Erosion (Hydraulic Action, Abrasion, Attrition)

Erosion is the process by which natural forces like water, wind, and ice wear away rocks and soil. In coastal environments, erosion shapes the landscape by breaking down cliffs and transporting sediment. Three main types of coastal erosion are hydraulic action, abrasion, and attrition.

Hydraulic Action

Hydraulic action is a powerful coastal erosion process where the force of the waves breaks rock apart. When waves crash against cliffs, water is forced into cracks and joints in the rock. This compresses air trapped inside these cracks, increasing pressure. When the wave retreats, the pressure is suddenly released, causing the rock to weaken and break off over time.

This process is especially effective on cliffs with many joints or faults. Over time, hydraulic action widens cracks, leading to pieces of rock breaking away and the cliff retreating.

For example, along the Jurassic Coast in Dorset, hydraulic action helps to erode the soft clays and limestones, contributing to the formation of features like caves and wave-cut platforms.

The power of hydraulic action depends on wave energy, which is influenced by wind strength, fetch (the distance over which the wind blows), and wave height.

For instance, during a storm, waves are stronger and can force more water and air into cracks, increasing erosion rates.

Abrasion

Abrasion, also called corrasion, occurs when waves carry sediment such as sand, pebbles, and larger rocks and smash them against the cliff face. This sediment acts like sandpaper, scraping and wearing down the rock surface.

This process smooths rough cliff surfaces and can create features such as wave-cut platforms. Abrasion is most effective when waves are powerful and carry large amounts of sediment.

For example, on the Holderness Coast in East Yorkshire, fast-eroding soft boulder clay cliffs are worn down by abrasive action of sediment carried by destructive waves.

Abrasion depends on:

• The amount and size of sediment carried by waves
• The strength and frequency of waves
• The hardness of the rock being eroded

For instance, harder rocks like chalk erode more slowly by abrasion than softer rocks like clay.

Attrition

Attrition is the process where rock fragments carried by waves collide with each other. These collisions cause the fragments to break into smaller, smoother, and rounder pieces.

Unlike hydraulic action and abrasion, attrition does not directly erode the cliff but reduces the size of sediment, making it easier for waves to transport. However, the smaller sediment produced by attrition can increase erosion indirectly by enhancing abrasion and other processes.

For example, on the beaches of Cornwall, large rock fragments are gradually worn down by attrition into smaller, rounded pebbles and sand.

Attrition depends on:

• The amount of sediment in the water
• The energy of the waves causing sediment to collide
• The hardness of the rock fragments

For instance, during storms, waves have more energy, increasing collisions and speeding up attrition.

An important effect of attrition is that it produces smaller sediment that can be transported further along the coast by processes like longshore drift.

For example, large boulders broken off cliffs are gradually worn down into sand-sized particles through attrition.