Coasts EQ2 Flashcards
four wave erosion processes:
Hydraulic action
Corrosion
Abrasion
Attrition
How wave erosional processes influences by wave type, size and lithology
They are most effective during high energy storm events with large destructive waves.
However, even coastlines composed of soft, unconsolidated sediment (e.g. boulder clay of Holderness Coast in Yorkshire), experience little erosion under normal conditions.
Most erosion (in the UK) occurs in the winter, in high energy storms.
It’s faster when the wind is blowing directly onshore
It’s faster when the tide is high (bringing deeper water closer to the cliff so less energy is lost to friction before impact)
The effect of erosion
The boulder clay of the Holderness coast has retreated by 120 m in the last 100 years.
The granite of Land’s End in Cornwall has retreated by only 10 cm in the last 100 years.
Hydraulic Action
Where the force of water itself breaks up rock
It can occur through:
the direct impact of the water itself
Plunging destructive waves can exert a force of 50 kg / cm^3
This is sufficient to break off material from unconsolidated material, e.g. boulder clay, or weak rocks like clay and shale.
the force of the breaking wave compressing air into cracks in rocks
when the wave energy is exhausted, the compressed air explodes outwards, causing micro-fractures in rock and expanding the main crack.
over time, small fragments of rock become weakened and break away
or, the main crack can extend until larger slabs of rock fall
The pressure forces the crack open, meaning more air is trapped and greater force is experienced in the next cycle of compression.
It dislodges blocks of rock from the cliff face.
In hard, resistant igneous rocks, hydraulic action attacking its cooling joints may be the only effective wave erosion process.
High energy waves with a large wave height are the most effective at erosion through hydraulic action. It is also accentuated when there is no debris at the cliff foot to absorb some of the wave energy and protect the cliff base.
Abrasion (a.k.a corrasion)
Abrasion is where a wave picks up sediment and throws these load items against a rock. The repeated impact chips away at the rock face until small fragments break away.
Most effective: High-energy destructive waves with a large wave height hurl load items with greater force, resulting in faster rates of erosion by abrasion.
They need a supply of hard load items close to the foot, e.g. shingle from the beach.
Rocks eroded most quickly by abrasion: Soft sedimentary rock such as chalk, mudstones, and clays, and unconsolidated material, e.g. boulder clay.
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.
Most effective waves: Constructive waves, as the force of impact is not relevant, and the spilling wave increases the time for the chemical reaction to occur. They are slow, and with a long wavelength (longer the better) it prolongs the contact of rock with the water.
Rocks eroded most quickly by corrosion: carbonate rocks like limestones (e.g. chalk, Jurassic limestone and carboniferous limestone) and sedimentary rocks with calcite sediment/cement.
Attrition
Attrition is where material transported by a wave is eroded through collision with other load items. It breaks down sediment into smaller sized particles, and the repeated collision blunts any of the particles’ sharp edges, making the sediment increasingly rounded. Even harder rocks, such as quartz and granite form larger rounded shingle pebbles.
It occurs in the foreshore and nearshore zones, where sediment is moved by swash and backwash.
Rocks eroded most quickly by attrition: soft rocks (e.g. poorly cemented sandstones, chalk and clay) are broken down quickly by attrition into silt and sand grains.
How a wave breaks:
When waves reaching the shore reach a wave depth of 1/2 their wavelength, the internal orbital motion of water within the wave touches the sea bed.
Friction between the sea bed begins to distort the wave particle orbit from circular to elliptical, and slows down the wave.
The wave has entered the offshore zone
The wave depth decreases further, and the wave velocity slows, wavelength shortens, and wave height increases. Waves ‘bunch’ together.
The wave crest begins to move forwards much faster than the wave trough
Eventually the wave crest outruns the trough and the wave topples forwards - breaking.
The wave breaks in the nearshore zone, and water flows up the beach as swash
The wave then losses energy and gravity pulls the water back down the beach as backwash
Constructive Waves features
Low energy waves
Low, flat wave height (<1m)
Long wavelength (up to 100 m)
Low wave frequency (about 6-9 per minute)
This means their swash is unimpeded by previous backwash
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.
Destructive (plunging) Waves features
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
this directs most energy downwards and backwards, so the particle orbit is more circular than constructive breakers(?)
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
what is beach morphology
Beach morphology is the shape of the beach.
what is a beach sediment profile
A beach sediment profile is the pattern of distribution of different sized or shaped deposited material.
beach morphology of constructive waves
Constructive waves alter beach morphology by causing net movement of sediment up the beach, steeping the beach profile.
They produce berms at the point where the swash reaches the high tide line. (A berm is a ridge of material across the beach)
Swash carries sediment of all sizes up the beach, but weaker backwash can only transport smaller particles down the beach.
This leads to a sorting of material in the foreshore zone, with larger, heavier shingle (pebble-sized sediment) at the back of the beach, and sand drawn back closer to the sea.
Since the backwash flows down the beach and loses energy through friction and depletion of water through percolation, sediment is further sorted as coarser sands are deposited in the middle of the beach and only fine sands are carried to the area of beach closest to the sea.
beach morphology of constructive waves
Weak swash and powerful backwash produces a net transport of sediment down the beach, reducing beach gradient.
Some sediment thrown forwards in detached spray of high impact breaking wave. Accumulates above high tide mark as storm ridge.
Large, pebble-sized sediment dragged down beach by backwash to form wide ridge of material below low tide mark at start of offshore zone.
Friction may be sufficient to cause backwash to down some sediment on middle or lower beach, with deposited sediment size decreasing towards sea.
bars, spits and other localised features develop where…
1.Abundant material is available, particularly shingle and sand
2.The coastline is irregular due to for e g geological variety
3.Deposition is increased by the presence of vegetation
4.There are estuaries and major rivers
