Coasts

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Erosion Processes

  • Hydraulic action. Air may become trapped in joints and cracks on a cliff face. When a wave breaks, the trapped air is compressed which weakens the cliff and causes erosion.
  • Abrasion. Bits of rock and sand in waves grind down cliff surfaces like sandpaper.
  • Attrition. Waves smash rocks and pebbles on the shore into each other, and they break and become smoother.
  • Solution. Acids contained in sea water will dissolve some types of rock such as chalk or limestone.
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Coastal transport

Coastal transport

ProcessDescription Solution Minerals are dissolved in sea water and carried in solution. The load is not visible. Load can come from cliffs made from chalk or limestone, and calcium carbonate is carried along in solution. Suspension Small particles are carried in water, eg silts and clays, which can make the water look cloudy. Currents pick up large amounts of sediment in suspension during a storm, when strong winds generate high energy waves. Saltation Load is bounced along the sea bed, eg small pieces of shingle or large sand grains. Currents cannot keep the larger and heavier sediment afloat for long periods. Traction Pebbles and larger sediment are rolled along the sea bed.

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Deposition

Deposition

When the sea loses energy, it drops the sand, rock particles and pebbles it has been carrying. This is called deposition. Deposition happens when the swash is stronger than the backwash and is associated with constructive waves.

Deposition is likely to occur when:

  • waves enter an area of shallow water.
  • waves enter a sheltered area, eg a cove or bay.
  • there is little wind.
  • there is a good supply of material.
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The action of waves

The power of waves is one of the most significant forces of coastal change. Waves are created by wind blowing over the surface of the sea. As the wind blows over the sea, friction is created - producing a swell in the water. The energy of the wind causes water particles to rotate inside the swell and this moves the wave forward.

The size and energy of a wave is influenced by:

  • how long the wind has been blowing
  • the strength of the wind
  • how far the wave has travelled (the fetch)
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Wave types

Waves can be destructive or constructive.

When a wave breaks, water is washed up the beach - this is called the swash. Then the water runs back down the beach - this is called the backwash. With a constructive wave, the swash is stronger than the backwash. With a destructive wave, the backwash is stronger than the swash.

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Destructive waves

The effects of a high wave (http://www.bbc.co.uk/staticarchive/8d4062a22a73715eb6abc6e21c88bbbba5eebf8c.gif)

  • Destructive waves are created in storm conditions.
  • They are created from big, strong waves when the wind is powerful and has been blowing for a long time.
  • They occur when wave energy is high and the wave has travelled over a long fetch. They tend to erode the coast.
  • They have a stronger backwash than swash. They have a short wave length and are high and steep.
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Constructive Waves

The effects of a low wave (http://www.bbc.co.uk/staticarchive/903cb89096e5824a7b28b5172135498d5bc2a193.gif)

  • They are created in calm weather and are less powerful than destructive waves.
  • They break on the shore and deposit material, building up beaches.
  • They have a swash that is stronger than the backwash.
  • They have a long wavelength, and are low in height.
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Transport

There are various sources of the material in the sea. The material has been:

  • eroded from cliffs
  • transported by longshore drift along the coastline
  • brought inland from offshore by constructive waves
  • carried to the coastline by rivers

Waves can approach the coast at an angle because of the direction of the prevailing wind. The swash of the waves carries material up the beach at an angle. The backwash then flows back to the sea in a straight line at 90°. This movement of material is called transportation.

Continual swash and backwash transports material sideways along the coast. This movement of material is called longshore drift and occurs in a zigzag.

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Cliffs, wave-cut platforms and notches

Soft rock, eg sand and clay, erodes easily to create gently sloping cliffs. Hard rock, eg chalk, is more resistant and erodes slowlyto create steep cliffs.

The erosion of cliffs (http://www.bbc.co.uk/staticarchive/64b86dfc8b63682a9640eed8336f64c36dc420da.gif)

The process of cliff erosion

  • Weather weakens the top of the cliff.
  • The sea attacks the base of the cliff forming a wave-cut notch.
  • The notch increases in size causing the cliff to collapse.
  • The backwash carries the rubble towards the sea forming a wave-cut platform.
  • The process repeats and the cliff continues to retreat.
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Headlands and bays

Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock.

The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant rock, such as chalk. This leaves a section of land jutting out into the sea called a headland. The areas where the soft rock has eroded away, next to the headland, are called bays.

Geology is the study of the types of rocks that make up the Earth's crust. Coastlines where the geology alternates between strata (or bands) of hard rock and soft rock are called discordant coastlines. A concordant coastline has the same type of rock along its length. Concordant coastlines tend to have fewer bays and headlands.

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Caves, Arches, Stacks and Stumps

Weathering and erosion can create caves, arches, stacks and stumps along a headland.

  • Caves occur when waves force their way into cracks in the cliff face. The water contains sand and other materials that grind away at the rock until the cracks become a cave. Hydraulic action is the predominant process.
  • If the cave is formed in a headland, it may eventually break through to the other side forming an arch.
  • The arch will gradually become bigger until it can no longer support the top of the arch. When the arch collapses, it leaves the headland on one side and a stack (a tall column of rock) on the other.
  • The stack will be attacked at the base in the same way that a wave-cut notch is formed. This weakens the structure and it will eventually collapse to form a stump.
  • Old Harry Rocks, a stack found off a headland in the Isle of Purbeck.
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Beaches

Beaches are a common feature of a coastline. Beaches are made up of eroded material that has been transported from elsewhere anddeposited by the sea.

Constructive waves help to build up beaches. The material found on a beach (ie sand or shingle) depends on the geology of the area and wave energy.

A cross-section of a beach is called a beach profile. The shingle ridges often found towards the back of a beach are calledberms.The material found on a beach varies in size and type as you move further away from the shoreline. The smallest material is deposited near the water and larger material is found nearer to the cliffs at the back of the beach. Large material is deposited at the back of the beach in times of high energy, for example during a storm. Most waves break near the shoreline, so sediment near the water is more effectively broken down by attrition.

Sandy beaches have gently sloping profiles and shingle and pebble beaches are steeper.

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Spits

Spits are also created by deposition. A spit is an extended stretch of beach material that projects out to sea and is joined to the mainland at one end.

Spits are formed where the prevailing wind blows at an angle to the coastline, resulting in longshore drift. An example of a spit is Spurn Head, found along the Holderness coast in Humberside.

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Conflicts of interest

There are many different groups of people who have an interest in how coastal areas are managed. These include:

  • local residents
  • environmental groups
  • developers
  • local councils
  • national governments
  • tourist boards
  • National Park Authorities, such as the Pembrokeshire National Park Authority

Each interest group may have a different view about what should be done to protect and manage coastal areas. A difference of opinion can cause conflict between interest groups. Reasons why groups of people might be concerned about the coast

  • Erosion may be threatening beaches or coastal settlements.
  • People may want to develop tourism in the area or existing tourism could be declining.
  • There is a danger of flooding if sea levels rise. There could be a problem with sewage and/or pollution.
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