Coastal Transport and Deposition

Spec mapping (AQA 7037): Paper 1, §3.1.3 Coastal Systems and Landscapes — transport: traction, saltation, suspension, solution; longshore (littoral) drift; the role of swash and backwash; conditions favouring deposition. These are the transfers (transport) and the precursor to the store-building (deposition) in the systems model of Lesson 1, taking the sediment that Lesson 3 detached and routing it to the landforms of Lesson 6. The lesson links synoptically to §3.1.1 (the energy-threshold control on entrainment and deposition, paralleled in the water cycle) and to §3.1.4 Glacial Systems (comparable competence/capacity ideas in glacial and fluvioglacial transport). The dominant Assessment Objectives are AO1 (knowledge of transport mechanisms and longshore drift) and AO2 (applying the energy–threshold relationship to explain where and why deposition occurs, including human disruption of sediment supply). The Hjulström-curve exercise exercises AO3 (interpreting a velocity–grain-size graph).

Why this lesson matters. Transport and deposition are where the systems approach earns its keep. A frontage erodes not only because its waves are strong but because sediment is no longer arriving from updrift; a beach grows not because deposition is mysteriously favoured there but because it sits in an energy shadow. Mastering longshore drift and the energy-threshold idea is the prerequisite for explaining the depositional landforms of Lesson 6 and the management conflicts of Lessons 9–10.

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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.

Two concepts borrowed from fluvial geomorphology sharpen the analysis of coastal transport and are worth deploying in exams. Competence is the largest particle a given flow can move — it depends on velocity, and because energy scales steeply with velocity, a small rise in wave or current speed sharply increases the size of material that can be entrained (this is why storms move boulders that fair-weather waves cannot budge). Capacity is the total quantity of sediment a flow can carry — it depends on both velocity and the supply of available material. The distinction matters: a high-energy but sediment-starved frontage has high competence but low actual load (little is available to move), whereas an abundant supply of fine sand can saturate a moderate flow's capacity. Recognising that transport is limited sometimes by energy and sometimes by supply is the key to explaining why two coasts with identical wave climates can have utterly different sediment throughput — and it underpins the sediment-starvation problem that recurs throughout this course.

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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

What controls the rate of longshore drift?

The volume of sediment moved alongshore per year is not fixed; it depends on four interacting controls, and explaining their interplay is a strong AO2 skill:

1. Angle of wave approach. Drift rate rises as the angle between the incoming wave crest and the shoreline increases — up to an optimum of about 30°, beyond which the alongshore component of swash energy actually falls again. This is why the residual angle surviving wave refraction (Lesson 2) is so important: a strongly refracting coast leaves little angle and little drift.
2. Wave energy. Higher-energy waves entrain and carry more sediment, so a stormy coast with a long fetch (Holderness, ~500,000 m³ yr⁻¹) drifts far more than a sheltered one.
3. Sediment supply and calibre. Drift can only move what is available; a sediment-starved frontage transports little regardless of wave conditions, while an abundant supply of mobile sand (rather than heavy shingle) maximises transport.
4. Beach gradient and permeability. On a permeable shingle beach much backwash percolates away, weakening the return flow and altering the swash–backwash balance that drives the zigzag.

Because these controls multiply rather than add, a single change — for example, a sea wall cutting sediment supply — can collapse drift rates even where wave energy is unchanged, propagating a deficit downdrift.

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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:

• Boulders and cobbles — deposited at the back of the beach or at the storm berm, moved only by the highest-energy waves
• Pebbles and gravel — deposited on the upper beach
• Sand — deposited on the lower beach and in the nearshore zone
• Silt and clay — carried furthest before deposition, settling in low-energy environments such as estuaries, lagoons and salt marshes

This process of size-sorting is clearly visible at Chesil Beach, Dorset, where sediment grades from pea-sized gravel at the western end (West Bay) to cobbles and large pebbles at the eastern end (Portland). Local fishermen have traditionally been able to determine their position along the beach in fog simply by examining the pebble size.

The Chesil grading is, in fact, geomorphologically debated and rewards a critical mention. The intuitive explanation is straightforward longshore sorting — that waves carry the larger material progressively to one end. But the gradient runs against the dominant drift in places, and an alternative view holds that the grading reflects the original glacial and fluvial sources of the shingle plus the differential mobility of different sizes under the local wave regime, rather than simple alongshore sorting. The honest position is that the precise mechanism is not fully resolved — and citing a contested example, with awareness that geomorphologists disagree, is exactly the kind of nuanced, evaluative knowledge that lifts an answer above textbook recall. It also reinforces a wider truth: real coasts rarely obey a single clean process, and the best geography acknowledges complexity rather than forcing a tidy story.

Sediment sinks and the end of the transport pathway

Deposition is ultimately about sediment reaching a sink — a store where it resides for a long time, sometimes permanently leaving the active system. The principal coastal sinks are offshore (material carried below the wave base into deep water, effectively lost to the system as an output), estuarine (fine sediment settling in flood-dominant estuaries to build mudflats and marshes), and terrestrial (sand blown inland into dune fields beyond the reach of normal waves). Distinguishing a temporary store (a beach, from which sediment will be remobilised) from a long-term sink (deep water, a stabilised dune) is important for sediment-budget reasoning: a sink represents a genuine loss from the cell's active budget, whereas a store is merely sediment in transit. Misclassifying the two leads to errors in budget questions — a beach is not an output, but sediment lost offshore is.

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Beach Morphology

Beaches are the most common depositional landform on the coast. Understanding their form (morphology) is essential for A-Level.

Beach Profiles

A typical beach profile includes several distinct features:

Feature	Location	Description
Storm berm	Uppermost part of beach	Ridge of coarse material deposited by storm waves; highest point on the beach
Berm(s)	Upper beach	One or more ridges marking the limit of normal swash; formed by constructive waves
Beach face	Sloping surface between berms	The main slope of the beach, where swash and backwash operate
Break point bar	Nearshore zone	Submerged ridge where waves break; formed by destructive wave backwash carrying sediment offshore
Runnel	Between bars or berms	Shallow trough or channel running parallel to the shore
Ridge	Between runnels	Low raised areas between runnels, created by wave action
Low tide terrace	At low water mark	Flat area exposed at low tide

Summer vs. Winter Beach Profiles

Beach profiles change seasonally in response to changing wave conditions:

Feature	Summer Profile	Winter Profile
Dominant waves	Constructive	Destructive
Beach width	Wide	Narrow
Gradient	Gentle	Steep (upper beach)
Berms	Well-developed	Eroded or absent
Offshore bar	Absent or small	Prominent breakpoint bar
Sediment location	Mainly on beach	Much sediment stored offshore