approaches.

Bill Schelling, an area manager with gabion manufacturer Maccaferri Inc., notes that riprap, which can be used in medium to high flow conditions, has an approximate shear resistance of 6.2 lbs per square foot when unvegetated, and 7.3 lbs per square foot when vegetated. That compares to a reno mattress, one type of gabion, which can be used in high to extreme flow conditions where its shear resistance is about 7.1 lbs per square foot when unvegetated and 9.4 lbs per square foot when vegetated.

“When designing erosion control solutions, it is vital to consider not only the shear stress applied but how long the material will provide the kind of resistance,” Schelling says. For example, some types of TRMs may withstand a shear stress of 10 lbs per square foot  but only for no longer than about 15 minutes. A rock-filled gabion will maintain its resistance for a much longer time.”

Reno mattresses are usually 6- to 9-inches high, 6-feet wide and 9- or 12-feet long. Generally, they’re not appropriate for lining landfill channels. Those applications call for the larger, heavier-duty gabion baskets, which are typically 12- to 18-inches high, 3-feet wide and 12-feet long. They can handle water flows in the range of 22 to 24 feet per second, Schelling notes.

“One big advantage of gabions is their flexibility,” Schelling says. “They can adjust to movements of the landfill surface caused by settling of the content without failing.”

A Maccaferri gabion project near Alexandria, VA, illustrates the versatility of gabions in landfill applications. This project involved a construction waste landfill that was converted to a nine-hole executive golf course following its closing in the late 1990s. In addition to lining drainage ways on the golf course, gabion baskets were used to control erosion of slopes on elevated tees and to support the roadway for golf cart paths.

“Gabions were a natural for this project because of their ability to withstand differential settlement of the landfill,” he says.

Articulated Concrete Blocks
Articulating concrete block systems are dry-cast, interlocking concrete blocks that are either cabled together into mattresses or simply hand-placed in the field, depending on site installation conditions. These blocks are placed on top of a geotextile, which controls erosion. A beveled relief on the side of the individual blocks allows the mat to flex in all directions.

Richard Kane, national sales manager of erosion control with Contech Construction Products Inc., of West Chester, OH, sells the Armorflex articulated concrete block system made by Armortec, a Contech company.

“Articulated concrete blocks are relatively new to the landfill market,” he says. “Because of their ability to articulate, the mats are able to conform perfectly to the trapezoidal geometry of the down-chute channel, holding the geotextile in place and keeping it in intimate contact with the soil. You can also place an apron of Armorflex at the top of the chute to funnel water into the channel. At the bottom you can install a splash pad made of 24-inch, interlocking concrete A-Jacks units to dissipate the energy of the water as it exits the down-chute. The A-Jacks are vastly superior to riprap in terms of energy dissipation and long term stability.”

Kane reports that a 4.75-in. thick Armorflex mat, usually used in landfill chutes, has a shear resistance of about 12 lbs per square foot and can withstand flows of up to 15 feet per second.

The mats can be custom fabricated to any size and dimension up to the size of an 8-foot-by-40-foot flat bed truck. The mats are delivered preassembled to the site, ready for picking and placing into the channel. The width of the mats can be varied depending on dimensions of the chute. A 20-foot-by-8-foot mat, for example, will cover the flat bottom of a channel 8 feet wide and extend 4 feet up each side slope with a 2-foot toe-in.

These systems eliminate the maintenance issues of riprap and install much faster than riprap or gabions as well, Kane says. “With one pick-and-place movement, an excavator and crew can install a 20-foot-long mat in about 20 minutes,” he says.

Greg Northcutt writes frequently on construction and business issues.

MSW - July/August 2006