Geography
Coastal processes
Coasts are shaped by the sea and the action of waves. Waves act in different ways, through the processes of erosion, transportation and deposition.
The action of waves
The power of waves is one of the most significant forces of coastal change. Waves are created by wind blowing over the surface of the sea. As the wind blows over the sea, friction is created - producing a swell in the water. The energy of the wind causes water particles to rotate inside the swell. This moves the wave forward.
The size and energy of a wave is influenced by:
- the length of time that the wind has been blowing.
- the strength of the wind.
- how far the wave has travelled (called the fetch).
The activity below shows the length of fetch along the south coast of England:
Waves can be destructive or constructive.
When a wave breaks, water is washed up the beach: this is called the swash. Then the water runs back down the beach: this is called the backwash. With a constructive wave, the swash is stronger than the backwash. With a destructive wave, the backwash is stronger than the swash.
Constructive and destructive waves
Destructive waves
The effects of a high wave
- operate in storm conditions.
- are created from big, strong waves when the wind is strong and has been blowing for a long time.
- occur when wave energy is high and the wave has travelled over a long fetch.
- tend to remove material from the coast and are associated with erosion. [erosion: the wearing away of rocks, soil or other solid materials ]
- backwash is stronger than the swash.
- have a short wave length and are high and steep.
Constructive waves
The effects of a low wave
- operate in calm weather.
- are less powerful waves.
- break on the shore and tend to deposit material, building up beaches.
- are responsible for transporting material.
- swash is stronger than the backwash.
- have a long wavelength, a low height and are less steep than destructive waves.
Coastal erosion
The sea changes and shapes the coastal landscape. Coastal erosion is the wearing away and breaking up of rock and beach material found along the coast. Destructive waves will erode the coastline in the following ways.
- The constant force of waves crashing on the shore damages it. In addition air may become trapped in joints and cracks on a cliff face. When the wave breaks the air becomes compressed, weakening the cliff and causing erosion. This is called hydraulic action.
- Waves bring with them bits of rock and sand. These grind down cliffs like the action of sandpaper. This is called abrasion.
- Waves cause rocks and pebbles on the shore to smash into each other and break down and become smaller and smoother. This is called attrition.
- Acids contained in sea water will slowly dissolve certain types of rock such as chalk or limestone. This is called corrosion or solution.
Hydraulic action, abrasion, attrition and solution are demonstrated in the diagram below:
Transport
Waves sometimes approach the coast at an angle; this is because of the direction of the prevailing wind. This causes the waves to break on the beach at the same angle as the prevailing wind. The swash of the waves carries material up the beach at an angle. The backwash then flows back to the sea in a straight line at 90 degrees due to the influence of gravity. This movement of material is called transportation.
On many coasts the combined effect of continually repeating swash and backwash is to transport material sideways along the coast. This movement of material along coasts is called longshore drift and occurs in a zigzag motion along the shoreline.
Coastal transport
There are four methods by which waves and tidal currents can transport sediment. These processes can then contribute to the movement of sediment by longshore drift.
| Process | Description |
|---|---|
| Solution | Soluble minerals are dissolved in sea water and carried along in solution. The load is not visible. Load can come from coastal cliffs made from chalk or limestone, and calcium carbonate is carried along in solution. |
| Suspension | Small particles are carried along in the water, eg silts and clays. When large amounts of sediment are carried in suspension, it can make the water look cloudy. Currents have the energy to pick up large amounts in suspension during a storm, for example when strong winds generate high energy waves. |
| Saltation | Load is bounced along the sea bed, eg small pieces of shingle or large sand grains. This size of sediment is larger and heavier, therefore the currents cannot keep them afloat for long periods. |
| Traction | Pebbles, and larger sediment, are rolled along the sea bed with the force of the water. |
Deposition
When the sea loses energy, it drops its load of sand, rock particles and pebbles, which it has been carrying. This is called deposition. Deposition happens when the swash is stronger than the backwash and is associated with constructive waves.
Deposition is likely to occur when:
- waves enter an area of shallow water.
- waves enter a sheltered area, eg a cove or bay.
- there is little wind.
- there is a good supply of material.
Sources of material include:
- erosion from cliffs.
- transported by longshore drift along the coastline.
- brought inland from offshore by constructive waves.
- rivers carrying sediment to the coastline.
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