Coastal Processes (Edexcel IGCSE Geography)

Revision Note

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Jacque Cartwright

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

Constructive and destructive waves

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

longshore-drift
The Process of Longshore Drift

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

types-of-mass-movement
Types of mass movement

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:

  1. 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)

  2. 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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Jacque Cartwright

Author: Jacque Cartwright

Expertise: Geography Content Creator

Jacque graduated from the Open University with a BSc in Environmental Science and Geography before doing her PGCE with the University of St David’s, Swansea. Teaching is her passion and has taught across a wide range of specifications – GCSE/IGCSE and IB but particularly loves teaching the A-level Geography. For the past 5 years Jacque has been teaching online for international schools, and she knows what is needed to get the top scores on those pesky geography exams.

Bridgette Barrett

Author: Bridgette Barrett

Expertise: Geography Lead

After graduating with a degree in Geography, Bridgette completed a PGCE over 25 years ago. She later gained an MA Learning, Technology and Education from the University of Nottingham focussing on online learning. At a time when the study of geography has never been more important, Bridgette is passionate about creating content which supports students in achieving their potential in geography and builds their confidence.