Coastal Processes: Weathering, Erosion & Transport

The coast is a dynamic environment where the land meets the sea. Understanding the processes that operate at the coast is essential for explaining how coastal landforms are created. This lesson covers weathering, erosion, mass movement, and transport processes.

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Waves

Waves are the primary agent of change at the coast. They are created by wind blowing across the surface of the sea. The characteristics of a wave depend on:

• Wind speed — stronger winds create larger waves.
• Wind duration — wind blowing for a longer time creates larger waves.
• Fetch — the distance of open water over which the wind blows. A longer fetch produces larger, more powerful waves.

Constructive vs Destructive Waves

Feature	Constructive Waves	Destructive Waves
Wave height	Low (< 1 m)	High (> 1 m)
Wave frequency	Low (6-8 per minute)	High (10-14 per minute)
Swash	Strong, long	Weak, short
Backwash	Weak, short	Strong, long
Effect on beach	Build up material (deposition)	Remove material (erosion)
Wave type	Spilling	Plunging
Typical conditions	Calm weather, low wind	Stormy weather, strong wind

Exam Tip: Constructive waves construct (build up) the beach. Destructive waves destroy (erode) the beach. The key to remembering is: constructive waves have a strong swash that pushes material up the beach; destructive waves have a strong backwash that drags material away.

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Weathering

Weathering is the in-situ breakdown of rock — the rock breaks apart where it is, without being moved. Three types of weathering affect coastlines:

Mechanical (Physical) Weathering

Type	Process
Freeze-thaw	Water enters cracks in rock, freezes and expands by about 9%, widening the crack. Repeated cycles shatter the rock. Common in winter on exposed cliffs.
Wetting and drying	Some rocks (especially clay and shale) expand when wet and contract when dry. Repeated cycles cause the rock to crack and crumble.
Salt crystallisation	Seawater evaporates in cracks and pores, leaving salt crystals that grow and exert pressure on the surrounding rock, causing it to break apart.

Chemical Weathering

• Carbonation — rainwater absorbs carbon dioxide from the atmosphere, forming a weak carbonic acid. This reacts with calcium carbonate in limestone and chalk, dissolving it. This is particularly important on coastlines made of these rocks.
• Oxidation — oxygen in water reacts with iron-rich minerals in rock, causing them to rust and weaken.
• Solution — slightly acidic water dissolves soluble minerals in rock.

Biological Weathering

• Root action — plant roots grow into cracks in rock and gradually widen them.
• Burrowing organisms — piddocks (boring molluscs) and sea urchins drill into rock, weakening it.
• Algae and lichen — produce acids that chemically weather the rock surface.

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Erosion

Erosion is the wearing away and removal of rock by a moving agent — at the coast, this is primarily waves. There are four main types of coastal erosion:

Type	Process	Analogy
Hydraulic action	The force of waves crashing against the cliff compresses air in cracks. When the wave retreats, the air expands explosively, weakening the rock. Over time, cracks widen and pieces of rock break off.	Like a pressure washer blasting into a crack
Abrasion (corrasion)	Waves hurl sand, pebbles, and rocks at the cliff face, wearing it away like sandpaper. This is the most effective form of coastal erosion.	Like sandpaper rubbing against wood
Attrition	Rocks and pebbles carried by waves smash into each other, becoming smaller, smoother, and rounder over time. This does not erode the cliff itself but reduces the size of beach material.	Like shaking stones in a tumble dryer
Solution (corrosion)	Slightly acidic seawater dissolves soluble rocks such as limestone and chalk.	Like acid dissolving a tablet

Exam Tip: Abrasion is generally the most powerful form of coastal erosion. Examiners often ask you to explain which process is most effective — always discuss abrasion first and explain why it is so damaging (waves throw large, heavy material at the cliff face with great force).

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Mass Movement

Mass movement is the downslope movement of material under the influence of gravity. It is an important process on cliffs and contributes to coastal retreat. The main types are:

Type	Description	Speed
Rockfall	Fragments of rock break away from the cliff face (often after freeze-thaw weathering) and fall under gravity.	Very fast (instantaneous)
Landslide	A block of rock slides downward along a flat plane, such as a bedding plane or fault.	Fast
Rotational slump (slumping)	Saturated material slides downward along a curved surface. Common on clay cliffs after heavy rain.	Moderate
Mudflow	Saturated soil and clay flow downhill as a liquid mass.	Moderate to fast
Soil creep	Very slow downhill movement of soil particles.	Very slow

Factors affecting mass movement

• Rock type — soft rocks like clay are more prone to slumping.
• Rainfall — heavy rain saturates the ground, adding weight and reducing friction, making mass movement more likely.
• Undercutting by waves — erosion at the base of the cliff removes support, making the cliff unstable.
• Vegetation — roots bind the soil together, reducing the risk of mass movement. Removal of vegetation increases the risk.

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Transport

Once material has been eroded, it must be transported along the coast or out to sea. The main transport processes are:

Longshore Drift

Longshore drift (also called littoral drift) is the most important coastal transport process. It moves sediment along the coastline in the direction of the prevailing wind.

How it works:

1. Waves approach the beach at an angle (determined by the prevailing wind direction).
2. The swash carries sediment up the beach at this angle.
3. The backwash carries sediment straight back down the beach under gravity.
4. This creates a zigzag movement of sediment along the coast.
5. Over time, large quantities of sediment are transported in one direction.

In England and Wales, the prevailing wind is from the south-west, so longshore drift generally moves sediment from west to east along the south coast and from south to north along the east coast.

Exam Tip: When drawing a diagram of longshore drift, always label: the direction of the prevailing wind, the angle of the swash, the direction of the backwash (straight down the beach), and the overall direction of sediment transport. This is a very commonly examined diagram.

Other Transport Processes

Sediment is also transported by:

Process	Description
Traction	Large boulders and cobbles are rolled along the seabed by the force of the waves.
Saltation	Pebbles and gravel are bounced along the seabed by the waves.
Suspension	Fine particles of sand and silt are carried within the water itself.
Solution	Dissolved minerals are carried invisibly in the water.

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Deposition

Deposition occurs when waves lose energy and can no longer carry their load of sediment. This happens when: