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The steady and somewhat ominous growth rates of today's urban populations create significant challenges for professionals who design and build urban landscapes to meet increasing environmental, functional, and aesthetic needs. With important environmental issues such as water and air quality, recreation, wildlife habitat, and aesthetic values focused on the relatively small number of natural resources in urban areas, maintaining as many functional components of urban ecosystems as possible is imperative.

However, urban and metropolitan development creates large areas of non-permeable surfaces (roads, parking lots, roofs, etc.) and increases stormwater runoff and its associated impacts. The ability of the environment to cleanse this typically polluted runoff is severely impaired due in general to the reduction or elimination of local ecosystems and biotic communities. The discharge capacity of associated waterways can be quickly exceeded, leading in turn to flooding and erosion. Erosion control, because it seeks to preserve the soil component, plays a vital role in maintaining urban landscapes.

Erosion in Urban Landscapes

Defined in simple terms, erosion is the removal of soil by the action of wind or water. In undisturbed environments, except in very arid and truly desert areas, soil is usually held in place by vegetation. Once vegetative cover--the dominant form of erosion control--is removed, soil is susceptable to erosion.

On a global scale, 60%-80% of all erosion can be attributable to agricultural practices. Nevertheless, in terms of soil loss, the immediate impacts of land disturbing activities in urban areas can often be far greater. Typical soil losses from urban construction sites may be 10 - 20 times higher than the 8 - 25 tons/hectare (3 -10 tons/acre) lost from well managed agricultural land.

The most obvious impacts of erosion on the urban landscape are derived from construction activities. Negative impacts of high sediment discharges from urban construction sites range from high costs associated with sediment removal and disposal to degraded water quality to damaged recreational areas. As projects increase in size and duration, erosion can proceed at alarming rates, impacting a host of nearby and downstream environments.

A less obvious, but equally important erosion factor, is urban runoff. Urban area watersheds often suffer accelerated erosion when the reduced absorption capacity of the landscape leads to increased stormwater volumes. Due to the restricted waterways to which urban runoff is often confined, water velocities increase and are more likely to erode banks, abutments, and other watercourse features. Erosion of stream and river banks causes additional sediment loading, impacts property values, reduces access, creates safety hazards, and degrades wildlife habitat.

Road construction in urban areas, particularly in steeper terrain, can create steep cut and fill slopes, which can quickly erode with several adverse impacts. Sediment eroded from the slopes and deposited on the roads produce potential surface safety and dust hazards, clog culverts and drains, and contribute slope failures. Considering the added costs of removal and disposal, erosion of road slopes becomes a serious operational and economic concern. The aesthetic consequences of unsightly eroded slopes and muddy waterways can also escalate into economic consequences, especially where tourism is encouraged or where businesses rely on wildlife habitats. For instance, sedimentation in rivers, streams, and lakes can reduce biological diversity or favor undesirable species enough to destroy recreational fishing and fisheries.

While water borne sediment is an obvious undesirable by product of urban erosion, deposition of wind borne sediments require costly clean up measures from residential or business areas. Blowing dust from construction sites can impair visibility and create localized vehicle safety hazards--not to mention generate fines for exceeding air quality standards.

Erosion Control Technology

Erosion control is a relatively straightforward concept--keep soil in place by preventing the forces of water and wind from transporting it off-site where it can cause the environmental and economic impacts described above. Since the production of sediment is negligible in the absence of erosion, erosion control is the most effective means of sediment control.

The erosion control industry possesses a wealth of technology for addressing urban erosion concerns and offers a proliferation of tools and products. 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, while bioengineering techniques which combine living plant material and structural elements (such as retaining walls and geotextiles) to control erosion and stablize soil are gaining wider acceptance.

Typical practices for controlling slope erosion involve establishing vegetation using blankets or hydraulically applied mulches, though temporary erosion control on exposed flat areas may use tackifiers and other organic solutions to bind soil particles together. Filter fabrics can provide perimeter control by capturing sediment which may leave the site due to sheet or rill erosion processes. For channels with light to moderate flows, erosion control blankets and turf reinforcement mats are combined with vegetation to anchor soil. In channels with higher water velocities, articulated concrete blocks, cellular confinement systems, gabions, fabric formed revetments, and bioengineering techniques are used not only to prevent erosion, but to establish vegetation and create softer, greener channel or stream environments.

A current trend is to utilize biologically-oriented solutions rather than traditionl non-living erosion control systems. More and more frequently, erosion control practices are incorporating vegetation as structural and biological components, especially in riparian environments. Also, soil bioengineering techniques can provide long-lasting, self-repairing slope stabilization. Although unstable or oversteep slopes can be stabilized by using geotextiles layers, geogrids, and modular retaining wall systems, successful stabilization of critical grades often results from capitalizing on a combination of techniques to create a composite system with the strengths of several distinct components. Such approaches offer effective slope and soil stabilization while providing environmental benefits of improved wildlife and fishery habitat, improved ground water recharge capacity, visual enhancement, and greater utilization by local citizens.

Expecting total control of erosion is unrealistic--erosion is a natural process and healthy environments assimilate a certain amount of sediment. Yet, 95% efficiency in reduction of sediment production has been demonstrated in laboratory tests of erosion control products. Technology has reached the level where a sound erosion and sediment control planning in conjunction with sediment ponds, retention basins, flocculating agents, and other stormwater management technology--even created wetlands--can filter out sediment and the accompanying pollutants that enter a watershed.

Who Are The Experts?

Various forms of erosion control have been used worldwide for centuries for agricultural purposes or stabilization works on mountainsides or stream banks. As a science and an industry for solving the erosion problems of construction sites and other urban situations, though, the technology has only been around for 30-40 years. Although there is still no known academic degree in erosion control or standard construction practice, academic curricula have in general incorporated fundamentals, and special discipline erosion control principles continue to be developed and practiced by a broad spectrum of professionals--civil engineers, landscape architects, contractors, consultants, manufacturers, and government agencies. The 1994 opening of the Institute of Biological Engineering as part of the University of Vienna, Austria specifically reflects a growing interest in soil bioengineering practices.

However, one program that can help identify people with demonstrated erosion and sediment control skills is the Certified Professional Erosion and Sediment Control (CPESC) specialist certification, the first of its kind for erosion control professionals. Sponsored by three leading U.S. soil conservation organizations, CPESC designation recognizes professionals whose education, work experience, and examination evaluation have met established performance criteria. Qualifications criteria also reflect that well designed erosion control plans and policies should often involve a team approach for effective resource management results.

Responsible Resource Stewardship The role of erosion control is not a highly visible role nor currently one that lends itself well to cost/benefit analyses. Yet, as the world's populations continue to grow exponentially and urban areas convert more and more land to non-living structures, protecting soil through erosion control practices becomes critical to responsible resource stewardship. Education, regulation, and public concern must in combination create a "soil preservation ethic" and incentives to use available technology.

Soil that washes or blows away is, for all practical purposes, a lost resource. The failure to utilize erosion control practices in urban landscapes--to maximize the utility and function of soil, water, and air resources, while mimimizing the harmful impacts of the processes of urban development on these same resources--is a lost opportunity. If everyone involved with land disturbing activities would just say to themselves, "If I disturb the ground, it is my responsibility to protect it and insure its ecological function," the environment would quickly benefit. LASN

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June 18, 2019, 9:08 pm PDT

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