Sediment Transport & Deposition (Edexcel A Level Geography)

Influences on Sediment Transport

• 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

Direction of wave attack

• The direction that the waves are coming from is the main factor affecting the direction of sediment transport

Longshore (littoral) drift

• Longshore (littoral) drift is the main process of deposition and 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

• Offshore currents can contribute to the movement of sediment along or up the beach

Current

• Current is the flow of water in a specific direction which transports sediment

• Surface currents are formed mainly by wind

• Deepwater currents are also called thermohaline circulation and are caused by density differences 

• Tidal currents are associated with rising and falling tides, the greater the tidal range the greater the strength of the currents

• Rip currents are underwater currents that occur near the coastline and transport sediment a few metres out to sea

Tides

• The gravitational pull of the sun or moon causes tides 

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

• Larger tidal ranges produce stronger tidal currents and this can transport more sediment, particularly in the areas around estuaries

• Deposition occurs when waves lose energy 

• Waves lose energy when: 

  • The wind slows or changes direction 

  • The wave experiences friction, usually with the sea floor as the depth decreases 

  • When the coastline changes direction such as at an estuary or headland

• It is a gradual rather than immediate process 

• High energy coastlines deposit large rocks and shingle but generally maintain enough energy to carry smaller sediment particles 

  • This is why these types of coastlines tend to have rocky beaches, such as Brighton on the south coast of England

• Low energy coastlines deposit smaller sediment due to low wave velocity, creating sandy beaches

Transportation & Deposition Landforms

• Depositional landforms occur at low energy coastlines when the waves do not have enough energy to carry a large sediment load

• The most common depositional landform is a beach

  • Beaches build up due to constructive waves

  • They form in sheltered areas such as bays

  • Beach formation is more common in summer when there are fewer destructive waves

• Beaches can be drift-aligned or swash-aligned 

  • Drift-aligned beaches form where longshore drift moves the sediment along the beach as waves approach at an oblique angle 

  • This will often culminate in a spit where the coastline changes direction 

  • Swash-aligned beaches form where the energy is low 

  • The waves are more parallel to the shore in swash aligned environments so there is little horizontal or lateral movement of sediment 

• Spits are created due to longshore drift

  • A spit is an extended stretch of sand or shingle that extends out to sea from the shore 

  • Sediment is transported by waves along a coastline

  • When the coastline changes direction the waves no longer have the energy to carry the sediment and it is deposited 

  • The build-up of deposited sediment out to sea, often into an estuary, is called a spit 

  • The spit often has a curved end or hooks due to a secondary prevailing wind

• As spits form across an estuary or around a headland there can also be the formation of tombolos and bars where the sediment meets either an island or the opposite side of a headland respectively 

  • These are formed in the same way, through longshore drift and deposition

  • Bars can also form offshore

  • These are raised ridges of sediment away from the shore 

  • They form where sediment level is high, and the sea is shallow 

• Bars that form as spits extend to join two headlands are also known as barrier beaches

  • Water trapped behind the barrier beach is called a lagoon

• Sandbars can also form offshore due to the action of breaking waves

Lagoon

• A lagoon is where a small body of water is cut off from the sea 

• A lagoon may form behind a bar or tombolo

• Lagoons do not last forever and may fill with sediment and form new land

Tombolo  

• A tombolo is formed when a spit joins the mainland to an island

• Chesil Beach in Dorset is a tombolo, as the mainland is joined to the Isle of Portland 

Barrier island

• Barrier islands form parallel to the coast 

• The main difference between a bar and a barrier island is that a bar joins two headlands, whereas a barrier island is open at one or both ends

Salt marshes and sand dunes

• Behind spits and barrier beaches the area is very sheltered, tidal currents and river currents meet leading to lots of deposition 

• This can lead to the formation of salt marshes 

• Salt-tolerant species colonise these sheltered, flat muddy areas 

• They are covered at high tide and exposed at low tide 

• Deposition can also lead to the formation of sand dunes 

  • Sand dunes are formed in areas where there is a large quantity of sand, a large tidal range, and onshore prevailing winds 

  • When sand is trapped towards the back of the beach due to an obstacle then dunes will develop forming rows at right angles to the prevailing wind

  • Pioneer plant species hold the sand together and stabilise the dune 

  • Other plants are then able to thrive in the environment which leads to the development of the dune ecosystem, this is called plant succession

Sediment Cell Concept

• There are a range of sources of sediments in the coastal zone 

• Most coastal sediment is brought to the shore by rivers

• Cliff erosion is also an important source of sediment at most coastlines

  • Where coastlines are retreating by a few metres a year there is a large input of sediment to the coastal zone 

• Wind can also transport sediment to the shore and this can lead to the build-up of sand dunes 

• In high latitude coastal areas glaciers break off into the sea and sediment that was trapped within the ice is deposited at the coastal zone 

• Sediment Cells are an important way of understanding the coastal system

  • They can be considered a closed system as most sediment movement is contained within the cell 

  • There are 11 sediment cells around the UK coastline 

  • Sediment cells can be divided into sub-cells where the inputs (sources), stores (sinks), transfers, and outputs within the system can all be identified 

• The sediment budget is the balance between the inputs and outputs of sediment in the system 

• Coastal systems should be in a state where the sediment budget is in a state of dynamic equilibrium 

• However, human activity and natural changes like climate change can disturb the state of dynamic equilibrium.