physical geography - coasts

• Constructive wave


  A low, gentle wave that builds beaches
• Destructive wave


  Tall, high energy waves that erode beach material
• Wave


  A disturbance on the surface of the sea in the form of a moving ridge
• Waves are caused by energy transfer from the wind to the sea
• As the wind blows over the surface of the sea, it creates friction forming waves
• Wave Energy


  • Determined by: the strength of the wind, the duration of the wind, the distance of open water over which the wind blows (fetch)
• Characteristics of Waves


  • Destructive Waves
  • Constructive Waves
• Wave length


  Distance between crests
• Upper layer of air sinks
• Waves have a circular orbit
• Breaking wave gains height

  Orbit becomes elliptical due to shore friction slowing the base of the wave
• Destructive Waves


  Steep wave front, wave height >1m, wave plunges onto steep beach with energy directed downwards, weak swash, strong backwash erodes sand, offshore bar where sand is deposited, short wave length so high frequency (10-14 per minute)
• Constructive Waves


  Gentle wave front, wave height <1m, wave spreads a long way up the gently sloping beach, strong swash, weak backwash, wave gains little height, long wave length so low frequency (8-10 per minute)
• Chemical weathering


  The break down of rock due to chemical changes
• Landslide


  Rapid mass movement of surface material down a slope
• Mass movement


  The downhill movement of material under gravity
• Weathering is the process of breaking up or dissolving rocks in situ
• Types of Weathering


  • Chemical Weathering
  • Mechanical Weathering
• Carbonation


  Carbon dioxide, dissolved in rainwater forms a weak carbonic acid. This reacts with calcium carbonate (limestone and chalk) which forms calcium bicarbonate
• Hydrolysis


  Acidic rainwater reacts with minerals in granite, causing it to crumble
• Oxidation


  Oxygen dissolved in water reacts with iron-rich minerals causing rocks to crumble
• Salt weathering


  Crystals of salt grow in cracks and expand causing rock fragments to flake away
• Mechanical weathering


  The breakdown of rock without chemical changes
• Rockfall


  Fragments of rock break away from the cliff face
• Slumping


  Cliffs slide down a curved slip plane
• Weathering


  The breakdown of rock in situ
• Erosion


  1. Abrasion - Rocks carried along by a wave wear down cliff material

  2. Hydraulic Action - The force of water compressing air in cracks, weakening cliffs

  3. Solution - Soluble particles are transported by the sea

  4. Attrition - Rocks transported by a wave collide and become smaller and rounded
• Hydraulic Action


  The force of the waves hits the cliff and forces water and air into cracks in the bedrock
• Attrition


  Sediment particles knock against the bed or each other and break, and become more rounded and smaller
• Abrasion


  Material carried by waves wear away cliffs
• Deposition occurs when


  • Waves enter an area of shallow water
  • Waves enter a sheltered area, e.g. a cove or bay
  • There is little wind
  • A river or estuary flows into the sea, reducing wave energy
  • There is a good supply of material
• Deposition


  When material is dropped by constructive waves
• Longshore Drift


  The zig-zag movement of sediment along the coast
• Erosional Landforms


  • Arch - A wave eroded passage through a headland
  • Headland - Resistant rock that juts out into the sea
  • Discordant Coastline - Alternative bands of rock along the coastline
  • Wave-cut Platform - A wide, gently sloping surface found at the base of a cliff, extending to the sea
  • Bay - A broad inlet of the sea where the land curves inwards
  • Wave-cut Notch - A dent in the cliff usually at the level of high tide
  • Cave - A large hole in a cliff caused by waves enlarging cracks
  • Stack - A column of rock, often the remains of an arch
• Headlands and bays are characteristic features of a discordant coastline where rocks of different hardness are exposed at the coast
• As weaker rock is eroded faster bays form, leaving more resistant headlands jutting out into the sea
• Cliff retreat


  1. Wave-cut notch

  2. Cliff collapse

  3. Wave-cut platform
• Cave formation


  1. Cracks enlarged by hydraulic action

  2. Cave enlarged by hydraulic action and abrasion

  3. Arch forms when cave erodes through headland

  4. Roof of arch weakened by weathering and collapses leaving a stack

  5. Stump formed when stack collapses due to wave-cut notch
• Depositional Landforms


  • Bar - a ridge of sand or single that joins two headlands on either side of a bay
  • Deposition - The laying down of sediment
  • Lagoon - A shallow body of water protected from a larger body of water
  • Spit - stretch of beach material that sticks out to sea and is joined to the mainland at one end
  • Beach - The area between the lowest tide level and the point reached by storm waves in the highest tides
  • Salt marsh - A coastal ecosystem found between land and open salt water
  • Berm - A ridge often found towards the back of a beach
  • Sand dune - Ridges or hills of sand at the top of a beach
• Sand dune formation


  1. Sand is transported by the wind and gathers against an obstacle forming an embryo dune

  2. Dune slack - A trough (dip) that separates sand dunes