Coastal Geomorphological Processes Revision Notes for AQA A-Level Geography

Coastal Geomorphological Processes

What are coastal geomorphological processes?

Coastal systems evolve gradually over time, moving toward a state of dynamic equilibrium. A variety of processes work together to alter the features of coastal landscapes. These processes are powered by energy inputs that continuously reshape coastline characteristics.

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When these processes work together at the coast, they create distinctive landforms found nowhere else, making coastal environments unique from inland landscapes.

The coast is shaped by two distinct groups of processes:

Marine processes: operate directly on the coastline and are driven by the sea
Sub-aerial processes: occur on land but significantly influence coastal shape and form

Coastal sediment budget

The sediment budget determines whether a coastline is building up or wearing away. It represents the balance between sediment being added to and removed from a coastal area.

Positive budget (surplus)

When more material is added to the coast than is removed, a net accretion of sediment occurs. This creates a positive budget, or surplus, which causes:

The shoreline to build towards the sea
Coastal features to grow and expand
Beach levels to increase

Negative budget (deficit)

When more material is removed from the coast than is added, a deficit in sediment supply develops. This creates a negative budget, which results in:

The shoreline retreating landward
Coastal erosion
Loss of beach material

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Example: Calculating a Sediment Budget

For a coastal cell to have a positive budget:

Sediment inputs = $500,000$ $m^3$ per year (from rivers and cliff erosion)
Sediment outputs = $300,000$ $m^3$ per year (removed by longshore drift)
Net balance = $+200,000$ $m^3$ per year

This positive balance of $200,000$ $m^3$ leads to accretion and seaward shoreline growth.

Understanding sediment budgets

Erosion processes determine the extent of sediment loss by removing material from coastal areas. When the coastline loses more sediment than it gains, retreat occurs.

Sediment budgets help us trace where coastal materials come from (sources) and where they are stored (sinks). To calculate the budget for a coastal cell, we must:

Identify all sediment sources and sinks
Estimate the amount of material added each year
Estimate the amount of material removed each year

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Calculating these budgets is extremely challenging. Most attempts rely on complex computer models and estimates rather than precise measurements, as tracking every grain of sediment movement along a coastline is practically impossible.

Types of coastal processes

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Marine processes operate upon a coastline and are connected with the sea, such as waves, tides and longshore drift.

Marine processes include:

Wave action
Tidal movements
Longshore drift
Marine erosion

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Sub-aerial processes include processes that gradually deteriorate the coastline and reduce the strength of underlying rocks, allowing sudden movements or further erosion to occur more easily. Material is broken down in situ (in its original position). These affect the shape of the coastline and include weathering, mass movement and run-off.

Components of geomorphological processes

Weathering

Weathering involves the breakdown of rock and sediment. Three main types operate at the coast:

Chemical weathering: rocks decompose through chemical reactions with water, acids or oxygen
Biological weathering: organisms such as plant roots, burrowing animals or bacteria break down rock materials
Mechanical weathering: physical forces such as freeze-thaw action, salt crystallisation or thermal expansion fracture rocks

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Weathering types can be remembered as "CBM":

Chemical
Biological
Mechanical

All three types often work together at the coast, with chemical weathering being particularly effective in areas with high rainfall and salt spray.

Mass movement

Mass movement describes the downslope movement of material under gravity. Five key types affect coastlines:

Landslides: rapid movement of rock and soil down a slope
Rockfalls: individual rocks or rock fragments falling freely from a cliff face
Soil creep: very slow, gradual downslope movement of soil particles
Rotational slumping: material slides downward along a curved slip plane, rotating as it moves
Mudflows: rapid flow of water-saturated soil and debris

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Common Mistake to Avoid:

Students often confuse mass movement with erosion. Remember: mass movement is the downslope movement driven by gravity, while erosion involves the removal and transport of material by agents like waves or wind. Mass movement weakens the coastline, making it more susceptible to erosion.

Coastal processes

Three interconnected processes continuously reshape the coast:

Erosion

Processes of marine erosion remove material from the coastline
Waves carry tremendous energy that breaks down coastal features

Transport

Material is moved along the coast by waves
Wind can also transport sediment, particularly on beaches and dunes

Deposition

Sediment is deposited by waves when energy decreases
Wind deposition creates dune systems in coastal areas

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These three processes—erosion, transport, and deposition—work as an interconnected system. Material removed by erosion doesn't disappear; it's transported elsewhere and eventually deposited, forming new coastal features. This cycle maintains the dynamic equilibrium of coastal systems.

Marine erosion processes

Waves often break on a coastline with considerable energy. Research has estimated that waves breaking against the foot of a cliff can generate energy of $25-30$ tonnes $m^{-2}$ .

This substantial wave energy erodes coastlines through several mechanisms. When these processes work together at the coast, they create distinctive landforms found nowhere else.

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The enormous energy of $25-30$ tonnes per square metre is equivalent to the weight of several large vehicles crashing into every square metre of cliff face with each wave. This explains why marine erosion is such a powerful force in shaping coastlines, capable of dramatically transforming coastal landscapes over time.

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Key Points to Remember:

Coastal sediment budgets determine whether shorelines advance seaward (positive budget) or retreat landward (negative budget)
Two main process groups shape coasts: marine processes (connected to the sea) and sub-aerial processes (operating on land but affecting the coast)
Weathering breaks down materials through chemical, biological and mechanical means, whilst mass movement transports material downslope
Marine processes include erosion, transport and deposition powered by waves, tides and currents
Wave energy reaching $25-30$ tonnes $m^{-2}$ makes marine erosion a powerful force in shaping coastlines
Coastal systems move toward dynamic equilibrium through the interaction of all these processes

Coastal Geomorphological Processes (AQA A-Level Geography): Revision Notes