Sediment Transport and Deposition

While erosion removes material from the coastline, transport moves it and deposition accumulates it to create some of the most distinctive and economically important coastal landforms — beaches, spits, bars, tombolos, barrier islands, salt marshes and sand dunes. This lesson addresses Edexcel A-Level Geography Enquiry Question 1 by explaining the processes of sediment transport and deposition, and the conditions under which depositional landforms develop.

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Sediment Sources

Before sediment can be transported and deposited, it must be supplied to the coastal system. The main sources of coastal sediment are:

Source	Contribution	Sediment Type
Cliff erosion	The dominant source on many UK coasts; collapsed cliff material is reworked by waves	Variable — from boulders and cobbles (resistant rock) to sand and clay (soft rock)
River input	Rivers carry fluvial sediment to the coast at their mouths	Predominantly sand, silt and clay (fine-grained)
Offshore sources	Waves and tidal currents move sediment from the seabed onshore	Sand and gravel deposited during lower sea levels (relict sediment)
Longshore drift	Sediment transported along the coast from adjacent areas	Variable — depends on source
Wind (aeolian)	Wind blows sand inland from beaches	Fine to medium sand
Biological sources	Shell fragments, coral debris, algal deposits	Calcium carbonate fragments

On the Holderness coast, cliff erosion supplies an estimated 2 million cubic metres of sediment per year — mostly glacial till that is quickly broken down into sand and clay.

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Sediment Transport Processes

Sediment is moved along the coast and between the nearshore and offshore zones by several mechanisms:

Longshore Drift (Littoral Drift)

Longshore drift is the most important process for transporting sediment along the coast. It occurs when waves approach the shore at an oblique angle (determined by the prevailing wind direction). The swash carries sediment diagonally up the beach in the direction of wave approach. The backwash returns sediment straight down the beach under gravity, perpendicular to the waterline. This produces a zigzag movement of sediment along the coast.

graph TD
    A["Prevailing wind direction<br/>↘ oblique to coast"] --> B["Swash carries sediment<br/>diagonally UP the beach"]
    B --> C["Backwash returns sediment<br/>straight DOWN the beach"]
    C --> D["Net sediment movement<br/>along the coast →"]
    D --> E["Longshore drift continues<br/>until interrupted by headland,<br/>groyne or estuary"]

On the south coast of England, the dominant wave approach is from the south-west (Atlantic swell waves with long fetch), so longshore drift generally moves sediment from west to east. On the east coast, waves from the north-east (North Sea fetch) drive sediment generally from north to south.

Modes of Sediment Transport

Sediment is transported by waves and currents in four modes, depending on particle size and flow energy:

Mode	Description	Sediment Size
Traction	Large particles (boulders, cobbles) are rolled along the seabed by strong currents	> 64 mm
Saltation	Sand-sized particles bounce along the seabed in a series of short jumps	0.06–2 mm
Suspension	Fine particles (silt, clay) are carried within the water column without touching the bed	< 0.06 mm
Solution	Dissolved material (e.g., calcium bicarbonate from limestone dissolution) is carried invisibly in the water	Dissolved ions

The competence of a flow (the maximum size of particle it can transport) increases with velocity. During storms, currents are strong enough to move boulders by traction. In calm conditions, only fine suspended sediment and dissolved material are transported.

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Deposition: Conditions and Processes

Deposition occurs when the energy available to transport sediment falls below the threshold needed to keep it moving. Sediment is deposited when:

• Wave energy decreases (e.g., in sheltered bays, behind headlands, in estuaries)
• Longshore drift is interrupted (e.g., by a headland, change in coastal orientation, or groyne)
• Water velocity slows (e.g., where a river enters the sea and current speed drops)
• Sediment supply exceeds transport capacity (e.g., during calm weather after a storm that mobilised large volumes of material)

The largest and heaviest particles are deposited first (boulders, then pebbles, then sand), while the finest particles (silt, clay) remain in suspension longest and are deposited only in very calm conditions — typically in sheltered estuaries and mudflats.

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Depositional Landforms

Beaches

A beach is the most fundamental depositional landform — an accumulation of sediment between the lowest tide level and the highest point reached by storm waves. Beaches were covered in detail in Lesson 3, but their role as a sediment store within the coastal system should be emphasised here.

Beaches act as a buffer between the sea and the land. A wide, well-developed beach dissipates wave energy through friction and percolation, protecting the cliff or land behind from erosion. Beach removal (by storms or human extraction) exposes the coast to increased erosion.

Spits

A spit is an elongated ridge of sand or shingle that extends from the mainland into the sea, usually at a point where the coastline changes direction (e.g., at the mouth of a river or estuary). Spits are formed by longshore drift continuing to transport sediment beyond a change in coastal orientation.

Formation of a Spit