2B.4 Marine Erosion Flashcards
2B.4a waves and beach morphology
wave types: constructive
Low energy waves
Low, flat wave height (<1m)
Long wavelength (up to 100 m)
Low wave frequency (about 6-9 per minute)
A strong swash that pushes sediment up the beach, but a weaker backwash is unable to transport all particles back down, so they are deposited it as a ridge of sediment (berm) at the top of the beach
A backwash that percolates into the beach material
encouraged by a long, shallow nearshore, so friction slows down the wave and releases energy
Constructive (spilling or surging) waves have a stronger swash than backwash due to a low angle of wave impact.
2B.4a waves and beach morphology
wave types: destructive
High energy waves
Large wave height (>1 m)
Short wavelength (about 20 m)
High wave frequency (13-15 per minute)
They’re encouraged by a short, steep nearshore zone, quickly dropping away into deeper water, so that there is little energy loss through friction
They have strong backwash and weak swash due to the steep angle of impact
Strong backwash erodes material from the top of the beach, carrying down the beach to the offshore zone
it’s often deposited as a offshore ridge or berm
2B.4a waves and beach morphology
waves
a wave is created through friction between the wind and water surface, transferring energy from the wind into the water
wave size depends on
- the strength of the wind
- the duration for which the wind blows
- water depth
- wave fetch
2B.4a waves and beach morphology
beach morphology
beach morphology is the shape of a beach, including its width and slope (the beach profile) and features such as berms, ridges and runnels, as well as different types of sediment (shingle, sand and mud) at different locations on the beach
2B.4a waves and beach morphology
variations at a variety of temporal scales
Decadal variation:
climate change expected to produce more extreme weather, reducing beach size
Seasonal variation:
destructive, high energy waves dominate in winter
Monthly variation:
tide varies over the course of a lunar month
Daily variation:
storm events can reshape beach profiles in a few hours
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2B.4a waves and beach morphology
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2B.4a waves and beach morphology
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2B.4b wave erosion
wave erosion processes: hydraulic action
Air is trapped in cracks and fissures is compressed by the force of waves crashing against the cliff face causing cavitation. Pressure forces cracks open and dislodges blocks of rock from the cliff face.
how are they influenced by wave type and size?
High energy waves with a large wave height are the most effective at erosion through hydraulic action
how are they influenced by lithology?
heavily jointed/fissured sedimentary rocks are the most vulnerable as well as metamorphic rocks
greatest wave energy when there is no debris at the cliff foot to absorb some of the wave energy and protect the cliff base
2B.4b wave erosion
wave erosion processes: abrasion
sediment is picked up by breaking waves and thrown against the cliff face
how are they influenced by wave type and size?
Constructive waves, as the force of impact is not relevant, and the spilling wave increases the time for the chemical reaction to occur
how are they influenced by lithology?
suitably loos sediment must be available
softer sedimentary rocks are the most vulnerable
2B.4b wave erosion
wave erosion processes: attrition
as sediment is moved around by waves, the collisions between sediment particles erode the rocks causing sediment to get smaller and more rounded over time
how are they influenced by wave type and size?
how are they influenced by lithology?
softer rocks are very rapidly reduced in size by attrition
2B.4b wave erosion
wave erosion processes: corrosion
Corrosion is where water in waves dissolves rock minerals. Minerals are immediately carried away by the wave in solution. They are also vulnerable to erosion by rainwater and sea spray.
how are they influenced by wave type and size?
how are they influenced by lithology?
2B.4b wave erosion
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2B.4c coastal landforms
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2B.4c coastal landforms
wave-cut notches
A wave cut notch is a curved indentation along the base of a cliff. It forms between the high tide and low tide marks, where destructive waves impact against the cliff.
The notch is formed by hydraulic action and abrasion, and in some cases corrosion.
The depth of the notch varies depending upon the resistance of the rock at different points.
2B.4c coastal landforms
wave-cut platforms
A wave cut platform is a flat rock surface exposed at low tide, extending out to sea from the base of a cliff.
A wave cut platform forms when a wave cut notch deepens by further erosion until the overlying material collapses by mass movement due to gravity. The process repeats and the cliff retreats (coastal recession).
Wave cut platforms then dissipate wave energy before reaching cliff base, acting in a negative feedback loop to reduce the rate of coastal recession.
