Landforms and Landscape Processes » Coastal processes and erosional landforms

What you'll learn this session

Study time: 30 minutes

The main coastal processes of erosion, transportation and deposition
How waves form and their different types
The four main types of coastal erosion
Formation of erosional landforms: headlands and bays, cliffs and wave-cut platforms, caves, arches, stacks and stumps
Real-world case studies of erosional landforms

Coastal Processes: The Power of Water

Coasts are dynamic environments where the land meets the sea. They're constantly changing due to the relentless action of waves, tides and currents. These forces shape our coastlines into spectacular landforms that attract millions of tourists each year, but they can also threaten coastal communities.

Key Definitions:

Coastal processes: The natural actions that shape coastlines, including erosion, transportation and deposition.
Erosion: The wearing away and removal of rock and soil by natural forces.
Transportation: The movement of eroded material along the coast.
Deposition: The dropping of eroded material when the sea loses energy.

Waves: The Driving Force

Waves are the main force shaping our coastlines. They form when wind blows across the surface of the sea, transferring energy to the water.

🌊 Constructive Waves

Features: Low frequency (6-8 per minute), strong swash, weak backwash

Effect: Build up beaches by depositing material

When: Calm weather conditions

🌊 Destructive Waves

Features: High frequency (10-14 per minute), weak swash, strong backwash

Effect: Erode coastlines by removing material

When: Stormy weather conditions

Coastal Erosion: Breaking Down the Coast

Erosion is the process by which the sea wears away the land. It's particularly powerful during storms when wave energy is at its highest. There are four main types of erosion:

💧 Hydraulic Action

When waves crash against the cliff, they force air into cracks. The pressure can break off rock fragments. Think of it like squeezing air into a balloon until it pops!

💧 Abrasion (Corrasion)

Waves throw sand, pebbles and larger rocks against the cliff face, wearing it away like sandpaper. This is often the most effective type of erosion.

💧 Attrition

Rocks and pebbles in the sea knock against each other, becoming smaller and more rounded over time - like stones in a tumble dryer!

💧 Solution (Corrosion)

Seawater dissolves certain rocks (especially limestone) through chemical action. The slightly acidic seawater slowly dissolves the rock.

Erosional Landforms: Nature's Coastal Sculptures

The processes we've learned about create distinctive coastal landforms. Let's explore how they form:

Headlands and Bays

Coastlines rarely consist of the same rock type. When waves attack a stretch of coast with alternating bands of hard rock (like granite) and soft rock (like clay), they erode the soft rock more quickly, forming bays. The hard rock sticks out as headlands.

Did You Know?

Waves refract (bend) around headlands, concentrating wave energy on them. This is why headlands often feature dramatic erosional features like cliffs, caves and arches.

Cliffs and Wave-Cut Platforms

Cliffs form when waves erode the base of a slope, creating an overhang that eventually collapses. This process repeats, causing the cliff to retreat inland. At the base of the cliff, a wave-cut platform forms - a flat area exposed at low tide.

⛰ How Cliffs Form

Waves attack the base of the cliff (wave-cut notch)
The notch deepens, creating an overhang
Without support, the overhang collapses
Waves remove the fallen material
The process repeats and the cliff retreats

⛰ Wave-Cut Platform Features

Gently sloping flat surface
Exposed at low tide
Often covered with rock pools
May have jagged edges from differential erosion
Grows wider as the cliff retreats

The Lifecycle of Coastal Features: Caves, Arches, Stacks and Stumps

These dramatic features form in sequence as erosion attacks headlands and cliffs:

🗻 Caves

Waves exploit weaknesses in the cliff face (like joints or faults), eroding them to form caves. Hydraulic action is particularly important here.

🗻 Arches

When caves on opposite sides of a headland meet, they form an arch. The roof is supported by the remaining rock.

🗻 Stacks & Stumps

When an arch collapses, it leaves an isolated pillar of rock called a stack. Further erosion reduces it to a low stump.

Case Study: Old Harry Rocks, Dorset, UK

Old Harry Rocks are a perfect example of these erosional features in action. Located at the eastern end of the Jurassic Coast, they show the complete sequence of formation:

Old Harry is a chalk stack standing in the sea
Next to it is Old Harry's Wife (actually a stump) - all that remains after a stack collapsed in 1896
Along the headland, you can see caves and arches forming - future stacks in the making
The whole formation shows how a coastline retreats over time through erosion

The rocks are made of chalk (a soft, white limestone) which is vulnerable to both mechanical erosion and solution.

Factors Affecting Erosion Rates

Not all coastlines erode at the same rate. Several factors influence how quickly erosion happens:

🔬 Rock Type and Structure

Hard rocks like granite resist erosion better than soft rocks like clay or chalk. Rocks with many joints, bedding planes, or faults erode faster as waves exploit these weaknesses.

🌞 Weather and Climate

Stormy conditions create larger, more powerful waves with greater erosive power. Freeze-thaw weathering in cold climates can weaken cliff faces, making them more vulnerable to erosion.

Case Study: Holderness Coast, Yorkshire, UK

The Holderness Coast in Yorkshire is Europe's fastest-eroding coastline, retreating at an average rate of 1.5-2.5 metres per year.

Why so fast?

The cliffs are made of soft boulder clay (till) deposited by glaciers during the last ice age
The coast faces northeast, exposed to powerful waves from the North Sea
Prevailing winds create strong longshore drift that removes eroded material
Human activity has reduced natural defences like beaches

Impact: Since Roman times, over 30 villages have been lost to the sea. Properties that were once several kilometres inland now teeter on cliff edges.

Human Interaction with Coastal Erosion

Coastal erosion presents significant challenges for people living near the coast:

Property loss: Homes and businesses can be damaged or destroyed
Infrastructure damage: Roads, railways and utilities may be threatened
Economic impacts: Tourism can be affected, either positively (scenic views) or negatively (beach loss)
Coastal management: Authorities must decide whether to defend the coast or allow natural processes to continue

In our next session, we'll explore how humans manage coastal erosion and the different strategies used to protect vulnerable coastlines.