Coasts 🏖️🌊

• constructive waves

  • Wave height-small
  • Wavelength-long
  • Wave period-long (16-8 per minute)
  • Wave steepness-gentle, tend to spill
  • Wave energy-low
• Constructive waves


  1. Push sediment forward

  2. Build sandy beaches

  3. Sometimes backed by sand dunes
• Destructive waves

  • Wave height-big
  • Wavelength-short
  • Wave period-short (10-14 per minute)
  • Wave steepness-steep, tend to plunge
  • Wave energy- high
• Destructive waves


  1. Strong backwash

  2. Move material down beach

  3. Create a steep upper beach (storm beach)
• Tides


  Result from the gravitational attraction on water of the moon and sun
• Moon


  Has twice the impact of the sun on tides
• Tidal frequency


  • Most coastlines have semi-diurnal tides (two high and two low tides every 24 hours)
• Spring tides


  • Occur when the sun and moon align, giving a stronger gravitational pull and higher than average tides
• Neap tides


  • Occur when the sun and moon are at right angles with the earth, causing lower than average tides
• Tidal range


  • The height difference between high water and low water during the monthly tidal cycle
  • Micro tidal - less than 2m
  • Macro tidal - in excess of 6m
• Tidal currents

  1. Flood the intertidal zone causing entrainment and deposition of material

  2. Ebb tide carries material in the reverse direction
• Shore normal currents

  Occur when waves approach the shore with their crests parallel to the coastline
• Coastal sediment budget

  The balance between the system and volume of sediment entering or leaving
• Positive budget


  • More inputs than outputs
• Negative budget


  • More outputs than inputs
• Balanced budget


  • Volume of sediment in = volume of sediment out
• Sediment cell


  A length of coastline where the movement of sediment is self contained
• Equilibrium


  The stability in a system
• Dynamic equilibrium


  • Some change in a system but in a more gradual and long term way
• Steady-state equilibrium


  • A system is broadly unchanging and its relationship between inputs and outputs are similar, may be seasonal variation but on average the system is stable
• Meta-state equilibrium


  • A place switches between types of equilibrium, usually triggered by an event such as a storm or human activity
• Positive feedback


  Amplifies initial change in the system, makes it worse
• Negative feedback


  Dampens the effect of change
• Waves


  • Most affecting coastal zones are wind-generated
• Fetch


  The distance over which wind blows to generate waves
• Orientation of the coast

  Greatest energy occurs when strong winds blow in the same direction as the waves over a long period of time
• Wave approach


  Waves are modified as they approach land due to the decreasing water depth
• Wave refraction


  Causes the wave energy from the breaking waves to vary along the coastline
• Wave reflection


  Along rocky coastlines with deep water offshore, the waves are reflected back from the cliffs
• Lithology of the coastline

  • The make up of the rock, including hardness, chemical composition, and permeability
• Igneous and metamorphic rocks

  • Harder and more resistant to erosion
• Sedimentary rocks


  • Often made up of softer, unconsolidated materials like sand, clay, and gravel, more easily eroded
• Chemical composition


  • Some rocks are chemically inert, others prone to chemical weathering
• Permeability


  • If there are pores, fissures, cracks or joints in the rock, surface water will seep through which will increase resistance to subaerial processes
• Rock structure


  • The way rocks are geologically arranged, including joints, faults, and folds
• Physical/mechanical weathering


  • Rock breaks apart and falls to the foot of the cliff, can protect the cliff from erosion
• Freeze-thaw


  • Water enters cracks and joints, repeating freezing and thawing exerts pressure, causing pieces to break off
• Crystal growth


  • Seawater connects in cracks, when the water evaporates the salt crystals are left which exerts pressure on the rock
• Wetting and drying


  • Expansion and contraction of minerals, most effective on clay and in macro-tidal environments
• Mass movements


  • The downslope of material under the influence of gravity, type depends on geology