Coastal Erosion: Processes and Rates

Coastal erosion is the wearing away of land by the action of the sea. At A-Level, AQA requires a detailed understanding of the specific processes involved, the factors that control rates of erosion, and the resulting cliff profiles and landforms.

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Marine Erosion Processes

Four main processes of marine erosion operate at the coast. These often work together, and their relative importance depends on rock type, wave energy, and local conditions.

1. Hydraulic Action

Hydraulic action is the mechanical force of water striking the rock face. It operates in two ways:

• Direct wave impact — waves striking the cliff exert pressures of up to 30 tonnes per m² during severe storms. Even during moderate conditions, wave pressures of 10–15 kN/m² are common
• Compressed air — as waves enter cracks, joints and faults, they trap and compress air. The repeated compression and decompression weakens the rock, gradually widening fractures. Pressures within joints can exceed 100 kN/m²

Hydraulic action is most effective on:

• Heavily jointed or faulted rock
• Rock with bedding planes exposed to wave attack
• Coastlines with high wave energy (large fetch, deep water close to shore)

2. Abrasion (Corrasion)

Abrasion is the wearing away of the cliff face by sediment carried in the waves. It operates like sandpaper — the wave hurls sand, pebbles and boulders against the rock surface, scratching, scouring and chipping it away.

• Most effective when waves carry a heavy sediment load (coarse material is more abrasive)
• Evidence: smooth, polished rock surfaces, scratches and grooves on wave-cut platforms
• Rate depends on: wave energy, sediment availability, rock hardness
• Can erode rock at rates of 1–5 mm per year on harder lithologies

3. Attrition

Attrition is the wearing down of sediment particles as they collide with each other and with the rock surface. It does not directly erode the cliff but reduces the size of beach material:

• Angular rocks become rounded and smaller over time
• Pebbles on beaches show progressive rounding with distance from their source (measured using Cailleux's Roundness Index)
• Produces increasingly fine material: boulders → cobbles → pebbles → sand → silt

4. Corrosion (Solution)

Corrosion is the chemical dissolution of rock by seawater. It is particularly effective on:

• Limestone (calcium carbonate dissolves in slightly acidic seawater): CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
• Chalk — a porous, relatively soft limestone
• The process is enhanced by the slightly acidic nature of seawater (pH approximately 8.1, decreasing with higher CO₂ absorption)
• The rate of corrosion increases with water temperature and acidity
• Creates features such as solution pools, pitted rock surfaces and enlarged joints

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Sub-aerial Processes

Sub-aerial processes operate on the cliff face above the direct reach of waves. They work alongside marine erosion to shape the coastal landscape.

Weathering

Mechanical weathering:

• Freeze-thaw (frost shattering) — water enters cracks, freezes (expanding by 9%), and exerts pressures of up to 2,100 kN/m², shattering the rock. Most effective where temperatures fluctuate around 0°C
• Salt crystallisation — sea spray enters pores and cracks; as water evaporates, salt crystals grow and exert pressure, breaking the rock apart. Common on porous rocks like sandstone
• Wetting and drying — clay minerals expand when wet and contract when dry, causing surface flaking. Important on clay and shale cliffs

Chemical weathering:

• Carbonation — rainwater absorbs CO₂ to form weak carbonic acid, which dissolves limestone
• Oxidation — iron-bearing minerals react with oxygen and water, forming weaker iron oxides. Visible as orange-brown staining on cliff faces
• Hydrolysis — minerals react with water, breaking down feldspars in granite into clay minerals

Biological weathering:

• Plant roots grow into cracks, widening them over time
• Burrowing organisms (piddocks, marine worms) bore into soft rock
• Algae and lichens produce organic acids that dissolve rock surfaces

Mass Movement

Mass movement is the downslope movement of material under the influence of gravity. The type depends on cliff material, water content, angle and structure.

Type	Material	Speed	Water Content	Cliff Type
Rockfall	Hard, jointed rock	Very fast	Low	Steep, fractured cliffs (chalk, limestone)
Toppling	Columnar-jointed rock	Fast	Low	Vertically jointed cliffs
Landslide (translational)	Rock/debris	Moderate to fast	Variable	Inclined bedding planes dipping seaward
Rotational slump	Clay, soft rock	Slow to moderate	High	Clay cliffs (e.g., London Clay)
Mudflow	Saturated clay/silt	Moderate	Very high	Impermeable clay cliffs after rainfall
Soil creep	Soil	Very slow	Moderate	Gentle clay slopes

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Factors Controlling Rates of Cliff Retreat

Cliff retreat rates vary enormously around the UK coastline. The key controlling factors are:

1. Rock Type and Resistance