Welcome to Coasts: Physical Processes in Action!

Have you ever stood on a beach, felt the salty breeze, and listened to waves crashing against the cliffs? The coast is one of the most exciting and dynamic environments on Earth. It is a battleground where land meets the sea, constantly being shaped, broken down, and rebuilt by powerful natural forces.

In this chapter, we will explore:

• How waves are made and why some build beaches while others destroy them.
• The powerful processes of erosion, weathering, and mass movement that wear cliffs away.
• How sand and shingle travel along the shoreline through longshore drift.
• The incredible coastal features created over time, from dramatic arches and sea stacks to sweeping sandy spits.

Don't worry if this seems like a lot to learn at first! We will break down every process step-by-step with clear real-world examples, memory tricks, and simple comparisons.

1. Waves: The Engines of the Coast

Almost everything that happens at the coast is driven by waves. But where do waves actually come from?

How Waves Are Formed

Waves are generated when the wind blows across the surface of open water. The friction between the moving air and the water pulls the surface into ripples, which grow into waves. The size and power of a wave depend on three key factors:

1. Wind speed: How fast the wind is blowing.
2. Wind duration: How long the wind blows for without stopping.
3. Fetch: The uninterrupted distance of open ocean over which the wind blows.

Memory Trick: Think of S.D.F. — Speed, Duration, Fetch! A storm blowing across thousands of miles of open Atlantic ocean creates massive, energetic waves, whereas a breeze blowing across a small pond barely creates a ripple.

Wave Anatomy: Swash and Backwash

When a wave enters shallow water near the shore, the bottom of the wave drags on the seabed. It slows down, becomes steeper, and eventually breaks. This creates two distinct movements of water:

• Swash: The rush of water up the beach slope after a wave breaks.
• Backwash: The water running back down the beach slope toward the sea under the pull of gravity.

Constructive vs. Destructive Waves

Not all waves behave the same way. Geographers classify waves into two main types based on how they shape the beach:

Constructive Waves (Beach Builders)

• Appearance: Low wave height (typically \(< 1\,\text{m}\)) with a long wavelength and gentle slope.
• Wave Frequency: Low frequency of around \(6\text{–}8\) waves per minute.
• Action: They have a strong swash and a weak backwash.
• Result: Because the swash is stronger than the backwash, they push sediment up the beach and leave it there, gradually building up wide, gentle beaches.

Destructive Waves (Beach Destroyers)

• Appearance: High wave height (typically \(> 1\,\text{m}\)) with a short wavelength and steep gradient.
• Wave Frequency: High frequency of around \(10\text{–}14\) waves per minute.
• Action: They have a weak swash and a strong backwash.
• Result: Because the backwash is much more powerful than the swash, destructive waves drag pebbles, gravel, and sand away from the shore, eroding the beach.

Quick Summary Table:

• Constructive: Low height (\(< 1\,\text{m}\)) | Low frequency (\(6\text{–}8\)/\(\text{min}\)) | Strong swash, weak backwash | Builds beach.
• Destructive: High height (\(> 1\,\text{m}\)) | High frequency (\(10\text{–}14\)/\(\text{min}\)) | Weak swash, strong backwash | Erodes beach.

Common Mistake to Avoid: Many students think constructive waves have "more energy" because they push sand up the beach. In reality, destructive waves have far more energy, which is why they tear sediment away and smash cliffs!

2. Coastal Processes: Wearing Away the Land

Coastlines are constantly attacked by two different sets of forces: marine processes (action from the sea) and sub-aerial processes (action from the weather and gravity on cliff faces).

A. The Four Main Marine Erosion Processes

The sea wears away solid rock through four specific mechanisms. You can remember them using the acronym HASH (Hydraulic Action, Abrasion, Solution, Hydraulic action / Attrition) or AASH:

1. Hydraulic Action: The sheer physical force of water crashing against the cliff. Waves force water into tiny cracks in the rock, trapping and compressing pockets of air. When the wave retreats, the compressed air expands explosively. Over time, this constant pressure change shatters the rock and widens the cracks.

2. Abrasion (also called Corrasion): Waves pick up sand, pebbles, and boulders from the seabed and hurled them violently against the cliff face. This acts like coarse sandpaper, gouging, scraping, and wearing the rock away.

3. Attrition: This process happens to the sediment carried in the water, not the cliff itself! Pebbles and rocks carried by waves smash into each other. As they collide, jagged edges break off, making the stones smaller, smoother, and rounder over time.

4. Solution (also called Corrosion): Seawater contains weak acids that chemically dissolve soluble minerals within certain rocks, especially calcium carbonate found in limestone and chalk cliffs.

Common Mistake to Avoid: Do not confuse attrition with cliff erosion. Attrition is rocks bumping into other rocks in the water. Cliffs are eroded primarily by hydraulic action, abrasion, and solution.

B. Sub-Aerial Processes (Weathering & Mass Movement)

Cliffs are not only attacked at the base by waves; the rock face above the water is also weakened by the weather, plants, and gravity.

Weathering

• Mechanical / Freeze-thaw Weathering: Rainwater gets into cracks in the cliff face. When the temperature drops below \(0^\circ\text{C}\), the water freezes and expands by approximately \(\sim 9\%\). This expansion exerts huge outward pressure on the rock. When it thaws, more water enters the widened crack. Repeated freezing and thawing eventually splits the rock apart.
• Biological Weathering: Plant seeds lodge in cliff joints, grow roots, and push rocks apart. Burrowing animals and nesting birds also weaken the cliff top.
• Chemical Weathering: Slightly acidic rainwater reacts directly with minerals in the cliff rock, causing it to decompose.

Mass Movement

When the base of a cliff is undercut by wave erosion and the cliff top is weakened by weathering and heavy rain, the rock becomes unstable. Under the pull of gravity, huge sections can collapse downhill. Types of mass movement include:

• Rockfalls: Rapid collapse of individual rock fragments falling vertically down a steep cliff.
• Landslides: Blocks of rock sliding quickly down a flat slope.
• Rotational Slumping: Saturated soil and weak rock (like clay) slipping downhill along a curved slip plane, often leaving a stepped, terraced cliff profile.

3. Transportation: How Sediment Moves

Once rock has been eroded into sand and pebbles, the sea transports this sediment around the coastline.

Four Ways Sediment is Transported

1. Traction: Large, heavy boulders are rolled along the seabed by the force of moving water.
2. Saltation: Small pebbles and stones are bounced along the seabed in a leapfrog motion.
3. Suspension: Fine, light particles (like silt and sand) are carried along suspended in the body of the water.
4. Solution: Dissolved minerals are carried invisibly in the seawater.

Longshore Drift (LSD): The Zigzag Journey

Have you ever noticed that if you leave your towel on a beach and go swimming, you often drift further down the beach? This is because of longshore drift, the main process that moves sand and shingle along our coastlines.

Step-by-Step Explanation of Longshore Drift:

1. Waves approach the beach at an angle, governed by the prevailing wind direction (the most common wind direction).
2. The swash pushes sand and pebbles up the beach at that exact same angle.
3. When the wave loses energy, the backwash pulls the sediment straight back down the beach slope at a \(90^\circ\) right angle due to gravity.
4. The next wave pushes the sediment up the beach at an angle again, and gravity pulls it straight down again.
5. This continuous cycle creates a net zigzag movement of material along the coast.

Key Exam Pitfall: Remember that swash moves at an angle (following the wind), but backwash ALWAYS returns straight down at \(90^\circ\) (following gravity)!

4. Coastal Landforms

The combination of erosion, transportation, and deposition creates stunning landforms along the coast.

A. Erosional Landforms

1. Headlands and Bays

Headlands and bays form along discordant coastlines, where alternating bands of hard rock (e.g., granite or chalk) and soft rock (e.g., clay or sand) meet the sea at right angles (\(90^\circ\)).

• The soft rock erodes quickly due to hydraulic action and abrasion, forming sheltered, curved inlets called bays.
• The hard rock is much more resistant and erodes much more slowly, sticking out into the sea as rocky projections called headlands.
• In the sheltered bays, wave energy drops, allowing constructive waves to deposit sand and form beaches.

Note: Along concordant coastlines, the rock bands run parallel to the shore, so headlands and bays do not form in this way.

2. Wave-Cut Notch and Wave-Cut Platform

1. Destructive waves attack the base of a rocky cliff between high and low tide marks via hydraulic action and abrasion.
2. Over time, an indent called a wave-cut notch forms at the cliff foot.
3. As the notch deepens, the cliff above is left unsupported (an overhang).
4. Weakened by weathering and gravity, the overhanging rock collapses into the sea.
5. The cliff retreats inland. As this process repeats over hundreds of years, a flat, gently sloping rocky ledge is left at the base of the retreating cliff, visible at low tide. This is called a wave-cut platform.

3. The Cave-Arch-Stack-Stump Sequence

When waves attack a headland, they create a famous sequence of erosional landforms over time:

• Cracks / Faults: Waves exploit natural weaknesses, joints, and faults on both sides of a headland using hydraulic action and abrasion.
• Cave: Continuous erosion widens and deepens these cracks into hollowed-out sea caves.
• Arch: Caves on opposite sides of a narrow headland may erode back-to-back, or a single cave may erode all the way through, forming a bridge-like arch.
• Stack: Sub-aerial weathering weakens the roof of the arch, while wave erosion widens its base. Eventually, the heavy arch roof collapses under gravity, leaving an isolated pillar of rock standing in the sea called a stack.
• Stump: Waves continue to attack the base of the stack. Eventually, it collapses to leave a low, submerged rocky base called a stump, often only exposed at low tide.
• Real-World Example: Old Harry Rocks on the Dorset coast in southern England is a classic example of this sequence.

B. Depositional Landforms

When waves lose energy, they drop (deposit) the sand, shingle, and mud they have been carrying. This creates depositional landforms.

1. Beaches

Beaches are deposits of sand and shingle formed in sheltered areas such as bays. They are created when constructive waves deposit material faster than backwash can remove it.

2. Spits

A spit is an extended ridge of sand and shingle projecting out from the mainland across a river estuary or bay:

1. Longshore drift moves sediment along the coastline in the direction of the prevailing wind.
2. When the coastline suddenly changes direction or reaches a river mouth, sediment continues to be deposited out into the open water in the original direction.
3. The ridge builds up above sea level over time.
4. Occasional changes in wind and wave direction can push the end of the spit inland, creating a recurved hook.
5. In the calm, sheltered water behind the spit, fine mud accumulates, allowing salt marshes to develop.

3. Bars

If a spit continues to grow across a bay from one headland until it reaches the other headland, it seals off the bay. This forms a bar, trapping a body of calm water behind it known as a lagoon.

Quick Chapter Check: Test Your Knowledge

1. What three factors determine the size and energy of a wave?
Answer: Wind speed, wind duration, and fetch.

2. Which wave type has a strong backwash and erodes beaches?
Answer: Destructive waves.

3. Why does backwash always travel at \(90^\circ\) straight down the beach?
Answer: Because of the pull of gravity.

4. What is the correct order of the headland erosion sequence?
Answer: Crack \(\rightarrow\) Cave \(\rightarrow\) Arch \(\rightarrow\) Stack \(\rightarrow\) Stump.

5. What forms behind a spit in the sheltered water?
Answer: A salt marsh.