Coastal Processes and Landforms

Waves are the primary agent of coastal change. They are generated by wind transferring energy to the surface of the sea. The characteristics of waves — size, frequency, and energy — determine whether a coast is being built up or broken down.

How waves break: As a wave approaches shallow water, the base slows due to friction with the seabed while the top continues at speed — the wave "breaks" and surges up the beach (swash). After breaking, water returns downslope under gravity (backwash).

Two wave types:

Feature	Constructive wave	Destructive wave
Origin	Distant storms (long fetch)	Local storms (short fetch)
Wave frequency	Low (6–8 per minute)	High (10–14 per minute)
Height	Low, gently sloping	Steep, tall
Swash	Strong swash, weak backwash	Weak swash, strong backwash
Effect	Deposits sediment; builds beach	Removes sediment; erodes cliff base
Example coasts	West-facing beaches (Atlantic swell)	East-facing coasts (North Sea)

Fetch is the distance over which wind blows across open water before reaching a coast. A longer fetch allows waves to gather more energy. The west coasts of the UK receive Atlantic swells from fetches of thousands of kilometres, producing powerful constructive and destructive waves.

Coastal erosion attacks cliffs and rock platforms through multiple simultaneous processes.

Marine erosion processes:

Process	How it works	Example
Hydraulic action	Waves crash against rock; air is trapped and compressed in cracks; when the wave retreats, the pressure release causes explosive expansion, widening joints and fractures	Most effective in joints and cracks in limestone and chalk
Abrasion (corrasion)	Waves pick up sand, pebbles, and boulders and hurl them against the cliff face, cutting and grinding the rock	The "sandpaper" effect; produces smooth, rounded cliff faces at the high-water mark
Attrition	Rock fragments and pebbles carried by waves collide with each other and the cliff, becoming progressively smaller, rounder, and smoother over time	The process by which angular cliff-fall boulders are reduced to smooth beach pebbles
Solution (corrosion)	Slightly acidic sea water dissolves soluble rock minerals, particularly in limestone and chalk coasts	Visible as pitting and hollowing on chalk cliffs

Weathering weakens cliffs before marine erosion removes them:

• Mechanical weathering — salt weathering: sea spray deposits salt crystals in rock pores; as crystals grow and expand, they widen cracks
• Mechanical weathering — freeze-thaw: water enters cracks, freezes and expands (by ~9%), then thaws; repeated cycles shatter rock
• Chemical weathering — carbonation: slightly acidic rainwater (carbonic acid) reacts with calcium carbonate in chalk and limestone cliffs; the rock dissolves and is removed in solution, weakening the cliff face over time
• Biological weathering: plant roots and burrowing organisms (piddock clams) break up rock (extra context — beyond AQA 8035 spec)

Mass movement transfers weakened material down the cliff face:

• Rock falls: fragments break off steep, bare cliffs (typical of chalk cliffs, e.g. Seven Sisters, East Sussex)
• Sliding: a mass of material moves along a flat (planar) failure surface; occurs when the weight of rock or regolith exceeds the friction holding it in place; common after cliff saturation by rainwater
• Slumping (rotational slides): a curved shear plane allows a mass of material to rotate downward; typical on clay cliffs (e.g. Barton-on-Sea, Hampshire)
• Mudflows: saturated material flows rapidly downslope after heavy rain; associated with soft cliffs (extra context — beyond AQA 8035 spec)

Material removed by erosion is transported along and up the coast before being deposited where energy falls.

Coastal transportation:

• Solution: dissolved minerals carried in seawater (e.g. calcium carbonate from chalk and limestone cliffs)
• Suspension: fine particles (fine sand, silt, clay) carried within the water body
• Saltation: sand-sized particles bounce along the seabed/beach in a leapfrog motion
• Traction: large pebbles and boulders are rolled along the seabed by strong waves

Longshore drift: Where waves approach the shore at an angle (driven by prevailing winds), swash moves sediment diagonally up the beach in the direction the wave is travelling. Backwash moves sediment straight down the beach slope (gravity). The repeated zigzag motion gradually moves sediment along the coast in the direction of the prevailing wind. On the south coast of England, prevailing SW winds drive longshore drift eastward.

Deposition occurs when wave energy falls below the level needed to transport sediment:

• In sheltered bays, where wave energy is low
• In the lee of headlands, where waves refract and lose energy
• Where a river mouth or bay interrupts the longshore drift pathway

The geology of a coastline determines its resistance to erosion and the landforms that develop.

Concordant coastlines:

• Rock bands run parallel to the coast — the same rock type faces the sea along the entire length
• The outer rock acts as a hard, resistant barrier; if breached, a sheltered lagoon forms behind the softer inner rock
• Example: Isle of Purbeck (Dorset) — a belt of Portland limestone runs parallel to the coast; a breach at Lulworth Cove has allowed erosion of the softer clays and sands behind

Discordant coastlines:

• Rock bands run perpendicular to the coast — alternating hard and soft rock is exposed to the sea at right angles
• Soft rock erodes faster, forming bays; hard rock resists erosion, forming headlands
• Example: Swanage Bay area (Dorset) — chalk and limestone headlands (Durlston Head, Handfast Point) flank the softer clay and sand of Swanage Bay

Wave refraction: Waves approaching a headland-and-bay coast refract (bend) around headlands. Energy concentrates on headlands (highest erosion) and disperses in bays (deposition). This explains why erosional landforms dominate headlands and depositional landforms dominate beaches in bays.

Erosion over time produces a characteristic sequence of landforms at cliff-fronted coasts.

Cliff and wave-cut platform (development sequence):

1. Waves attack the cliff at sea level, exploiting weaknesses through hydraulic action and abrasion
2. A wave-cut notch forms at the base of the cliff
3. The cliff above is unsupported and collapses (rock fall, slumping)
4. The cliff retreats landward; the collapsed material is removed by waves
5. A gently sloping wave-cut platform (also called a shore platform) is left at the base, covered at high tide, exposed at low tide
6. As the platform extends seaward, waves expend energy crossing it; cliff retreat eventually slows

Headlands: caves → arches → stacks → stumps:

1. Waves attack weaknesses (joints, faults) in a headland
2. Hydraulic action and abrasion cut a cave into the cliff face
3. The cave extends through the headland from both sides until the roof collapses, creating an arch (a natural rock bridge spanning the sea)
4. The arch roof is further weakened by weathering and erosion; it collapses, leaving an isolated pillar of rock — a stack
5. The stack is undercut at sea level; it collapses, leaving a low-lying stump, visible only at low tide

The cross-section below shows the four stages as they appear along a headland, with the most eroded feature furthest out to sea.

Dorset Coast examples:

• Old Harry Rocks (Handfast Point, near Studland): chalk stacks and stumps; Old Harry (the main stack) stands approximately 30 m high; Old Harry's Wife (an adjacent stack) collapsed in 1896 leaving a stump
• Durdle Door (near Lulworth): a natural limestone arch; one of the most photographed coastal landforms in England; the arch is approximately 30 m wide at its base
• Lulworth Cove: a near-circular bay behind a breach in the Portland limestone belt; the cove is approximately 750 m across; rapid erosion of the softer Wealden Clay inside has produced the distinctive rounded shape

Where wave energy falls, sediment accumulates to form depositional landforms.

Beaches: The most common coastal depositional feature. Divided into:

• Sandy beaches (low-energy bays): fine particles settle in calm conditions; typical of sheltered west-coast bays
• Shingle/pebble beaches (higher-energy coasts): only coarser material survives where smaller particles are removed; typical of English Channel and North Sea coasts

Sand dunes: Ridges of sand that form landward of the beach when prevailing onshore winds blow dry sand from the beach and trap it around obstacles (driftwood, vegetation). Dune systems progress inland:

• Embryo dunes (nearest sea): mobile, unstable; pioneer plants like Marram grass colonise (long roots bind sand; tolerates salt spray and burial)
• Yellow dunes: stabilised by vegetation; organic matter improves soils
• Grey dunes: mature; moss and heather indicate improved, less saline soil

Spits: Long, narrow strips of sand or shingle extending out from the coast into the sea, formed where longshore drift continues past a bend in the coastline or into an estuary. The spit grows in the direction of dominant longshore drift. The end often curves (a recurved spit) due to secondary wind or wave directions.

• Chesil Beach (Dorset): a 29 km barrier beach connecting Portland (technically a tied island/tombolo) to the mainland; a spit-like feature driven by complex longshore drift from the SW; separates the Fleet lagoon from the sea; shingle is graded from small pebbles in the west to large cobbles at the Portland end — historically used by fishermen at night to determine location by feel

Bars: A spit that has grown completely across a bay, sealing off a lagoon:

• Slapton Sands (Devon): a shingle bar approximately 5 km long that has sealed off Slapton Ley (a freshwater lagoon); formed by longshore drift across Start Bay

1. Confusing erosion processes

Hydraulic action is caused by wave pressure and compressed air in cracks — not "water hitting the rock." Abrasion involves fragments hitting the rock. Attrition involves fragments hitting each other. In extended answers, name the process, then describe the mechanism precisely.

2. Describing the cave-arch-stack sequence without a headland context

Caves, arches, and stacks form at headlands — not on straight cliff sections. Headlands jut into the sea and are attacked from two sides, which is why caves can form on both sides of the headland and meet in the middle to form an arch. Situate these landforms at a headland.

3. Forgetting to describe longshore drift direction and prevailing wind

Describing longshore drift as "sediment moves along the coast" is incomplete. State the prevailing wind direction, the direction of swash movement, and the direction sediment is transported. On the south coast of England, SW prevailing winds drive eastward longshore drift.

4. Mixing up concordant and discordant coastlines

Concordant: rock runs parallel to coast → uniform resistance → no alternating headlands and bays (until breached). Discordant: rock runs perpendicular → alternating hard/soft → headlands of hard rock, bays in soft rock. Lulworth Cove is a concordant coast where the outer Portland limestone has been breached; Swanage Bay is a discordant coast.

5. Treating Chesil Beach as a simple spit

Chesil Beach is more precisely described as a barrier beach (or tombolo, connecting Portland to the mainland). It is not a typical spit — it does not extend past a bend in the coastline. The AQA spec asks about spits and bars; use Chesil Beach carefully and note its specific formation rather than labelling it a standard spit.