Keeping urban land on solid ground
Today’s urban landscape must meet an increasing number of environmental, recreational and aesthetic needs. With the steady and somewhat ominous growth rates of urban populations, it is faced with the significant challenges of providing quality and functional environments.
But, as more land is covered with residential buildings, highways, shopping centers, industrial parks, athletic complexes, warehouses and office buildings, the natural environment becomes significantly altered.
Urban development creates large areas of non-permeable surfaces (roads, parking lots, roofs, etc.) which increase stormwater runoff. As a result, the discharge capacity of waterways can be quickly exceeded, which in turn can lead to flooding and erosion. In addition, the ability to clean this typically very polluted runoff is impaired because of the reduction or actual elimination of local ecosystems and biotic communities.
With important environmental issues such as water and air quality, recreation, wildlife habitat and aesthetic values juxtaposed against the relatively small chunks of natural resources found in urban areas, maintaining as many functional components of urban ecosystems as possible becomes imperative. Erosion control, which seeks to preserve the soil component, therefore plays a vital role in urban landscapes.
Factors affecting urban landscapes
Simply put, erosion is the removal of soil by the action of wind or water. In the undisturbed environment, soil is usually held in place by vegetation, which, except in very arid and truly desert areas, is the dominant form of erosion control. Once the vegetative cover is removed, soil becomes susceptible to erosion.
On a global scale, 60 to 80 percent of all erosion can be attributed to agricultural practices. Although the scope of erosion occurring in urban areas is significantly less, the rates of soil loss and the immediate impacts can often be far greater. For example, typical soil losses of well-managed agricultural land may range from eight to 25 tons per hectare, while erosion rates on urban construction sites may be 10 to 20 times higher. Such high rates of erosion can generate sufficient sediment to quickly choke channels and drains, fill ponds and flood-control storage basins and cause severe damage to property.
Results range from the high costs of sediment removal and disposal to degraded water quality to expensive liability suits.
Construction activities are responsible for the most obvious urban erosion. However, a less obvious, but equally important erosion factor is urban runoff. Urban watersheds often suffer accelerated erosion due to the increased volume of stormwater which is the result of the increased area of impermeable surfaces and reduced absorption capacity of the landscape. Erosion of stream- and riverbanks causes additional sediment loading, affecting property values, reducing access, creating safety hazards and degrading wildlife habitat. Because of the restricted channels to which urban waterways are often confined, water velocities increase and are more likely to erode banks, abutments and other watercourse features.
Road construction in urban areas, particularly in steeper terrain, can create steep cut and fill slopes, which can quickly erode. Results include safety hazards caused when eroded sediment from the slopes is deposited on the roads, clogged culverts and drains, dust hazards and slope failures. When the added costs of removal and disposal are considered, erosion on road slopes quickly becomes a serious operational and economic concern.
Waterborne sediment is one obvious undesirable by-product of urban erosion, yet windborne sediment can be equally damaging. Blowing dust from construction sites can impair visibility and create localized safety hazards for vehicles. It can also contribute to air pollution problems and may require costly cleanup measures.
Finally, degraded aesthetics are an important consequence of erosion in the urban environment. In areas where tourism is encouraged, unsightly eroded slopes and muddy waterways can be a deterrent to business. Wildlife habitat, although not typically viewed as an important resource, can be easily damaged by urban erosion. Sedimentation of rivers, streams and lakes can reduce biological diversity and destroy fisheries, as well as favor undesirable species.
Erosion control is a relatively straightforward concept — the idea is to keep soil in place by preventing the forces of water and wind from carrying this soil away. It is also the most effective means of controlling sediment since, without erosion, the production of sediment is negligible.
The field of erosion control involves a wealth of technological advances for addressing urban erosion concerns. In fact, there has never been a greater selection of techniques and products available to meet the challenges of urban erosion.
A proliferation of products, including natural and synthetic materials, is now available. Natural products range from mulches made of straw, hay, wood fiber, cotton, plant residues and recycled paper to soil binders made of glue-like solutions to blankets and mats made of straw, jute, paper, wood excelsior and coconut. Synthetic materials are commonly used for filtering sediment, stabilizing slopes and reinforcing vegetation. In addition, bio-engineering techniques, which make use of a combination of living plant material and man-made elements, such as retaining walls and geotextiles, have been gaining wider acceptance.
Typical practices for controlling erosion on slopes involve establishment of vegetation using blankets or hydraulically applied mulches. Temporary erosion control on exposed flat areas may use tackifiers and other organic solutions to bind soil particles together. Filter fabrics are used as a perimeter control to capture sediment that may leave the site via sheet or rill erosion processes.
Where relatively light to moderate channel flows occur, a variety of erosion control blankets and turf reinforcement mats can be combined with vegetation to firmly anchor soil. In channels conducting higher water velocities, methods like articulated concrete blocks, cellular confinement systems, gabions, fabric formed revetments and bio-engineering techniques are used not only to prevent erosion, but to establish vegetation and create softer, greener channel environments.
Unstable or over-steepened slopes can be stabilized by using geotextile layers, geogrids and modular retaining wall systems. Soil bio-engineering techniques can also be effectively used to provide long-lasting, self-repairing slope stabilization. Successful stabilization of critical slopes often involves a combination of techniques, creating a composite system that benefits from the strengths of several distinct components.
Erosion control technology has additional benefits important to urban environments. One current trend is to use biological solutions as opposed to traditional systems such as concrete and riprap.
More frequently, erosion control practices are using vegetation as both a structural and biological component, especially in riparian environments. Such approaches offer effective slope and soil stabilization while providing improved wildlife and fishery habitat, improved groundwater recharge capacity and visual enhancement.
Additionally, erosion control professionals are taking a page from stormwater management pros, using sediment ponds, retention basins, flocculating agents and even constructed wetlands to filter out sediment and accompanying pollutants.
Used in conjunction with a sound erosion and sediment control plan — and combined with good stormwater management — the technology is extremely effective. In fact, laboratory tests of erosion control products have demonstrated 95-percent reductions in the amount of sediment entering watersheds.
The Erosion Control Mindset
Current erosion control technology is more than adequate to solve the majority of urban erosion problems and to improve these environments. Actually putting to use the available technology is the more difficult challenge today.
Few of those involved with urban growth and development understand the implications of erosion in the urban environment. Consequently, erosion control does not usually become a standard component of urban landscape design and management.
It is, therefore, necessary to encourage awareness of the importance of erosion control in urban development. Many people are naturally concerned about the environment and will voluntarily commit to taking necessary steps to protect valuable soil and water resources.
In practice, however, few in authority have a “soil preservation ethic.”
Developers — even government highway departments — will generally not take it upon themselves to implement sound erosion and sediment control practices when the impacts are typically felt off-site. And, since budget considerations are always at the forefront of development decisions, incurring additional costs — which can be significant — to keep soil on a project site often does not make sense to most developers.
The most effective incentive for controlling erosion, though not necessarily the preferred one, is the regulatory incentive — legally requiring erosion control and enforcing the requirement with monetary and/or construction shutdown consequences.
Education is a more preferable alternative. While it requires a long-term commitment, its potential is far-reaching. Educating the entire spectrum of people responsible for the causes and prevention of erosion is, indeed, a formidable and probably unachievable task.
But, in the long run, empowering people to make sound environmental decisions based on the benefits of sound resource management will serve all urban landscapes and the citizens residing therein better than will a regulatory/compliance approach.
It is easy to forget about environmental issues, about the role that biological systems play in our lives and about the benefits nature can provide to a needy society.
Educating government leaders and citizens about the ecological function of urban landscapes and the critical role that erosion control plays, both in defined economic terms as well as intangible quality-of-life terms, will result in healthier, more resilient urban landscapes.
Ben Northcutt is executive director of the International Erosion Control Association, Steamboat Springs, Colo.
Wood excelsior mats help town keep the lid on landfill closure
When heavy rains deluged the Northeast last fall, they were successful in washing out the first World Series game between the New York Yankees and the Atlanta Braves in the Bronx. The rains were not able, however, to wash away erosion-control blankets installed during closure of the Port Washington Landfill, a 25-acre site owned and operated by the town of North Hempstead on Long Island, N.Y.
As part of the final cover project at the landfill, project manager James Hummel specified single-net mats of wood excelsior to be used on the landfill’s 3:1 slopes and double-net mats for the drainage swales on top of the landfill. Hummel’s firm, Breco Mechanical Group, Yonkers, N.Y., was the contractor for the closure, and subcontractor Old Oak Landscaping, Glenham, N.Y., laid down the erosion control blankets, manufactured by American Excelsior, Arlington, Texas.
Approximately five truckloads of materials — equivalent to 172,500 square yards — were installed during closure of the landfill.
Because of the large area to be covered in the project, rolls of blankets measuring eight feet by 90 feet were chosen to speed application and reduce labor time. Just three days after the blankets were installed, the New York area was doused with over five inches of heavy, continual rain. Nevertheless, the blankets remained in place on the landfill’s slopes and swales.
The single-net blankets were selected to cover the slope areas of the landfill because the wood excelsior mat offered a consistent thickness, with fibers evenly distributed over the entire blanket. The top of the blanket was covered with a photodegradable, extruded plastic mesh.
The dense mat of fibers and netting was designed to arrest the destructive energy of rain drops, while holding the soil and seed in place to help establish vegetation. Since the single-net blankets were dyed green, they gave the site a finished look while vegetation was taking root.
The double-net, high-velocity blankets were installed on the landfill’s drainage swales because of their heavyweight construction. Each side of the blanket was covered with a black, heavy-duty extruded plastic mesh designed to last for years while reinforcing the root system after the excelsior mat has decomposed.
The blankets can be installed in channels with a water velocity of up to 10 feet-per-second and on slopes that need long-term protection.
As the blankets’ fibers become wet, they expand. As moisture is released from the blankets, the fibers contract. This characteristic helps the blankets to, in effect, “dig in” to the soil, keeping both the blankets and soil in place as new vegetation takes root.
City shores up creek embankment
The Roseville (Calif.) Flood Control project involved bridge replacements, channel reconstruction and construction of flood walls and embankments along seven miles of Cirby Creek. The original streambank slopes varied from 4:1 to greater than 1:1, with channel water-flow velocities as high as nine feet-per-second. Existing soils consisted of densely compacted silty and clay-like sands.
The project included widening the channel and placing an 18-inch sewer line in the invert of the creek, which was closely bordered by residences. Finding a cost-effective solution that provided natural aesthetics was critical to local government officials as well as to the city’s residents. A cellular confinement system was chosen to provide channel embankment protection as well as a series of vehicle ramps, service roads and scour aprons.
The geocell system, manufactured by Presto Products, Appleton, Wis., is an expandable, honeycomb-like structure used to confine and improve the performance of specified infill materials. Stacked sections along the channel side slopes can withstand high flows and associated hydraulic stresses and tolerate differential settlement without loss of structural integrity.
In January 1995, two years after construction, the section of Cirby Creek’s channel protected by the geocell structure was under more than 10 feet of water during winter flooding. While many areas of the city suffered significant flood damage, the area bolstered by the geocell system made it through the flood well and has since required no maintenance, according to Garth Gaylord, associate civil engineer for Roseville’s department of public works and director of the city’s flood plain management section.
During a site visit following the flooding, engineer Sara Conner of CalTrans, the state transportation department, found no damage to the geocell channel protection system in spite of very obvious flooding evidence up the wall face.
Roseville decided against using concrete revetments and gabion walls, even though the required channel widening called for near vertical cuts. The use of the geocell material provided the city with the chance to install a naturally friendly alternative system for bank protection, thus meeting the desire to maintain the neighborhood’s aesthetically pleasing appearance.
The decision to use the geocell system was based on site constraints, poor foundation soils, a short installation timeframe and construction cost savings of $45,000 when compared to the use of gabions. Design flexibility allowed the system to meet specific site criteria as well as compatibility with local environmental and aesthetic requirements. Vegetation taking root in the tan-colored cells provided a natural appearance.
Roseville’s project at Cirby Creek was unique in that the geocell system was used in all four traditional applications: load support, channel protection, slope protection and earth retention.
Erosion Control Show
More than 70 speakers, including Dave Rosgen and Deron Austin, will address attendees at the International Erosion Control Association’s Conference and Trade Exposition in Nashville, Tenn., February 25-28. Rosgen will lead a one-day course on natural channel design, and Austin will speak on steep slope stabilization.
Other topics covered in technical sessions will include stormwater management, stream and channel stabilization, policies and management practices and wetlands design/construction. For those seeking designation as a Certified Professional in Erosion and Sediment Control, tutorials and exams will be offered.
In addition, 120 product and technology exhibits covering around 40,000 square feet will be featured. For more information, contact the IECA at (800) 455- 4322 or http://www.ieca.org.