2.5 - Transportation And Deposition Flashcards
(21 cards)
Traction
Where large, heavy load items are rolled along the seabed
Suspension
Where very light sediment is carried within a body of water
Saltation
Where lighter sediment bounces along
Solution
Where sediment is carried dissolved within the water
How does the direction of wave attack affect transportation
The direction that the waves are coming from is the main factor affecting the direction of sediment transport
How does Longshore drift affect rates of transportation
- 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
How does the current affect rates of transportation
- 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
How do tides affect rates of erosion
- 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
When does deposition occur
- 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
Beaches
- Beaches are created when sediment is deposited near the coastline when constructive waves lose their energy.
- Larger sediment is found toward the top of the beach where it has been left from winter storms.
- They form in sheltered areas such as bays
- Beach formation is more common in summer when there are fewer destructive waves
Double spits
- Double spits are where two spits extend out in opposite directions from both sides of the bay, towards the middle.
- They form where longshore drift is operating in different directions on opposite sides of the bay.
- They can also form when rising sea levels drive ridges of material onshore from the offshore zone.
Recurved spit
- A spit whose end is curved landwards, into a bay or inlet.
- This is because wave refraction transports and deposits sediment for a short distance in the landward direction.
- it could be because the wind and wave front are frequently at the opposite angle to the prevailing wind, generating short periods of longshore drift towards the land or a strong incoming tidal current
Off shore bars
- A region offshore where sand is deposited, as the waves don’t have enough energy to carry the sediment to shore.
- They can be formed as the wave breaks early, instantly depositing its sediment as a loose-sediment offshore bar
- They may also be formed as a result of backwash from destructive waves removing sediment from a beach.
- Offshore bars may absorb wave energy, reducing erosion in some areas.
Barrier beach
- A barrier beach occurs when a beach or spit extends across a bay to join two headlands.
- This traps water behind it leading to the formation of a lagoon which is separated from the sea.
- some barrier beaches may have formed due to rising sea levels after the last glacial period, when meltwater from glaciers deposited sediment near coasts
- If a barrier beach becomes separated from the mainland, it becomes a barrier island
Tombolos
- A tombolo is a spit that connects the mainland to an offshore island.
- Tombolos are formed due to wave refraction off the coastal island reducing wave velocity, leading to deposition of sediments.
- They may be covered at high tide if they are low lying.
Cuspate foreland
- A cuspate foreland is a low lying triangular shaped headlands, extending out from a shoreline, formed from deposited sediment.
- When longshore drift currents from opposing directions converge at the boundary of two sediment cells.
- The sediment is deposited out into the sea by both currents creating a triangular shaped area of deposited material.
What are sediment cells
A sediment cell (or littoral cell) is a linked system of sources, transfers and sinks of sediment along a section of coastline.
For example: Flanborough head - source region, Holderness coast - transfer zone, Spurn head - sink region
Examples of inputs, transfers and sinks
Inputs
- places where sediment is generated, such as cliffs or eroding sand dunes
- Cliff erosion, Onshore currents, River transport, Wind blown (aeolian) sediment from land, Subaerial processes, Marine organisms
Transfers
- Places where sediment is moving alongshore through longshore drift and offshore currents
- Longshore drift, Swash, Backwash, Tidal currents, Sea/ocean currents, Wind (onshore, offshore or along shore)
Sinks
- Sinks are locations where the dominant process is deposition and depositional landforms are created, including spits and offshore bars.
- sand dunes, beaches, bars, barrier islands
Sediment cells being a dynamic equilibrium
- Sediment cells are dynamic because the sediment is constantly generated in the source region, transported through the transfer region and deposited in the sink region
- Dynamic equilibrium is reached when inputs of sediment from the source region are balanced by the amount being deposited in sinks.
- It’s dynamic because even though its balanced the sediment in it is always changing and moving through the system
What is positive feedback on the coast
- when the change produces an effect that operates to increase the original change
- Eg. When wind erosion of a dune section during high velocity storms may removing stabilising vegetation, further wind erosion now occurs in later low velocity wind conditions, increasing the depletion of dune sand.
What is negative feedback on the coast
- when the change produced creates effects that operate to reduce or work against the original change.
- E.g. when erosion leads to rock fall mass movement. The collapsed debris acts as a barrier protecting the cliff base, slowing or preventing erosion for a period of time.
