Coastal Landscapes: How the Sea Shapes the Land

Coasts as systems

A coast is the strip where land meets sea. Geographers see it as an open system: inputs (energy from waves, wind, tides; sediment from rivers and cliffs), stores (beaches, dunes, sea bed), flows (sand moving along the shore) and outputs (sand lost to deep water).

A sediment cell is a length of coast where sand mostly stays inside, between two big headlands. The sediment budget is sand in minus sand out. If more leaves than arrives, the beach shrinks.

Feedback: if a beach loses sand, waves break closer to the cliff, the cliff erodes faster and gives more sand, which can rebuild the beach (negative feedback, keeps balance).

High-energy coasts face big waves and long fetch; erosion wins, cliffs and platforms form. Low-energy coasts are sheltered; deposition wins, beaches, mudflats and marshes form.

Waves, tides, currents and wind

Waves form when wind drags the sea surface. Their size depends on wind speed, how long it blows and the fetch (distance of open water). Near the shore the wave base drags on the sea bed, the top topples and breaks. The water rushing up the beach is the swash; the water going back is the backwash.

• Constructive waves: low, long, 6–8 per minute; swash stronger than backwash; build beaches.
• Destructive waves: high, steep, 10–14 per minute; backwash stronger; remove sand, often in storms.

Tides are the rise and fall of the sea, about twice a day, caused mainly by the Moon's pull. The tidal range decides how wide a strip waves can attack. Currents (like rip currents and longshore currents) and wind also move sand.

Weathering and mass movement

Sub-aerial processes act on the land above the sea.

• Mechanical weathering: freeze-thaw (water in cracks freezes, expands about 9% and splits rock); salt weathering (salt crystals grow in cracks).
• Chemical weathering: rainwater (weak acid) dissolves chalk and limestone.
• Biological weathering: plant roots and burrowing animals open cracks.

Mass movement is material moving downhill due to gravity: rockfall (blocks fall from steep cliffs), landslide (a block slides down a flat surface), slumping (saturated clay slides along a curved surface), and slow soil creep.

Erosion, transport and deposition

Erosion

• Hydraulic action: waves force air and water into cracks; pressure breaks rock.
• Abrasion: waves throw sand and pebbles at the cliff like sandpaper.
• Attrition: rocks knock each other and become smaller and rounder.
• Solution: seawater dissolves some rocks such as chalk.

Transport

Traction (rolling big stones), saltation (bouncing), suspension (carried in the water), solution (dissolved). Longshore drift: waves reach the beach at an angle set by the wind; swash carries sand up at that angle, backwash pulls it straight down; sand zig-zags along the coast.

Deposition

When waves lose energy (in bays, behind spits, where water is calm) they drop sediment.

Erosional landforms

• Headlands and bays: where hard and soft rock bands meet the sea, soft rock erodes into bays, hard rock sticks out as headlands.
• Wave-cut notch and platform: waves cut a notch at the cliff foot; the overhang collapses; the cliff retreats and leaves a flat, rocky platform seen at low tide.
• Cave → arch → stack → stump: a crack in a headland widens into a cave; caves on both sides meet to form an arch; the arch roof falls leaving a stack; the stack erodes to a stump.

Depositional landforms

• Beach: sand (in bays) or shingle (pebbles) laid down by constructive waves.
• Spit: longshore drift carries sand past a bend in the coast; it builds out into the sea; wind changes curve the end (hooked spit); calm water behind becomes saltmarsh.
• Bar: a spit grows across a bay and joins two headlands, trapping a lagoon (Chilika in India is a lagoon behind a bar).
• Tombolo: a bar that joins an island to the mainland.
• Sand dunes: dry sand blown inland is trapped by plants like marram grass; dunes grow from embryo to yellow to grey dunes.
• Estuaries, mudflats and saltmarsh: in a river mouth the sea and river meet, water slows and fine mud settles; salt-tolerant plants grow and trap more mud.

Sea level change and climate change

• Eustatic change: the whole ocean's volume changes (ice ages lock water in ice, sea falls; melting ice and warming water raise it).
• Isostatic change: the land moves; after heavy ice melts, land slowly rises back up.
• Tectonic change: earthquakes raise or drop the coast suddenly.

Emergent coasts (land rose or sea fell): raised beaches and old cliffs stranded above today's sea. Submergent coasts (sea rose): drowned river valleys called rias, drowned glacial valleys called fjords.

Climate change: sea level is rising (about 3–4 mm a year now), storms may get stronger, and low coasts, deltas and islands (for example the Sundarbans and the Maldives) face flooding and salty water.

Coastal management

Hard engineering

• Sea wall: concrete wall that reflects waves; strong but very costly.
• Groynes: wooden or rock fences across the beach that trap longshore drift; can starve beaches further along.
• Rock armour (rip-rap): big boulders that absorb wave energy.
• Gabions: wire cages of rocks at the cliff foot.

Soft engineering

• Beach nourishment: adding sand to widen the beach.
• Dune regeneration: fencing and planting dunes.
• Mangrove planting: roots hold mud and reduce waves.

Managed retreat

Letting low, cheap land flood on purpose so that saltmarsh forms and absorbs waves; people and roads are moved inland with help.

Plans

A shoreline management plan decides for each stretch: hold the line, advance the line, managed retreat or no active intervention, based on cost and value. Integrated coastal zone management (ICZM) plans a whole sediment cell together, including people, wildlife, tourism and jobs, so that protecting one place does not harm another.

Example

On a soft clay coast losing about 1–2 m a year, towns are protected with sea walls and groynes, but this can starve beaches down-drift and speed erosion of farmland there, which shows why whole-cell planning matters.

Skills and fieldwork on coasts

• Measure beach profiles with ranging poles and a clinometer; compare both sides of a groyne.
• Measure pebble size and roundness down the beach to test for attrition and longshore drift.
• Count waves per minute to tell constructive from destructive.
• Use old maps and photos to work out cliff retreat rates; draw field sketches.
• Case studies: one local coast (processes and how it is managed) and one contrasting coast elsewhere (for example a delta coast like the Sundarbans facing cyclones and sea level rise, and how people respond).

Try it at home

Fill a tray with damp sand at one end, add water at the other and make waves with a ruler held at an angle. Watch sand drift sideways (longshore drift). Push a small stick into the sand as a groyne and see sand pile up on one side. Then try the plans in the last 3D step.

Key formulas and definitions

• Wave size depends on wind speed, wind duration and fetch
• Constructive: 6–8 waves/min, swash > backwash
• Destructive: 10–14 waves/min, backwash > swash
• Erosion: hydraulic action, abrasion, attrition, solution
• Sediment budget = sediment in − sediment out
• Retreat rate = distance lost ÷ years

Worked examples

Common mistakes

• Mixing up abrasion and attrition. Abrasion is rocks wearing the cliff; attrition is rocks wearing each other.
• Saying the backwash in longshore drift also goes at an angle. It goes straight down the beach because of gravity.
• Writing the sequence as cave, stack, arch. The correct order is crack, cave, arch, stack, stump.
• Thinking hard engineering is always best. It is costly and can increase erosion further along the coast.