Edexcel IGCSE Geography 复习笔记 2.1.1 Coastal Processes

• Coasts are the meeting point of the 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 will 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. They have a short wavelength, high-frequency rate and a steep wave gradient. Their backwash is stronger than their swash, which scours the beach, dragging material out to sea
  • Constructive waves are beach builders. They have a long wavelength, low-frequency rate and a shallow wave gradient. The swash is stronger than its backwash, which carries material up onto the beach and deposits it there

Comparison of Wave Type

Erosion

• Destructive waves are responsible for the majority of erosion that happens along a coast. They cut into the coastline in four ways:
  • Hydraulic Action 
  • Attrition
  • Corrosion 
  • Abrasion 

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
• 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

Weathering

• This is the break-down of rock in-situ. Weathering does not involve the movement of material and this 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 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 prizes 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 destabilizes the rock above the burrow
    • Increasing pressure on any cracks
    • Eventually leading to pieces falling off the rock

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

• The type of movement is influenced by:
  • Angle of slope (steeper is faster)
  • Nature of regolith
  • Amount and type of vegetation
  • Water
  • Type and structure of rock
  • Human activity
  • Climate

• 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:
  • 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 collapse of the slope
    • Earthquakes can dislodge unstable rocks
    • Hot weather can dry out soil causing it to shrink and allow rocks to fall

• Slump:
  • Usually found on weaker rock types (i.e. clay), that becomes 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

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