Where waves get their energy

Waves are made when wind blows across the sea surface, transferring energy to the water. A wave has two parts as it breaks:

• Swash — water rushing up the beach after the wave breaks.
• Backwash — water draining back down the beach under gravity.

The energy of a wave depends on the fetch — the distance of open water the wind has blown over. A longer fetch (plus stronger, longer-lasting wind) makes bigger, more powerful waves.

Key term — fetch: the uninterrupted distance of open sea over which the wind blows. The Atlantic-facing coasts of the British Isles have a huge fetch, so their waves are powerful.

Constructive vs destructive waves

Two kinds of wave shape the coast in opposite ways:

Misconception: the constructive wave is the one that builds beaches (its swash is stronger than its backwash). Don't assume big, dramatic waves build up the coast — the tall destructive ones tear it down.

Which wave builds a beach?

Breaking down the cliff

Weathering is the breakdown of rock in place (not by moving water). At the coast you meet all three types:

• Mechanical (physical) — e.g. freeze–thaw: water in cracks freezes, expands and prises the rock apart. Salt crystallisation also widens cracks.
• Chemical — e.g. carbonation: weak acid in rainwater and seawater slowly dissolves limestone.
• Biological — plant roots and burrowing animals loosen the rock.

Weathering weakens the cliff, and then mass movement shifts loose material downslope under gravity:

• Sliding — a block of rock or soil slips down a slope.
• Slumping — saturated material rotates and slides on a curved surface (common in clay cliffs after heavy rain).

Four ways the sea erodes

Moving waves erode the coast by four marine processes:

• Hydraulic action — the sheer force of water (and trapped, compressed air) pounding into cracks blasts the rock apart.
• Abrasion (corrasion) — waves fling pebbles and sand against the cliff, sandpapering it away.
• Attrition — rock fragments carried by the sea knock against each other, becoming smaller and rounder over time.
• Solution (corrosion) — slightly acidic seawater dissolves soluble rock such as limestone.

Watch out: attrition wears down the sediment itself (making rounder pebbles); abrasion uses that sediment to wear down the cliff. Don't mix them up.

Match the process to its meaning

Tap a process on the left, then tap its correct description on the right.

Which process?

Longshore drift moves the sediment

The sea moves eroded material along the coast. The key process is longshore drift:

Misconception: longshore drift moves sediment along the shoreline, not straight out to sea. It's the sideways, zig-zag transport of material — not the loss of a beach offshore.

Your turn — longshore drift

Headlands and bays

Where bands of hard and soft rock meet the coast, the sea erodes them at different rates (differential erosion):

The headland now sticks out into the sea, so waves refract onto it and concentrate their erosive energy there — carving the sequence of landforms on the next screen.

Cave → arch → stack → stump

On a headland, erosion (mainly hydraulic action and abrasion) attacks lines of weakness, forming a famous sequence:

Getting the order right

Cliffs and wave-cut platforms

Where the sea attacks a cliff, it erodes a wave-cut notch at the base (between high and low tide). As the notch deepens, the rock above is left unsupported and collapses, so the cliff retreats inland.

Where the sea drops its load

When waves lose energy — in sheltered bays, or where the coast changes direction — they can no longer carry sediment, so they deposit it. This builds:

• Beaches — sand and shingle deposited by constructive waves, usually in bays.
• Spits — a long ridge of sand/shingle that grows out from the coast where the shoreline bends (see next screen).
• Bars — where a spit grows right across a bay, joining two headlands and trapping a lagoon behind it.

How a spit forms

Your turn — spit formation

Erosion or deposition landform?

Tap a landform, then tap the box it belongs in.

What controls how fast a coast changes

The rate of coastal erosion and retreat is not the same everywhere. Edexcel expects you to know four influences:

• Geology — resistant hard rock (e.g. granite) erodes slowly; soft rock and clay (e.g. Holderness) erode fast. Rock structure and lines of weakness also matter.
• Vegetation — plants (e.g. marram grass, mangroves) bind sediment with their roots, stabilising it and slowing erosion.
• People — building on cliffs, removing beach material, or protecting one stretch can starve another of sediment and speed its erosion.
• Sea-level change — rising sea level (with climate change) increases the reach of waves and the risk of coastal flooding.

Ecosystems of the coastline

Coasts are home to important, and often fragile, ecosystems:

• Coral reefs — high biodiversity but very sensitive; threatened by warming seas (bleaching), pollution and tourism.
• Mangroves and salt marshes — thrive in salty, tidal water; absorb wave energy and protect the coast, but are cleared for development.
• Sand dunes — built and stabilised by marram grass; act as a natural sea defence.

Hard vs soft engineering

Coasts are managed to reduce erosion and flooding. Two broad approaches:

• Hard engineering — sea walls reflect waves; groynes trap sediment moved by longshore drift and widen the beach; rock armour (rip-rap) and gabions absorb wave energy.
• Soft engineering — beach nourishment adds sand to build up the beach; managed retreat deliberately lets low-value land flood or erode.

Misconception: managed retreat is a deliberate decision to stop defending a stretch of coast and let it flood/erode naturally — it is a chosen strategy, not a failure or an accident.

Hard or soft engineering?

Tap the correct type of engineering for each defence.

Choosing a defence

Case study: the Holderness Coast, UK

The Holderness Coast in East Yorkshire is one of the fastest-eroding coastlines in Europe, retreating on average about 1.5–2 metres every year.

• Why so fast? The cliffs are made of soft boulder clay (glacial till), which slumps easily when saturated. Powerful destructive waves have a long North Sea fetch.
• Impacts: around 29 villages have been lost to the sea since Roman times; farmland, homes and roads continue to be lost.
• Management: the town of Mappleton is protected by two rock groynes and rock armour (built 1991), which trap sediment and shield the cliff.
• Conflict: the groynes trap sediment, so the coast down-drift (e.g. Great Cowden) is starved of beach material and erodes even faster — a source of conflict between residents. Elsewhere, low-value land is left to erode (managed retreat).

Exam tip: a good case-study answer names the place, gives facts/figures (e.g. ~2 m/yr, 29 lost villages, Mappleton groynes 1991) and explains a conflict between different users of the coast.

Understanding the case study

The key ideas to know

You've covered the whole Edexcel International GCSE Coastal Environments unit. Press Finish to see your score.