Waves
- Waves are made by the wind blowing over the sea. The fetch is the distance of open water the wind blows over: a longer fetch makes bigger waves.
- Constructive waves are low and gentle, with a strong swash and weak backwash, so they build up beaches.
- Destructive waves are high and frequent, with a weak swash and strong backwash, so they erode beaches.
Erosion and landforms
- The sea erodes by hydraulic action, abrasion, attrition and solution. Weathering and mass movement (such as rockfalls and slumping) also shape cliffs.
- Headlands and bays form where bands of hard and soft rock meet the coast: the soft rock is eroded into bays, leaving hard-rock headlands.
- On a headland, a crack is eroded into a cave, the cave becomes an arch, the arch collapses to leave a stack, and the stack is eroded into a stump.
- As cliffs retreat, a flat wave-cut platform is left at their base.
Longshore drift and deposition
- Longshore drift: waves approach the beach at an angle, following the prevailing wind. The swash carries sediment up the beach at that angle, and the backwash carries it straight down, so it zigzags along the coast.
- Deposition forms beaches, spits (long ridges of sand or shingle stretching out from the coast), bars and tombolos.
Managing the coast
- Hard engineering: sea walls, groynes (which trap sediment moved by longshore drift), rock armour and gabions. They are effective but expensive, and can cause erosion further along the coast.
- Soft engineering: beach nourishment (adding sand), dune regeneration and managed retreat (letting low-value land flood).
- Guernsey has high cliffs on its south coast and lower bays on the west coast, where sea walls help protect the land from storms.
Key terms
- Fetch
- The distance of open water the wind blows over.
- Swash
- Water moving up a beach after a wave breaks.
- Backwash
- Water flowing back down a beach.
- Constructive wave
- A gentle wave that builds up beaches.
- Destructive wave
- A powerful wave that erodes beaches.
- Headland
- Land that sticks out into the sea.
- Bay
- A curved inlet of the sea between headlands.
- Stack
- A pillar of rock separated from a headland.
- Longshore drift
- The zigzag movement of sediment along a coast.
- Spit
- A long ridge of sand or shingle stretching out from the coast.
- Groyne
- A wooden or rock barrier that traps sediment on a beach.
- Managed retreat
- Letting the sea flood low-value land in a controlled way.