Coastal Processes (AQA GCSE Geography): Revision Note

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

  • deposit

• Waves are formed as winds blow over the surface of the sea 

• The height and strength of a wave are dependent on three 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 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

Constructive wave

• Swash: strong

• Backwash: weak

• Wavelength: long with a low height

• Frequency: low (6–8 per minute)

• Beach type: depositional sandy

Destructive wave

• Swash: weak

• Backwash: strong

• Wavelength: short with a high height

• Frequency: high (10-12 per minute)

• Beach type: erosional shingle

  

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 enters cracks and joints in the rock

    •  As the water freezes, it expands, causing the cracks to open wider

    • As the water thaws, the crack shrinks, easing the pressure on it

    •  Repeated freezing and thawing over time widens the crack, causing pieces of rock to split off the rock face

    • Big boulders break into smaller rocks and gravel

• When a chemical process breaks down rocks, chemical weathering occurs.

  • Rainwater absorbs carbon dioxide from the atmosphere, making it slightly acidic

  • This interacts with minerals in the rock and forms new material

  • Rock type affects weathering rates. For example, limestone weathers chemically faster than granite

  • Higher temperatures speed up chemical reactions

• When living things wear away rocks, this is known as biological weathering

  • Trees and other plants grow in the cracks of a rock formation

    • The roots grow bigger and push open cracks in the rocks, making them wider and deeper

    • The growing tree forces the rock apart over time

  • Burrowing animals like rabbits disturb the ground above their burrows

  • This disturbance puts pressure on cracks, causing pieces of rock to fall off

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:

  • It often occurs in humid areas, moving less than 1 cm each year.

  • Soil expands when it freezes, gets wet, or heats up in the sun

  • The soil expands and lifts at right angles to the slope

  • When the soil shrinks, it drops straight down

  • Soil creep happens slowly as the soil shifts just a millimetre to a few centimetres at a time

• Flow:

  • Happens on slopes from 5° to 15° at speeds of 1 to 15 km per year

  • It usually occurs after the soil gets saturated with water flowing across the surface

  • Flattened vegetation gets carried away with the soil

• Slide:

  • A mass of material moves together until it reaches the bottom of a slope

• Fall:

  • Slopes rise sharply, and movement happens quickly

  • Several reasons caused this.

    • Extreme weathering: Freeze-thaw action loosens rocks, making them unstable and causing collapse

    • Excess rain will soften the surface and cause the slope to collapse

    • Earthquakes dislodge unstable rocks

    • Hot weather dries out soil, causing it to shrink and rocks to fall

• Slump:

  • Typically located on weaker rock types like clay that get saturated and heavy

  • This occurs often at the coast and is called rotational slip

  • A large area of land moves down the slope in one piece

  • It slumps and leaves 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

• This 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 occurs when material is dropped from seawater

• This happens when the water flow's speed (velocity) decreases

• This means the force is no longer strong enough to keep the material suspended in the water, so it drops to the ground

• Sediment deposits in bays form beaches

• Salt marshes and mudflats form in sheltered estuaries, typically behind spits