🌊 Hydraulic action
Waves crash into the cliff and force water and air into cracks. The air is squeezed, and the pressure widens the crack until pieces of rock break off. It attacks the cliff.
Coastal Processes » Coastal Erosion, Transportation and Longshore Drift
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 2.1.1
- The four ways waves erode the coast: hydraulic action, corrosion, corrasion and attrition
- How wave energy, fetch and rock type change the rate of erosion
- How material is transported at the coast, and how longshore drift moves it along a beach
- Why material is deposited, and where
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Unlock This CourseThe coast: where waves do the work
At the coast the sea is the main force that shapes the land. Waves hit cliffs and beaches all day long. Like a river, the sea has three jobs: it erodes (wears things away), it transports (carries material) and it deposits (drops material). Which job it does depends on how much energy the waves have.
Some process names are the same as in rivers, but two of them have a different name at the coast. Look out for them below.
Coastal erosion: four processes
Key terms:
- Corrosion: the coastal name for erosion by solution. Seawater slowly dissolves certain rocks.
- Corrasion: the coastal name for abrasion. Waves hurl rocks and sand at the cliff and wear it away.
🔨 Corrasion
Waves pick up pebbles, sand and boulders and throw them against the cliff, like sandpaper. This is a very powerful process in a storm. It attacks the cliff.
⚡ Attrition
Rocks carried by the waves knock against each other and the beach. They break into smaller, rounder pieces, and shingle slowly becomes sand. It acts on the load, not on the cliff.
🧪 Corrosion
Water slowly dissolves some rocks, such as limestone and chalk, and carries them away in the sea. This is a chemical process. It attacks the cliff.
What controls how fast the coast erodes?
Some coasts retreat by a metre or more every year. Others hardly change. Three things explain the difference.
| Factor | What it is | Effect on erosion |
|---|---|---|
| Wave energy | The power of the waves hitting the coast | Higher energy waves, such as in a storm, erode much faster |
| Fetch | The distance of open water the wind blows over | A long fetch makes bigger waves with more energy, so more erosion |
| Rock type | How hard the cliff is and how many cracks it has | Soft rock (clay, sand) erodes quickly. Hard rock (granite) erodes slowly |
A coast facing an open ocean has a very long fetch. A coast inside a bay or facing a narrow sea has a short fetch, and so gentler waves.
Key terms:
- Wave energy: the power in waves, which decides how much work they can do.
- Fetch: the distance of open water over which the wind blows to make waves.
Transportation at the coast
Waves carry material in the same four ways as a river, which you met in the lesson on river erosion, transportation and deposition. At the coast they work like this:
- Traction: large boulders are rolled along the sea bed.
- Saltation: pebbles are bounced along the sea bed.
- Suspension: fine sand and silt are carried within the water, which can make the sea look cloudy.
- Solution: dissolved minerals are carried invisibly.
Longshore drift
Longshore drift is the movement of sand and shingle (small stones) along a coast in a zigzag pattern. It happens because waves usually hit the beach at an angle. Here is how to explain it, step by step:
- The prevailing wind (the most common wind direction) blows waves towards the coast at an angle.
- The swash carries sand and pebbles up the beach at the same angle.
- Gravity pulls the water back down the beach at 90 degrees to the shore, by the steepest route. This is the backwash, and it carries the material with it.
- The next wave carries the material up the beach at an angle again, so it ends up a little further along.
- Over time the material moves along the coast in a zigzag, in the direction the waves are travelling.
How to sketch it
Draw a beach from above with the sea at the top and a straight line for the shoreline. Draw arrows running up the beach at an angle (swash) and arrows going straight back down (backwash). Join them to make a zigzag, and add a big arrow along the shore for the direction of drift. Label the prevailing wind, swash, backwash and direction of drift.
Key terms:
- Longshore drift: the zigzag movement of material along a beach by swash and backwash.
- Swash: the movement of a wave up the beach.
- Backwash: the movement of water back down the beach.
- Prevailing wind: the most common wind direction in an area.
Deposition at the coast
Deposition happens when waves lose energy and can no longer carry their load. At the coast this happens:
- In sheltered bays, where land sticking out into the sea protects the water, so the waves are small and have low energy.
- Where the coastline changes direction and the waves slow down.
- When there is a lot of material, more than the waves can move.
The biggest, heaviest pieces are dropped first and the finest last. This is how beaches build up. The landforms made by deposition are taught later in this course.
Worked example
A cliff made of clay faces an ocean 3,000 km wide. A cliff made of granite faces a small sea 50 km wide. Which erodes faster, and why? The clay cliff. Clay is a soft rock, so it is worn away easily. The long fetch gives the waves lots of energy, so hydraulic action and corrasion are strong. The granite cliff has hard rock and a short fetch, so erosion is slow.
Common mistakes
Do not say that longshore drift moves material straight along: it zigzags. Do not say that backwash goes at an angle: it goes straight down the beach. Do not mix up fetch (a distance of open water) with wave height. Know both names for the processes, because corrasion means the same as abrasion and corrosion means the same as solution.
Exam-style question
Fig. 1.1 shows made-up data about three cliffs.
| Site | Rock type | Fetch (km) | Cliff retreat (m per year) |
|---|---|---|---|
| A | Granite | 1,000 | 0.1 |
| B | Clay | 1,000 | 1.5 |
| C | Clay | 50 | 0.3 |
Fig. 1.1
(a) Identify the site with the fastest rate of cliff retreat. [1 mark]
(b) Explain why Site B has a faster rate of retreat than Site C. [2 marks]
(c) Explain how longshore drift moves material along a beach, and why some material is later deposited in a sheltered bay. [6 marks]
Model answer
(a) Site B (1)
(b) Site B has a much longer fetch (1,000 km compared with 50 km), so the waves have more energy (1). More powerful waves cause more hydraulic action and corrasion, so the clay cliff is worn away faster (1).
(c) Waves arrive at an angle because of the prevailing wind (1). The swash carries sand and pebbles up the beach at an angle (1). The backwash pulls the material straight back down the beach at 90 degrees (1). Repeated waves move the material in a zigzag along the coast (1). In a sheltered bay the waves are small and have low energy (1), so they cannot carry the material and it is deposited (1).
Exam tip
For a question on how longshore drift works, write numbered steps in order and use the words swash, backwash and prevailing wind. A labelled sketch can earn the same marks.