Coastal erosion

Many stretches of the East Anglia, England coastline are prone to heavy of erosion, such as this collapsed section of cliffs at Hunstanton, Norfolk.

Coastal erosion is the wearing away of land or the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage (see also beach evolution). Waves, generated by storms or fast moving motor craft, cause coastal erosion, which may take the form of long-term losses of sediment and rocks, or merely in the temporary redistribution of coastal sediments; erosion in one location may result in accretion nearby. The study of erosion and sediment redistribution is called 'coastal morphodynamics'. It may also be caused by hydraulic action, abrasion, and corrosion.

On rocky coasts, coastal erosion results in dramatic rock formations in areas where the coastline contains rock layers or fracture zones with different resistances to erosion. The softer areas become eroded much faster than the harder ones, which can result in typical landforms such as tunnels, bridges, columns, and pillars.

Pacifica, California coast before major storms in 1997 destroyed the houses shown above.

On sedimentary coasts, coastal erosion typically poses more of a danger to human settlements than it does to nature itself. Dunwich, the capital of the English medieval wool trade, disappeared over the space of a few centuries due to redistribution of sediment by waves. Human interference can also increase coastal erosion: Hallsands in Devon, England, was a coastal village that was washed away overnight, an event possibly exacerbated by dredging of shingle in the bay in front of it.

The California coast, which has soft cliffs of sedimentary rock and is heavily populated, regularly has incidents of housing damage as cliffs erode. Damage in Pacifica is shown at left. Devil's Slide, Santa Barbara and Malibu are regularly affected.

The Holderness coastline on the east coast of England, just north of the Humber Estuary, is the fastest eroding coastline in Europe due to its soft clay cliffs and powerful waves. Groynes and other artificial measure to keep it under control has only sped up the process further down the coast, because longshore drift starves the beaches of sand, leaving them more exposed.

Wave action - basic

Beach erosion at Cabrillo National Monument, California.

The four main types of wave action can be remembered in this simple way; (by the use of the word "HACC")

• Hydraulic action - this is when air in cracks on the cliff face becomes compressed by the power of the waves striking the cliff face. As this happens the air inside the crack is compressed, putting a lot of pressure on the surrounding rock. The air then expands explosively, forcing out pieces of rock. Over time, the cliff face crack breaks causing a larger crack or cave to form. The rock from the cliff face which was removed falls to the bottom of the sea bed and is used for another two wave action (Attrition and Corrasion (Abrasion)).

• Attrition - this is the when the sea grinds the rocks, and also the rock on the cliff, together, causing it to become smoother and reduced in size. As the sea rocks from side to side it moves the scree causing pieces of scree to collide with other pieces of scree thus causing them to become reduced in size and smoothed and rounded. As well as colliding with other pieces of scree the scree also collides with the cliff face base causing pieces of rock to be broken of the base of the cliff face contributing to this wave action and one more (Corrasion (Abrasion)).

• Corrasion (Abrasion) - this is when the waves break on the cliff face pounding the cliff face slowly eroding it. Along with the cliff face being eroded by the power of the sea the sea also uses the scree from other wave actions. As the sea pounds the cliff faces it also uses the scree to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action or one other (Attrition).

• Corrosion or solution - this is when the sea uses its low pH (anything below pH 7.0) to corrode away the rocks on the cliff face. Usually the only cliff faces to be greatly eroded in this manner would be limestone cliff faces as they have a high pH and would be easily eroded by a low pH. The rocking action of the sea also makes it easier for the sea to erode limestone cliff faces as the rocking action of the sea acts as the stirring motion in a chemistry experiment which helps to speed up a corrosive experiment.

Wave action - extra detail

The ability of waves to cause erosion of the cliff face depends on number of factors, which include:

• The hardness or ‘erodibility’ of the rocks exposed at the base of the cliff
  • The key factors in determining erodibility include the rock strength along with the presence of fissures, fractures, and beds of non-cohesive materials such as silt and fine sand.
• The rate at which cliff fall debris is removed from the foreshore
  • Debris removal from the foreshore is dependent on the power of the waves crossing the beach, and this energy must reach a critical level or material will not be removed from the debris lobe. On many cliffs these debris lobes can be very persistent and may take many years before they are completely removed.
• The presence or absence of a beach at the base of the cliff
  • Beaches help dissipate wave energy on the foreshore and can provide a measure of protection to the cliff from marine erosion.
• The stability of the foreshore, or its resistance to lowering
  • Lowering of the beach or shore platform through wave action is a key factor controlling the rate of cliff recession. If the beach is not lowered the foreshore should widen and become more effective at dissipating the wave energy, so that fewer and less powerful waves reach the cliff.
• The adjacent bathymetry
  • The nearshore bathymetry controls the wave energy arriving at the coast, and can have an important influence on the rate of cliff erosion.
• The supply of beach material in the coastal cell from updrift
  • The provision of material eroded updrift coming onto the foreshore beneath the cliff will help ensure a stable beach, thus providing a measure of protection.

Factors affecting the erosion rate

First order (most important)

• Geological structure and lithology: hardness, height, fractures/faults
• Wave climate: prevailing wave direction
• Sub-aerial climate: weathering (frost, etc.), stress relief swelling/shrinkage
• Water-level change: groundwater fluctuations, tidal range
• Geomorphology

Second order

• Weathering and transport slope processes
• Slope hydrology
• Vegetation
• Cliff foot erosion
• Cliff foot sediment accumulation
• Resistance of cliff foot sediment to attrition and transport

Third order

• Coastal land use
• Resource extraction
• Coastal management

See also

• Coastal and oceanic landforms
• Natural arch
• Blowhole
• Bioerosion
• Modern recession of beaches
• Coastal defense
• Beach nourishment
• Streisand effect#Etymology
• Submersion

External links

• Coastal Erosion Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution
• Wave Erosion
• Examine an example of wave erosion
• Erosion & Flooding in the Parish of Easington
• Some interesting teaching resources
• Examples of coastal landforms

Images:

• Work to reduce coastal erosion at Lyme Regis in Dorset 2006
• Images of Coastal features