Geo 2026: Coastal processes

Waves are made by wind blowing over the sea. Their size depends on wind strength, how long it blows and the fetch (the distance of open water it blows over).
Destructive waves: tall and frequent, with a strong backwash; they erode beaches and cliffs.
Constructive waves: low and less frequent, with a strong swash; they deposit material and build beaches.

Erosion: hydraulic action (waves force air into cracks), abrasion (waves throw rocks at the cliff), attrition (rocks knock together, becoming smaller and rounder) and solution (seawater dissolves rocks such as chalk and limestone).
Transport: traction (rolling large rocks), saltation (bouncing), suspension (carried in the water) and solution (dissolved).
Longshore drift: waves approach at an angle set by the prevailing wind; the swash carries sediment up the beach at that angle and the backwash drags it straight back down, so sediment zigzags along the coast.
Deposition: when waves lose energy, for example in sheltered bays.

Weathering breaks rock down in place: mechanical (freeze-thaw: water freezes in cracks, expands and splits the rock), chemical (rainwater, which is slightly acidic, dissolves limestone and chalk), biological (plant roots and burrowing animals).
Mass movement: rock and soil move downhill under gravity. Sliding: material moves down in one piece along a slip surface. Slumping: saturated soft rock (such as clay) slips along a curved surface, rotating as it moves.
Other influences: geology (hard rock erodes slowly, soft rock quickly), weather (storms bring big waves; rain saturates cliffs), people (sea walls, building, removing sand) and sea-level change.

The hardest process here is longshore drift, because it is a sequence.
1. The prevailing wind (the wind that blows most often) drives waves towards the beach at an angle.
2. The swash carries sand and pebbles up the beach at that same angle.
3. The backwash runs straight back down the beach at right angles to the coast, because gravity pulls it down the steepest slope.
4. This repeats with every wave, so the sediment moves along the beach in a zigzag.
Why does this matter? Over time, longshore drift can move huge amounts of sediment along the coast. If people build structures that trap it, beaches further along can become starved of sand and erode faster.
Example: drop a coloured pebble at one end of a beach and you would expect it to move gradually along the beach in the direction of the prevailing wind.

Saying the backwash goes back at an angle: it goes straight down the beach under gravity.
Forgetting the prevailing wind: it sets the angle and direction of longshore drift.
Mixing up transport types: traction rolls, saltation bounces, suspension carries, solution dissolves.
Confusing sliding and slumping: sliding is along a straight slip surface; slumping is a rotational movement on a curved surface, often in saturated clay.
Treating weathering as a sea process: weathering and mass movement act on the cliff from above, on land.

Your class TIF slide used pictures to hint at factors that affect the rate of erosion:
Storm waves: storms bring big, destructive waves with a strong backwash, so erosion is fastest during storms.
Seasons: winter brings more storms, and cold nights cause freeze-thaw weathering that weakens cliff tops.
Fetch and prevailing wind: a coast facing a long stretch of open sea (a long fetch) gets bigger, more powerful waves than a sheltered coast. In the UK the prevailing wind is from the south-west, so coasts facing the Atlantic receive large waves.
Geology: soft rock such as clay and sand erodes faster than hard chalk or limestone (the Swanage map; see the erosional landforms lesson).
Sea-level change: a higher sea level lets waves reach higher up the cliff and attack it more often.
People: removing beach sand or building defences can increase erosion further along the coast.