Threat of Coastal Retreat (Edexcel A Level Geography)

Human Actions & Coastal Recession

• 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 

Sub-aerial Processes

• 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

Changing Rates of Recession

• 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