Coastal Processes (Cambridge (CIE) O Level Geography)

Coastal Processes

Coastal regions

• Where the land meets the sea is called the coast

• The coastline is the edge of the land marked through the high-water mark on a low-lying coast or the foot of steep sloped coasts

• The area between the lowest tide point and the highest point is known as the shore

• Tides are usually twice a day, but vary from coast to coast and with the time of the year

• The difference between low and high tide is known as the tidal range

• The tide controls high low and high the waves can work 

• It is the action of waves and currents that contribute to coastal features

Waves

• Waves are marine processes that erode, transport and deposit material

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

• The size of a wave depends on:

  • The speed of the wind

  • The fetch (distance the wind travels)

  • The amount of time the wind blows (in the same direction)

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

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

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

• There are two types of waves:

  • Destructive waves erode the beach. They have a short wavelength, high-frequency rate and a steep wave gradient. Their backwash is stronger than their swash, which scours the beach, dragging material out to sea 

  • Constructive waves are beach builders. They have a long wavelength, low-frequency rate and a shallow wave gradient. The swash is stronger than its backwash, which carries material up onto the beach and deposits it there  

Comparison of Wave Type

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 

  • Attrition

  • Corrosion 

  • Abrasion 

• The effects of attrition are enhanced when the waves move sediment further and longer

  • A large, rough bolder is eventually eroded into round sand grains (quartz) the longer it stays in the water and the further it travels along the coast

• Rounded pebbles on a beach are known as shingle

Marine Transportation

• The sea transports sediment that it gets from erosion in the same way as a river does

• 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

• Once in the water, the material is moved in different ways:

  • Traction 

  • Saltation

  • Suspension

  • Solution 

Longshore Drift

• It is the main process of transportation along the coast 

• Influenced by the prevailing wind, waves approach the beach at an angle

• 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

Process of longshore drift

• On coasts where longshore drift in one direction, beach sediment is transported further down the coast

• If obstructed, sediment is prevented from moving and the area further along the coast is deprived of sediment

• This causes two issues:

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

  • Removes natural coastal protection 

Marine Deposition

• The movement of waves carries sand or shingle with them

  • Swash carries onto a beach

  • Backwash carries it away

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

• As the backwash moves back down the beach, it loses water and therefore energy as it travels due to the porosity of the sand

• Consequently, the deposition of sediment gets progressively smaller, and the beach is therefore, sorted by wave deposition, with the smallest mud particles settling in the low-energy environment offshore

• If a destructive waveform due to a storm, then a large shingle is thrown above the usual high tide level to form a ridge at the top of the beach called a berm

Sediment Deposition