Coasts
Wave processes, coastal landforms of erosion and deposition, coral reefs and mangroves, and how coasts are managed.
📌 Key points
- Wave size depends on wind speed, how long the wind blows and the fetch (the distance of open water the wind blows over).
- 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 strong backwash, so they erode beaches.
- Coastal erosion processes are hydraulic action, abrasion, attrition and solution, the same as in rivers.
- Where hard and soft rocks alternate along the coast, the soft rock erodes faster to form bays and the hard rock is left as headlands.
- Waves erode a wave-cut notch at the base of a cliff; the cliff above collapses and retreats, leaving a gently sloping wave-cut platform.
- On headlands, erosion of weaknesses forms a cave, then an arch, then a stack when the arch collapses, then a stump.
- Longshore drift moves sediment along the coast: swash carries material up the beach at an angle (the direction of the prevailing wind), and backwash carries it straight back down under gravity.
- Depositional landforms include beaches, spits (ridges of sand or shingle extending from the coast, often with a curved end and a salt marsh behind), bars (a spit across a bay) and tombolos (linking an island to the mainland).
- Coral reefs grow in warm (above about 18 °C), shallow, clear, salty water where sunlight reaches the algae living in the coral; mangroves are salt-tolerant trees growing on muddy tropical coasts between high and low tide.
- Coasts can be protected by hard engineering (sea walls, groynes, rock armour, gabions) or soft engineering (beach nourishment, dune regeneration, managed retreat).
📖 Key terms
- Fetch
- The distance of open water over which the wind blows to form waves.
- Swash
- The movement of water up the beach after a wave breaks.
- Backwash
- The movement of water back down the beach towards the sea.
- Longshore drift
- The movement of sediment along a coast by waves approaching at an angle.
- Wave-cut platform
- A gently sloping rocky surface left at the base of a retreating cliff.
- Spit
- A long, narrow ridge of sand or shingle joined to the land at one end.
- Groyne
- A wooden or rock barrier built at right angles to the beach to trap sediment.
- Coral reef
- A ridge built by tiny marine animals (polyps) in warm, shallow, clear tropical seas.
✏️ Worked example
Explain how a stack is formed on a headland.
- Waves attack a weakness (such as a crack or joint) in the headland by hydraulic action and abrasion.
- The crack widens into a cave.
- Erosion continues until the cave breaks through the headland, forming an arch.
- The base of the arch is eroded and weathering weakens the roof until it collapses.
- Answer: the isolated pillar of rock left standing in the sea is a stack (which later erodes into a stump).
In longshore drift answers, always say that the swash moves up the beach at an angle (direction of the prevailing wind) and the backwash returns straight down (at right angles) because of gravity.
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