Coastal Landforms (OCR GCSE Geography)

Wave action

• Waves are marine processes that erode, transport and deposit material
• Waves are formed as winds blow over the surface of the sea 
• The height and strength of a wave is dependent on 3 factors:
  • The fetch 
  • The amount of time the wind blows
  • The strength of the wind
• The greater the strength, time and fetch of the wind, the larger the wave
• As a wave enters the shallow water of the coast, friction from the seabed causes the wave to lean forward. It will eventually crest and break onto the beach 
• The movement of water up the beach is called the swash, and the return movement is the backwash

Types of waves

• There are two types of waves:
  • Destructive waves erode the beach
    • They have a short wavelength, high-frequency rate and a steep wave gradient
    • Their backwash is stronger than their swash, which scours the beach, dragging material out to sea 
  • Constructive waves are beach builders
    • They have a long wavelength, low-frequency rate and a shallow wave gradient
    • The swash is stronger than its backwash, which carries material up onto the beach and deposits it there  

Comparison of Wave Type

	Constructive	Destructive
Swash	Strong	Weak
Backwash	Weak	Strong
Wavelength	Long with low height	Short with high height
Frequency	Low (6-8 per minute)	High (10-12 per minute)
Type of beach	Sandy - depositional	Shingle - erosional

Diagram of Wave Types

Constructive waves are beach builders and destructive waves are the destroyers

• Destructive waves are responsible for the majority of erosion that happens along a coast
• They cut into the coastline in four ways:
  • Hydraulic Action 
  • Attrition
  • Corrosion 
  • Abrasion 
• The effects of attrition are enhanced when the waves move sediment further and longer
  • A large, rough bolder is eventually eroded into round sand grains (quartz) the longer it stays in the water and the further it travels along the coast
• Rounded pebbles on a beach are known as a shingle

Headlands and bays

• Occur where there are alternating bands of hard and soft rocks run perpendicular to oncoming waves (discordant coastline)
• At first, the soft rock (e.g. clay) is eroded backward, forming an inlet
• As the inlet continues to erode it curves inwards, and a bay is formed, usually with a beach
• The hard rock (e.g. limestone) is left protruding out to sea as a headland

• A headland usually features:
  • Cliffs along its sides
  • Projects out to sea
  • Usually longer than it is wide
  • Geology is of resistant rock
• A bay usually has:
  • A wide, open entrance from the sea
  • A roughly, semi-circular shape extending into the coastline
  • Land that is lower than the headlands surrounding it
  • A bay may or may not have a beach

Diagram of Headland and Bay Formation

The formation of headlands and bays on discordant coastlines

Cliffs and wave-cut platforms

• Cliffs are shaped through erosion and weathering processes
• Soft rock erodes quickly and will form sloping cliff faces
• Steep cliffs are formed where there is hard rock facing the sea
• A wave-cut platform is a wide gently sloped surface found at the foot of a cliff:
  • As the sea attacks the base of a cliff between the high and low water mark, a wave-cut notch is formed
  • Abrasion, corrosion and hydraulic action further extend the notch back into the cliff
  • The undercutting of the cliff leads to instability and collapse of the cliff
  • The backwash of the waves, carries away the eroded material, leaving behind a wave-cut platform
  • The process repeats and the cliff continues to retreat, leading to a coastal retreat

Diagram showing the Formation of a Cliff and Wave-cut Platform

The process of cliff retreat and wave-cut platform formation

Caves, arches, and stacks 

• These form in a headland because of wave action and sub-aerial weathering
• As waves approach the shore, their speed is reduced as they move along the sea floor
• This changes the angle of the waves, and they will turn so the crest becomes parallel to the coast - known as wave refraction
• This refraction concentrates the erosive action on all sides of the headland
• The erosional processes of hydraulic power, abrasion and some corrosion begin to attack any weaknesses in the headland
• As the crack begins to widen, abrasion will begin to wear away at the forming cave
• The cave will become larger and eventually breaks through the headland to form an arch
• The base of the arch continually becomes wider and thinner through erosion below and weathering from above
• Eventually, the roof of the arch collapses, leaving behind an isolated column of rock called a stack
• The stack is undercut at the base by wave action and sub-aerial weathering above, until it collapses to form a stump

Diagram of Cave, Arch, Stack and Stump Formation

The formation of a cave, arch, stack and stump

Beaches

• Beach formation usually occurs in the summer months when the weather is calmer
• Form in sheltered areas such as bays through deposition via constructive wave movement, where the swash is stronger than the backwash
• Blown sand can create sand dunes at the backshore of a beach
• When a constructive wave carries sediment up the beach, the largest material is deposited along the upper reach of the swash
• As the backwash moves back down the beach, it loses water and therefore energy as it travels due to the porosity of the sand
• Consequently, the deposition of sediment gets progressively smaller, and the beach is therefore, sorted by wave deposition, with the smallest mud particles settling in the low-energy environment offshore
• If a destructive wave forms due to a storm, then large shingle is thrown above the usual high tide level to form a ridge at the top of the beach called a berm

Diagram showing Names of Deposition Types on a Beach

Sediment deposition

Spits

• An extended stretch of sand or shingle that extends out to sea from the shore
• Spits occur when there is a change in the shape of the coastline
• Or the mouth of a river, which prevents a spit from forming across the estuary
• A spit may or may not have a 'hooked' end, depending on opposing winds and currents
• A good example is Spurn Point, which stretches for three and a half miles across the Humber Estuary in the northeast of England

Stages of formation

• Sediment is transported by the action of longshore drift
• Where the coastline changes direction, a shallow, sheltered area allows for the deposition of sediment
• Due to increased friction, more deposition occurs 
• Eventually, a spit slowly builds up to sea level and extends in length
• If the wind changes direction, then the wave pattern alters and results in a hooked end
• The area behind the spit becomes sheltered
• Silts are deposited here to form salt marshes or mud flats

Diagram Explaining the Formation of a Spit

Spit formation

Bars

• When a spit grows across a bay, and joins two headlands together
• A bar of sand is formed (sandbar)
• Sandbars can also form offshore due to the action of breaking waves from a beach

Image Showing How Longshore Drift Contributes to Spit and Bar Formation

Image showing how longshore drift contributes to spit and bar formation