Sediment cells and the sediment budget

The coast is an open system: energy (wave, tide, wind) and matter (sediment) both cross its boundaries. Around the coast of England and Wales the system is divided into sediment cells (littoral cells) — stretches within which sediment movement is largely self-contained, bounded by major headlands. In practice cells are only semi-closed: some sediment leaks past headlands and offshore.

• Inputs: rivers (the largest source on most coasts), cliff erosion, offshore/seabed material driven onshore, longshore drift from an adjacent cell, wind-blown sand, and biological material (shell, coral).
• Outputs: longshore drift out of the cell, offshore losses to deep water, wind transport inland into dunes, and human dredging or aggregate extraction.
• Stores: beaches, dunes, spits, bars, offshore bars, salt marshes.
• Littoral zone: backshore (above normal high tide, affected only in storms) → foreshore (the intertidal zone, where most process operates) → nearshore (breaker zone) → offshore (beyond wave influence on the bed).

Why this is the single most useful idea in the topic: it converts "the beach is disappearing" into a testable statement — outputs must exceed inputs. Then you ask which term changed: a dam trapping river sediment, a sea wall stopping cliff erosion, a groyne intercepting drift updrift of you. That is A-level analysis.

The unintended consequence

Rock decides the shape of the coast

Before you talk about waves, talk about rock. Edexcel wants geology treated as a control, at two levels.

• Lithology — what the rock is made of. Resistant (granite, basalt, chalk when massive) versus weak (clay, sands, unconsolidated glacial till). Weak lithology erodes fast and gives low, slumping cliffs; resistant rock gives high, steep cliffs.
• Structure — how the rock is arranged. Joints and faults are lines of weakness that erosion exploits; the dip of the beds controls the cliff profile (seaward-dipping beds produce unstable cliffs that fail along bedding planes; landward-dipping beds give steeper, more stable faces).
• Concordant coast — rock bands run parallel to the coastline, giving a relatively straight coast. Breach the outer band and the sea excavates a cove. A drowned concordant coast produces the offshore-island Dalmatian pattern; a haff coast is the low-lying variety with sand bars and lagoons.
• Discordant coast — bands run at right angles to the coast, so differential erosion produces the alternating headland and bay pattern.
• Energy: high-energy coasts (long fetch, exposed, destructive waves) tend to be erosional and rocky; low-energy coasts (short fetch, sheltered, constructive waves) tend to be depositional coastal plains with estuaries and marsh.

Vegetation is a control too. Marram grass on dunes and halophytes such as Spartina in salt marsh bind and trap sediment, stabilising the store and reducing wave energy. Remove the vegetation and the landform destabilises — which is why dune regeneration counts as coastal management.

Erosion, deposition, or sea-level change?

Tap a landform, then tap the group it belongs to. Photo and OS-map questions live on exactly this three-way split.

Waves, refraction and longshore drift

Wave energy depends on wind strength, duration and fetch (the open water distance the wind blows over). Two wave types do very different work:

• Constructive — low, long wavelength, low frequency (roughly 6–8 per minute). Swash > backwash, so they push sediment up the beach, building berms and a gentle profile.
• Destructive — high, short wavelength, high frequency (roughly 10–14 per minute), plunging. Backwash > swash, so they comb sediment offshore, steepening the upper beach and building an offshore bar.

Wave refraction: as waves approach an irregular coast they drag in the shallow water off a headland first, so they slow and bend. Energy is concentrated on the headland (high-energy, erosional) and dissipated in the bays (low-energy, so deposition builds a beach). This is why headlands wear back and bays fill in — the coast tends towards a straighter line over time.

Erosion processes: hydraulic action (air compressed into cracks by breaking waves; the pressure shatters rock — includes cavitation), abrasion/corrasion (sediment hurled at the cliff — usually the most effective), attrition (clasts collide, becoming smaller and rounder — this does not erode the cliff, it modifies the sediment), and corrosion/solution (chemical, dissolving carbonate rock).

Why are the bays sandy?

The cliff is attacked from above as well as below

Sub-aerial processes operate on the cliff face and are often the reason a cliff actually fails — the sea then simply removes the debris. This is the distinction between marine and sub-aerial that so many candidates blur.

• Weathering — mechanical: freeze–thaw in joints; salt crystallisation as spray evaporates and crystals grow in pores; wetting and drying (especially in clays, which swell and shrink).
• Weathering — chemical: carbonation (weak carbonic acid dissolving limestone and chalk), oxidation, hydrolysis.
• Weathering — biological: root wedging, boring molluscs, birds nesting in cliff faces.
• Mass movement: rockfall (rapid, on steep, well-jointed faces), slumping (rotational failure along a curved slip plane — the classic in saturated clay, producing a terraced, hummocky cliff), landslide (translational, along a planar bedding surface), and soil creep (imperceptibly slow).

Water is the trigger. Heavy rainfall raises pore-water pressure in permeable material sitting on impermeable clay. That reduces shear strength, and the cliff fails. This is why most large slumps happen after a wet winter, not during a storm surge — and why cliff drainage counts as coastal management.

The cliff profile therefore records the balance: marine-dominated cliffs are steep with a fresh wave-cut notch; sub-aerially dominated cliffs are lower-angled, vegetated and terraced by slumping.

Name that process

Tap a description on the left, then the process it defines. Naming the mechanism, not the outcome, is the AO2 mark.

Where the sediment ends up

Deposition happens where energy falls — in a bay, in the lee of an obstacle, or where the coastline changes direction and longshore drift can no longer be sustained.

• Beach and berm: constructive waves build ridges at successive high-tide levels. A storm beach of coarse material sits at the back; a sandy beach has a gentler profile than shingle because water percolates less readily, so backwash stays strong.
• Spit — drift continues past an abrupt change in coastline direction (an estuary mouth, a headland) and sediment is deposited in the sheltered water. The distal end is often recurved by wave refraction or by a secondary wind direction. Behind it, low energy allows mud to settle and a salt marsh to develop.
• Bar — a spit that grows across a bay, sealing off a lagoon behind it. Tombolo — a spit that joins the mainland to an island. Cuspate foreland — a triangular projection where two drift directions converge.
• Sand dunes — a psammosere succession: embryo dune (pioneer species trap wind-blown sand) → fore-dune → yellow dune (marram grass, with deep roots and rolled leaves, is the key stabiliser) → grey dune (humus builds, soil develops) → dune slack (damp hollows) → mature dune with scrub and woodland. Blowouts occur where vegetation is stripped, often by trampling.
• Salt marsh — a halosere: pioneer halophytes (e.g. Salicornia, Spartina) trap sediment on mudflats, raising the surface, reducing tidal inundation, allowing less salt-tolerant species in. A textbook case of vegetation creating the landform, not just occupying it.

Systems link: every one of these is a store. Cut the input (dam the river, wall the cliff, dredge the offshore bank) and the store shrinks — the spit erodes, the marsh drowns, the dune blows out. Always finish a landform explanation with what happens if the budget goes negative.

Reading a spit

Your turn — the rate of cliff retreat

A fieldwork group uses a fixed marker post set back from a soft-rock cliff and compares two surveys (hypothetical data).

Your turn — balancing the sediment budget

A sediment sub-cell is monitored for one year (hypothetical data, all in cubic metres per year).

Eustatic and isostatic — and why Britain tilts

Two mechanisms, and the exam wants them kept apart:

• Eustatic — a change in the volume of water in the ocean (or the volume of the basin). It is global. Glacials lock water up in ice, so sea level falls; deglaciation returns it, so sea level rises. Warming water also expands thermally, adding to the rise.
• Isostatic — a change in the height of the land. It is local/regional. The weight of an ice sheet depresses the crust; when the ice melts, the crust slowly rebounds (isostatic recovery), which produces a relative fall in sea level. Local subsidence (sediment compaction, groundwater abstraction) does the reverse.

Because northern Britain was under the thickest ice, it is still rebounding while the south-east is sinking — so the same global eustatic rise produces very different relative sea-level change at Oban and at Southend. Always specify relative sea level when you discuss risk.

• Emergent landforms (relative fall): raised beach with a fossil wave-cut platform, and a relict (abandoned) cliff stranded inland with a fossil notch, cave or stack.
• Submergent landforms (relative rise): a ria (a drowned river valley — sinuous, shallowing seaward, dendritic in plan), a fjord (a drowned glacial trough — U-shaped in cross-section, deep, with a shallow rock threshold at its mouth), a fjard (a drowned glaciated lowland) and the Dalmatian coast (drowned concordant ridges, giving long islands parallel to the shore).

Contemporary rise is driven principally by thermal expansion of the warming ocean plus the addition of water from melting land ice (glaciers and ice sheets). It matters less through gradual inundation than through storm surge: a higher baseline means a given surge reaches further inland and overtops defences designed for the old baseline. Rates and projections vary between studies — describe the mechanism confidently and hedge the numbers.

Ria or fjord?

Why some coasts retreat fast

Rapid coastal recession is rarely one cause. Build the explanation in layers:

• Physical: weak lithology (glacial till, clay); high wave energy and long fetch; a narrow or absent beach, so waves reach the cliff foot at every high tide; a steep offshore gradient allowing large waves close inshore; and storm frequency.
• Human — sediment starvation: dams and reservoirs trap river sediment that would have nourished the coast; sea walls stop cliff erosion, which sounds good until you remember eroding cliffs are a major sediment input; groynes starve the downdrift frontage; dredging removes offshore stores.
• Human — subsidence: groundwater and hydrocarbon abstraction, and drainage of organic soils, lower the land surface and raise relative sea level. Several major delta cities are subsiding faster than the sea is rising.
• Flood risk is greatest where a storm surge (low pressure lifting the sea surface, plus wind piling water against the coast) coincides with a high spring tide on a low-lying, funnel-shaped coast. Add a subsiding delta with dense population and you have the classic high-risk configuration.

The evaluative point: coastal defence is a zero-sum problem within a sediment cell. Protecting one frontage almost always removes sediment from another. This is precisely why management moved from ad-hoc local schemes to cell-wide Shoreline Management Plans.

The counter-intuitive one

Hard, soft, and the four SMP policies

• Hard engineering: sea wall (effective, very expensive, reflects energy and can cause toe scour), groynes (build the beach locally, starve downdrift), rip-rap / rock armour (cheap, absorbs energy, ugly and can shift in storms), revetments, offshore breakwaters (force waves to break early).
• Soft engineering: beach nourishment (works with the system, looks natural — but needs repeating and the sediment must come from somewhere), dune regeneration and fencing, cliff regrading and drainage (attacks the sub-aerial cause), salt-marsh creation.
• Shoreline Management Plans assess the whole sediment cell and assign one of four policies to each frontage: hold the line · advance the line · managed realignment (deliberately allow the shoreline to move landward, creating intertidal habitat that absorbs energy) · no active intervention.
• Cost–benefit analysis decides which frontages are worth defending — and this is where geography becomes political. Property values and population density drive the numbers, so high-value urban frontages are defended and low-value rural ones are not. Intangible losses (community, heritage, a family home) are extremely hard to price.
• ICZM (Integrated Coastal Zone Management) is the holistic alternative: manage the whole coastal zone — land and sea, all players, all activities — over the long term, with sustainability and adaptation rather than resistance as the goal.

Players and conflict: residents facing a "no active intervention" designation lose their homes and cannot insure or sell them; businesses want the beach protected; environmental groups may welcome realignment because it creates intertidal habitat; national government must ration a limited budget. There is no solution that satisfies all four — say so, and say whose criteria you are judging by.

Writing the evaluative essay

Edexcel Paper 1 rewards AO1 (knowledge), AO2 (application and analysis — the bulk of the extended-answer marks) and AO3 (working with sources). For this topic:

• Use the system as your spine. Sediment cell, budget, inputs, outputs, stores, dynamic equilibrium. It converts description into explanation and it is the language of the mark scheme.
• Separate marine from sub-aerial — and then argue about which dominates in this case, using the cliff profile as evidence.
• Chain the causes. "Dam → river sediment trapped → negative budget in the cell → beach thins → less wave energy absorbed → cliff-foot erosion accelerates." Each arrow is a mark.
• Bring players into every management question, and state the criterion you are judging against (cost, sustainability, equity, habitat, community).
• Conclude conditionally. Say to what extent, and on what your judgement depends — scale (frontage vs cell), timescale (decades vs a century of sea-level rise), and whose values.

Try it: "Evaluate the view that managed realignment is a more sustainable response to coastal flood risk than hard engineering." Plan: (1) hard engineering — immediate, quantifiable protection of high-value assets, but expensive, ages, needs maintenance, disrupts the sediment budget and encourages further development behind it (the levee effect); (2) managed realignment — works with the system, creates energy-absorbing intertidal habitat, cheaper over the long run, and adapts to rising sea level; (3) but it sacrifices land and homes, is politically toxic and cannot be used to protect a city centre. Judgement: more sustainable in low-value rural frontages and over a long timescale; hard engineering remains rational where asset value is high — so the answer depends on the frontage, not on the technique.

Sharpening the judgement

The big ideas to know

That is Edexcel Topic 2 (Coastal Landscapes) — system, process, landform, risk, response. Press Finish to see your score.