Coasts
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Wave types
Waves are created by wind blowing across the surface of the sea. There are two main types.
The balance between constructive and destructive waves determines whether a beach grows or shrinks.
Wave types — Key Knowledge
- Constructive waves low, long waves; strong swash, weak backwash; build up beaches by depositing material
- Destructive waves tall, steep waves; weak swash, strong backwash; erode beaches by removing material
Coastal erosion processes
The sea erodes the coastline through four main processes.
Same four processes as river erosion — but driven by wave energy rather than river flow.
Coastal erosion processes — Key Knowledge
- Hydraulic action force of waves compresses air into cracks, breaking rock apart
- Abrasion/corrasion waves hurl rocks and pebbles at the cliff face, wearing it away
- Attrition rocks smash together in the waves, becoming smaller and rounder
- Solution/corrosion slightly acidic seawater dissolves soluble rock like limestone and chalk
Coastal transport — longshore drift
The main process by which material is moved along a coastline.
Longshore drift explains why beaches build up on one side of a groyne and why spits form.
Coastal transport — longshore drift — Key Knowledge
- Waves approach beach at an angle determined by prevailing wind direction
- Swash carries material up the beach at an angle, Backwash pulls material straight back down under gravity, Net movement of material along the coast in a zigzag pattern
Erosional landforms — cliffs and wave-cut platforms
Cliffs retreat inland as the sea erodes their base.
Wave-cut platforms prove that the coastline has retreated — the platform shows where the cliff used to be.
Erosional landforms — cliffs and wave-cut platforms — Key Knowledge
- Wave-cut notch indent at the base of a cliff caused by erosion at high tide
- Cliff collapses as the notch deepens, the rock above becomes unsupported and falls
- Wave-cut platform flat rock shelf left behind as the cliff retreats — visible at low tide
- Process repeats cliff continues to retreat, platform extends
Erosional landforms — caves, arches, stacks and stumps
A sequence of landforms that develop from the erosion of a headland.
This is a sequence — each stage leads to the next. The classic example is Old Harry Rocks, Dorset.
Erosional landforms — caves, arches, stacks and stumps — Key Knowledge
- Crack weak point in the rock is attacked by hydraulic action and abrasion
- Cave crack is widened and deepened into a cave
- Arch cave erodes through the headland — opening on both sides
- Stack roof of the arch collapses, leaving an isolated pillar of rock
- Stump stack is eroded and collapses to a low stump, visible at low tide
Headlands and bays
Form where bands of hard and soft rock meet the coast.
Over time, headlands are eroded back and bays fill with sediment — the coastline straightens.
Headlands and bays — Key Knowledge
- Soft rock erodes faster forming a bay — sheltered, curved inlet
- Hard rock erodes slower left jutting out as a headland
- Wave refraction waves bend around headlands, concentrating energy on them
- Bays are sheltered deposition occurs, forming beaches
Depositional landforms — beaches
Beaches form where sand and pebbles are deposited by constructive waves.
Beaches act as a natural defence — they absorb wave energy and protect the land behind.
Depositional landforms — beaches — Key Knowledge
- Sandy beaches gently sloping, fine material
- Pebble/shingle beaches steeper, coarser material
- Material comes from cliff erosion, longshore drift, river sediment
Depositional landforms — spits
A spit is a long, narrow ridge of sand or shingle extending from the coast into the sea.
Spurn Point (Humber estuary) is a well-known UK example.
Depositional landforms — spits — Key Knowledge
- Forms where coastline changes direction or at a river mouth
- Built by longshore drift material deposited in the direction of drift
- Curved end caused by wave refraction or secondary wind direction
- Salt marsh sheltered water behind the spit — low-energy environment allows mud and vegetation to accumulate
Coral reefs
Underwater structures built by tiny organisms (coral polyps) in warm, shallow seas.
Coral reefs are threatened by rising sea temperatures (bleaching), pollution and ocean acidification.
Coral reefs — Key Knowledge
- Conditions needed warm water 25–29°C, shallow water with sunlight, clear and clean water, salty water
- Importance huge biodiversity, coastal protection from storms, tourism income, fishing grounds
Mangrove swamps
Dense forests growing in coastal tidal zones in tropical regions.
Mangroves are being destroyed for aquaculture, tourism and development — reducing natural coastal protection.
Mangrove swamps — Key Knowledge
- Conditions needed tropical coastlines, sheltered muddy shores, tidal areas
- Importance coastal protection from storms and tsunamis, nursery habitat for fish, carbon storage, prevent coastal erosion
Coastal management
Strategies to protect coastlines from erosion — divided into hard and soft engineering.
Protecting one stretch of coast can starve the next stretch of sediment — known as the "terminal groyne effect".
Coastal management — Key Knowledge
- Hard engineering sea walls — reflect wave energy, expensive; groynes — trap sediment to build up beach; rock armour/rip-rap — large boulders absorb wave energy; gabions — wire cages filled with rocks
- Soft engineering beach nourishment — adding sand to replace eroded material; managed retreat — allowing the sea to flood low-value land; dune regeneration — planting marram grass to stabilise dunes
Case study required
The spec requires a named coastal case study.
The case study should cover opportunities, hazards and management together.
Case study required — Key Knowledge
- Opportunities presented by an area of coastline tourism, fishing, ports, wildlife
- Associated hazards erosion, tropical storms, flooding
- Their management specific strategies used