Coastal Processes (Edexcel IGCSE Geography)

Wave action & erosion

• Coasts are the meeting point of land and sea

• They 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 it 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 is the sheer force of the waves hitting the coast 

  • Attrition occurs when material carried in the waves bumps against each other and becomes smaller and smoother. This doesn't erode the coast but forms the sand and shingle

  • Corrosion is when seawater is slightly acidic and gradually dissolves some types of coastal rock (e.g. limestone) 

  • Abrasion occurs when waves pick up material and hurl it at the coast 

Rates of erosion

• The rates of erosion on the coast are affected by three factors:

  • Energy

  • Materials

  • Shore geometry

Energy

• The greater the energy available the more erosion takes place

• More energy is available when:

  • waves are steep, destructive waves

  • waves breaking onshore

  • there are strong tidal currents

  • rip currents occur

  • there are higher winds

Materials

• The type and amount of materials at the coast affect the rate of erosion

  • Large amounts of material increase abrasion

  • When more material comes into the system than leaves the system this leads to the formation of larger beaches

  • Large beaches absorb wave energy reducing erosion

  • More resistant rocks erode more slowly than less resistant rocks

Shore geometry

• Higher, steeper waves are created by a steep seabed, these lead to more erosion

• Bars offshore lead to waves breaking reducing their energy when they reach the shore

Transportation & deposition

Transportation

• Material at the coast arrives from:

  • Eroded cliffs

  • Longshore drift 

  • Constructive waves

  • River discharge

• In the water, material is moved through:

  • Traction is when large heavy material is dragged along the sea floor 

  • Saltation occurs when smaller material bounces along the sea floor

  • Suspension is the fine material held in the water

  • Solution is dissolved material carried in the water 

Longshore drift

• Longshore drift is the main process of deposition and transportation along the coast 

• The prevailing wind pushes the waves at an angle to the beach

• As the waves break, the swash carries material up the beach at the same angle

• The backwash then carries the material down the beach at right angles (90°) 

• The process repeats, transporting material along the beach in a zig-zag movement

Deposition

• Deposition occurs when:

  • wave energy decreases

  • there is increased friction between the water and the seabed

  • large amounts of sediment are being carried by the water

  • the water encounters obstacles causing the waves to break

Weathering

Weathering

• Weathering is the breakdown of rock in-situ

  • It 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

• Weathering weakens cliffs and makes them more vulnerable to erosion

Types of weathering

• There are three types of sub-aerial weathering:

  • Mechanical

  • Chemical

  • Biological

• Mechanical weathering physically breaks up rock:

  • 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

  • Salt weathering

    • Water in cracks evaporates leaving salt crystals

    • The salt crystals expand and the cracks become larger

    • 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 overland flow 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