Erosion Control

Summary

Preventing or eliminating the erosion of soil and rock protects water and air quality and the integrity and usability of public and private lands. As a result, erosion control requires both engineering and land-use management techniques. Erosion control is often seen as primarily an agricultural problem because the land area affected by farming practices is large. However, erosion control is also necessary in coastal regions and areas used for forestry, transportation, development, and recreational purposes because natural or human actions can change landscapes or soil covers and result in increased erosion.

Definition and Basic Principles

Erosion control is the practice of preventing the movement of soil or rock by the action of wind or water. Uncontrolled erosion by either natural or human actions can cause water or air pollution or damage to property. Erosion is a natural result of the action of water or wind. However, human activity can accelerate this process by removing protective vegetation or creating instabilities in existing soil and rock structures.

Wind erosion acts by selectively transporting soil particles. In other words, the higher the wind velocity, the larger the soil particles that can be transported. Water erosion works both as precipitation hits the ground surface and as water flows over the land. Erosion-control techniques and structures work to reduce the potential for soil or rock transport either by reducing the exposure of land surfaces to the effects of wind and water or by modifying the landscape or installing structures that increase the stability of the landform. For example, mulching, revegetation, and the application of geotextiles are all approaches that work to reduce the vulnerability of soil surfaces to erosion. Gabions, retaining walls, and terracing are examples of structures or modifications to landforms.

Lack of erosion control can result in declines in soil productivity for agricultural purposes and loss of land stability for land development and uses. Examples of the large-scale effects of erosion include landslides and desertification.

Background and History

Before 1920, erosion was noted during farming practices but not necessarily considered reparable, and eroded lands were abandoned in favor of new fields. This mindset had its roots in the colonial days of the United States, where potential new farmland stretched to the horizon and was available to those willing to clear the land. Decades of intensive farming led to the loss of soil productivity and then the soil itself to water and wind erosion, with dramatic examples of wind erosion occurring during the Dust Bowl era of the 1930s.

In 1935, the United States Congress established the Soil Conservation Service (now the Natural Resources Conservation Service of the US Department of Agriculture) to provide for ongoing work to conserve the nation's soils. This act established soil conservation as a national priority independent of agricultural programs, which had far-reaching advantages borne out by more recent concerns about erosion related to storm-water runoff, coastal wave and storm action, and river movement.

Erosion-control practices are required in many different circumstances. Agricultural practices for animal husbandry or to produce food crops can have significant adverse impacts on soil health if soil conservation techniques are not applied. Land development for residential housing or commercial and industrial facilities can have short-term erosion impacts during construction and long-term effects from rainwater or snowmelt on the sites. Recreational uses near lakes, streams, and oceans can affect the stability of shorelines. Transportation systems may create unstable slopes with increased potential for landslides from cuts through hills for roads and rail lines.

How It Works

Agriculture. Erosion-control practices in agriculture focus primarily on nonstructural techniques to retain and improve soil productivity. The intent of erosion-control measures is not only to protect the soil from raindrop impacts and wind transport but also to increase the infiltration capacity of the soil to reduce the amount of water that runs off over land. One way that infiltration capacity is increased is to reduce the length of the slope that is available for water to travel by using terraces or contour plowing. Another way to increase infiltration is to slow the movement of water and physically protect the soil surface with mulch or vegetation, practices that also increase soil fertility by increasing organic content and biological activity.

Coastal Zone Management. Wave action and storm surges in coastal environments cause beach and headland erosion that is typically controlled through revegetation of coastal dunes and barrier islands or hardened structures like riprap, gabions, and retaining walls. An understanding of beach dynamics is needed, particularly of the seasonal transport of sand on- and offshore or the transverse migration of sediments along shorelines. Hard structural erosion-control features must be evaluated and designed to ensure that erosion is not increased elsewhere as an unintended consequence.

Transportation. The development and maintenance of transportation systems can increase erosion potential in the landscape by altering the stability of landforms through either physically changing the landscape or by increasing water runoff. For example, road cuts can increase the potential for erosion and mass wasting by creating new steep slopes in previously stable hillsides. Unless control measures, such as gabions, geotextiles, or terraces, are used, erosional forces will work to reduce the steep slope to a more stable form. This erosive action can take the form of catastrophic landslides. Increased water runoff is also generated by road systems because of the impermeable nature of most road surfaces. The runoff, if not addressed, can increase the erosion of road beds or nearby stream drainages, which can increase road maintenance requirements or property damage.

Storm-Water Management. Residential, commercial, industrial, and public facility uses generate erosion-control needs during both the development phase and long-term occupation of the site. Site preparation includes activities such as tree and vegetation clearing and grading that expose soils and subsoils to erosion and alter the topography of the site. Erosion-control features during construction include phasing the work to minimize the amount of land exposed at any particular time, rapid revegetation of the site, and temporary approaches such as mulching.

Long-term land uses can also increase erosion potential if rainfall or snowmelt is not adequately accommodated on the site. In a similar manner to road systems, the impermeable surfaces created by roofs, driveways, parking areas, and walkways means that overland flow is increased and the infiltrative capacity of the site is reduced. Increased off-site erosion can result if excess overland flow is discharged to adjacent properties or the road system.

Recreation Management. The effects of recreational activities on landscapes may be dealt with through other disciplines and programs mentioned above. Effects of recreation often include soil compaction and vegetation removal through foot or vehicle travel. When the effects are limited in area, the overall effect on erosion potential can be slight. However, when the effects are widespread or occur on steep slopes or near bodies of water, the erosion potential can be high. Mitigating approaches may be to limit the area of travel, for example, designated trails or sites for activities, which will allow revegetation to occur naturally or through replanting. In some instances, the volume of traffic may be so great that structural approaches are needed. For example, a shoreline hardened with a retaining wall can allow increased access for fishing when foot traffic would heavily impact a naturally vegetated shoreline.

Applications and Products

Agriculture. Many agricultural erosion-control measures emphasize changing farming practices rather than necessarily installing a structure or feature on the site. For example, conservation tillage works to minimize soil tilling to leave as much crop residue in place as possible on the surface of the soil. This approach requires that the crop residue is not plowed into the soil at the end of the growing season and the new crop is planted in rows plowed through the residue at the beginning of the next season. The crop residue acts as a mulch to help stabilize the soil, retain soil moisture, and increase the soil's organic content. In this approach, chemical weed control is often used to eliminate undesirable plants or the crop from the previous year. Alternative methods of controlling weeds are available if a reduction in use of herbicides is needed.

Other nonstructural approaches to erosion control on agricultural land include contour plowing and crop or pasture rotation. Crop rotation is a practice where a sequence of crops are cultivated on the land over a series of seasons or years. Rotating crops improves soil structure and makes it more resistant to erosion, particularly when paired with conservation-tillage techniques. In addition, varying the crops grown can reduce reliance on herbicides and pesticides by, for example, changing the crop to one that creates field conditions that are less favorable to the weeds or pests of concern.

Structural erosion-control measures for agriculture include terraces and large windbreaks. Although windbreaks are not constructed, they usually are composed of trees planted in rows perpendicular to the prevailing wind direction. They do require planning for location and long-term maintenance to ensure continued effectiveness. Terraces are constructed to created areas of flat land in an otherwise sloped terrain. Terraces retain water behind a dyke or berm at the edge of the terrace and can help manage water on a site after heavy rains. Terraces cost more than other erosion-control measures in terms of labor and equipment to construct but can be effectively farmed with large equipment when properly designed, located, and constructed.

Coastal Zone Management. Erosion control in coastal zones can be necessary because the action of wind and waves erodes coastal lands so that it threatens structures or features such as roads, buildings, or navigation channels. Erosion control can also be needed when structures encroach into areas that are vulnerable to erosion or are otherwise unstable landforms. Examples include the construction of buildings on barrier islands or on exposed headlands.

Nonstructural erosion-control methods in coastal environments can be modeled after natural features such as coastal dune environments or saltwater marshes to create buffers that absorb the energy of waves or storm surges. These approaches include limiting access for foot and vehicle traffic in areas vulnerable to erosion and revegetating with native plants. The plants trap wind-borne sediment and sand and anchor coastal soils with their root systems. Revegetation measures may include the use of geotextiles or mulches to protect the soil surface while the vegetation becomes established.

Structural erosion-control measures in coastal environments include riprap, gabions, and retaining walls. Riprap is rock placed to protect the toe of a slope or the base of a vertical wall. The rock absorbs the energy of wave action and prevents the undermining of existing structures. Gabions are large cages filled with rock, often the same rock that would be used for riprap, that armor the shoreline. Gabions can be stacked to create a retaining wall and can be used in place of riprap where space is limited. Gabions have an advantage over other armoring approaches in that the rock-filled cages drain water freely and are flexible enough to accommodate ground movement. In general, retaining walls need to be properly designed and installed to ensure that the pressure of the ground and subsurface water behind the wall can be withstood for the long term by creating weep holes for moisture to escape and using devices to prevent the soil load from transferring to the wall. Retaining walls can be constructed using either non-proprietary methods or proprietary products such as interlocking concrete blocks and grid materials.

Transportation. Transportation facilities, in particular roads and railways, are often constructed through hilly terrain using cuts into steep slopes. The resulting wall often creates an instability in the landscape that needs to be addressed to avoid damage to facilities and delays in shipping and travel. Slope-stabilization techniques are commonly used in road cuts to prevent rock fall and landslides. These techniques include structural solutions (such as gabions, retaining walls, and spray-on concrete) and less structural approaches incorporating geotextiles and revegetation.

In addition, transportation facilities need to be designed and constructed to ensure that unintended erosion impacts are not created by runoff of storm water from impermeable surfaces. In these circumstances, facility designs need to include consideration of how storm water can be infiltrated.

Storm-Water Management. Construction and other site-development activities can significantly increase site erosion potential by removing vegetation and changing the topography by grading. Erosion control on construction sites can be achieved either temporarily during construction or for the long-term operation of the site after construction is complete. Temporary measures include phasing construction activities to minimize how much of the soil on the site is exposed at any one time and the use of stabilizing materials such as mulches or geotextiles. Products such as sedimentation fences or storm-drain filters are not erosion-control devices. Instead, they control the movement of eroded sediments to ensure they do not create off-site impacts on roads or adjacent lands.

Long-term storm-water management on developed sites includes the use of structural approaches, such as retaining walls, or nonstructural approaches, such as revegetation, to stabilize sites and prevent erosion as the site is used.

Recreation Management. Recreation activities can be dispersed or focused in terms of impacts on a site's erosion potential. Activities such as hiking or canoeing can have small impacts on sites if traffic is light. However, heavily used trails or boat launches can become significantly eroded. Other activities, such as off-road recreational vehicles, can significantly reduce vegetation and produce bare compacted soils vulnerable to wind and water erosion even with light usage. Erosion-control measures related to recreation areas will vary by need and facility. For example, boat launches for light craft in a rural area may consist of a graveled surface along the bank. In contrast, heavily used launches for heavy craft may consist of a concrete ramp extending into the water.

Erosion control along river channels historically emphasized engineering solutions such as riprap, retaining walls, and channelization where the stream channel was lined with concrete. Because of the unintended erosion and flooding effects experienced as a result of these approaches, modern stream-erosion projects have mimicked natural systems by reestablishing riparian vegetation and features such as floodplains and meanders. These approaches include removing barriers to river movement and using products such as coconut fiber bales and geotextiles to help revegetate areas and improve infiltration.

Careers and Course Work

Coursework that supports careers in erosion control or soil conservation is based on the physical and applied sciences. Degree programs include soil science, agronomy, hydrology, hydrogeology, and geology. Civil engineering is another alternative degree program, although coursework from the other disciplines is required to create a focus on erosion control. A solid foundation of coursework in soil science, geology, and hydrogeology is required to understand the basic physical processes involved in this subject, as is coursework in mathematics, physics, and engineering.

Students may focus on the engineering, land-use management, or regulatory approach to erosion control. A career oriented toward engineering or structural solutions to erosion control requires an engineering degree, including physics, mathematics, and engineering dynamics coursework. This career may also require professional certifications, such as being certified as a professional engineer, which potentially requires additional study for the examination that must be passed to achieve the certification. A career focusing on land-use management or a regulatory approach to erosion control may be founded on any of the degree programs listed above and includes additional coursework in land use and public policy analysis.

A bachelor's or master of science degree is typically required for career opportunities designing and implementing erosion control practices. A degree is typically not required for a career as a construction contractor installing or building erosion control structures or features, such as retaining walls or regrading and revegetation projects. However, certain professional certifications or licenses may be required for construction contractors, depending on the jurisdiction.

Social Context

Erosion-control requirements affect many sectors of society and have become integrated into a wide variety of occupational disciplines. Given the potential for significant damage if erosion-control measures fail—for example, in areas with steep slopes or coastal zone environments—the emphasis on and expectations for high levels of success increase with increasing development pressures. Increasing development and market pressures can also affect food production in that farmers seek to maximize the productivity of their land while reducing costs. Soil-conservation measures can increase productivity while reducing the need for some herbicides and pesticides. Soil-conservation measures that increase the moisture-holding capacity of sites and make those sites less dependent on irrigation are also important for agriculture and urban or suburban areas where water supplies may be stressed by demands for domestic use.

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