Coastal Definitions

Erosion Processes

Hydraulic Action

Erosion caused by the sheer force of waves striking a coastline

Compressed Air

Waves approaching the coastline force air into the joints in the rocks which make up the coastline.  This exerts pressure on the joints and eventually weakens them and makes them wider.


Stones and pebbles carried by sea water strike the coastline and chip away at the rocks which make up the coastline


Stones and pebbles are worn away by constantly striking off each other.

Destructive Waves

Waves which remove material from a beach.  The incoming wave (swash) is weaker than the returning wave (backwash).


Transportation Processes

Longshore Drift

This is the movement of material along a beach.

When waves approach a coastline at an angle the waves washes sand upwards on a beach at an angle.

The returning wave brings material back down the beach at a right angle.  This happens constantly and moves material along the beach.

Deposition Processes


Constructive Waves

Waves which add material to a beach.

The approaching wave (swash) is stronger than the returning wave (backwash).

Longshore Drift

As above – material is transported from one location on a beach to another – it is eroded at one point and deposited at another.


This is the bending of sea waves as they approach a coastline.

When waves approach a broken, irregular or indented coastline, the waves bend.  This concentrates erosion at certain points on the coastline and also causes deposition at other points. Remember – when water slows down it loses energy  and so it deposits.


The processes of coastal erosion and deposition lead to the formation of a variety of features which make each coastline uniquely different from others.  This essay examines these processes and discusses the formation of two features typical of coastal erosion and two features typical of coastal deposition.

Coastal erosion results from the interaction of four processes namely hydraulic action, abrasion, attrition and compression.  Hydraulic action causes erosion through the sheer force with which water lashes a coastline.  When combined with abrasion and compression, this force can be very considerable.  Compression leads to erosion when waves force air in between rock joints and bedding planes causing rock to weaken and shatter.  Abrasion erodes a coastline through the force with which the sea hurls rock fragments, stone and other materials against the coastline.  Attrition is the continual wearing down of rock debris when it contacts other debris. This process is responsible for the eventual creation of sand and shingle.

Destructive waves and wave refraction aid erosion.  The formation of waves depends on the fetch – the distance over which wind blows.  Stronger winds and long fetches lead to stronger waves and greater erosion potential.  Destructive waves are the strongest waves and occur during storms with a frequency of 12 waves per minute.  The backwash of these waves are stronger than their swash consequently material eroded by the four erosion processes is carried out to sea.

Wave refraction occurs when waves approach an irregular coastline in a parallel fashion.  In this case, wave energy is concentrated on promontories such as headlands, erosion occurs and the consequent loss of energy leads to deposition between headlands.  Refraction along a straight coastline such as a beach occurs when waves approach the coastline obliquely (at an angle).  This form of refraction can aid erosion and Longshore drift.

Marine deposition occurs whenever the swash or incoming wave moving up a beach is greater than the backwash – the volume of water moving down the beach through percolation.  Whenever the swash is great and the percolation gentle, the incoming waves are termed constructive waves.  This is because material will be added to the beach whilst little or none will be removed by the backwash.

Longshore drift aids deposition.  When an oblique wave reaches a coastline, material is swashed diagonally up the beach.  The backwash carries material vertically back down the beach where it meets another incoming wave which moves it diagonally up the beach again.  In this way, longshore drift moves material along a beach.


The interaction of these processes produce well defined coastal landforms