Coastal Processes

The coast is one of the most dynamic environments in the UK. It is constantly changing as waves, tides, and currents interact with the rocks and sediments of the shoreline. This lesson examines the key coastal processes — wave action, erosion, transport, and deposition — that shape our coastline.

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Waves: The Engine of Coastal Change

Waves are the primary force shaping the coastline. They are created by wind blowing across the surface of the sea, transferring energy to the water.

How Waves Form

1. Wind blows across the ocean surface, creating friction with the water.
2. Energy is transferred to the water, causing it to move in a circular orbit.
3. As waves approach the shore, the water becomes shallower. The circular motion is distorted: the base of the wave slows down due to friction with the seabed, while the top continues forward.
4. Eventually, the wave breaks — the top collapses forward and water rushes up the beach as swash.
5. Water then flows back down the beach under gravity as backwash.

Factors Affecting Wave Size and Energy

Factor	Description	Effect on Waves
Fetch	The distance of open water over which the wind blows	Longer fetch = larger, more powerful waves. The Atlantic fetch to the south-west coast can exceed 5,000 km
Wind speed	How fast the wind is blowing	Stronger wind = more energy transferred = larger waves
Wind duration	How long the wind has been blowing	Longer duration = more energy transferred = larger waves
Water depth	Depth of water near the coast	Shallow water causes waves to break earlier; deep water allows waves to travel further before breaking

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Constructive vs Destructive Waves

This is one of the most fundamental concepts in coastal geography.

Feature	Constructive Waves	Destructive Waves
Wave height	Low (typically under 1 m)	High (typically over 1 m)
Wave frequency	Low — 6 to 8 waves per minute	High — 10 to 14 waves per minute
Swash	Strong — pushes material up the beach	Weak — limited material pushed up
Backwash	Weak — little material pulled back	Strong — drags material back down the beach
Dominant process	Deposition — builds up the beach	Erosion — removes material from the beach
Wave shape	Low, long wavelength; spilling breaker	Tall, short wavelength; plunging breaker
Associated weather	Calm, settled conditions	Stormy conditions with strong onshore winds
Beach profile effect	Creates a wide, gently sloping beach	Creates a narrow, steep beach (or removes material entirely)

graph LR
    subgraph Constructive
        A["Long, low wave"] --> B["Strong swash<br/>pushes material UP"]
        B --> C["Weak backwash"]
        C --> D["Net effect: DEPOSITION<br/>Beach builds up"]
    end
    subgraph Destructive
        E["Tall, steep wave"] --> F["Weak swash"]
        F --> G["Strong backwash<br/>pulls material DOWN"]
        G --> H["Net effect: EROSION<br/>Beach eroded"]
    end

Exam Tip: The key distinction is which is stronger — the swash or the backwash. Constructive waves: strong swash, weak backwash (net deposition). Destructive waves: weak swash, strong backwash (net erosion). Examiners frequently test this.

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Coastal Erosion Processes

Four main processes erode the coastline:

1. Hydraulic Action

When a wave crashes against a cliff face, it traps and compresses air in cracks and joints in the rock. The enormous pressure of the compressed air forces the cracks apart. When the wave retreats, the air expands rapidly, creating an explosive effect. Over time, this widens cracks, loosens blocks, and can enlarge caves.

Hydraulic action is most powerful during storms, when wave heights can exceed 10 m and the force against the cliff can reach 30 tonnes per m².

2. Abrasion (Corrasion)

Waves pick up rocks, pebbles, and sand from the seabed and hurl them against the cliff face. This acts like sandpaper, scraping and wearing away the rock surface. Abrasion is particularly effective at the base of cliffs, where it creates wave-cut notches.

The effectiveness of abrasion depends on:

• The size and quantity of sediment available
• The power of the waves
• The hardness of the cliff rock

3. Attrition

Rock fragments carried by waves collide with each other and with the cliff face. These collisions chip off sharp edges, making particles progressively smaller, smoother, and more rounded. This is why pebbles on a beach are typically smooth and rounded — they have been worn down by attrition over many years.

4. Solution (Corrosion)

Seawater is slightly alkaline, and certain rock types (especially chalk and limestone, which contain calcium carbonate) dissolve slowly in it. This chemical process weakens the rock and creates distinctive features such as pitted rock surfaces, enlarged joints, and notches.

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Coastal Transport

Once material has been eroded or weathered from the coast, it must be transported. The main transport process at the coast is longshore drift.

Longshore Drift

Longshore drift is the movement of sediment along the coast by wave action.

How it works:

1. Waves approach the shore at an angle, determined by the prevailing wind direction (in the UK, this is typically from the south-west).
2. The swash carries sediment up the beach at this angle.
3. The backwash carries sediment straight back down the beach under gravity, at right angles to the shoreline.
4. This creates a zig-zag movement of sediment along the coast.
5. Over time, large quantities of material are transported laterally.