Coastal Processes (Edexcel IGCSE Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Wave Action & Erosion
Coasts are the meeting point of land and sea and are an open system with inputs (sediment), transfers (longshore drift), stores (beach) and outputs (water).
Coastal processes are divided into two parts:
Marine processes: offshore (water-based)
Terrestrial processes: onshore (land-based)
These processes are then sub-divided into:
Wave action
Erosion
Transportation
Weathering
Mass movement
It is these activities that are responsible for producing distinctive landforms found on the coast
Wave Action
Waves are marine processes. They erode, transport and deposit material
Waves are formed by winds blowing over the surface of the sea
The height and strength of a wave is 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 approaches the coast and enters shallower water, friction from the sea bed 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
Types of Waves
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 |
Examiner Tips and Tricks
Make sure you are familiar with the way waves are formed and their different characteristics. Don't be surprised if you are asked to identify the type of wave from a list of identifiers.
Worked Example
Which statement below, best describes the characteristics of a destructive wave? (1)
A | long wavelength & weak backwash |
B | short wavelength & weak backwash |
C | short wavelength & strong backwash |
D | long wavelength & strong backwash |
Answer
The answer is C (1):
A destructive wave has a short wavelength, high frequency rate, steep wave gradient & a strong backwash
Erosion
Destructive waves erode the coastline in four ways:
Hydraulic Action
Attrition
Corrosion
Abrasion
Examiner Tips and Tricks
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.
Deposition & Transportation
Transportation
Material arrives from:
Eroded cliffs
Longshore drift
Constructive waves
River discharge
In the water, material is moved through:
Traction
Saltation
Suspension
Solution
Longshore Drift
It is the main process of deposition and transportation along the coast
The prevailing wind pushes the waves at angle to the beach
As the waves break, the swash carries material up the beach at the same angle
As the swash retreats, 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 Tips and Tricks
You can gain full marks using well-annotated diagrams to support your answer. Just as you like having a visual prompt, it helps the examiner to see that you do know the answer. Sometimes a diagram is easier than actually writing it all out. After all, a picture paints a thousand words!
Weathering
Weathering
This is the breakdown of rock in-situ. Weathering does not involve the movement of material, making 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)
Salt weathering
Weathering weakens cliffs and makes them more vulnerable to erosion
Mechanical weathering physically breaks up rock:
One example is freeze-thaw or frost shattering
Water gets into cracks and joints in the rock
When the water freezes it expands and the cracks open a little wider
Over time, pieces of rock split off the rock face, whilst big boulders are broken into smaller rocks and gravel
Chemical weathering occurs when rocks are broken down by a chemical process:
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
Biological weathering takes place when rocks are worn away by living organisms:
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 forces the rock apart
Tiny organisms like bacteria, algae and moss can grow on rocks
These produce chemicals that break down the surface layer of the rock
Burrowing animals, such as rabbits, disturb the ground
This destabilises the rock above the burrow
Increasing pressure on any cracks
Eventually, pieces fall off the rock
Mass Movement
Mass Movement
The downhill movement of material under the influence of gravity
Throughflow and runoff caused by heavy rain can also make cliffs more unstable and increase the likelihood of mass movement
It includes landslides, slumping and rockfalls
What influences the type of movement?
The angle of slope (steeper is faster)
Nature of regolith
Amount and type of vegetation
Water
Type and structure of rock
Human activity
Climate
Types of movement
Soil Creep:
Speed is below 1cm per year
Common in humid climates
When soil expands, individual particles are lifted up at right angles to the slope
Soil also expands when it freezes, gets wet or is heated up in the sun
When the soil shrinks again, the particles fall straight back down
Soil creep takes a long time because the soil moves only a millimetre to a few centimetres at a time
Flow:
It occurs on slopes between 5° and 15°
Usually, after the soil has become saturated with a flow of water across the surface
Vegetation can be flattened and carried away with the soil
Speeds range from 1 to 15km per year
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
Due to the nature of the slip, it leaves behind a curved 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
Answer:
One sub aerial 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, freeze and expand, exerting pressure on the crack. When temperatures rise, water melts, pressure is released and the crack contracts. Repeated cycles eventually breaks 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). This means that the cliff is weakened and can then be eroded more easily by the waves (1)
Chemical weathering (1) is another sub-aerial process and the rock type, will decide on 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)
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
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