Coastal Processes (AQA GCSE Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Wave Types & Characteristics
The coast can be defined as:
The meeting point between land and sea
Coastal processes are divided into two parts:
Marine processes: Offshore (water-based)
Terrestrial processes: Onshore (land-based)
These processes are further divided into:
Wave action
Erosion
Transportation
Weathering
Mass movement
These processes produce the distinctive landforms found around our coastlines
Examiner Tip
When you are asked to describe the formation of a landform, you must refer to the physical processes involved in its formation.
You must also use the correct terminology and show your understanding of each of the processes involved.
Wave action
Waves are marine processes that erode, transport and deposit material
Waves are formed as winds blow over the surface of the sea
The height and strength of a wave are dependent on 3 factors:
The fetch
The amount of time the wind blows
The strength of the wind
The greater the strength, time and fetch of the wind, the larger the wave
As a wave enters the shallow water of the coast, friction from the seabed causes the wave to lean forward and eventually crest and break onto the beach
The movement of water up the beach is called the swash, and the return movement is the backwash
There are two types of waves:
Destructive waves erode the beach
Constructive waves are beach builders
Comparison of Wave Type
| Constructive Wave | Destructive Wave |
---|---|---|
Swash | Strong | Weak |
Backwash | Weak | Strong |
Wavelength | Long with low height | Short with high height |
Frequency | Low (6–8 per minute) | High (10–12 per minute) |
Type of beach | Sandy: depositional | Shingle: erosional |
Worked Example
Identify which statement below best describes the characteristics of a destructive wave?
| A | long wavelength and weak backwash |
| B | short wavelength and weak backwash |
| C | short wavelength and strong backwash |
| D | long wavelength and strong backwash |
[1 mark]
Answer:
The answer is C [1]:
A destructive wave has a short wavelength, high frequency rate, steep wave gradient and a strong backwash
Weathering
Weathering is best defined as:
The break-down of rock in-situ
Weathering does not involve the movement of the material; this is what makes it different from erosion
Sub-aerial weathering describes coastal processes that are not linked to the action of the sea
It includes freeze-thaw weathering (mechanical) and chemical weathering
Weathering weakens cliffs and makes them more vulnerable to erosion
Mechanical weathering physically breaks up rock
Freeze-thaw or frost shattering is where:
Water gets into cracks and joints in the rock
When the water freezes, it expands and the cracks open a little wider
When the water thaws, the crack contracts, releasing pressure on the crack
Over time, repeated freezing and thawing widens the crack until pieces of rock split off the rock face, whilst big boulders are broken into smaller rocks and gravel
When a chemical process breaks down rocks, chemical weathering occurs.
Rainwater is slightly acidic through absorbing carbon dioxide from the atmosphere
This reacts with minerals in the rock, creating new material
Rock-type affects the rate of weathering; e.g. limestone chemically weathers faster than granite
The warmer the temperature, the faster the chemical reaction
When living things wear away rocks, this is known as biological weathering
Trees and other plants can grow within the cracks in a rock formation
As the roots grow bigger, they push open cracks in the rocks, making them wider and deeper
Over time, the growing tree eventually prizes the rock apart
Burrowing animals, such as rabbits, disturb the ground above the burrow, which puts pressure on any cracks, eventually leading to pieces falling off the rock
Mass Movement
Mass movement is:
The downhill movement of material under the influence of gravity
Heavy rain can also make cliffs more unstable and increase the likelihood of mass movement through throughflow and runoff
It includes landslides, slumping and rockfalls
The following factors determine the type of movement:
Angle of slope (the steeper the slope, the faster the movement)
Nature of regolith
Amount and type of vegetation
Water
Type and structure of rock
Human activity
Climate
Soil Creep:
Common in humid climes with the movement of less than 1 cm per year
Soil expands when it freezes, gets wet or is heated up in the sun
As the soil expands, it lifts at right angles to the slope
When the soil shrinks, it falls straight back down
Soil creep takes a long time because the soil moves only a millimetre to a few centimetres at a time
Flow:
Occurs on slopes between 5° and 15° with speeds between 1 and 15 km per year
Usually happens after the soil has become saturated with a flow of water across the surface
Vegetation is flattened and carried away with the soil
Slide:
A movement of material 'en masse' which remains together until hitting the bottom of a slope
Fall:
Slopes are steep and movement is rapid
Caused by a number of reasons:
Extreme weathering: Freeze-thaw action can loosen rocks that become unstable and collapse
Rainfall: Too much rain will soften the surface, leading to the collapse of the slope
Earthquakes can dislodge unstable rocks
Hot weather can dry out soil, causing it to shrink and allowing rocks to fall
Slump:
Usually found on weaker rock types (i.e. clay) that become saturated and heavy
This is common at the coast and is also known as rotational slip
It involves a large area of land moving down the slope in one piece
Because of the way it slumps, it leaves behind a curved indented surface
Worked Example
Outline two ways that sub-aerial processes can affect the shape of a cliff
[4 marks]
There will be 2 marks available for each point
1 mark for the processes
1 mark for the explanation
Remember that there are three sub-aerial processes that you can use to answer this question
Freeze-thaw, chemical and biological
You need to explain how each process works and then link that to how it would change the shape of a cliff
Answer:
One subaerial process is freeze-thaw weathering [1], where temperatures need to go above and below freezing 0°C. Any water trapped in cracks of a rock freezes and expands, exerting pressure on the crack. When temperatures rise, water melts, pressure is released and the crack contracts. Repeated cycles eventually break the rock apart. Therefore, there will be more freeze-thaw occurring in winter than in summer, resulting in more weathering of the cliff face [1]. As a result, the cliff is weaker and the waves can erode it more easily [1].
Chemical weathering [1] is another sub-aerial process and the rock type will decide how quickly the rock will dissolve. Rainwater and seawater are both slightly acidic. Less resistant rock, such as limestone, will react with the acid in the water faster than granite. Therefore, a cliff made of softer, less resistant rock will weather faster than a cliff made of harder, more resistant rock [1]
Erosion
Destructive waves are responsible for the majority of erosion that happens along a coast
They carve the coastline in three main ways:
Hydraulic Power/Action
Attrition
Abrasion
There is also a fourth way that waves erode along the coast and that is by corrosion
Examiner Tip
Make sure you know the difference between the four types of erosion, particularly between abrasion (corrasion) and attrition. So many students confuse these two terms.
A tip for you is to think of abrasion as rubbing with sandpaper or maybe you have grazed your knees or elbows when you fell off your bike/skateboard? Those grazes were abrasions on your knees, elbows, etc.
Transportation
Material in the sea arrives from many sources:
Eroded from cliffs
Transported by longshore drift along the coastline
Brought inland from offshore by constructive waves
Carried to the coastline by a river
Once in the water, the material is moved in different ways:
Traction
Saltation
Suspension
Solution
Longshore drift
It is the main process of deposition and transportation along the coast
Waves approach the beach at an angle due to the prevailing wind
As the waves break, the swash carries material up the beach at the same angle
As the swash dies away, the backwash carries the material down the beach at right angles (90°)
The process repeats, transporting material along the beach in a zig-zag movement
Worked Example
Describe and explain the process of longshore drift
[4 marks]
Identify the command words and link to the key term
Command words are 'describe and explain'—say what you see and why
Your focus is on 'longshore drift'—what is it?
Answer:
Longshore drift is the process where the waves transport material [1], such as sand, along the beach in the direction of the prevailing wind [1]. The swash moves material up the beach at an angle [1] as the waves approach in a similar direction to the wind. The material then moves back down the beach at 90° due to gravity [1]. This is the backwash. This movement continues along the beach in a zig-zag motion [1] in the direction of the prevailing wind
Examiner Tip
When describing a feature, make sure you are clear on the correct sequence/stage. Use annotated sketches or diagrams to help you explain the steps.
Deposition
Deposition will happen when transported material is dropped from the seawater
This occurs when the velocity (speed) of the water flow slows down
This means there is not enough force to hold/suspend the material in the water anymore, and it is dropped onto the ground
Beaches are formed of sediment deposited in bays
Saltmarshes and mudflats are formed in sheltered estuaries, usually behind spits
Worked Example
Study Figure 9, a photograph taken along the stretch of coastline.
Suggest one type of mass movement that is affecting these cliffs.
[1 Mark]
Answer:
Remember the question asks about mass movement and not weathering; therefore, your answer should be one of the following:
Rock fall [1]
Slumping [1]
Landslip [1]
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