Coastal Erosion

Coastal Erosion

The power of the sea has shaped Ireland’s coast into what we can see today. Two main processes are responsible for this; erosion and deposition. Coastal erosion is the breaking down and carrying away of materials by the sea. Deposition is when material carried by the sea is deposited or left behind on the coast.

Destructive Waves
Coastal erosion takes place with destructive waves. These destructive waves are very high in energy and are most powerful in stormy conditions. The swash is when a wave washes up onto the shoreline and the backwash is when the water from a wave retreats back into the sea. Destructive waves have stronger backwashes than swashes. This strong backwash pulls material away from the shoreline and into the sea resulting in erosion.

Constructive Waves
Constructive waves, on the other hand, are low energy waves that result in the build-up of material on the shoreline. Constructive waves are low energy and have stronger swashes than backwashes. This means that any material being carried by the sea is washed up and begins to build up along the coastline. The material that is deposited by constructive waves can most often be seen by the creation of beaches.

How Waves Erode
Destructive waves erode through four main processes; Hydraulic Action, Compression, Abrasion and Attrition.

coastal erosion in San Francisco
Image credit: Jeff Hansen, U.S. Geological Survey.

Hydraulic Action

Hydraulic Action is the sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea.


Compression occurs in rocky areas when air enters into crack in rock. This air is trapped in cracks by the rising tide, as waves crash against the rock the air inside the crack is rapidly compressed and decompressed causing cracks to spread and pieces of rock to break off. Compression is one of the main processes that result in the creation of caves.

Abrasion is when rocks and other materials carried by the sea are picked up by strong waves and thrown against the coastline causing more material to be broken off and carried away by the sea.


Attrition is when material such as rocks and stones carried by waves hit and knock against each other wearing them down. As these materials are worn down sand and rounded beach pebbles are formed.

Sea cliffs are one of the clearest examples of sea erosion that we can see. Sea cliffs are steep faces of rock and soil that are formed by destructive waves. Waves crashing against the coastline erode until a notch is formed. The erosion of this notch undercuts the ground above it until it becomes unstable and collapses. This process repeats itself and the sea cliff will continue to retreat. Overtime a wave-cut platform will be formed in the sea just beneath the cliffs. This wave-cut platform will form at the low-tide level and is evidence of where the cliff face once stood before erosion caused the cliff face to retreat.

Sea Caves
Sea Caves form when cracks in rock at the base of cliffs are eroded and expanded by the sea. The processes of compression and hydraulic are key to the creation of sea caves.

Sea Arch
Sea Arches are formed when a cave continues to be eroded and expanded until it cuts right through a headland.

Sea Stack
A sea Stack forms when a sea arch continues to be eroded and widened until the rock becomes too weak to support the roof of the sea arch and collapses into the sea. The remaining pillar of rock is known as a sea stack.

Sea Stump
A sea stump is formed when a tall sea stack is eroded and worn down until it juts just above the surface of the sea.