Marine Erosion (Edexcel A Level Geography)

• The main source of energy at the coast is the waves 
• • Waves are mainly generated by wind 
  • The friction between wind and the water surface which transfers energy from the wind to the water
• Wind occurs because of the rotation of the earth and differences in atmospheric pressure when different parcels of air are heated differently by the sun
• Various features of the wind will affect how much energy a wave has: fetch, duration and strength of the wind.  

Wave characteristics

• Wave height is the distance from the peak to the trough
• Wave length is the distance from crest to crest
• Wave frequency is the number of waves which pass a specific point over a given time

Types of wave

• There are two main types of wave, constructive and destructive

Constructive and destructive waves

Impact on beach morphology

• Beaches are formed from loose material 
• Waves alter the morphology (form/shape) of the beach
• The morphology is also affected by the size and shape of the beach material
• The type of wave in an area can vary depending on the time of year or coastal management
  • Beach profiles are steeper in summer when there are fewer destructive waves and gentler in winter
  • Storm berms may develop in winter when large destructive waves carry larger sediment further up the beach than normal
  • In winter beaches may also have a greater variation in pebble size as larger pebbles are deposited by the destructive waves and smaller pebbles are removed
  • Offshore ridges/bars are formed due to material being eroded from the beach by destructive waves and deposited offshore

Types of Wave and Their Characteristics

Wave Type	Formation	Wave Form	Break Characteristics	Beach
Constructive	Distant weather systems, calm local weather, short fetch	Long wavelength (up to 100m), low, spilling waves, low energy, low wave frequency	Strong swash, weak backwash	Build up the beach with a gentle beach profile
Destructive	Local storms, strong winds, high fetch	Short wavelength (Approx. 20m), high, plunging waves, high energy, large wave height	Weak swash, strong backwash	Beach is limited. Steeper beach profile is likely in the short term

• Erosion leads to the formation of a range of landforms at the coast
• There are four types of erosion
  • Abrasion (corrasion) - sediment and stones are picked up by the waves and wear away at the cliff/headland
  • Hydraulic action - this is the shear force of the waves forcing air at high pressure into cracks in the cliff over time this weakens the rock and causes the joint to widen
  • Corrosion - weak acids in seawater dissolve the rock particles
  • Attrition - as rocks are moved around by the water they knock into each other gradually becoming smaller and rounder

Factors affecting erosion

• The amount of erosion is influenced by a range of factors
  • Wave type
  • Wave size
  • Lithology
• Wave type and size affects the seasonal changes in the amount of erosion 
  • Erosion tends to happen more during the winter due to a greater number of destructive waves
  • The size and type of waves affects the amount of hydraulic action and abrasion
• Lithology influences the rate of erosion
  • Weaknesses in rocks erode more quickly
  • More resistant rocks erode more slowly

Wave cut platforms

• At high-energy coastlines, hydraulic action and abrasion can cause the formation of wave-cut platforms 
  • Powerful destructive waves attack the base of the cliff at high tide 
  • The hydraulic action and abrasion create a wave-cut notch which over time increases in size 
  • This is called undercutting 
  • Eventually, the overhang created by the undercutting collapses due to weathering and gravity 
  • The cliff retreats, leaving a wave cut platform that is exposed at low tide

A wave cut platform in Portugal

Diagram to show the formation of a wave cut platform 

• At a headland, where the rock is hard, erosion is slow and can lead to a variety of landforms
• Wave refraction concentrates wave energy onto the headland and can contribute to the formation of caves, arches, stack and stumps
  • Joints in the headland are susceptible to erosion by hydraulic action
  • Over time the joints widen forming a cave that is enlarged by hydraulic action and abrasion
  • Eventually, erosion cuts through the headland forming an arch
  • The roof of the cave will eventually collapse due to gravity and the lack of support
  • This leaves a stack that will over time be eroded by weathering, abrasion, and hydraulic action to form a stump

 

Erosional features at Durdle Door on the Dorset Coast

Erosional landforms, cave, arch and stack