Coastal Erosion and Deposition

Shaping coastlines

Waves are the primary drivers in shaping coastlines. Destructive waves (waves that have a stronger backwash) are the most common types of waves that cause erosion . These waves occur during stormy conditions and are characterised by big, strong waves that have high energy. Constructive waves (with a limited backwash) occur during calmer weather, have low energy, and cause deposition rather than erosion. Because of their characteristics, destructive waves can erode the coastlines in several ways:

• Hydraulic action – air in cracks in the cliff is compressed when waves crash in. The pressure caused by this action breaks off pieces of rocks.
• Attrition – bits of rock in water smash against each other and break into smaller pieces.
• Abrasion – bits of rock and sediment transported by the waves, smash and grind against rocks and cliffs, breaking bits off and smoothing the surface.
• Corrosion – soluble rocks get gradually dissolved by the seawater.

Coastal formations

The principle marine processes responsible for shaping the coastline are erosion , transportation , and deposition. Erosion is where the force of waves breaks down the land. Transportation is when waves and tides transfer the broken material somewhere else. Meanwhile, deposition is when waves and tides lose their energy, cease transporting the eroded material, and deposit it. Each coastline has its balance between the processes of erosion, transportation, and deposition.

Erosional coastal formations include:

1. Headlands and bays.
2. Cliffs and wave-cut platforms.
3. Stacks and stumps.
4. Shoreline platforms.

Duncansby Stacks Image: Bill C CC BY-SA 3.0

When waves no longer have any energy left to transport the sediment, deposition occurs. What features are formed by the sediment will depend on how and where the sediment is deposited.

Depositional coastal formations include:

1. Spits.
2. Beaches.
3. Offshore bars and tombolos.
4. Cuspate forelands.
5. Salt marshes and estuarine mudflats.
6. Sand dunes.

Sediment transportation

The energy provided by waves, tides, and currents transport the eroded material. There are four main processes involved in sediment transportation:

• Solution – substance that is dissolved and carried along in the water.
• Saltation – larger particles are too heavy to be carried, so they are bounced along the sea bed.
• Suspension – this is where very fine material is carried along in the water.
• Traction – enormous particles are pushed along the sea bed by the force of the water.

These processes can transport sediment along the shore; this is known as longshore drift or littoral drift .

Sediment is moved along the coast in sediment cells. Within each cell, the sediment moves between the beach, cliffs, and the sea through the process of erosion, transportation, and deposition. Any action taken in one place has an impact elsewhere in the cell. Each cell operates between physical barriers that prevent the sediment from moving any further along the coast. There are 11 principal cells along the coastline of England and Wales.

Sub-aerial processes

Sub-aerial processes are land-based processes that alter the shape of the coastline. The main sub-aerial processes are weathering and mass movement.

Weathering

The gradual breakdown of rocks in situ at or close to the ground surface is known as weathering. This can be divided into three different types - mechanical, chemical and biological.

Mass movement

This is the movement of materials downslope at a range of speeds. Water acts as the common lubricant involved in mass movement.

Types of mass movement are:

• Soil creep

• Solifluction

• Earth and mudflows

• Rock falls

• Rock / debris slides

• Slumps