February 8, 2010
Erosion

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Erosion Overview

Erosion is the process by which water, wind, ice, and gravity shape the Earth's landforms. Running water is one of the most significant erosional processes. Not only does the action of moving water itself erode soil and rock, sediment transported by running water abrades the surfaces that the water contacts. Specifically, rivers, streams, and waves transport various sizes of clay, silt, sand and gravel, and even cobbles and boulders at times. As these materials are swept along by the water, they grind against one another and any surface they contact. The result of such abrasion varies based the material being eroded. For example, a stream in soft sediments will erode more quickly than a stream through solid rock; the same is true for the erosion of a sandy beach versus that of a rocky coastline.


The Grand Canyon is an impressive example of stream erosion through rock. For additional information on its formation visit Erosion Examples. Image courtesy of Encyclopedia Britannica.

Stream Erosion - the loosening and transportation of soil and rock from one location to another by streams and rivers. Stream erosion is accomplished by three main actions:
  1. Downcutting - incision of a stream or river into underlying materials caused by the abrasion of transported sediment;
  2. Headward erosion - extension of the river or steam bed up slope (lengthens the stream in the direction of its source);
  3. Sediment transport - the transportation of sediment in rivers and streams for deposition farther downstream;
Rivers and streams commonly flow in meanders, characterized by back-and-forth curves that slowly migrate in the direction of the outside bends. At each meander bend, erosion occurs on the outer bank of the bend and material is deposited on the inner bank. In some cases, erosion causes oversteepening of outer bank, resulting in slope failures and more sediment entering the stream channel. Developments located adjacent to the outer banks of a meandering stream are subject to loss of land as the bank erodes.

Geologists evaluate stream erosion and the site-specific factors that influence the type and rate of erosion. Maps, aerial photographs, and satellite images are often used to evaluate a stream's migration history and to assess hazards to nearby development from future erosion. In previously developed areas experiencing stream erosion, mitigation measures can be implemented to reduce the rate of erosion and protect nearby structures. For example, slope stabilization methods such as rip-rap (boulders), erosion- resistant vegetation, reduction of slope steepness, and retaining structures are used to project stream banks from erosion.

Above: Aerial photograph of a meandering stream. Notice the sand bars on the inside banks of the meander bends indicating zones of deposition. Erosion is occurring on the outer banks, which are located opposite the sand bars. Photograph courtesy of USGS.

Left: Schematic of meandering stream indicating locations of erosion and deposition. Image courtesy of Maine Geological Survey.

Severe stream erosion on the South Branch of the Potomac River in West Virginia. Photograph courtesy of Canaan Valley Institute.
Coastal Erosion - the loosening and transportation of soil and rock from one location to another by wave action. In addition to the abrasion of coastal surfaces by sediment carried by the waves, wave energy plays an important role in coastal erosion. Because of the influence of wave energy, coastal erosion is often most significant during large storm events. This is why hurricanes can quickly erode a beach that had been relatively stable for months or years. Even during calm weather, wave energy is highly effective at eroding surfaces composed of unconsolidated materials (sand) and highly fractured/bedded rock. Because waves and currents are dependent on the weather, coastal erosion processess at a given location can change from season to season.

The rate of coastal erosion is accelerated by rising sea level. As the seas move inland, new surfaces are abraded by waves and debris. Initially, these surfaces form sea cliffs, which eventually transition to beaches. The material eroded from the sea cliffs is redeposited as beach sediment elsewhere along the coast or carried into deeper water. Because of the dynamic nature of wave action and ocean currents, beaches are constantly being eroded and deposited. Construction in such an actively changing environment should consider the potential loss of land when planning developments in coastal areas. Even those located further inland are at risk of property loss caused by ongoing coastal erosion.

Geologists study coastal erosion processes to understand the relationship between wave action, ocean currents, and coastal deposits and to better assess areas prone to erosion. In previously developed areas experiencing coastal erosion, protection structures such as riprap, revetments and jetties have been used to preserve the coastline. However, while these structures may reduce further erosion at their location, they can accelerate erosion and re-deposition on adjacent shorelines. Alternatively, artificially nourishing an eroding beach with new sand helps protect an eroding coastline but is a temporary, and expensive, solution to coastal preservation.

Schematic of sea cliff erosion and exposed beach. Image courtesy of North Carolina State University.

Houses undermined by coastal erosion in Pacifica, California. Photograph courtesy of NASA.

Map of coastal erosion indicating annual shoreline change. Image courtesy of NASA.

Erosion Examples Back to top

Erosion is a constant process occurring around the world. Some notable examples of erosion include:
  • Coastal Erosion at Cape Hatteras Lighthouse, North Carolina: Cape Hatteras is located on a barrier island of North Carolina's Outer Banks. At the time of its construction in 1870, the lighthouse was approximately 460 meters from the shoreline. Rising sea level and westward migration of the barrier islands slowly reduced the distance between the lighthouse and the Atlantic Ocean such that, by 1987, only 49 meters of beach stood between the structure and the ocean. At that distance, a large storm event could rapidly erode much of the beach and undermine the lighthouse's foundation. The National Park Service began pursuing aggressive solutions to preserve the lighthouse. By 1996, plans were put in motion to relocate the lighthouse. The lighthouse was finally moved during the summer of 1999 to a new position approximately 490 meters from the shoreline. Visit the National Park Service for more information about saving Cape Hatteras Lighthouse from coastal erosion.

  • Coastal Erosion at Southern Lake Michigan: In recent years, rising sea level has caused an increase in erosion of bluffs surrounding Lake Michigan, resulting in loss of beaches and properties. Analysis performed by USGS scientists revealed that bluffs between Willmette and Waukegan, Illinois are retreating at rates between 10 and 75 centimeters per year, and erosion rates north of Waukegan approach 300 centimeters per year. While the eroded sediment typically feeds the nearshore beaches, structures constructed to protect the bluffs reduce the amount of available sand. In addition, the lack of deposited sediments has increased the effects of wave action on the lakebed, further accelerating coastal retreat. Visit the USGS Fact Sheet on Southern Lake Michigan for additional details.

  • Stream Erosion and Formation of the Grand Canyon: The Grand Canyon may be the Earth's best example of stream erosion. The Grand Canyon formed as the result of an incised meander caused by aggressive downcutting combined with uplift of the Colorado Plateau. Because of the arid climate, local area soils do not absorb much water during rain events; consequently, runoff rapidly enters the Colorado River and causes flash flooding, which is a likely contributor to erosion of the area. In addition, vegetation in the Grand Canyon consists of shallow root systems, offering little stabilization to the steep slopes. This results in large amounts of sediment and rock being transported into the river and down the canyon during storm events. The combination of water volume and high sediment load abrade the rock, widening and deepening the river channel. Visit Grand Canyon Explorer for additional information formation of the Grand Canyon.

Links to More Information Back to top

For more in-depth information about erosion, check out AEG's Technical References page. Back to top