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Sediment Transportation

Geography: Edexcel A Level, topic 3.2.3

Longshore Drift

Longshore drift transports material along coastlines when waves approach the beach at an angle.

Swash direction

Swash direction

  • Wind approaches the coast at an angle because of a prevailing wind direction.
  • Waves are controlled by the wind and so this angle will be the direction the swash moves up the beach.
Backwash direction

Backwash direction

  • Gravity is the only force that acts on the backwash, so it falls back to the sea at right angles to the coastline.
Overall effect

Overall effect

  • Because of the difference between the angle of the swash and the angle of the backwash, sediment repeatedly moves in the shape of a right-angled triangle.
  • Over time, sediment is carried along a beach.

Depositional Landforms

Coastal landforms caused by deposition include beaches, spits, bars, tombolos and cuspate forelands.

Spits

Spits

  • When the coast changes direction at an estuary (where a river meets the sea), longshore drift continues to move sediment across the inlet.
  • The river doesn't let the spit completely join to the coast on the other side because the river has the energy to move the sediment.
  • Spits are the long fingers of sand sticking out from one side in a coastline that have been curved by secondary winds. They often have salt marshes behind them.
Bars

Bars

  • A bar is formed when a spit grows across a bay.
  • Lagoons often form behind bars.
Beaches

Beaches

  • Beaches are made by constructive waves moving and depositing sand or shingle inland.
  • Generally, a more gently sloping beach tends to be formed from sand, whereas a steeper sloping beach is formed from pebbles.
Tombolos

Tombolos

  • If a spit stretches across the mainland and joins an offshore island, this causes the formation of a tombolo.
Cuspate forelands

Cuspate forelands

  • Cuspate forelands form when sediment is deposited across a bay caused by longshore drift transporting sediment in two directions.
  • This leads to the formation of two spits which eventually meet and this then results in the trapping of sediment until eventually new land is formed.

The Sediment Cell Concept

The sediment cell concept describes a closed system operated by sources, transfers and sinks driven by erosion, transportation and deposition processes.

Sources

Sources

  • The sources are subaerial processes, erosional processes (breaking down cliffs) and sediments brought to the coastline by rivers.
Transfers

Transfers

  • Transfers are longshore drift, onshore and offshore winds and tides.
Sinks

Sinks

  • The sinks are depositional landforms (spits, bars, beaches and sand dunes).
The UK

The UK

  • The UK has 11 key sediment cells and sub-cells, each one with its own Shoreline Management Plan (SMP).

Test yourself on Sediment Transportation

  • The Process of Longshore Drift

  • What process transports material along coastlines?

  • Which of these coastal landforms is NOT caused by deposition?

  • Which of the following coastal landforms are caused by deposition?

  • What are the coastal landforms caused by deposition?

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