Coasts

• Coastal system


  An open system that receives inputs from outside the system and transfers outputs away from the coast into other systems (terrestrial, atmospheric or oceanic)
• Cycles the coastal system is part of

  • Rock cycle
  • Water cycle
  • Carbon cycle
• Coasts can be considered as a closed system in some circumstances such as during scientific research and coastline management planning
• Sediment cell


  Sections of the coast often bordered by prominent headlands, where the movement of sediment is almost contained and the flows of sediment act in dynamic equilibrium

  
• Dynamic equilibrium


  The maintenance of a balance in a natural system, despite it being in a constant state of change, where inputs and outputs constantly change to maintain the balance
• Dynamic equilibrium may be upset in the long term by human interventions, or in the short term it may be interrupted by natural variations
• Inputs to the coastal system

  • Marine (waves, tides, salt spray)
  • Atmosphere (sun, air pressure, wind speed and direction)
  • Humans (pollution, recreation, settlement, defences)

  
• Outputs from the coastal system

  • Ocean currents
  • Rip tides
  • Sediment transfer
  • Evaporation
• Stores/sinks in the coastal system

  • Beaches
  • Sand dunes
  • Spits
  • Bars and tombolos
  • Headlands and bays
  • Nearshore sediment
  • Cliffs
  • Wave-cut notches
  • Wave-cut platforms
  • Caves
  • Arches
  • Stacks
  • Stumps
  • Salt marshes
  • Tidal flats
  • Offshore bands and bars
• Transfers/flows in the coastal system

  • Wind-blown sand
  • Mass-movement processes
  • Longshore drift
  • Weathering
  • Erosion (hydraulic action, corrosion, attrition, abrasion)
  • Transportation (bedload, in suspension, traction, in solution)
  • Deposition (gravity settling, flocculation)
• Energy sources in the coastal system

  • Wind
  • Gravitational
  • Flowing water
• Negative feedback loop


  A mechanism that balances changes, taking the coastal system back towards dynamic equilibrium
• Positive feedback loop


  A mechanism that exaggerates changes, taking the coastal system more unstable and away from dynamic equilibrium
• Most of the sediment in the coastal zone is a result of an input from rivers, especially in high-rainfall environments where significant river erosion occurs
• Sediment may be deposited in estuaries which are brackish areas where rivers flow into the sea, and they are important wildlife habitats
• Cliff erosion is very important in areas with unconsolidated cliffs that are eroded easily, with some coastlines retreating by up to 10m per year, providing a significant sediment input
• Wind is a coastal energy source and can cause sand to be blown along or up a beach, with sediment transport by winds occurring where there are sand dunes or in glacial and desert environments
• In some coastal systems such as in Antarctica, Greenland, Alaska and Patagonia, glaciers flow directly into the ocean depositing sediment that was stored in the ice when they calve
• Sediment is transferred to the coastal zone when waves, tides and currents erode offshore sediment sinks such as offshore bars, and storm surges or tsunami waves may also transfer sediment into the coastal zone
• Longshore drift


  Sediment is moved along the beach due to prevailing winds which alter the direction of the waves, allowing sediment to be transported from one section of coastline to another

  
• Sediment budgets use data of inputs, outputs, stores and transfers to assess the gains and losses of sediment within a sediment cell
• Littoral zone


  The area of land between the cliff's or dunes on the coast and the offshore area that is beyond the influence of the waves, which is constantly changing due to short-term factors like tides and storm surges and long-term factors like changes in sea level and human intervention

  
• The main energy source at the coast is from waves which are most commonly generated by wind, or less frequently tectonic activity or underwater landslides causing tsunami waves
• Wave formation


  Winds move across the surface of the water, causing frictional drag which creates small ripples and waves, and as the seabed becomes shallower towards the coastline, the orbit of the water particles becomes more elliptical, leading to more horizontal movement of the waves and causing the wave to break and surge up the beach

  
• Factors affecting wave energy

  • Strength of the wind
  • Duration of the wind
  • Size of the fetch
• Wave types


  • Constructive waves (formed by weather systems in the open ocean, have long wavelength, low frequency, strong swash and weak backwash, deposit material)
  • Destructive waves (formed by localised storm events closer to the coast, have short wavelength, high frequency, weak swash and strong backwash, remove depositional landforms)

  
• The type of waves in a coastal environment may vary, with constructive waves dominating in summer and destructive waves dominating in winter, and climate change potentially increasing storm frequency

  
• Constructive waves


  Act to increase the size of beaches
• Destructive waves


  Act to remove depositional landforms through erosion, which work to decrease the size of a beach
• Wave characteristics


  • Constructive: Long wavelength, low frequency, low waves that surge up the beach
  • Destructive: Short wavelength, high frequency, high waves that plunge onto the beach

  
• Swash characteristics


  • Constructive: Strong swash, weak backwash
  • Destructive: Weak swash, strong backwash
• Effect on beach


  • Constructive: Occurs on gently sloped beaches
  • Destructive: Occurs on steeply sloped beaches
• In summer, constructive waves dominate


  But destructive waves dominate in winter
• Constructive waves may become destructive waves


  If a storm begins
• Climate change


  May increase the storm frequency within the UK
• Coastal management


  May affect the type of waves that occur

  
• Constructive waves cause deposition on the beach


  Which in turn leads to the beach profile becoming steeper
• Steeper beaches


  Favour the formation of destructive waves
• Destructive waves erode the beach


  Reducing the beach profile and leading to the formation of constructive waves
• This should lead to a state of dynamic equilibrium though in reality this may not occur due to external factors such as the wind strength and direction