Coasts

• Coastal landscapes are shaped by the power of waves
• Waves


  Created by wind blowing over the sea surface
• Size and energy of a wave


  Determined by the fetch (how far the wave has travelled) and how long (in time) the wind has been blowing
• Wave types


  • Constructive: long wavelength, low wave height, strong swash, weak backwash, cause deposition of material
  • Destructive: short wavelength, high wave height, weak swash, strong backwash, cause erosion of coast
• Chemical weathering


  Occurs when acidic rainfall falls on rocks, can cause breakdown of certain rocks like limestone over time
• Mechanical weathering processes


  • Cracking caused by expansion and contraction of rocks as they heat up and cool down
  • Biological weathering
  • Freeze-thaw weathering
• Freeze-thaw weathering


  Rainwater freezes in cracks in rocks, expanding the cracks, repeated freezing and thawing causes the rock to break down
• Coastal erosion processes


  • Hydraulic power: waves crashing against a cliff, compressing air in cracks which causes the rock to break down
  • Abrasion: rock fragments carried by waves chip away at the cliff face
  • Attrition: waves cause rocks and pebbles to collide and become smaller and more rounded
  • Solution/Corrosion: acids and salts in seawater gradually dissolve some rocks
• Wave-cut platforms


  Waves crashing against a cliff will lead to the formation of a notch

  1. Continued erosion will cause the notch to increase in size
  2. Eventually, the overhanging cliff will collapse
  3. Repetition of this sequence will complete the wave-cut platform
• Arch and stack formation

  Waves attack areas of weakness on a headland

  1. Over time, caves form
  2. If two caves form on either side of the headland, erosion will cause the neck to widen, forming an arch
  3. The arch will eventually collapse, leaving a stack
• Headlands and bays are formed when bands of more resistant rock (like granite) and less resistant rock (like clay) meet the sea
• Types of mass movement

  • Sliding: sudden movement of large volumes of rock and soil along a zone of saturated soil

  Slumping: rapid mass movement of permeable rock and soil such as clay which has become heavily saturated, lying on top of impermeable material

  Rock fall: free-fall movement of rock fragments due to mechanical weathering
• Coastal transport processes


  • Solution: dissolved minerals carried within the water

  Traction: the rolling of large pebbles along the sea floor

  Suspension: particles carried within the water

  Saltation: the bouncing of small pebbles along the sea floor
• Longshore drift


  Waves approach the beach at an oblique angle

  1. This causes the sediment to be carried along the coast in a zig-zag pattern
• Coastal landforms

  • Beaches: formed by constructive waves depositing material, typically in sheltered bays

  Sand dunes: form when the wind blows sand onshore, trapped by obstacles and vegetation

  Spits: long expanse of sand attached to the land, formed by longshore drift

  Bars: sand deposited across the mouth of a bay by longshore drift
• Hard engineering strategies

  • Sea walls: concrete structures designed to reflect wave energy, can provide a promenade

  Rock armour: large boulders that absorb wave energy, relatively cheap and quick to implement

  Groynes: barriers stretching out into the sea at right angles, trap sediment and reduce longshore drift

  Gabions: wired cages of rock that absorb wave energy, used on sandy beaches
• Soft engineering techniques

  • Beach reprofiling: mechanical transfer of sand and shingle to change the shape of the beach to absorb more wave energy

  Dune regeneration: artificial creation or restoration of sand dunes, using plants and fences to stabilise them

  Beach nourishment: depositing sand or shingle onto an eroded beach to replace what has been washed away
• Managed retreat


  Allowing designated low-value coastal areas to flood, often involving the removal of current defences, to create new habitats and reduce long-term costs
• Lyme Regis coastal management scheme used both hard and soft engineering techniques, completed in 2015 at a cost of over £43 million
• Swanage has a discordant coastline with alternating bands of soft rock (clay) and hard rock (limestone/chalk) perpendicular to the coastline, creating headlands and bays
• Coastal landforms at Swanage

  • Old Harry Rocks: stack of chalk headland

  Studland Bay: sand spit formed by longshore drift

  Swanage Bay: broad, sandy beach sheltered by the limestone headland to the south