Coastal Physical Processes

How coastal processes work together

A coastline changes through processes acting both on land and in the sea. Weathering breaks down rock in place. Mass movement transfers material downhill under gravity. Waves then erode, transport and deposit material, changing the shape of the coast.

These processes are connected rather than separate. Weathering can weaken a cliff; heavy rain can make its material unstable; a section can move downhill; and waves can remove the fallen debris. Coastal retreat is the resulting movement of the cliff edge or coastline inland.

Weathering weakens the cliff

Mechanical weathering physically breaks rock into smaller pieces without changing its chemical composition. Freeze–thaw weathering is one example. Water enters cracks and joints in the rock. When temperatures fall below freezing, the water freezes and expands, putting pressure on the sides of the cracks. It thaws when temperatures rise. Repeated freezing and thawing widens the cracks until pieces of rock break away. This requires both water and temperatures moving above and below freezing, so lower winter temperatures can increase its importance on UK coasts. Unlike erosion, weathering does not itself carry the broken material away.

Chemical weathering changes rock through chemical reactions. Rainwater absorbs carbon dioxide from the atmosphere and becomes slightly acidic. It can then react with minerals in rock. Rock type matters: limestone is more vulnerable to this chemical weathering than granite. Warm, wet conditions generally encourage faster chemical weathering.

Biological weathering is caused by living organisms. A plant can take root in an existing crack. As its roots grow thicker, they push against the sides, widening the crack until pieces of rock break apart. Burrowing animals can also disturb the ground and contribute to its breakdown.

Weathering therefore prepares material for removal. A weakened cliff is more vulnerable to wave erosion and collapse, even though the weathering itself may occur above the reach of the sea. Processes acting on the exposed land rather than through direct sea action are called sub-aerial processes.

Sliding and slumping move material downhill

Mass movement occurs when gravity moves material down a slope. The steepness of the slope, rock type and structure, vegetation and water content all influence its stability.

In sliding, a body of material moves downhill along a relatively flat surface. Much of the material can remain together during its movement.

In slumping, the material moves along a curved surface and rotates as it descends. Slumping is common in weaker material such as clay. Heavy or prolonged rainfall can saturate the slope and make it heavier, increasing the likelihood of movement. The movement leaves a curved indentation in the slope.

Rainfall and waves can work together. Rain makes cliff material less stable, while waves can undercut the cliff base and leave the material above unsupported. A collapse is therefore not necessarily the result of just one process.

Constructive and destructive waves have different effects

Waves form as wind blows over the sea. Stronger winds, winds blowing for longer and a longer fetch — the distance the wind blows across water — can produce larger, more powerful waves. As waves enter shallow water, interaction with the seabed slows them and they eventually break. Water running up the beach is swash; water returning down it under gravity is backwash.

Constructive waves have a stronger swash than backwash. They carry sediment up the beach and leave more there than they remove, building the beach through deposition. They are generally low waves with long wavelengths and a low frequency.

Destructive waves have a stronger backwash than swash. Their backwash removes beach sediment, so they tend to erode the beach. They are generally higher, steeper waves with short wavelengths and a high frequency, and are common during stormy conditions.

Changes in wave conditions can therefore change a beach's shape and sediment store. A substantial beach can absorb wave energy and help protect the cliff behind it; removing beach sediment can leave the cliff more exposed to wave attack.

Four ways coastal material is eroded

Coastal erosion wears away and removes material. Destructive waves are responsible for much coastal erosion, but the individual processes act in different ways:

• Hydraulic action: waves strike the coast and force water and air into cracks. Air is compressed, exerting pressure on the rock. Repeated wave impacts help widen cracks and break pieces away.
• Abrasion: sand, pebbles and other material carried by waves scrape or strike the cliff and seabed, wearing them away. The sediment acts like sandpaper.
• Attrition: transported rocks and pebbles collide with one another. Pieces chip off, making the fragments smaller, smoother and rounder. This breaks down the sediment rather than directly wearing away the cliff.
• Solution: water dissolves soluble minerals in coastal rocks, including limestone and chalk. This is chemical removal rather than scraping or impact.

Chemical weathering and solution can involve similar reactions. The distinction here is their role: weathering breaks down rock in place, while coastal solution removes dissolved material into the water.

Sediment is transported in several ways

Sediment in coastal water can come from eroding cliffs, rivers delivering material to the sea, movement along the coastline, or material brought towards the shore from offshore. Its movement is called transport.

Traction moves large, heavy particles by rolling or dragging them along the seabed. Saltation moves smaller particles in short hops or bounces along the bed. Suspension carries fine particles within the water, rather than along the bed. Solution carries dissolved material in the water; unlike suspended sediment, it is not present as separate solid particles.

These describe how material is carried. Longshore drift describes its overall movement along the coast. When waves approach a beach at an angle, their swash carries sediment diagonally up the beach. Gravity pulls the backwash down the slope, approximately at right angles to the shoreline. Repetition produces a zig-zag path and a net movement along the beach.

The direction of approaching waves, influenced by wind direction, determines the direction of drift. If the waves approach from the opposite angle, the direction of sediment movement can change.

Deposition stores material along the coast

Deposition occurs when water slows down and no longer has sufficient energy to carry its sediment. Material is then laid down. Sheltered areas have lower wave energy, making deposition more likely. Beaches contain deposited sediment, while sheltered estuaries can accumulate the finer material that forms mudflats and saltmarshes.

The important connection is that material removed from one place can become a deposit elsewhere. Longshore drift transfers sediment; deposition occurs when the moving water can no longer carry it. Erosion and deposition are therefore parts of the same coastal system, rather than unrelated events.

Rock type and structure guide coastal change

Rock type influences resistance to erosion. Hard, resistant rocks such as granite generally erode more slowly than weak material such as clay or sand. Resistant rocks can support steep cliffs and projecting headlands, while weaker material is more readily eroded into bays or affected by slumping. Resistance depends on the process: limestone can resist physical erosion yet still be vulnerable to chemical weathering and solution.

Geological structure means how rocks are arranged and fractured. On a discordant coastline, rock bands run at right angles to the shoreline, exposing different rocks to the sea. Softer bands retreat faster, forming bays, while harder bands remain projecting as headlands. At Swanage Bay in Dorset, the bay lies in softer clay and sand, while nearby Ballard Point is formed from more resistant chalk.

On a concordant coastline, rock bands run parallel to the shoreline. An outer band of resistant rock can protect weaker rock behind it. If the sea breaks through that outer band, it can erode the weaker rock more rapidly and widen an inlet behind the opening. Lulworth Cove in Dorset illustrates this relationship. A concordant coast is not automatically resistant: what matters is which rock faces the sea and whether it remains intact.

Joints are cracks without significant displacement; faults are fractures along which rock has moved. Both provide weaknesses that water can enter and waves can exploit. Hydraulic action and abrasion can enlarge these weaknesses into caves, even in otherwise resistant headlands. Thus, rock type affects the general rate of erosion, while structure helps determine where erosion is concentrated.

Seasonal weather changes the rate of retreat

Weather describes short-term atmospheric conditions; climate describes their longer-term pattern. The UK has a temperate maritime climate, with generally mild conditions and rainfall throughout the year. Seasonal changes in temperature, rainfall and wind affect how quickly coastal processes operate.

Lower winter temperatures may increase freeze–thaw weathering where temperatures cross freezing. Higher winter rainfall can saturate cliffs and increase mass movement. Stronger winter winds supply more energy to waves, increasing erosion. These effects combine to weaken slopes and make retreat more likely, rather than producing a constant rate of change throughout the year.

During calmer conditions, often in summer, constructive waves can rebuild beaches through deposition. Rain also supports chemical weathering, whose rate depends partly on temperature and rock type. Seasonal contrasts are tendencies, not a rule that every winter day is erosive or every summer day is calm.

Storms can cause rapid coastal change

Storms bring strong winds and heavy rainfall. Strong winds increase wave power and therefore erosion. Heavy rainfall saturates cliff material and increases the risk of mass movement. Both the cliff face and the slope above it can be affected during the same event.

More frequent storms give the coast repeated opportunities to erode. Stronger storms can cause greater changes during individual events. Coastal retreat can therefore be episodic: considerable change may occur during a short stormy period rather than gradually at an even rate. Storms can also remove beach and dune sediment, changing the landscape without a cliff collapse.

Prevailing winds connect climate and coastal processes

The UK's prevailing winds come from the south-west. They commonly bring warm, moist air and frequent rainfall. This moisture supports weathering and increases the risk of mass movement. Wind also generates waves, so a coast's exposure to approaching winds and the available fetch affect wave energy.

To assess the effect of weather and climate, connect atmospheric conditions to a physical process and then to landscape change. For example, repeated heavy rainfall can saturate weak cliff material, trigger slumping and move the cliff edge inland. However, weather is not the only control: slope angle, rock type, rock structure, beach protection and exposure to waves affect how readily that cliff fails. The same storm can therefore cause very different amounts of retreat along different stretches of coast.