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Coastal erosion

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, wind, or fast moving motor craft, cause coastal erosion, which may take the form of long-term losses of sediment and rocks, or merely 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 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. Softer areas become eroded much faster than harder ones, which typically result in landforms such as tunnels, bridges, columns, and pillars.

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

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

  • Hydraulic action - occurs when waves striking the cliff face compresses air in cracks on the cliff face. This puts tremendous pressure on the surrounding rock. The air then expands explosively, forcing out pieces of rock. Over time, the cliff face crack grows, sometimes forming a cave. 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 - occurs when the sea grinds rocks together, causing them to become smoother and reduced in size. As the sea rocks (scree) 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, smoothed and rounded. As well as colliding with other scree, the scree also collides with the cliff face base causing pieces of rock to break off the base of the cliff face contributing to this wave action and to (Corrasion (Abrasion)).
  • Corrasion (Abrasion) - occurs when the waves break on the cliff face pounding the cliff face and slowly eroding it. As the sea pounds the cliff faces it also uses the scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and to (Attrition).
  • Corrosion or solution - occurs when the sea uses its low pH (anything below pH 7.0) to corrode the rocks on the cliff face. Usually the cliff faces to be greatly eroded in this manner are limestone cliff faces, which have a high pH. The rocking action of the sea also increases the rate of reaction by removing the reacted material.

Wave action - extra detail

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

  • The hardness or ‘erodibility’ of the rocks exposed at the base of the cliff
    • The key factors in determining erodibility include the rock strength and 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. This energy must reach a critical level or to remove material from the debris lobe. On many cliffs debris lobes can be very persistent and may take many years to completely disappear.
  • The presence/absence of a beach at the cliff base.
    • Beaches 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 updrift material coming onto the foreshore beneath the cliff helps ensure a stable beach.

Factors affecting the erosion rate

First order (most important)

  • Geological structure and lithology: hardness, height, fractures/faults
  • Wave climate: prevailing wave direction, wave breaking point
  • 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

  • Resource extraction
  • Coastal management

See also

External links

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