2B Marine erosion, sediment transport, deposition and subaerial processes

Welcome to the Coastal Engine Room!

Have you ever wondered why some beaches are sandy while others are pebbly, or why some cliffs look like they’ve been sliced with a giant knife? In this chapter, we are looking at the processes—the "engine" of the coast. We will explore how waves, wind, and weather work together to erode, transport, and deposit material to create the landscapes we see today.

Quick Note: This chapter focuses on the work the sea and weather do. To understand how the rock type (geology) affects this, check out the chapter on The littoral zone, geological structure and lithology.

1. Marine Erosion: The Power of the Waves

Marine erosion is the wearing away of the land by the force of the sea. Not all waves are the same, and the way they hit the shore determines how much damage they do.

Constructive vs. Destructive Waves

The "energy" of the coast depends on the wave type:

Constructive Waves: These are "calm" waves. They have a strong swash (the water moving up the beach) and a weak backwash (water pulling back). They build beaches by depositing sediment.

Destructive Waves: These are "angry" waves. They have a weak swash and a very strong backwash. They have high energy and are the main cause of coastal erosion, as they "steal" sediment from the beach.

The Four Horsemen of Erosion

There are four main ways the sea breaks down the land. Think of these as the sea’s toolkit:

1. Hydraulic Action: This is the sheer power of the water. As waves smash against a cliff, air is trapped and compressed into cracks. When the wave retreats, the air expands explosively, shattering the rock from the inside. Analogy: Imagine blowing air into a tiny crack in a wall until it pops!

2. Abrasion (or Corrasion): The sea picks up pebbles and rocks and hurls them against the cliff base. This acts like sandpaper, scouring the rock away. This is usually the most effective form of erosion on British coasts.

3. Attrition: This doesn't erode the cliff; it erodes the sediment. Rocks and pebbles carried by waves smash into each other, breaking into smaller, smoother, and rounder pieces. This is why pebbles at the beach are often so smooth!

4. Corrosion (Solution): Seawater is slightly acidic. It can dissolve certain types of rocks, like limestone or chalk. It’s a chemical reaction, not a physical smash.

Key Takeaway: Erosion is most intense during storms when wave energy is highest. The rate of erosion is influenced by fetch (how far the wind has blown over the water)—the longer the fetch, the more powerful the wave.

2. Marine Transport: Moving the Goods

Once the sea has eroded the rock, it doesn't just sit there. It gets moved! This movement is called transportation.

Methods of Transport

Depending on the size of the "luggage," the sea moves sediment in four ways:

Traction: Large boulders are rolled along the seabed.

Saltation: Small pebbles "bounce" along the seabed.

Suspension: Fine light material (like silt or clay) is carried along in the water, making it look cloudy.

Solution: Dissolved minerals are carried in the water (you can’t see this).

Longshore Drift (LSD): The Great Coastal Zig-Zag

This is a crucial process for your exams! Longshore Drift is the movement of sediment along the coastline in a zig-zag pattern.

Step 1: Waves approach the beach at an angle (driven by the prevailing wind).

Step 2: The swash carries sediment up the beach at that same angle.

Step 3: Gravity pulls the backwash straight down the beach (at a \(90^{\circ}\) angle).

Step 4: This repeats, moving sediment along the shore in a "zig-zag."

Quick Tip: If you are asked to draw this, always include an arrow for the "Prevailing Wind" and label the Swash and Backwash clearly!

3. Marine Deposition: Dropping the Load

Deposition happens when the sea loses energy. If the water slows down or the waves become less powerful, they can no longer carry their "luggage," so they drop it.

Deposition is likely to happen when:
• Waves enter a sheltered bay.
• There is a large amount of sediment available.
• The swash is stronger than the backwash (Constructive waves).
• There is an obstacle, like a groyne or a breakwater.

Key Takeaway: Deposition creates "landforms of accumulation" like beaches, spits, and bars. Without deposition, we wouldn't have many of our favorite seaside holiday spots!

4. Subaerial Processes: The "Hidden" Shapers

Students often forget that the sea isn't the only thing shaping the coast. Subaerial processes happen on the cliff face, above the reach of the waves. They "prepare" the cliff for erosion.

Weathering

Weathering breaks down rock in situ (where it is).
Mechanical (Physical) Weathering: e.g., Freeze-thaw. Water gets into cracks, freezes, expands by roughly \(9\%\), and wedges the rock apart.
Chemical Weathering: e.g., Carbonation. Rainwater absorbs \(CO_{2}\) to become a weak carbonic acid, which reacts with calcium carbonate in rocks like limestone.
Biological Weathering: Plant roots growing into cracks or birds/animals burrowing into the cliff.

Mass Movement

This is the movement of material downslope under the force of gravity. It often happens when cliffs become saturated with rainwater.
Rockfall: Bits of rock fall vertically (common on steep, hard rock cliffs).
Landslides/Slumps: Large "blocks" of land slide down. In rotational slumping, the land slides down a curved surface. You’ll often see this on "soft" rock coasts like Holderness.

Did you know? Many "coastal" collapses are actually caused by heavy rain weakening the cliff from above, rather than the sea attacking from below!

5. Summary: How it all fits together

To master this topic, remember that the coastline is a system:

1. Inputs: Weathering and Erosion break the rock down.

2. Processes: Longshore Drift and Marine Transport move that material around.

3. Outputs: Deposition creates new landforms (like spits), or material is lost to the deep ocean.

Quick Review Questions:

1. Which wave type has a stronger backwash than swash? (Answer: Destructive)

2. What is the name of the "zig-zag" movement of sediment? (Answer: Longshore Drift)

3. Is freeze-thaw an example of marine erosion or subaerial weathering? (Answer: Subaerial weathering)

Common Mistake to Avoid: Don't confuse erosion (the sea attacking the land) with weathering (the atmosphere attacking the land). In an exam, mentioning both will show the examiner you understand the whole picture!