Geosynthetics in Stormwater Management
These relatively new materials offer for filtration and erosion control.
Buried beneath the engineering marvel that is the city of Amsterdam, Netherlands, is a predecessor of today's water management workhorse: geosynthetic materials.
"Geosynthetics are new, but the concept is not," explains Thomas Levin, a Tampa, FL, landscape architect. "A few years ago, there was a plan to construct a parking facility in Amsterdam. When they started digging, they found that hundreds of years ago their ancestors had stabilized the soil by using animal skins. There were layers: Skins then soil, more skins then more soil."
|Pyramat following installation|
The Dutch were making use of the best materials available some five centuries ago, Levin notes. In the late 20th century, an explosion of new materials and new applications created stormwater management techniques beyond the wildest imagination of the mythical Hans Brinker, whose finger in the dike averted a tidal wave of seawater and saved a grateful community.
Levin and his peers around the world employ techniques and materials that would amaze the Dutch of 1500, yet in many ways the concept is the same.
In some instances, geosynthetic materials mimic the animal skins of Holland by allowing engineers and contractors to hold onto land that water fights to consume. More commonly, geosynthetic materials reduce or eliminate soil erosion and chemical leaching that threaten to degrade surface waters or underground reservoirs.
|Preparing anchor trench|
|Pyramat partially installed|
Geosynthetics come in a variety of types and functions, and they can be incorporated into projects in varying degrees.
"Their rise in growth during the past 15 years has been nothing short of awesome," notes Grace Hsuan, a Drexel University professor at the Geosynthetic Research Institute in Philadelphia, PA, who became involved in the field through her peer Robert Koerner, one of the pioneers in geosynthetics.
Hsuan attributes three factors to the explosive growth of geosynthetic products and applications. "During those 15 years, a group of engineers, dedicated academia, and manufacturers did a great deal of educational work by holding technical courses and international conferences. Second, geosynthetics have become recognized as engineering materials. We know their properties; they are no longer unknown to us. Once a material becomes an engineering material, once it has specifications for use, for instance, engineers become comfortable using it. And, finally, the landfill industry was a big factor in the development and use of these materials. There were requirements to use geomembranes as a liner, and once you have a regulation that a material must be used, that's it. You have to do it."
Hsuan does not expect the materials' development to increase so dramatically as in the last 15 years, though she believes its usage will continue to grow. Nor does she see strong federal pressure driving the development of new products or applications. "The role of the US Environmental Protection Agency has diminished tremendously," Hsuan says. "EPA always used to say, 'We need to remediate.' Now 'containment' is the word."
She explains that the geosynthetics family comprises seven subgroups (see sidebar). The variety of geosynthetics provides a dazzling array of choices - literally hundreds of choices, according to Hsuan - for engineers, landscape architects, contractors, and surface-water professionals.
Projects in Florida, Idaho, Virginia, and Pennsylvania demonstrate the versatility of geosynthetics. In one project hailed by the Natural Resources Defense Council as a "low-tech approach that minimized the degree of disturbance to the site while meeting all the original economic objectives of development," Levin's minimalist use of geosynthetic materials enhanced its success.
In the creation of the Magdelene Reserve, a subdivision northwest of Tampa, FL, the first goal was to protect as many of the site's numerous and varied trees as possible. Stormwater management was also a consideration.
Levin, owner of Ekistics Design Studio, and his colleagues lessened the subdivision's impact on the environment by altering road designs from the norm, positioning each of the 36 houses on its own lot and elevating them above the floodplain with stem-wall footings.
|Geotextile tube cut open, exposing dried material|
|Conventional equipment can be used to excavate and transport dried material |
The preservation of 800 other trees and native ground cover - and the incorporation of other landscaping features - reduced stormwater and chemical runoff, Levin says. As a result, the two stormwater retention ponds are smaller and less expensive than they would otherwise have been, and they have the capacity to contain the runoff from a 100-year storm.
In the stormwater pond, built next to an existing lake, Levin's plan called for a berm strengthened by a cellular confinement product. The open-cell material anchored soil and grass. "In a flooding event, the water will flow over the soil and grass instead of the concrete. The idea, of course, is that natural filtration of stormwater through vegetation is better for the environment, and aesthetically it blends into the natural surroundings," he explains. "This site is relatively flat and the soils are relatively stable. Usually there's more need for geotextiles when there are steep embankments or mucky soil. I'd say the main reason we used geosynthetics on this project was to control soil erosion."
In Sun Valley, ID, geomembranes were employed on a large scale in the transformation of a 20-ac. sheep ranch into an exclusive development with million-dollar homes, four lined lakes, and streambeds capable of sustaining trout.
"The developer had a finite amount of water available, and the area is very gravelly, so the project's plan for several lakes posed a problem for us," explains Doug McCoy of Doug McCoy Construction in Twin Falls, ID, the project's contractor. "We didn't want to lose the water into the ground. We had to do everything we could to make the best use of the water there was."
|Pipe inlets and outlets before (top)and after six months (bottom)|
The solution was a 30-mil PVC liner produced in Denver, CO, by Lange Containment Systems. The company created a panel layout that reduced field seaming. Large panels were fabricated at the factory using a dielectric welding process and delivered for installation following substrate preparation. "We installed over 100,000 square feet of the liner in three weeks," McCoy says. "Dip Creek Ranch has been everything that was expected and more."
The geotextile tube is a versatile tool. A reliable method of protecting and restoring shorelines, riverbanks and streambanks, and beach dunes, geotextile tubes also were a cost-effective, environmentally friendly solution in the management of dredged materials at a Rappahannock River, VA, marina. The use of the high-strength polypropylene and polyester geotextile tubes reduced both the time and space required for dewatering operations. The tubes retained fine-grained solids while allowing water to pass through their fabric. When the materials inside dried, the tubes were cut open. The materials were removed and reused as backfill behind a retaining wall. The tubes minimized the environmental impacts common to dewatering: conventional containment, transportation, and disposal.
When stormwater runoff contributed to a shallow surface failure on a western Pennsylvania property, the combination of a geogrid system and geotextiles put things back in place, according to Greg Kramer in the Pittsburgh, PA, office of ACF Environmental, which sells geosynthetic products.
The failure occurred when topsoil, imported over a 1.5:1 rock cut face slope, became saturated. The property owner was faced with a limited number of solutions to anchor the slope because of its steepness, its composition, and its short distance from a structure.
"The geogrid, in this case ACF's cellular confinement system, confines, reinforces, and retains the vegetated topsoil. As a result, it prevents downslope move caused by hydrodynamic and gravitational forces," explains Kramer, who gathered the pertinent design information for the project. "The walls of the [Presto] Geoweb [cellular confinement system] are specific in size, spacing, and quantity as to increase lateral drainage and root lock-up. What happens inside the cells is that you might get a little settlement [of soil], but everything is confined around it and it just can't go anywhere."
With a design in hand, workers removed vegetation and soil from the slope and installed a French drain system. They installed anchors at the top of the slope to hold the geogrid in place. A geotextile liner was placed over the slope, followed by the geogrid system. Soil was replaced and reseeded.
"We've gone through several winters and springs - the most demanding time of the year for a confinement system - and everything is functioning very well," Kramer says. "The property owner is very happy."
Warren Cohn, a watershed protection specialist in ACF Environmental's Philadelphia office, echoes Hsuan's belief about the present and future of geosynthetic materials. "We're seeing greater use of geosynthetics such as TRMs [turf reinforcement mats], but it continues to be an educational process because many people just don't know about them yet."
Cohn says a stormwater channel project in Loudoun County, VA, demonstrates the superiority of geosynthetics to outdated materials and methods. The use of a high-performance TRM - defined by EPA as a combination of high-end hydraulic performance characteristics and 3,000-lb. tensile strength - produced water-quality enhancements through the prevention of soil erosion while providing stormwater runoff filtration and infiltration.
Often project managers are faced with a Hobson's choice: Cost-effective materials can carry a higher up-front price tag. Especially in public-sector projects, the low-bid materials win the day. Cohn points out that high-performance TRMs provide value at the front end and over the long haul.
"The TRMs used in the Loudon County project, for instance, generally provide twice the factor of safety at half the cost going in," he explains. "These mats, which are 1-inch thick, replace 2 feet of riprap. When you're installing 2 feet of rock, you generally have to dig 2 feet of sediment out of the channel. Not only are the mats cheaper up-front, but they [also] save the disposal costs of all of that soil. One roll of TRM can eliminate seven dump trucks [driving] into the site to haul the dirt out and five dump trucks [driving] into the site to haul in the rock. The cost benefits are significant when you multiple that factor by five mats for this particular project."
He continues, "If a rock liner had been used in this project, we would have needed to widen the channel. TRMs can reduce the project's footprint." Cohn cites a New York project that would have required the state Department of Transportation to purchase an extra 6 ft. of land from one property owner if a channel had been lined with rock instead of TRMs. "That saved a tremendous amount of money.
"There's a reduction in noise pollution [during the project], which is important in populated areas. Reducing the amount of site disturbance prevented erosion from taking place and the sediment pollution that goes along with it," he says. "Once the mats are revegetated, they provide better filtration and infiltration, so you have less water leaving the area, and what water does leave is cleaner.
"Something I learned from public workers' supervisors when they talk about the real problems they have is that rock liners breed vermin, and vermin breed snakes," Cohn relates. "That's a problem around schools, day-care centers, or camps. Vandalism is another issue with rocks when kids are in the vicinity." Some state regulations require the use of larger rocks in such areas to minimize the likelihood of vandalism.
Cohn notes that TRMs provide hydraulic benefits too. He indicates that designers and contractors should consider more than velocity - the number of square feet of water moving through the channel per second - when working through the hydraulic factors of a project.
"Velocity measures how fast the water moves through the channel, but it doesn't measure what takes place in the channel," he explains. "Sheer stress measures the pull on the channel lining. The Federal Highway Administration, in its Hydraulic Engineering Circular 15, says that sheer stress is a more realistic measurement.
"A rock channel liner is a bunch of individual units, and the discrete particle theory is in force. Basically the liner is only as stable as its smallest rock. In steeper channel beds, rocks become less stable. On a flat surface, the vectors of gravity are holding the rock straight down. On a slope, if there is 10, 15, or 20% less gravity holding it down, then you have a situation with more sheer stress. You lose the stability of the stones. With a turf reinforcement mat, you don't have all those individual stones, so even though more sheer stress is created, the mat doesn't lose its effectiveness," Cohn points out.
That's especially important in such places as Pennsylvania, where most of the prime, flat properties have been developed. New development is occurring on properties with steeper grades, posing greater environmental threats because of stormwater runoff and drawing increased attention from state officials.
Another benefit is in the reduction of thermal pollution, which increasingly is coming under the scrutiny of government officials. "When you take big rocks and put them in a channel, the rocks absorb heat when the sun beats down on them. The rocks get hot, and the result is thermal pollution," Cohn says. "TRMs eliminate thermal pollution big time."
Dane County, WI, is a governmental entity that recognizes the effect of thermal pollution as part of a comprehensive strategy to protect surface-water and groundwater supplies. Its Erosion Control and Stormwater Management Ordinance's thermal control requirements, which govern runoff that could harm cold-water trout streams in the southwest part of the county, are particularly innovative for the Midwest.
One way to introduce the public to such stormwater management techniques as thermal pollution prevention and to encourage the use of best management practices (BMPs) is via the Internet, notes Susan Jones, Dane County watershed management coordinator. In addition to identifying practices that can be used to reduce runoff temperatures, the commission's Web site includes a locator that helps the public determine if their homes, businesses, or planned construction are in sensitive watersheds.
The Dane County Lakes and Watershed Commission site (www.co.dane.wi.us/commissions/lakes/index.shtml) includes a link to the county's Erosion Control and Stormwater Management Manual, which includes 38 BMPs that can be used to meet the standards of the Erosion Control and Stormwater Management Ordinance that went into effect in August 2002. For example, Dane County residents who might never have heard of TRMs are served a user-friendly lesson that includes explanations of natural and synthetic TRMs, their advantages and disadvantages, application and installation, construction and maintenance, and methods to determine practice efficiency.
"We have performance-based standards, and we say, ‘It's really up to you, as the landowner, to decide how you're going to meet those standards.' We don't prescribe the number or combination of practices you employ," says Jones. "I think they appreciate the flexibility of an approach that says, ‘Here's your target, and you decide what steps you take to meet it.'"
The ordinance, developed over three years with substantial input from elected officials, citizens, developers, and technical experts, recently received a citation when the Minnesota Erosion Control Association presented Dane County with its 2003 Achievement Award. A critical component in that success, Jones says, was the communication between the commission and its staff and the numerous townships and smaller communities in the county, as well as others who would be affected by the ordinance.
Dane County staff worked closely with the Wisconsin Department of Natural Resources to ensure that its ordinance - like state regulations - meets or exceeds the standards set forth in the National Pollution Discharge Elimination System Phase II that took effect in March 2003.
Author's Bio: Eric Woolson is based in West Des Moines, IA.