Coastal Transport and Deposition

Once material has been eroded from cliffs or delivered to the coast by rivers, it must be transported and eventually deposited to create the distinctive landforms of deposition. Understanding sediment transport mechanisms is essential for explaining coastal landscapes and for evaluating the effectiveness of coastal management strategies.

---

Sediment Transport Mechanisms

Sediment is transported in the coastal environment by waves, currents, tides and wind. The specific mechanism depends on particle size, available energy and the nature of the transporting medium.

Transport by Waves and Currents

Four mechanisms operate in the water:

Mechanism	Description	Particle Size	Energy Required
Traction	Large particles rolled along the sea bed by wave or current action	Cobbles, boulders (> 64 mm)	Very high
Saltation	Particles bounced along the sea bed in a series of hops	Pebbles, coarse sand (0.5-64 mm)	High
Suspension	Fine particles carried within the water column, held up by turbulence	Silt, fine sand (< 0.5 mm)	Moderate
Solution	Dissolved minerals carried invisibly in the water	Dissolved ions	Low

The relationship between flow velocity and sediment transport was quantified by Hjulström (1935) in his famous diagram, which remains one of the most important tools in physical geography:

The Hjulström Curve

The Hjulström curve shows the relationship between flow velocity and sediment particle size for erosion, transport and deposition:

Key features of the curve:

1. Erosion velocity — the minimum velocity needed to pick up (entrain) a particle from the bed. Counter-intuitively, the very finest particles (clay and silt) require higher velocities to erode than medium sand, because their flat shapes and electrostatic cohesion cause them to stick together
2. Transport velocity — once entrained, particles can be transported at velocities lower than those needed to erode them. This is because particles in motion are easier to keep moving than to initially dislodge
3. Deposition velocity — the velocity below which particles can no longer be carried and settle out. Large particles are deposited first (at higher velocities), while fine silts and clays remain in suspension until velocities drop very low

Key Definition: The Hjulström curve (1935) is a graph showing the relationship between stream velocity and the erosion, transport and deposition of sediment of different grain sizes. It demonstrates that very fine and very coarse particles both require relatively high velocities for erosion, while medium-sized particles are most easily eroded.

Aeolian (Wind) Transport

Wind is a significant transport agent on beaches and in sand dune systems:

Mechanism	Description	Conditions
Surface creep	Large grains rolled along the surface by wind or by the impact of saltating grains	Wind speeds > 4-5 m/s
Saltation	Grains bounced along the surface in short hops (typically 1-2 cm high for sand)	Wind speeds > 4-5 m/s; dry, loose sand
Suspension	Fine particles (silt, very fine sand) carried high into the air	Strong winds; disturbed surface

Research by Bagnold (1941) in The Physics of Blown Sand and Desert Dunes established the fundamental principles of aeolian transport that remain standard today. Bagnold showed that approximately 75% of sand transport by wind occurs through saltation.

---

Longshore Drift

Longshore drift (also called littoral drift) is the net movement of sediment along the coastline, and it is arguably the single most important transport process in the coastal system.

How Longshore Drift Works

1. Waves approach the shore at an angle, determined by the prevailing wind direction
2. The swash carries sediment up the beach at this oblique angle
3. The backwash, governed by gravity, carries sediment straight back down the steepest gradient (perpendicular to the shore)
4. The net result is a zigzag movement of sediment along the beach in the direction of the prevailing waves

graph LR
    subgraph "Longshore Drift Process"
    A["Incoming wave at angle"] --> B["Swash: sediment moves up beach at angle"]
    B --> C["Backwash: sediment moves straight down slope"]
    C --> D["Net movement along beach"]
    D --> A
    end

Rates of Longshore Drift

The rate of longshore drift varies enormously depending on wave energy, angle of wave approach, sediment availability and beach characteristics:

Location	Direction	Estimated Rate
Holderness coast, East Yorkshire	Southward	500,000 m³/year
Spurn Head, East Yorkshire	Southward	~250,000 m³/year
Chesil Beach, Dorset	Eastward	Approximately 15,000 m³/year
East Anglia coast	Southward	Up to 800,000 m³/year
Dungeness, Kent	Eastward	~35,000 m³/year

On the Holderness coast, the predominant wave approach is from the north-east (fetch across the North Sea), driving sediment southward. Valentin (1954) was among the first to quantify this drift, estimating that the Holderness coast feeds approximately 1 million m³ of sediment per year into the Humber Estuary system.

Evidence of Longshore Drift

Several features provide evidence of the direction and rate of longshore drift:

• Accumulation on the updrift side of groynes — sediment builds up on the side facing the approaching waves
• Spit orientation — spits grow in the direction of longshore drift (e.g., Spurn Point grows southward, Hurst Castle Spit grows south-eastward)
• Beach material grading — pebbles tend to become smaller and more rounded in the direction of drift (through attrition)
• Sediment traps — artificial structures used by researchers to measure transport rates directly
• Tracer experiments — using painted, fluorescent or radioactive pebbles to track sediment movement. Kidson and Carr (1961) pioneered fluorescent tracer studies on the Somerset coast

---

Processes of Deposition

Deposition occurs when the energy available to transport sediment falls below the level needed to carry it. This happens when:

1. Waves lose energy — entering sheltered areas behind headlands, inside bays, or in the lee of structures
2. Water depth increases — offshore, reducing wave and current velocities
3. Friction increases — as water flows over wide, shallow areas such as mudflats
4. Opposing forces meet — e.g., river currents meeting tidal currents in estuaries, creating slack water
5. Wave type changes — constructive waves with strong swash deposit more sediment than they remove
6. Wind speed drops — reducing the capacity of aeolian transport

Key Definition: Deposition is the laying down of sediment that was previously being transported, occurring when the energy of the transporting medium (waves, currents, wind) falls below the threshold needed to move particles of that size.

Selective Deposition (Sorting)

Not all sediment is deposited at the same time or place. As energy decreases, the largest and heaviest particles are deposited first, followed progressively by smaller particles. This produces graded deposits: