Marine Processes (AQA A Level Geography)

• Destructive waves erode the coastline 
• Less resistant (softer) rock, such as sandstone, erodes faster than more resistant (harder) rock, such as limestone, but all rocks will erode over time 
• Hydraulic action is the name for a group of erosion processes that occur due to the power of the waves, with no involvement of rocks at all 
  • Sea water forces air into cracks and joints in the rock
  • The pressure from the waves on this air causes the cracks and joints to expand 
  • When larger, loose blocks of rocks are eroded in this way, it is called wave quarrying 
  • If the waves are very large, cavitation will occur 
  • High-pressure air bubbles in the wave collapse on the rock, generating shock waves that erode the surface like a hammer 
• Abrasion occurs when breaking waves that are carrying sediment scrape against the rock surface
  • This is also known as corrasion 
  • It is dependent on how much sediment is available in a particular area of the coastal system 
• The size of sediment particles changes as sediment held in the waves rubs together 
  • This is called attrition
• Acids in the seawater can also wear away the rock, this is solution (corrosion)

• Material in the sea arrives from many sources:
  • Eroded from cliffs
  • Transported by longshore drift along the coastline
  • Constructive waves bringing material inland from offshore 
  • Carried to the coastline by a river
• Once in the water, the material is moved in different ways:
  • Traction
  • Saltation
  • Suspension
  • Solution 

Longshore (littoral) drift

• Longshore (littoral) drift 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
• Offshore currents can contribute to the movement of sediment along or up the beach

The process of longshore (littoral) drift

• Deposition occurs when waves lose energy 
• Waves lose energy when: 
• The wind slows or changes direction 
• The wave experiences friction, usually with the sea floor as the depth decreases 
• When the coastline changes direction such as at an estuary or headland
• It is a gradual rather than immediate process 
• High energy coastlines deposit large rocks and shingle but generally maintain enough energy to carry smaller sediment particles 
• This is why these types of coastlines tend to have rocky beaches, such as Brighton on the south coast 
• Low energy coastlines deposit smaller sediment due to low wave velocity, creating sandy beaches