Coastal Landforms (OCR GCSE Geography B)

Coastal Landforms - Erosion

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

Diagram of Wave Types

• 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

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

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

Coastal Landforms - Deposition

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

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

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