The coast as an open system

The coast is an open system: it exchanges both energy and matter (sediment) with its surroundings. We describe it using inputs, stores, transfers and outputs.

• Inputs — energy from waves, wind, tides and currents, plus sediment delivered by rivers, cliff erosion and offshore sources.
• Stores (components) — beaches, dunes, spits and salt marshes that hold sediment.
• Transfers (flows) — processes that move sediment, above all longshore drift, plus erosion, transport and deposition.
• Outputs — sediment and energy lost from the cell, e.g. sediment carried offshore into deep water or deposited beyond the cell.

Systems language: an open system trades energy and matter across its boundary; a closed system would trade energy only. Treating the coast this way lets us track where sediment comes from and where it goes.

Input, store or transfer?

Sediment cells & the sediment budget

A sediment cell (littoral cell) is a stretch of coast, usually bounded by prominent headlands, within which the movement of sediment is largely self-contained. Around the coast of England and Wales there are 11 major sediment cells, each subdivided into smaller sub-cells.

• Sources — where sediment enters: eroding cliffs, rivers, offshore banks.
• Transfers — sediment moved along the coast, mainly by longshore drift.
• Sinks (stores) — where sediment accumulates: beaches, spits, offshore bars.

Your turn — sediment budget

Dynamic equilibrium & negative feedback

Left alone, a coast tends towards dynamic equilibrium — a balance between inputs and outputs where the overall form stays roughly stable while sediment continually cycles through it.

When something disturbs that balance, negative feedback acts to counteract the change and nudge the system back towards equilibrium:

• A storm strips sediment from a beach.
• The lowered, gentler beach dissipates later wave energy, encouraging deposition.
• Sediment is returned and the profile is gradually rebuilt — the change is damped out.

Contrast: positive feedback amplifies a change (e.g. sediment loss steepening a beach so waves erode it faster still). Negative feedback restores balance; positive feedback drives the system further from it.

Which feedback?

Constructive & destructive waves; refraction

Wave energy depends on fetch (the open distance over which wind blows), wind speed and duration. Two contrasting wave types shape the coast:

• Constructive waves — low, long wavelength, low frequency (6–8 per minute); strong swash, weak backwash, so they build beaches.
• Destructive waves — tall, steep, high frequency (10–14 per minute); weak swash, strong backwash, so they erode and comb material offshore.

Wave refraction: as waves approach an irregular coast they slow in shallower water off headlands, bending so that energy is concentrated on headlands (erosion) and dissipated in bays (deposition).

Spot the destructive wave

Weathering at the coast

Weathering is the in-situ breakdown of rock (no transport). It prepares material for erosion and mass movement.

• Mechanical (physical) — e.g. freeze-thaw: water in cracks freezes, expands about 9%, prises the rock apart; also salt crystallisation as spray evaporates.
• Chemical — e.g. carbonation: rainwater plus dissolved CO₂ forms a weak carbonic acid that slowly dissolves limestone and chalk; also oxidation and hydrolysis.
• Biological — plant roots widen joints; boring molluscs and burrowing organisms break down rock; organic acids attack minerals.

Weathering vs erosion: weathering happens where the rock sits; erosion involves the wearing away and removal of material by a moving agent such as waves.

Mass movement

Mass movement is the downslope movement of material under gravity. It is common where cliffs are undercut by waves or lubricated by water.

• Rockfall — fragments loosened by weathering fall from a steep, well-jointed cliff face.
• Rotational slumping — saturated material slips along a curved failure plane, common in clays; produces a stepped, back-tilted cliff.
• Soil creep — the very slow (mm per year) downslope movement of soil particles; the slowest form.

Trigger: heavy rain adds weight and reduces friction along the failure plane, while wave action removes support at the cliff foot — together these make slumping far more likely.

Marine erosion & sediment transport

Marine erosion processes:

• Hydraulic action — the sheer force of water, and air compressed into cracks, shattering the rock.
• Abrasion (corrasion) — waves fling sediment against the cliff, wearing it away like sandpaper.
• Attrition — sediment particles collide and grind, becoming smaller and rounder (it wears the sediment, not the cliff).
• Solution (corrosion) — chemical dissolving of soluble rock such as chalk.

Transport processes: traction (rolling), saltation (bouncing), suspension (carried in the water) and solution (dissolved load). Along the shore, longshore drift moves sediment in a zig-zag: swash carries it up the beach at the angle of the wind, backwash returns it straight down under gravity.

Which kind of process?

Tap a process, then tap the group it belongs to.

Erosional landforms

• Headlands and bays — differential erosion of alternating hard and soft rock (a discordant coast) leaves resistant rock jutting out as headlands and erodes weaker rock into bays.
• Wave-cut platform — waves undercut the cliff at a wave-cut notch; the overhang collapses and the cliff retreats, leaving a gently sloping rocky platform.
• Cave → arch → stack → stump — hydraulic action and abrasion exploit a weakness in a headland to cut a cave, then an arch through it; the arch roof collapses to leave a stack, which is undercut to a stump.

Your turn — cliff retreat rate

Order the sequence

Depositional landforms

• Beaches — the main store; steep shingle berms built by constructive waves, gently sloping sand lower down.
• Spit — a ridge of sand or shingle extending across a bay or estuary where the coast changes direction; the tip is often recurved by wave refraction, and salt marsh develops in the sheltered water behind it.
• Tombolo — a beach or bar of sediment that links an offshore island to the mainland.
• Bar — a spit that grows right across a bay, trapping a lagoon behind it.
• Salt marsh — fine sediment accumulating in sheltered water (behind spits, in estuaries), colonised by salt-tolerant plants.

Sheltered water is key: spits, bars and tombolos create low-energy zones where fine sediment can settle and salt marshes can develop.

Match each landform to its description

Tap a description on the left, then its matching landform on the right.

Eustatic & isostatic change

• Eustatic change is a global change in the volume of water in the oceans — e.g. water locked into ice sheets during a glacial (fall), or meltwater and thermal expansion during warming (rise).
• Isostatic change is a local change in the level of the land — e.g. the crust rebounding upward (isostatic recovery) after the weight of ice is removed.

These produce contrasting coasts:

• Emergent coasts (relative fall): raised beaches and relict cliffs stranded above the modern shore.
• Submergent coasts (relative rise): rias (drowned river valleys) and fjords (drowned glacial troughs, deep and steep-sided).

Eustatic or isostatic?

Coastal management

Human activity (development, dredging, dams starving beaches of sediment) alters the coastal system, and managers must decide how to respond:

• Hard engineering — sea walls, groynes, rock armour, revetments. Effective but expensive, and groynes can starve down-drift coasts of sediment.
• Soft engineering — beach nourishment, dune stabilisation, working with natural processes; cheaper and more sustainable but needs maintenance.
• Managed realignment — deliberately allowing the sea to flood low-value land, recreating salt marsh that absorbs wave energy naturally.

Shoreline Management Plans (SMPs) plan management cell-by-cell using four policies: hold the line, advance the line, managed realignment, and no active intervention.

The big ideas to know

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