Coastal Systems: How Erosion, Transportation and Deposition affect the Coastline

The processes of erosion, transportation and deposition occur as a result of differing wave and current energy. Where marine energy is high, erosion, removal and transportation is more likely to take place. Where energy is reduced, deposition of transported material is more of a feature. It is important to recognise that the wave type affects the process that occur.

Erosion

The different methods of erosion are summarised below:

Hydraulic action – this process occurs where the power of the waves hits the cliff face directly and loosens the interior of joints and bedding planes. Cavitation is a similar process. As the wave smashes into a crevice, it compresses air bubbles that effervesce (fizz) powerfully as the wave (and pressure) begins to recede, and creates a mini explosion. Rock within the crack will be loosened and over time this process will widen the opening further.

Wave quarrying – when high energy, tall waves hit the cliff face they have the power to enlarge joints and remove large chunks of rock in one go through vibration. This occurs due to the intense force of these waves.

Corrasion / abrasion – this process occurs when high energy waves have the energy to be able to carry pebbles with force. As the wave breaks at the foot of the cliff, material is thrown at the cliff face and wears it away by chipping fragments off.

Solution – this process occurs when weak carbonic acid in seawater dissolves rock at the coastline and breaks it down (particularly that containing calcium carbonate such as limestone and chalk).

Attrition – this is when rocks and boulders which have already been loosened from the coastline are ground over each other in the water – usually by repeated swash and backwash - and over time the angular edges become smoothed and rounded and boulders/fragments become pebbles, pebbles become shingle, and shingle may become sand.

Run-off – this is when rivers & streams erode away their channel as they approach the mouth. Increased erosion of coastal landscapes can occur because of this. However, runoff can interfere with other process. Often, at river estuaries there is an increase in coastal deposition as the river current entering the sea increases turbulence and friction with marine currents, reduces the energy of both, resulting in deposition of fluvial and marine sediment.

Transportation

Most material is moved along the coastline by a process called longshore drift (littoral drift). This occurs when the waves approach the coastline at an oblique angle as a result of the wind driving them onwards at the same angle. The waves push material at an angle up the beach via the swash and it is then deposited on the beach. The backwash will then collect the material and pull it down the beach in a vertical direction back out to sea. Another wave will then collect that material and carry it obliquely towards shore again.

The process continues until material is moved along the coastline in a zig-zag movement. Some of this material is held within the water itself as suspension.

When waves are predominantly constructive it is this process that creates and enlarges beaches. Larger material which has not been reduced down by attrition is unlikely to be picked up and carried by the waves, unless there are storm conditions and the waves are high energy. This material will be moved along the sea bed by the power of the current. This is called traction.

Deposition

This process occurs when wave energy is reduced and friction overcomes the available energy. The wave has to drop material at this point. This can occur in a number of circumstances.

Firstly, during the process of longshore drift, where the coastline changes and longshore drift continues to move material, energy will reduce and the transportation will stop, depositing material offshore as a spit.

Sometimes when the coastline changes, there is a river estuary. The flow of water from the river into the sea can further reduce the energy of the waves; hence deposition may occur – requiring constant dredging to keep port routes open.

At a headland, wave energy is intensified at the promontory due to refraction and low energy waves then splay out into the bays either side. Material is then deposited in the bays due to reduced wave energy.

The formation of constructive waves also leads to deposition. Constructive waves have a low frequency, a long wavelength and a low amplitude. When they break on a beach, they have a strong swash and a weak backwash. This means they deposit more material on a beach as the strength in the swash allows the wave to travel far up the beach before deposition takes place. When they have deposited material on the beach, the energy in the wave is lost and much of the water percolates into the beach material and therefore the backwash is weak. This means material will be left on the beach with a net accumulation. The repeated action of pushing material up a beach eventually leads to the development of ridges, or berms on a beach.