Longshore drift, depositional landforms and sediment cells

Longshore drift

When waves approach at an angle (driven by the prevailing wind), the swash carries sediment up the beach at that angle, and the backwash drags it straight back down under gravity. The result is a zigzag movement of sediment along the coast: longshore drift. Groynes trap it, which is why sand builds up on one side of a groyne.

Depositional landforms

Deposition happens when waves or currents lose energy:

• Spit: a ridge of sand or shingle extending from the land into the sea where the coastline changes direction; longshore drift keeps depositing; the end often curves (recurved) because of wave refraction or secondary winds. Salt marsh forms in the sheltered water behind.
• Bar: a spit that grows across a bay, enclosing a lagoon. Barrier beaches and barrier islands lie parallel to the coast, separated by a lagoon.
• Tombolo: a bar joining an island to the mainland.
• Delta: where a river deposits sediment faster than waves and tides can remove it. The river slows on entering the sea and loses capacity; fine clay particles also flocculate (clump) in salt water and sink. Channels split into distributaries.

The June 2024 examiners' report noted few candidates knew deltas form by deposition, or the term distributary.

Sediment cells

A sediment cell is a stretch of coast, usually bounded by headlands, within which sediment moves in a largely closed system:

• Sources: cliff erosion, rivers, offshore sediment.
• Transfers: longshore drift, onshore and offshore movement.
• Sinks: beaches, spits, dunes, offshore banks.

The concept treats the coast holistically: defending a cliff (cutting off a source) can starve beaches downdrift. The January 2024 mark scheme credited the sediment cell as a useful but oversimplified model, since cells are not fully closed and boundaries shift.

Worked example

A council builds a sea wall along an eroding till cliff at the updrift end of a sediment cell. Predict and explain one effect further along the cell.

1. The cliff was a sediment source.
2. Protecting it cuts off supply to longshore drift.
3. Beaches downdrift receive less sand, become narrower and give less protection, so erosion increases there.

Answer: Downdrift beaches are starved and erosion increases, showing why cells must be managed as a whole.

Check yourself

1. In longshore drift, the backwash moves sediment
   • straight down the beach under gravity
   • along the beach at the angle of the wind
   • offshore into deep water only
   • up the beach
   Show answer

   straight down the beach under gravity. Backwash runs perpendicular to the shore under gravity.

2. Where does a spit usually form?
   • In front of a resistant headland
   • Only at river mouths with high tides
   • Where the coastline changes direction and longshore drift continues
   • Where waves are strongest
   Show answer

   Where the coastline changes direction and longshore drift continues. Drift carries sediment past a bend and deposits it into open water.

3. Which process best explains how a delta forms?
   • Deposition as a river slows and loses capacity at the coast
   • Erosion by destructive waves
   • Weathering of cliffs
   • Longshore drift onto a headland
   Show answer

   Deposition as a river slows and loses capacity at the coast. Deltas are depositional (June 2024 mark scheme).

4. A channel that splits off from the main river on a delta is a
   • tributary
   • geo
   • berm
   • distributary
   Show answer

   distributary. Distributaries carry water away from the main channel.

5. In a sediment cell, a sand dune is an example of a
   • source
   • sink
   • transfer
   • boundary
   Show answer

   sink. Dunes store sediment.

6. A bar of sediment joining an island to the mainland is a
   • spit
   • barrier island
   • stack
   • tombolo
   Show answer

   tombolo. A tombolo connects the island to the shore.

7. Explain why the sediment cell concept is useful for coastal managers.
   Show answer

   It shows the coast as a system in which sediment moves from sources to sinks, so managers can predict that protecting one place, for example by cutting off a source, will cause erosion downdrift and can plan for the whole cell.. System, consequences, planning.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Geography specification, Issue 3 (September 2025)
• Pearson IAL Geography Getting Started Guide
• Pearson WGE02/01 mark scheme, June 2024
• Pearson WGE02/01 examiners' report, June 2024
• Pearson WGE02/01 mark scheme, January 2024

Last updated 8 November 2026. Spotted a mistake? Tell us.