Coastal Processes: Waves, Weathering & Transport
Two Kinds of Wave
All coastal change starts with the wave. Waves are formed when wind blows across the sea surface, dragging energy from the air into the water. The distance the wind blows over — the fetch — controls how big the wave gets. Atlantic-facing UK coasts like Cornwall and Pembrokeshire have a fetch of thousands of kilometres, so they receive much bigger waves than sheltered North Sea coasts.
Geographers split waves into two types. Destructive waves are tall, steep and frequent (10–14 per minute). Their backwash — the water rushing back down the beach — is more powerful than their swash, so they drag sand and shingle away from the coast. Constructive waves are lower, longer and less frequent (6–8 per minute). Their swash beats their backwash, so they push material up the beach and build it. Storm conditions favour destructive waves; calm weather favours constructive waves.
Destructive waves are tall, steep and frequent (10–14 per minute) with strong backwash that erodes. Constructive waves are low, long and less frequent (6–8 per minute) with strong swash that builds beaches.
Weathering at the Coast
Before waves can erode a cliff, weathering often weakens the rock first. Weathering happens before any material is moved. Two types matter at the UK coast. Mechanical weathering includes freeze–thaw: water seeps into cracks, freezes overnight, expands by 9% and prises the rock apart. The chalk cliffs of the South Downs lose chunks each winter this way. Chemical weathering includes carbonation: rainwater absorbs CO₂ and forms a weak carbonic acid that slowly dissolves limestone and chalk, widening cracks and joints.
Mass Movement
Once weathered, weakened cliff material falls under gravity. This is mass movement. There are three forms to know:
- Sliding — rock and soil slip down a flat surface in a single block, usually after heavy rain saturates the ground.
- Slumping — saturated soft cliffs (like the boulder clay of Holderness) rotate downwards along a curved surface, leaving a stepped cliff face. Slumping is the dominant process on East Yorkshire's coast.
- Rock falls — chunks of rock break off vertical or near-vertical cliffs and tumble to the beach. Common on the chalk cliffs at Beachy Head.
Wave Erosion
Destructive waves erode the coast in four ways:
Hydraulic power. Waves slam into a cliff and force air into cracks at high pressure. As the wave retreats the trapped air expands explosively, prising the rock apart. This is the dominant process during storms on hard-rock coasts.
Abrasion (sometimes called corrasion). Waves pick up sand, pebbles and rocks and hurl them at the cliff base, sandpapering it away. You can see abrasion's smoothed, polished base on most UK sea cliffs.
Attrition. The pebbles and rocks already in the water knock against each other, slowly becoming smaller, smoother and rounder. Attrition does not erode the cliff — it shapes the sediment that eventually becomes beach material.
Solution (corrosion). Slightly acidic seawater dissolves alkaline rock such as limestone and chalk. Slow but steady — it widens joints and helps caves form.
Transportation: Longshore Drift
Eroded sediment doesn't stay still. The most important transport process at the UK coast is longshore drift. Waves usually approach the beach at an angle (driven by the prevailing wind), so their swash carries sand up the beach diagonally. Gravity then pulls the backwash straight back down the slope. The result is a zigzag movement of sediment along the coast, often hundreds of metres a year.
On the Holderness coast, longshore drift moves sediment southwards from Flamborough Head towards Spurn Head at around 1.5 metres per day. On the south coast of England, sediment moves eastwards along the English Channel. Where this drift is interrupted by a structure such as a groyne or harbour wall, sand piles up on one side and the beach shrinks on the other.
Longshore drift moves sediment along the coast in a zigzag pattern. The swash arrives at an angle (set by the prevailing wind), the backwash returns straight down the slope. Net movement on UK east and south coasts is southwards or eastwards.
Deposition
Waves drop their sediment when they no longer have the energy to carry it. Deposition happens where the water is sheltered (inside a bay, behind a headland, or in a river estuary), where waves are constructive, or where transport is interrupted (against a groyne, for example). The heaviest material is dropped first, so pebbles and shingle settle where wave energy is still high; fine sand is carried further and settles in calmer water.
UK beaches change shape with the seasons. In summer, calm conditions and constructive waves rebuild the beach — sand and shingle are pushed up to form a high berm. In winter, frequent Atlantic storms send destructive waves that drag material back offshore, lowering the beach profile and exposing the cliff base to attack. The same beach can look completely different in July and January. Coastal managers use this to time soft engineering: beach nourishment is usually done in spring, before summer's natural rebuild begins.