Coastal Processes (Edexcel IGCSE Geography)

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

Erosion

• Destructive waves erode the coastline in four ways:

  • Hydraulic Action 

  • Attrition

  • Corrosion 

  • Abrasion 

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

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