Coastal Processes (AQA GCSE Geography)

• 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

Constructive waves are beach builders and destructive waves are the destroyers

• 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
• 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 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 is:

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

• The type of movement is determined by:
  • 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 1cm 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 to 15km 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

Major mass movement examples

• 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 corrosion 

• 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 
• Influenced by the prevailing wind, waves approach the beach at an angle
• 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

The process of longshore drift

• Deposition will happen when transported material is dropped from the sea water
• 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