Marine and Sub-aerial Coastal Processes

How the sea and land work together

A cliff can be attacked from two directions. Waves erode its foot, while rainfall, temperature changes and living organisms weaken the exposed rock above. Gravity then moves unstable material downhill. Together, these processes change the coastal landscape and cause coastal retreat: the cliff edge or coastline moves inland.

The distinction between the processes matters. Erosion wears away and removes material. Weathering breaks rock down in place. Mass movement carries rock or soil downhill under gravity. Weathering can prepare material for removal, but it does not itself transport it away.

Wind supplies the energy for destructive waves

Wind blowing across the sea transfers energy to the water, generating waves. Wave size depends on three connected factors: wind strength, how long the wind blows and the fetch, which is the distance over open water that the wind travels. Strong winds blowing for a long time over a long fetch can produce large, powerful waves.

When a wave breaks, water moves up the beach as swash, then returns towards the sea as backwash. Destructive waves are relatively high and steep, with short wavelengths and a high frequency. Their backwash is stronger than their swash, so they tend to drag beach material away.

This affects cliffs as well as beaches. A beach can absorb wave energy and provide protection at the cliff foot. Removing beach sediment can leave the cliff more exposed to subsequent wave attack.

How waves wear away coastal rock

Destructive waves erode cliffs through several processes:

• Hydraulic action: the force of water striking the cliff and exerting pressure in cracks helps loosen and break rock.
• Abrasion: waves carry sand, shingle and larger fragments against the cliff, scraping and battering its surface. These fragments act like tools used by the waves.
• Solution, also called corrosion: soluble minerals in coastal rock are dissolved and removed. The importance of this process depends on the rock’s composition.

Attrition happens when fragments carried by waves collide with each other, becoming smaller and rounder. Unlike abrasion, it breaks down the transported fragments rather than directly wearing away the cliff.

Repeated wave attack can undercut a cliff, removing support from the rock above. Eventually, unstable material may fall or slide. Waves can then remove the fallen debris, exposing the cliff foot to renewed erosion.

Why UK climate makes erosion seasonal

The UK’s prevailing winds blow from the south-west. Winds crossing the Atlantic can bring moist air and storms. Where a coast faces incoming waves across a long fetch, it can receive considerable wave energy; a sheltered coast may experience much less.

Stronger winds and stormier conditions are generally more common in autumn and winter. More frequent storms mean repeated periods of powerful destructive waves, increasing opportunities for erosion. A storm surge raises sea level temporarily; if it coincides with a high tide, waves can reach higher parts of the shore and cliff.

Winter rainfall also affects the land. Heavy or prolonged rain can saturate cliff material, making it heavier and less stable. Where temperatures repeatedly rise above and fall below freezing, water in cracks can cause freeze–thaw weathering. Cold conditions alone are not enough: both water and repeated freezing and thawing are needed.

Warmer conditions can speed up chemical weathering when moisture is available. Summer is therefore not a period when all coastal change stops, although destructive wave activity is generally less intense than during stormier months. Seasonal patterns describe a tendency, not a fixed timetable for every cliff collapse.

Weathering weakens the exposed cliff

Sub-aerial weathering acts on the land rather than directly through waves. Three types help prepare coastal rock for erosion or collapse.

Mechanical weathering physically breaks rock without changing its chemical composition. In freeze–thaw weathering, water enters cracks, freezes and expands, putting pressure on the surrounding rock. Repeated cycles widen cracks until pieces break away. Salt weathering can also occur: salty water enters cracks, evaporates and leaves crystals whose growth puts pressure on the rock.

Chemical weathering changes or dissolves minerals. Rainwater absorbs carbon dioxide and becomes weakly acidic. It can dissolve limestone and chalk, gradually weakening the rock. Its effectiveness depends on rock composition as well as temperature and moisture.

Biological weathering involves living organisms. Plant roots grow into cracks and force them wider. Burrowing animals can loosen soil and destabilise material on a cliff slope.

Gravity moves weakened material downhill

Mass movement changes cliff profiles and can move the cliff edge inland suddenly. The type of movement depends on factors including slope steepness, rock structure and water content.

• Rockfall: loosened fragments detach from a steep cliff and fall rapidly. Weathering can weaken them, while wave undercutting can remove their support.
• Landslide: a mass of material slides downhill along a slip surface, often remaining together during the movement.
• Rotational slumping: a section of cliff moves down along a curved slip surface. It is common where weak material such as clay becomes saturated and heavy, leaving a curved scar and rotated blocks.

Other movements include slow soil creep and flows of saturated soil and debris. Although gravity drives these movements, rainfall and marine erosion can help trigger them.

Why retreat happens in bursts

Consider a clay cliff during a wet, stormy winter. Rain saturates the clay, while destructive waves erode the cliff foot. With heavier material above and less support below, a section may slump. Waves then remove the debris, allowing the cycle to begin again.

This interaction explains why retreat is often uneven: a cliff may change little for a time, then lose a substantial section during one event. It also explains why climate alone cannot determine the rate. Resistant rock, fewer weaknesses, shelter from waves or a protective beach can slow erosion. Less resistant material, strong wave exposure and repeated saturation can speed it up. Coastal retreat results from geology interacting with marine and sub-aerial processes, rather than any one factor acting alone.