Coastal erosion

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Many stretches of the coastline of East Anglia, England, are prone to high rates of erosion, as illustrated by this collapsed section of the cliffs at Hunstanton, Norfolk.

Coastal erosion (or shoreline erosion/beach erosion see also beach evolution) is the local loss of subaerial coastal landmass due to natural processes such as waves, winds and tides, or even due to human interference. Large storm-generated waves often 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 the processes of erosion and sediment redistribution is called 'coastal morphodynamics'. On rocky coasts, coastal erosion can result in dramatic rock formations in areas where the coastline contains stones 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 of 1997. The houses shown did not survive the storm season.

[edit] Effect on Human Establishments

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 right. 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.

[edit] Wave action

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.

[edit] Rate of Erosion

The factors which control the rate of erosion:

[edit] See also

• Coastal and oceanic landforms
• Natural arch
• Blowhole
• Bioerosion
• Coastal defense
• Beach nourishment

[edit] 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

[edit] Useful Glossary of Terms

Arch a hollow cut by wave erosion through a rocky headland

Bedrock the rock, usually solid, underlying soil and other unconsolidated surface material

Cave any cavity large enough to be entered by a person

Chalk usually a weak friable and porous limestone

Chine a sharply incised valley intersected by a sea cliff

Cliff any slope greater than 45 deg Cliff erosion involves detachment of particles or blocks of material, transport of this through the cliff system, its deposition on the foreshore and its removal by marine action.

Cliff recession the landward retreat of the cliff (from cliff foot to cliff top) in response to the cliff erosion process.

Cliff behaviour unit (CBU) the fundamental unit for cliff investigation and management, reflecting the interrelationships between process and form over time. CBU’s comprise three interrelated systems: cliff tops, cliffs and the foreshore.

Cliff face a high, steep to perpendicular or overhanging face of rock or soil.

Cliff foot the junction of the cliff face and the foreshore

Cliff profile cross section taken perpendicular to the cliff face contours and may include cliff top and foreshore.

Cliff top the junction of the cliff face and the undisplaced material.

Erosion the wearing away of the land’s surface by mechanical processes such as the flow of water, ice or wind

Granite a hard coarse grained igneous rock, resistant to erosion

Lithology descriptive of the constitution of a sediment or rock, including texture, composition and colour, and size, shape and mineral composition of constituent crystals

Littoral the area of seashore between the high and low water

Mud fine grained sediment particles of silt or clay with a diameter below 0.0625mm

Pebble a rock particle with a diameter between 4 and 64mm

Sand rocky particles in size between 0.0625 and 4mm

Sediment loose material derived from the weathering and erosion of pre-existing rocks

Shale a fine grained sedimentary rock, composed of clay particles, that splits easily into thin layers

Stack a pillar of rock, often formed when an arch collapses

Talus an accumulation of rocky debris at the foot of a slope or cliff

by Isle of Wight Centre for the Coastal Environment, UK