8.1 Waves, Marine, Sub-Aerial Processes

• Wave height / amplitude

  The distance between the trough and the crest.

  
  

  The height of a wave is usually an indication of energy.
• Wavelength


  Distance between two successive crests or troughs.
• Wave Frequency


  The number of waves per minute.
• Wave refraction


  Wave approaching shore will reduce their speed as they 'feel bottom'.

  
  

  Waves approach shore obliquely - they bend around the coast to break parallel with the shore.
• Wave refraction on coasts

  Energy is distributed along a coast with alternating headlands and bays wave refraction concentrates erosional activity on headlands while energy is dispersed in bays.

  
  

  Deposition tends to occur in bays.
• Spilling Breakers


  Steep waves associated with gentle beach gradients.

  
  

  Wave gradually peaks before crest grows unstable and gently spills forward.
• Plunging Breakers


  Intermediate steepness on steeper beaches.

  
  

  Shoreward face of wave become vertical, curls over and plunges downward as an intact mass of water.
• Surging Breakers


  Low steepness and found on steep beaches.

  
  

  Wave remains intact and slides up the beach without breaking.
• Constructive Waves


  Occur when frequency is low and wen waves advance over a gently shelving sea floor.

  
  

  Low frequency means backwash is able to return to the sea and swash of next wave is not impeded.

  
• Destructive Waves


  Caused by locally generated winds which create waves of high frequency. Backwash is stronger than swash - material is eroded.

  
• Marine Processes


  Hydraulic Action

  Abrasion

  Attrition

  Solution
• Hydraulic Action


  Waves break onto cliffs and trap air in joints and bedding planes - air is pressurised and expands.
• Abrasion (Corrasion)


  Where breaking wave can hurl pebbles and shingle against a coast, thereby abrading it.
• Attrition


  Takes place when eroded material itself is worn down.
• Solution (Corrosion)

  Chemical erosion affecting calcareous (lime-rich) rock.

  
• Sub-aerial processes (Cliff Face)

  Salt Weathering

  Freeze-thaw

  Biological

  Solution Weathering

  Slaking
• Salt Weathering


  Sodium and Magnesium compounds expand in joints and crack and weaken structures.

  
  

  Expansion can be up to 300%
• Freeze-Thaw Weathering


  Water freezes and expands by 10% or so and degrades jointed rocks.
• Biological Weathering


  Done by molluscs, sponges, urchins.

  
  

  Important on low energy coasts.
• Solution Weathering


  Chemical weathering of compounds due to the presence of organisms secreting organic acids.
• Slaking


  Materials disintegrating when exposed to water, which can be caused by hydration cycles.
• Mass Movements


  Slumping and Rock Falls common on coasts.
• Sources of sediment


  Onshore transport by waves.

  Offshore transport by waves

  Rivers

  Glacial and periglacial deposits.

  Wind-blown deposits.

  Artificial beach replenishment
• Bedload

  Grains are transported withcontinuous contact(traction or dragging) or bydiscontinuous contact(saltation) with the sea floor.
• Suspended Load


  Grains are carried by turbulent flow and generally held up by water.
• Littoral Cell System


  A simplified model that examines coastal processes and patterns in a given area.

  
  

  They operate on scales from a single bay like Bournemouth but also on regional scales like Californian Coast.

  
  

  Longshore Drift occurs if wave refraction is not complete.