Coastal Processes (AQA GCSE Geography)

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 

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

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

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 solution (corrosion) 

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

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