Coastal Erosion Processes

Erosion is the wearing away and removal of material from the coastline by natural processes. At A-Level, you must understand not only the marine processes that erode the coast directly, but also the sub-aerial processes (weathering and mass movement) that weaken rock and contribute material to the coastal system. The interplay between marine and sub-aerial processes determines the rate and pattern of coastal retreat.

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Marine Erosion Processes

Marine erosion occurs where waves attack the coastline. There are four principal mechanisms:

1. Hydraulic Action (Wave Quarrying)

Hydraulic action is the sheer force of water striking rock surfaces. When waves crash against a cliff face, water is driven into cracks, joints and bedding planes under enormous pressure. As the wave retreats, the compressed air within these cracks expands explosively, progressively widening the fissures and eventually causing blocks of rock to fracture and detach.

• The pressure exerted by storm waves can exceed 30 tonnes per m² (measured by Thomas Stevenson in the 1840s using a wave dynamometer at Skerryvore lighthouse, Scotland)
• Modern measurements by Wolters and Müller (2004) at the Elbe estuary recorded pressures of up to 100 kPa during storm events
• Hydraulic action is most effective in well-jointed rocks such as limestone, chalk and sandstone
• It is the dominant erosion process on exposed, high-energy coastlines

Key Definition: Hydraulic action is the erosive force of water and compressed air being driven into cracks and joints in rock by wave impact, causing mechanical fracturing and removal of material.

2. Abrasion (Corrasion)

Abrasion is the sandpapering effect of sediment carried by waves being hurled against rock surfaces. Pebbles, sand and gravel act as tools (the load) that scour, scrape and chip away at the cliff face and wave-cut platform.

• The effectiveness of abrasion depends on: the hardness of the load, the energy of the waves, and the resistance of the rock being eroded
• Abrasion is responsible for the smoothing and polishing of wave-cut platforms — studies by Trenhaile (1987) showed that platform surfaces are typically lowered by 1-3 mm per year through abrasion
• It is often the dominant erosion process where there is an abundant supply of sediment (e.g., beaches backed by cliffs)
• Robinson (1977) used micro-erosion meters on the Yorkshire coast and recorded abrasion rates of up to 3.2 mm/year on limestone platforms

3. Attrition

Attrition is the process by which the sediment itself is worn down. As rocks and pebbles are transported by waves, they collide with each other and with the sea bed, becoming progressively smaller, smoother and more rounded.

• Attrition does not directly erode the coastline, but it reduces the size of the erosive tools available for abrasion
• It also produces the progression from angular boulders at the base of cliffs to rounded pebbles and eventually fine sand further along the beach
• Wentworth's (1922) grain size classification remains the standard scheme for describing sediment:

Category	Grain Size
Boulder	> 256 mm
Cobble	64-256 mm
Pebble	4-64 mm
Granule	2-4 mm
Sand	0.0625-2 mm
Silt	0.004-0.0625 mm
Clay	< 0.004 mm

4. Solution (Corrosion)

Solution is the chemical dissolution of rock by seawater. It is most significant where the rock contains calcium carbonate (CaCO₃), which dissolves in the mildly acidic conditions created when CO₂ dissolves in seawater.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

• Solution is the dominant erosion process on chalk and limestone coastlines — for example, at the White Cliffs of Dover and along the Jurassic Coast in Dorset
• The rate of solution increases with: lower water temperature (cold water holds more dissolved CO₂), higher wave energy (greater surface area of rock exposed), and higher biological activity (organisms produce CO₂)
• Trudgill (1985) measured limestone dissolution rates of 0.5-1.0 mm/year on exposed coastal surfaces in County Clare, Ireland
• Solution can create distinctive landforms such as limestone pavements, karren (solution channels) and enlarged joints

Exam Tip: In exam answers, always use the correct terminology — "hydraulic action" not "hydraulic power," "abrasion" not "erosion by waves." Examiners reward precise use of geographical vocabulary. Also, remember to name specific researchers and data to support your points.

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Sub-aerial Processes

Sub-aerial processes operate on the land surface and cliff face, weakening rock and preparing it for removal by marine processes. They include weathering and mass movement.

Weathering

Weathering is the in situ breakdown of rock — material is broken down but not transported. Three types of weathering affect the coast:

Mechanical (Physical) Weathering

Process	Mechanism	Significance at the Coast
Freeze-thaw	Water enters cracks, freezes (expands by ~9%), thaws, refreezes — progressively widening the crack	Important on chalk and limestone cliffs in winter; responsible for scree slopes at the base of cliffs
Salt crystallisation	Seawater enters pores and cracks; evaporation causes salt crystals to grow, exerting pressure on the rock	Very effective in the splash zone; particularly damaging to porous rocks like sandstone
Wetting and drying	Repeated cycles of water absorption and loss cause swelling and contraction, leading to flaking	Important for clay-rich rocks and shales; contributes to cliff instability
Thermal expansion	Repeated heating and cooling of rock surfaces causes stress and eventual fracturing	Most significant in hot climates but also operates on south-facing cliffs in the UK
Pressure release	Removal of overlying rock (through erosion) reduces confining pressure, causing rock to expand and fracture	Important in areas of recent cliff retreat

Chemical Weathering

Process	Mechanism	Rocks Affected
Carbonation	CO₂ dissolves in rainwater forming weak carbonic acid (H₂CO₃), which reacts with calcium carbonate	Limestone, chalk
Oxidation	Oxygen reacts with iron-rich minerals, weakening the rock structure	Rocks containing iron (e.g., some sandstones, basalt)
Hydrolysis	Water reacts with minerals (especially feldspars) to form clay minerals	Granite, gneiss
Hydration	Water molecules are absorbed into the mineral structure, causing expansion	Clays, some igneous rocks

Biological Weathering

• Root action — plant roots grow into cracks and joints, exerting pressure and widening them. Studies by Gabet and Mudd (2010) demonstrated that tree roots can generate pressures of up to 15 MPa
• Burrowing organisms — piddocks (boring bivalves) drill holes in soft rock; sea urchins scrape rock surfaces; worms and crabs burrow into sediment
• Algal and lichen growth — produce organic acids that dissolve rock surfaces through bioerosion
• Guano — bird droppings contain phosphoric acid, which attacks rock surfaces

Key Definition: Weathering is the in situ breakdown of rock by mechanical, chemical or biological processes without the involvement of transport. It differs from erosion, which involves both breakdown and removal of material.

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Mass Movement

Mass movement is the downslope transfer of material under the influence of gravity. It is a critical process in cliff retreat, as it delivers weathered material from the cliff face to the beach, where it is removed by wave action.

Types of Mass Movement