Wave Types and Coastal Processes

A coast is a changing boundary

The coast is where land meets the sea. Waves can wear away rock, move the resulting sediment and deposit it elsewhere. Meanwhile, weathering weakens rock on land and gravity moves unstable material downhill. These processes work together: a weathered cliff can collapse, supplying fragments that waves then transport.

Sediment is loose material such as sand, mud and pebbles. Whether it is removed or deposited depends partly on the energy of the water moving it.

How wind produces waves

Wind blowing over the sea transfers energy to the water and generates waves. Waves generally become larger when the wind is stronger, blows for longer or blows across a greater fetch — a longer distance of open water.

As a wave enters shallow water, interaction with the seabed slows its lower part. The wave steepens and eventually breaks. Water then rushes up the beach as swash and returns down it under gravity as backwash.

The balance between these two movements helps determine whether a wave builds up or removes beach material.

Wavelength is the distance between successive wave crests; frequency is how many waves arrive in a given time. The frequencies above are typical characteristics, not fixed limits for every wave.

Constructive waves carry sediment up the beach, and their weaker backwash leaves more of it behind. Destructive waves have a stronger backwash that carries more material back towards the sea. A beach can therefore change as wave conditions change.

Weathering weakens coastal rock in place

Weathering breaks down rock without transporting it away. At the coast, it can weaken cliffs before waves or gravity remove the loosened material.

Mechanical weathering physically breaks rock into pieces without changing its chemical composition. Freeze–thaw weathering is an example:

1. Water enters cracks and joints in the rock.
2. When temperatures fall below freezing, the water freezes and expands, putting pressure on the sides of the cracks.
3. When temperatures rise, the ice melts and the pressure is released.
4. Repeated freezing and thawing widen the cracks until fragments become detached.

This needs both water and repeated changes above and below freezing; cold conditions alone are not enough.

Chemical weathering changes or dissolves minerals through chemical reactions. Rainwater absorbs carbon dioxide and becomes slightly acidic. It reacts with some minerals in rocks, gradually weakening them. Limestone is more vulnerable to this type of chemical weathering than granite. Rock type therefore affects how quickly a cliff weathers.

Gravity moves unstable material downhill

Mass movement is the downhill movement of rock or soil under gravity. Unlike weathering, it involves movement. Heavy rain can make cliff material saturated and heavier, increasing instability. Weathering also loosens rock, making it more likely to move.

The three important forms are:

• Sliding: a mass of material moves together down a slope along a sliding surface.
• Slumping: a mass moves down along a curved surface and rotates as it moves. This is also called rotational slip. It commonly affects weaker material such as clay when it becomes saturated and heavy, leaving a curved scar in the slope.
• Rock falls: loosened rock fragments fall rapidly from a steep cliff. Freeze–thaw weathering can prepare the fragments by widening cracks and separating them from the cliff face.

Rock type, geological structure, slope steepness and water content influence which movement occurs. These processes can cause cliff retreat even though gravity, rather than the direct force of a wave, causes the movement itself.

Waves erode rock and wear down sediment

Coastal erosion wears away and removes material. Three processes are especially important:

Hydraulic power is the force of waves striking coastal rock. Waves attack cracks and other weaknesses, gradually widening them and loosening material.

Abrasion occurs when sediment carried by waves scrapes or strikes against the cliff. Think of sandpaper rubbing a surface: the moving particles act as tools that wear away the rock.

Attrition occurs when the sediment particles themselves collide. Pieces chip off, so the particles become smaller, smoother and rounder. For example, angular fragments released from a cliff can become rounded pebbles after repeated collisions.

These processes connect: erosion and cliff collapse supply sediment; that sediment can abrade cliffs while also being worn down by attrition.

Longshore drift moves sediment along the coast

Waves often approach a beach at an angle because of the prevailing wind — the wind direction that occurs most often. Their swash carries sediment diagonally up the beach. Gravity then pulls the backwash straight down the beach, at right angles to the shoreline.

Repeating these movements carries a particle along the beach in a zig-zag path. This transport process is longshore drift.

Follow one particle through the diagram: each swash moves it both up the beach and along the coast, whereas the backwash takes it down the slope. After several waves, it has travelled sideways along the shoreline. Longshore drift is transportation; deposition happens only when sediment is left behind.

Why sediment is deposited

Water needs sufficient energy to transport sediment. When its speed and energy decrease, it can no longer carry as much material, so sediment is deposited.

Sheltered coastal areas, such as bays and estuaries, often have calmer water that allows sediment to settle. Constructive waves also encourage deposition because their stronger swash brings sediment up the beach and their weaker backwash removes less of it. Fine mud can settle in particularly low-energy, sheltered water.

Sediment may have come from eroded cliffs, rivers, offshore deposits or another stretch of coast. A depositional area can therefore depend on material supplied from elsewhere.

Rock type and structure shape the coastline

Rock type affects resistance to weathering and erosion. More resistant rocks, such as granite, generally erode more slowly than weak material such as clay. Resistant rocks often support steep, rugged cliffs; weaker material commonly produces gentler slopes with evidence of slumping. These are general tendencies, not rules that determine every cliff's shape.

Geological structure concerns how the rocks are arranged, including their layers, cracks and joints. Waves can exploit weaknesses, so even resistant rock is not equally strong everywhere.

Where different rock types are exposed along a coast, weaker sections retreat faster. Resistant sections are left projecting into the sea as headlands, while more rapidly eroded sections become bays. The arrangement of the rock bands therefore influences the outline of the coast.

A resistant outer band can also protect weaker rock behind it. At Lulworth Cove in Dorset, erosion opened a gap in resistant limestone, allowing waves to reach and erode the weaker clay behind. The resistant limestone helps keep the entrance narrow while erosion has widened the area behind it. This illustrates why both rock resistance and the position of the rock bands matter, rather than rock type alone.

Wave contrasts

• Constructive: low waves, long wavelength, lower frequency; strong swash and weak backwash build up the beach.
• Destructive: steep, higher waves, short wavelength, higher frequency; strong backwash removes beach material.
• Stronger wind, longer duration and greater fetch generally produce larger waves.

Weathering and mass movement

• Freeze–thaw: water enters cracks → freezes and expands → repeated cycles widen cracks → rock fragments detach.
• Chemical weathering: slightly acidic rainwater reacts with minerals; limestone is more vulnerable than granite.