Threat of Coastal Retreat (Edexcel A Level Geography)

• Rapid coastal recession is caused by physical factors (geological and marine) but can also be influenced by human actions such as dredging, dams or coastal management
• These actions affect the processes within the coastal system including those in the sediment cell

Dredging

• The removal of sediment from rivers or the sea
• This impacts on the amount of sediment being deposited at the coast

Dams

• The construction of dams, traps river sediment behind the dam wall 
  • It is estimated 100 billion tonnes of sediment is stored behind the world's dams and this increases by a billion tonnes each year
• Like dredging, this starves the coast of sediment
• It is estimated that dams on the river Ebro in Spain have led to a 93% reduction in sediment downstream
• The reduction of sediment supplied to the coast due to dams and dredging leads to greater coastal erosion because:
  • Beaches decrease in size as they are not being supplied with sediment
  • Destructive waves have more impact by increasing the rate of coastal erosion

Coastal management

• Coastal management can impact on the rate of recession because it affects the rate of erosion
• In some areas coastal defences such as in places like Hornsea sea walls and rip rap reduce the amount of erosion
  • This slows recession of the coast
• In other areas coastal management affects the movement of sediment
  • Groynes at Hornsea on the Holderness coast reduce longshore drift
  • This starves areas down the coast such as Mappleton of sediment also known as terminal groyne syndrome
  • As with dams and dredging the reduced supply of sediment decreases beach size and increases the impact of destructive waves 

• Weathering and mass movement are sub-aerial processes:
• The sub-aerial processes depend on the geology and work together to impact on the rate of erosion

 Holderness coast

• The Holderness coast is predominately boulder clay
• This leads to particular sub-aerial processes:
  • Wetting and drying
    • At high tide the particles expand when covered with water, at low tide they dry out and contract
    • Repeated wetting and drying causes the clay to crumble
  • Freeze-thaw
    • During the winter months, water enters the fractures in the boulder clay
    • At night this water freezes and expands - due to Arctic maritime air masses
    • Repeated expansion and contraction, causes the cliff to be weakened
  • Slumping 
    • The Holderness coast regularly experiences wet weather
    • Weathering leaves cracks in the boulder clay
    • Water enters these cracks, causing the clay to become heavier, and lubricated (slippery)
    • This leads to a large area of land moving downslope in one piece - known as rotational slip
• Along the Holderness coast, weathering and mass movement work together, causing the fastest rate of coastal erosion in Europe
  • The rate of erosion is approximately 1.8 metres a year

• Rates of recession are not constant
• They are influenced by different factors both short and longer term which include:
  • Wind direction/fetch
  • Tides
  • Seasons
  • Weather systems
  • Occurrence of storms

Wind direction and fetch

• Wind direction at the coast varies and may change every day
• The wind can be onshore or offshore
  • Rates of erosion (and so recession) are greater when winds are blowing onshore
• In most areas, wind generally comes from one main direction - this is known as the prevailing wind direction
• When prevailing wind direction is also the direction of the largest fetch this can lead to the build-up of large destructive waves causing rapid erosion

Tides

• Tides are the result of the gravitational pull of the moon and the sun
• The difference between high tide and low tide is the tidal range
• High tide occurs twice a day
  
  • Twice a month the Sun, Moon and Earth are in alignment increasing the gravitational pull; this causes the highest tide known as a spring tide
• Rates of recession are greatest during high tide because this is the time when the water and waves reach the backshore 
• The waves also have more energy when they reach the backshore leading to more erosion

Seasons

• Rates of recession are likely to be greater in winter than in summer because, storm events that cause destructive waves are more common in winter months

Weather systems

• The UK is located between warm tropical air and cold polar air
• It experiences periods of both high pressure (anticyclones) and low pressure (depressions)
  • During anticyclones there are gentle winds and low waves, so rates of recession are low
  • During depressions the winds are much stronger, leading to high waves and greater rates of recession

Storms

• Storms lead to high energy, destructive waves 
• These waves increase the rate of erosion and therefore, coastal recession
  • In Cornwall, over a two-week period, 1,350 cubic metres of cliff face was eroded along a 300-metre stretch of coastline, as a result of the 2013/14 storms