Coastal Processes

Coastal processes

Coastal processes

• The coast is the area where land and sea meet

• Coastal processes are divided into two parts:

  • Marine processes: offshore (water-based)

  • Terrestrial processes: onshore (land-based)

• These processes are further divided into:

  • Erosion

  • Transportation

  • Deposition by wave action

• Together with the local geology, these processes produce distinctive landforms around the coastline

  • A coastline made up of softer rocks such as sands and clays will be easily eroded by destructive waves to form low, flat landscapes such as bays and beaches

  • Coastlines of more resistant, harder rock will take longer to erode and produce rugged landscapes such as headlands

Marine erosion

• Destructive waves are responsible for the majority of erosion that happens along a coast

• They cut into the coastline in four ways:

  • Hydraulic action is the sheer force of the waves hitting the coast 

  • Attrition occurs when material carried in the waves bumps against each other and becomes smaller and smoother

    • This does not erode the coast but forms the sand and shingle

  • Corrosion, also known as solution, occurs because seawater is slightly acidic and gradually dissolves material. Some types of coastal rock such as limestone and chalk, are particularly prone to corrosion

  • Abrasion, also known as corrasion, occurs when waves pick up material and hurl it at the cliffs 

Marine transportation

• The sea transports sediment that it gets from erosion in the same way rivers do

• Material in the sea arrives from many sources:

  • eroded from cliffs

  • transported by longshore drift along the coastline

  • brought inland from offshore by constructive waves

  • carried to the coastline by a river

• In the water, material is moved through:

  • Traction when large, heavy material is dragged along the sea floor 

  • Saltation occurs when smaller material bounces along the sea floor

  • Suspension is the fine material held in the seawater

  • Solution is dissolved material carried in the seawater 

Longshore drift (LSD)

• Longshore drift (LSD) is the main process of transportation along the coast 

• Waves approach the beach at an angle due to the prevailing wind

• As the waves break, the swash carries material up the beach at the same angle

• As the swash dies away, the backwash carries the material down the beach at right angles (90°) 

• The process repeats, transporting material along the beach in a zig-zag movement

• Beach sediment is carried farther down the coast on coasts where longshore drift is one way; if it is blocked, sediment cannot move, and the area farther down the coast is deprived of sediment. This results in two problems:

  • Smaller beaches are less attractive to tourists, causing a loss of income

  • Removes natural coastal protection 

Marine Deposition

• The waves carry sand or shingle as they travel

  • Swash carries sediment onto the shore

  • Backwash carries sediment away

• The largest material is deposited along the upper reach of the swash when a constructive wave carries sediment up the beach

• The backwash loses water and energy as it travels because of the sand's porosity, resulting in smaller and smaller deposits.

• When a constructive wave carries sediment up the beach, the largest material is deposited along the upper reach of the swash

• During a storm, large shingle is thrown above the usual high tide level to form a ridge at the top of the beach called a berm

Wave types

Wave types

• Waves are marine processes

  • They erode, transport and deposit material

• Waves are formed by winds blowing over the surface of the sea 

• The height and strength of a wave are dependent on three factors:

  • the fetch 

  • the amount of time the wind blows

  • the strength of the wind

• The greater the strength, time and fetch of the wind, the larger the wave

• As a wave approaches the coast, it enters shallower water; friction from the seabed causes the wave to lean forward and eventually crest and break onto the beach 

• The movement of water up the beach is called the swash, and the return movement is the backwash

Types of waves

• There are two types of waves:

  • Destructive waves erode the beach

  • Constructive waves are beach builders

Constructive waves

• Constructive waves, also known as beach builders, have the following characteristics:

  • a long wavelength with low height

  • a low-frequency wave rate of 6-8 per minute

  • a shallow wave gradient

  • low energy

  • a stronger swash that carries material up onto the beach and deposits a gently sloping, sandy beach

Destructive waves

• Destructive waves erode the beach and have:

  • a steep wave gradient

  • a short wavelength with high height

  • a high-frequency wave rate of 10-12 per minute

  • high energy

  • a strong, abrasive backwash that drags material out to sea and forms steep, shingle beaches

Wave refraction

• Coastlines have different-depths of water and reflect waves

• The depths vary because headlands and bays have formed over time

• As waves travel into shallower water near the beach, they slow down

• This makes the wave bend or refract to fit the shape of the shoreline

• When waves are refracted, the energy is focused on the headland

• This cause more erosion to the headland than bays, where sediment is deposited