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Coastal Erosion and Deposition

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Waves, winds, currents, tides, and storms are the major forces on the coastline. The results of these actions and interactions by natural forces on the shoreline and near-shore seabed are coastal processes, which include erosion and deposition.

What factors are involved in coastal erosion and deposition?

There are several factors involved in coastal erosion and deposition. The main processes are:

Shaping coastlines

Waves are the primary drivers in shaping coastlines. Destructive waves (waves that have a stronger backwash) are the most common types of waves that cause erosion . These waves occur during stormy conditions and are characterised by big, strong waves that have high energy. Constructive waves (with a limited backwash) occur during calmer weather, have low energy, and cause deposition rather than erosion. Because of their characteristics, destructive waves can erode the coastlines in several ways:

  • Hydraulic action – air in cracks in the cliff is compressed when waves crash in. The pressure caused by this action breaks off pieces of rocks.
  • Attrition – bits of rock in water smash against each other and break into smaller pieces.
  • Abrasion – bits of rock and sediment transported by the waves, smash and grind against rocks and cliffs, breaking bits off and smoothing the surface.
  • Corrosionsoluble rocks get gradually dissolved by the seawater.

Coastal formations

The principle marine processes responsible for shaping the coastline are erosion , transportation , and deposition. Erosion is where the force of waves breaks down the land. Transportation is when waves and tides transfer the broken material somewhere else. Meanwhile, deposition is when waves and tides lose their energy, cease transporting the eroded material, and deposit it. Each coastline has its balance between the processes of erosion, transportation, and deposition.

Erosional coastal formations include:

  1. Headlands and bays.
  2. Cliffs and wave-cut platforms.
  3. Stacks and stumps.
  4. Shoreline platforms.

Coastal Erosion and Deposition Stacks and Stumps StudySmarterDuncansby Stacks Image: Bill C CC BY-SA 3.0

When waves no longer have any energy left to transport the sediment, deposition occurs. What features are formed by the sediment will depend on how and where the sediment is deposited.

Depositional coastal formations include:

  1. Spits.
  2. Beaches.
  3. Offshore bars and tombolos.
  4. Cuspate forelands.
  5. Salt marshes and estuarine mudflats.
  6. Sand dunes.

Sediment transportation

The energy provided by waves, tides, and currents transport the eroded material. There are four main processes involved in sediment transportation:

  • Solution – substance that is dissolved and carried along in the water.
  • Saltation – larger particles are too heavy to be carried, so they are bounced along the sea bed.
  • Suspension – this is where very fine material is carried along in the water.
  • Traction – enormous particles are pushed along the sea bed by the force of the water.

These processes can transport sediment along the shore; this is known as longshore drift or littoral drift .

Sediment is moved along the coast in sediment cells. Within each cell, the sediment moves between the beach, cliffs, and the sea through the process of erosion, transportation, and deposition. Any action taken in one place has an impact elsewhere in the cell. Each cell operates between physical barriers that prevent the sediment from moving any further along the coast. There are 11 principal cells along the coastline of England and Wales.

Sub-aerial processes

Sub-aerial processes are land-based processes that alter the shape of the coastline. The main sub-aerial processes are weathering and mass movement.

Weathering

The gradual breakdown of rocks in situ at or close to the ground surface is known as weathering. This can be divided into three different types - mechanical, chemical and biological.

Types of Weathering
Mechanical (physical)Freeze-thaw (frost shattering) Salt weathering (salt crystallisation) Wetting and drying (sea weathering)
Chemical Carbonation
BiologicalPlants WaterMicrobes

Mass movement

This is the movement of materials downslope at a range of speeds. Water acts as the common lubricant involved in mass movement.

Types of mass movement are:

  • Soil creep

  • Solifluction

  • Earth and mudflows

  • Rock falls

  • Rock / debris slides

  • Slumps

Coastal Erosion and Deposition - Key takeaways

  • Constructive and destructive waves are the primary drivers in shaping the coastline.

  • Constructive waves are depositional.

  • Destructive waves are erosive.

  • Destructive waves can erode the coastlines through hydraulic action, corrosion, attrition, abrasion, and corrosion.

  • The four main processes involved in transporting material are solution, saltation, suspension, and traction.

  • The sub-aerial processes involved in shaping the coastline are weathering and mass movement.

Coastal Erosion and Deposition

Both coastal erosion and deposition are caused by the action of waves. There are two main types of waves responsible for this, destructive waves and constructive waves

Coastal erosion is the wearing away of land by destructive waves, currents, and wind. As a result of coastal erosion, the shoreline will retreat causing land loss. Globally, this is a massive issue. Deposition happens when the sea loses energy and it drops the sand, rock particles, and pebbles it has been carrying. Constructive waves are responsible for this as the swash is stronger than the backwash.

The four processes of coastal erosion are corrosion, abrasion, attrition, and hydraulic power.

The most significant danger to coastal areas comes from natural events such as hurricanes, coastal storms (storm surge), tsunamis, and landslides (mass movement), as well as longer-term risks of coastal erosion and sea-level rise.

 The negative effects of coastal erosion are the damage it can cause to transport and infrastructure through storm surges and mass movement. In addition, changes to the soil structure result from seawater contaminating the farming land through flooding, causing economic loss and the destruction of property.


Final Coastal Erosion and Deposition Quiz

Question

Waves are responsible for shaping the coastline. There are two types: what are they?

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Answer

Constructive and destructive waves.

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What are the characteristics of a constructive wave?

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They have a low frequency and a weak backwash.

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What are the characteristic of a destructive wave?

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They are high and deep, have a high frequency and they have a strong backwash and a weak swash.

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Erosional coastal formations include what?

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Answer

Headlands and bays, Cliffs and wave-cut platforms, Stacks and stumps, and Shoreline platforms

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Depositional coastal formations include what? 


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Spits, beaches, offshore bars and tombolos, cuspate forelands, salt marshes, estuarine mudflats, and sand dunes.

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Question

There are four processes whereby the waves transport material; what are they?


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Answer

Solution, saltation, suspension, and traction.

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Add the missing words to the following sentences.  

Solution  - Substance that is ……….. and ………... along in the water.

Saltation  - Larger particles are ………. to be carried, so they are ……….. along the sea bed. 

Suspension  - This is where very ……….. is ……….. along in the water.

Traction  - …………. particles are ………… ... the sea bed by the force of the water.


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Answer

Solution - Substance that is dissolved and carried along in the water.

Saltation - Larger particles are too heavy to be carried, so they are bounced along the sea bed. 

Suspension - This is where very fine material is carried along in the water.

Traction - Enormous particles are pushed along the sea bed by the force of the water.

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Question

What are the main sub-aerial processes?

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Answer

The main sub-aerial processes are weathering and mass movement.


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What is weathering?


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Answer

The gradual breakdown of rocks, in situ, at or close to the ground surface.

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Weathering can be divided into three different types. What are they?


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Mechanical, chemical, and biological.

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 What is mass movement?


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This is the movement of materials downslope, because of gravity, at a range of speeds. Water acts as the common lubricant involved in mass movement.

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What are the types of mass movement?


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Types of mass movement are soil creep, solifluction, earth and mudflows, rock falls, rock / debris slides, and slumps.

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How are waves produced?

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Waves are produced by friction between the wind and water, resulting in an energy transfer from the wind to the sea. Ripples are created; these ripples will become waves if the wind is sustained.

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When do waves move forward?

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Waves move up and down – there is no horizontal movement. It is only when the water enters shallower areas that the water itself moves forward.

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What is the highest point of a wave called?


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wave crest

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What is the distance between trough and crest called?

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The wave height.

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Explain wave refraction.

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As a result of the coastal feature, the depth of water around a coast varies. As the wave approaches a coast, its progress is modified due to friction from the seabed, halting the motion of waves. In addition, as waves approach a coast, they are refracted so that their energy is concentrated around headlands but reduced around bays.

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What are the two types of waves?


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Constructive and destructive waves.

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Describe a constructive wave.


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They are also called spilling or surging waves. They are gentle, flat waves with a strong swash and a weak backwash; as a result, constructive waves build beaches. As the wave breaks, it carries material up the beach in its swash. Beach material is deposited as a bridge of sediment (berm) at the top of the beach. The relatively gentle profile of the beach with a steep ridge at the top of the beach means that the backwash soaks into the sand or slowly drains away. When the following wave breaks, its swash will deposit more material without it being 'captured' by the backwash of the preceding wave.

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Describe a destructive wave.

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These waves are also known as plunging waves. Not surprisingly, destructive waves destroy beaches. As the wave approaches the coast, it gains height and drops onto a steep beach. As a result, it does not travel far up it. The swash of a destructive wave is much weaker than its backwash. This means that these waves transport beach material back into the sea, lowering the height of a beach. The more the waves crash onto the beach, the more sediment is washed out to sea. Because the waves are so frequent, the backwash has less time to soak into the sand. During a storm, the most common waves are destructive.

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Question

The following are facts about constructive waves. Add in the missing words

Their wavelength is …….and a….. frequency (8-10 waves per minute).

They have a….... gradient, typically under ………. in height.

They have low ……. and an …... orbit.

Have a larger ……. than ………..

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Answer

Their wavelength is long and a low frequency (8-10 waves per minute).

They have a low gradient, typically under one meter in height.

They have low energy and an elliptical orbit.

Have a larger swash than backwash

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The following are facts about destructive waves. Add in the missing words.

Destructive waves are usually ……. over 1m.

They have a more ……… .cross profile.

They are most common where …… .. is short.

They have a mainly circular orbit, a …… .. gradient, 

These act as agents of erosion because their ………. is greater than their .......

They are more common in ……. than in summer

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Answer

Destructive waves are usually very high, over 1m.

They have a more circular cross profile.

They are most common where fetch is short.

They have a mainly circular orbit, a steep gradient.

These act as agents of erosion because their backwash is greater than their swah.

They are more common in winter than in the summer

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Question

There are four processes involved in the eroding coastlines. What are they?

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Answer

Hydraulic action, attrition, abrasion, and corrosion.

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Question

What is hydraulic action?

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Answer

This is where waves approach the cliff; air may become trapped and compressed in joints and cracks along a cliff face. Then when the wave retreats, the compressed air expands again. This continual process can weaken the joints and cracks in the cliff, causing pieces of rock to break off.

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What is abrasion?


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Bits of rock and sand that the waves have picked up are thrown against the cliff face. The sediment acts as a tool on the cliff, chiselling away at the surface and gradually wearing it down by removing small particles.

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What is attrition?


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When a wave breaks upon the shore, rocks and pebbles that are being carried collide with each other breaking them and eventually making them smaller and smoother. The net result is that the sediment gets smaller and smaller over time.

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What is corrosion?


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Corrosion is also known as solution. It is where salts and acids within the seawater will gradually dissolve some types of rock found along the coast. This process happens over thousands of years.

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What are the processes of weathering at the coast?

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Mechanical, chemical, and biological.

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What is mechanical weathering?

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Mechanical weathering is the breaking down of rocks above the waterline. During mechanical weathering, the chemical composition of the rocks is unchanged. 

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What is chemical weathering?


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Chemical weathering is the erosion of rock through a chemical reaction. Water reacts with the chemical components in the rocks, and this is done through either carbonation, hydrolysis, or oxidation.

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What is biological weathering?

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Biological weathering is the breaking down of rocks by living organisms, such as plants and animals.

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What is mass movement?


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Mass movement is the movement of soil and rocks in large quantities downslope as a result of gravity.

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Explain slumping.


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Slumping occurs in saturated conditions where softer rock retains rainwater and runoff on moderate to steep slopes. It is also common where softer clay or sand overlies more resistant or impermeable rocks. As the rock becomes saturated, a slip plane forms and the weight causes the cliff to slump.

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Explain a landslide.


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Answer

In areas of more resistant cliff material, erosion is more significant at the base where the continuous breaking of waves produces a wave-cut notch. The bigger the notch becomes, the heavier the cliff above the base becomes. Eventually, the base can no longer support the material above it and a landslide occurs. 


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Question

 Explain freeze-thawing.


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Answer

Freeze-thaw occurs in rocks that are porous or permeable. This allows the water to enter the rock and freeze. When water freezes, it expands by approximately 10%. This expansion exerts pressure on the rocks, forcing cracks in the rocks to widen. With repeated freezing and thawing, rock fragments break away, fall to the cliff face, and collect at the bottom of the cliff as scree. 

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Explain salt weathering.


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Answer

Salt crystallisation happens when sea spray penetrates cracks in the rocks. When salt evaporates, it leaves behind salt crystals. These grow over time and exert stresses on the rock, causing it to break up. The salt can corrode rocks, especially if they contain a trace of iron. 

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In which conditions does carbonation occur?


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In warm, wet conditions.

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Explain hydrolysis weathering.


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Answer

Hydrolysis is when acidic rainwater breaks down the rock, causing it to rot.

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What is oxidation weathering?


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Oxidation is when rocks are broken down by oxygen and water.


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Name a coastline in the UK that is a good example of mass movement?


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Answer

The Holderness Coast is the fastest eroding coastline in Europe.

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What's the name usually given to the rockfall at the bottom of a cliff face?


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Scree.

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Explain what is meant by the term subaerial processes.


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Answer

Subaerial processes include weathering and mass movement. Weathering weakens rocks above the high tide mark. It is the gradual breakdown of rocks in situ. It does not involve any movement. Mass movement is the movement of materials downslope at a range of speeds.

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Question

Where does coastal sediment come from?

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Answer

The seabed, beaches, river channels, estuaries, and erosion from cliffs. It also comes from material with a biological origin.

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Explain how coastal deposition occurs.

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Coastal deposition occurs when waves lose their energy. As a result, deposition and settlement of materials vary in size depending on how much energy is lost.

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What are the main processes of transportation?

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Traction, saltation, suspension, and solution.


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 What is traction?

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Traction is where large pebbles and boulders are rolled along the seafloor.


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What is saltation?


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Saltation is where sand-sized material is bounced along the seafloor either through the forces of water or wind. On a dry, windy day, a layer of saltation can often be seen 2-10cm above the beach surface.

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What is suspension?

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Suspension is where silt and clay are suspended and carried by the waves. The sea is often a muddy, brown colour due to the suspended sediment.


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What is solution? 


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Solution is where the material is dissolved and carried by the water as a solution.

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Explain what coastal currents are?


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Coastal currents are water masses in motion, and they are found between the coastline and the edge of the continental shelf. They are often made up of two components: alongshore (or parallel to the coast) and cross-shore (or perpendicular to the coast).


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