2B.4B Wave Erosion Processes
The importance of wave erosion processes (hydraulic action, corrosion, abrasion, attrition) and how they are influenced by wave type, size and lithology.
There are four wave erosion processes:
How these are influences by wave type, size and lithology
There are four wave erosion processes:
- Hydraulic action
- Corrosion
- Abrasion
- Attrition
How these are influences by wave type, size and lithology
- They are most effective during high energy storm events with large destructive waves.
- However, even coastlines composed of soft, unconsolidated sediment (e.g. boulder clay of Holderness Coast in Yorkshire), experience little erosion under normal conditions.
- Most erosion (in the UK) occurs in the winter, in high energy storms.
- It's faster when the wind is blowing directly onshore
- It's faster when the tide is high (bringing deeper water closer to the cliff so less energy is lost to friction before impact)
The effect of erosion
- The boulder clay of the Holderness coast has retreated by 120 m in the last 100 years.
- The granite of Land's End in Cornwall has retreated by only 10 cm in the last 100 years.
Hydraulic Action
- Where the force of water itself breaks up rock
- It can occur through:
- the direct impact of the water itself
- Plunging destructive waves can exert a force of 50 kg / cm^3
- This is sufficient to break off material from unconsolidated material, e.g. boulder clay, or weak rocks like clay and shale.
- the force of the breaking wave compressing air into cracks in rocks
- when the wave energy is exhausted, the compressed air explodes outwards, causing micro-fractures in rock and expanding the main crack.
- over time, small fragments of rock become weakened and break away
- or, the main crack can extend until larger slabs of rock fall
- the direct impact of the water itself
The pressure forces the crack open, meaning more air is trapped and greater force is experienced in the next cycle of compression.
It dislodges blocks of rock from the cliff face.
In hard, resistant igneous rocks, hydraulic action attacking its cooling joints may be the only effective wave erosion process.
High energy waves with a large wave height are the most effective at erosion through hydraulic action. It is also accentuated when there is no debris at the cliff foot to absorb some of the wave energy and protect the cliff base.
Abrasion (a.k.a corrasion)
Abrasion is where a wave picks up sediment and throws these load items against a rock. The repeated impact chips away at the rock face until small fragments break away.
Most effective: High-energy destructive waves with a large wave height hurl load items with greater force, resulting in faster rates of erosion by abrasion.
They need a supply of hard load items close to the foot, e.g. shingle from the beach.
Rocks eroded most quickly by abrasion: Soft sedimentary rock such as chalk, mudstones, and clays, and unconsolidated material, e.g. boulder clay.
Most effective: High-energy destructive waves with a large wave height hurl load items with greater force, resulting in faster rates of erosion by abrasion.
They need a supply of hard load items close to the foot, e.g. shingle from the beach.
Rocks eroded most quickly by abrasion: Soft sedimentary rock such as chalk, mudstones, and clays, and unconsolidated material, e.g. boulder clay.
Corrosion
Corrosion is where water in waves dissolves rock minerals. Minerals are immediately carried away by the wave in solution. They are also vulnerable to erosion by rainwater and sea spray.
Most effective waves: Constructive waves, as the force of impact is not relevant, and the spilling wave increases the time for the chemical reaction to occur. They are slow, and with a long wavelength (longer the better) it prolongs the contact of rock with the water.
Rocks eroded most quickly by corrosion: carbonate rocks like limestones (e.g. chalk, Jurassic limestone and carboniferous limestone) and sedimentary rocks with calcite sediment/cement.
Most effective waves: Constructive waves, as the force of impact is not relevant, and the spilling wave increases the time for the chemical reaction to occur. They are slow, and with a long wavelength (longer the better) it prolongs the contact of rock with the water.
Rocks eroded most quickly by corrosion: carbonate rocks like limestones (e.g. chalk, Jurassic limestone and carboniferous limestone) and sedimentary rocks with calcite sediment/cement.
Attrition
Attrition is where material transported by a wave is eroded through collision with other load items. It breaks down sediment into smaller sized particles, and the repeated collision blunts any of the particles' sharp edges, making the sediment increasingly rounded. Even harder rocks, such as quartz and granite form larger rounded shingle pebbles.
It occurs in the foreshore and nearshore zones, where sediment is moved by swash and backwash.
Rocks eroded most quickly by attrition: soft rocks (e.g. poorly cemented sandstones, chalk and clay) are broken down quickly by attrition into silt and sand grains.
It occurs in the foreshore and nearshore zones, where sediment is moved by swash and backwash.
Rocks eroded most quickly by attrition: soft rocks (e.g. poorly cemented sandstones, chalk and clay) are broken down quickly by attrition into silt and sand grains.